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9.4: Early Evolution of Plants - Biology

9.4: Early Evolution of Plants - Biology


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Which moved onto land first, plants or animals?

This fossilized fern may be millions of years old. Over 200 million years ago, the first evidence of ferns related to several modern families appeared. The "great fern radiation" occurred in the late-Cretaceous, which ended 65 million years ago, when many modern families of ferns first appeared. And if animals were the first on land, would many have starved?

Evolution of Plants

As shown in Figure below, plants are thought to have evolved from an aquatic green alga protist. Later, they evolved important adaptations for land, including vascular tissues, seeds, and flowers. Each of these major adaptations made plants better suited for life on dry land and much more successful.

From a simple, green alga ancestor that lived in the water, plants eventually evolved several major adaptations for life on land.

The Earliest Plants

The earliest plants were probably similar to the stonewort, an aquatic algae pictured inFigure below. Unlike most modern plants, stoneworts have stalks rather than stiff stems, and they have hair-like structures called rhizoids instead of roots. On the other hand, stoneworts have distinct male and female reproductive structures, which is a plant characteristic. For fertilization to occur, sperm need at least a thin film of moisture to swim to eggs. In all these ways, the first plants may have resembled stoneworts.

Modern stoneworts may be similar to the earliest plants. Shown is a field of modern stoneworts (right), and an example from the Charophyta, a division of green algae that includes the closest relatives of the earliest plants (left).

Life on Land

By the time the earliest plants evolved, animals were already the dominant organisms in the ocean. Plants were also constrained to the upper layer of water that received enough sunlight for photosynthesis. Therefore, plants never became dominant marine organisms. But when plants moved onto land, everything was wide open. Why was the land devoid of other life? Without plants growing on land, there was nothing for other organisms to feed on. Land could not be colonized by other organisms until land plants became established.

Plants may have colonized the land as early as 700 million years ago. The oldest fossils of land plants date back about 470 million years. The first land plants probably resembled modern plants called liverworts, like the one shown in Figure below.

The first land plants may have been similar to liverworts like this one.

Colonization of the land was a huge step in plant evolution. Until then, virtually all life had evolved in the ocean. Dry land was a very different kind of place. The biggest problem was the dryness. Simply absorbing enough water to stay alive was a huge challenge. It kept early plants small and low to the ground. Water was also needed for sexual reproduction, so sperm could swim to eggs. In addition, temperatures on land were extreme and always changing. Sunlight was also strong and dangerous. It put land organisms at high risk of mutations.

Vascular Plants Evolve

Plants evolved a number of adaptations that helped them cope with these problems on dry land. One of the earliest and most important was the evolution of vascular tissues. Vascular tissues form a plant’s “plumbing system.” They carry water and minerals from soil to leaves for photosynthesis. They also carry food (sugar dissolved in water) from photosynthetic cellsto other cells in the plant for growth or storage. The evolution of vascular tissues revolutionized the plant kingdom. The tissues allowed plants to grow large and endure periods of drought in harsh land environments. Early vascular plants probably resembled the fern shown in Figure below.

Early vascular plants may have looked like this modern fern.

In addition to vascular tissues, these early plants evolved other adaptations to life on land, including lignin, leaves, roots, and a change in their life cycle.

  • Lignin is a tough carbohydrate molecule that is hydrophobic (“water fearing”). It adds support to vascular tissues in stems. It also waterproofs the tissues so they don’t leak, which makes them more efficient at transporting fluids. Because most other organisms cannot break down lignin, it helps protect plants from herbivores and parasites.
  • Leaves are rich in chloroplasts that function as solar collectors and food factories. The first leaves were very small, but leaves became larger over time.
  • Roots are vascular organs that can penetrate soil and even rock. They absorb water andminerals from soil and carry them to leaves. They also anchor a plant in the soil. Roots evolved from rhizoids, which nonvascular plants had used for absorption.
  • Land plants evolved a dominant diploid sporophyte generation. This was adaptive because diploid individuals are less likely to suffer harmful effects of mutations. They have two copies of each gene, so if a mutation occurs in one gene, they have a backup copy. This is extremely important on land, where there’s a lot of solar radiation.

With all these advantages, it’s easy to see why vascular plants spread quickly and widely on land. Many nonvascular plants went extinct as vascular plants became more numerous. Vascular plants are now the dominant land plants on Earth.

Summary

  • The earliest plants are thought to have evolved in the ocean from a green alga ancestor.
  • Plants were among the earliest organisms to leave the water and colonize land.
  • The evolution of vascular tissues allowed plants to grow larger and thrive on land.

Review

  1. What were the first plants to evolve?
  2. What are vascular tissues of a plant? What is their function?
  3. Explain why life on land was difficult for early plants.
  4. Why did plants need to become established on land before animals could colonize the land?

A genome for gnetophytes and early evolution of seed plants

Gnetophytes are an enigmatic gymnosperm lineage comprising three genera, Gnetum, Welwitschia and Ephedra, which are morphologically distinct from all other seed plants. Their distinctiveness has triggered much debate as to their origin, evolution and phylogenetic placement among seed plants. To increase our understanding of the evolution of gnetophytes, and their relation to other seed plants, we report here a high-quality draft genome sequence for Gnetum montanum, the first for any gnetophyte. By using a novel genome assembly strategy to deal with high levels of heterozygosity, we assembled >4 Gb of sequence encoding 27,491 protein-coding genes. Comparative analysis of the G. montanum genome with other gymnosperm genomes unveiled some remarkable and distinctive genomic features, such as a diverse assemblage of retrotransposons with evidence for elevated frequencies of elimination rather than accumulation, considerable differences in intron architecture, including both length distribution and proportions of (retro) transposon elements, and distinctive patterns of proliferation of functional protein domains. Furthermore, a few gene families showed Gnetum-specific copy number expansions (for example, cellulose synthase) or contractions (for example, Late Embryogenesis Abundant protein), which could be connected with Gnetum’s distinctive morphological innovations associated with their adaptation to warm, mesic environments. Overall, the G. montanum genome enables a better resolution of ancestral genomic features within seed plants, and the identification of genomic characters that distinguish Gnetum from other gymnosperms.

The seed plants today are represented by five distinct lineages: the species-rich angiosperms (flowering plants, approximately 352,000 species) and four gymnosperm lineages (which together comprise approximately 1,000 species and encompass cycads, Ginkgo biloba, conifers and gnetophytes). It is apparent from their long fossil record (dating back to the Late Devonian approximately 360 million years ago (Ma)) that considerably greater seed plant diversity existed in the past 1 . Nevertheless, widespread extinctions among many gymnosperm lineages mean that today’s gymnosperms are only a relic of their former diversity, and this has presented a major challenge for reconstructing evolutionary relationships between the extant lineages 2 . Probably the most controversial outstanding question in plant evolution is the phylogenetic position of gnetophytes 3 (comprising the genera Gnetum, Welwitschia and Ephedra, Fig. 1) in relation to the other seed plant lineages. Apparent morphological similarities with angiosperms, such as vessel-like water-conducting cells, double fertilization and leaf morphologies with reticulate venation, have historically led to the proposition that gnetophytes form a group that is sister to angiosperms (termed the ‘Anthophyte hypothesis’) 4,5 . That hypothesis has, however, largely been rejected by molecular phylogenetic data and a deeper understanding of the developmental pathways that lead to similar morphological features. Nevertheless, the use of molecular data has also been problematic in inferring the exact phylogenetic position of gnetophytes, with topologies differing depending on the type of sequence data (for example, plastid versus nuclear genes, nucleotide versus amino acid data) and analytical approach used (for example, maximum parsimony, maximum likelihood, Bayesian, multispecies coalescent based methods) 6,7,8 . Consequently, several possible hypotheses have been put forward that place gnetophytes as sister to (1) Pinaceae (‘Gnepine’ hypothesis) (2) cupressophytes (‘Gnecup’ hypothesis) (3) all conifers (‘Gnetifer’ hypothesis) (4) all other gymnosperms or (5) all seed plants 9 . Currently, the emerging consensus, based on both older and more recent studies, and recently released data from the 1KP initiative (see https://sites.google.com/a/ualberta.ca/onekp/, and Wickett et al. 8 ), indicates that gnetophytes are sister to, or within, the conifers.

Top, left to right, female cones of G. montanum, male cones of W. mirabilis and female cones of E. equisetina. Scale bars, 5 cm. Bottom, pantropical distribution of the three gnetophyte genera, compared with three conifer species that are most abundant at higher latitudes and altitudes. The range of genomes sizes (1C-values) found in the three genera comprising gnetophytes and the three conifer species are also shown (data taken from http://data.kew.org/cvalues/ and unpublished data).

So far, the availability of whole genome sequences for gymnosperms has been limited to conifers (specifically to Pinaceae) 10,11,12,13 and G. biloba 14 , with no whole genome assemblies available for the two remaining major seed plant lineages—cycads and gnetophytes. This deficiency, together with the conflicting phylogenetic evidence for relationships among these groups, is impeding our understanding of genome evolution across all seed plants. Here, we present a high-quality draft genome of Gnetum montanum, the first for gnetophytes. The availability of this genome, as well as survey sequence data and transcriptome data from other vascular plants (including novel data from gnetophytes Ephedra and Welwitschia), enables us to compare genomic characters with G. biloba, conifers, angiosperms and non-seed plants. Comparisons within gymnosperms, and between gymnosperms and angiosperms, highlight the unique nature of the Gnetum genome, providing new insights into patterns of genome divergence across seed plants.


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EARLY EVOLUTION OF LAND PLANTS: Phylogeny, Physiology, and Ecology of the Primary Terrestrial Radiation

AbstractThe Siluro-Devonian primary radiation of land biotas is the terrestrial equivalent of the much-debated Cambrian “explosion” of marine faunas. Both show the hallmarks of novelty radiations (phenotypic diversity increases much more rapidly than species diversity across an ecologically undersaturated and thus low-competition landscape), and both ended with the formation of evolutionary and ecological frameworks analogous to those of modern ecosystems. Profound improvements in understanding early land plant evolution reflect recent liberations from several research constraints: Cladistic techniques plus DNA sequence data from extant relatives have prompted revolutionary reinterpretations of land plant phylogeny, and thus of systematics and character-state acquisition patterns. Biomechanical and physiological experimental techniques developed for extant plants have been extrapolated to fossil species, with interpretations both aided and complicated by the recent knowledge that global landmass positions, currents, climates, and atmospheric compositions have been profoundly variable (and thus nonuniformitarian) through the Phanerozoic. Combining phylogenetic and paleoecological data offers potential insights into the identity and function of key innovations, though current evidence suggests the importance of accumulating within lineages a critical mass of phenotypic character. Challenges to further progress include the lack of sequence data and paucity of phenotypic features among the early land plant clades, and a fossil record still inadequate to date accurately certain crucial evolutionary and ecological events.


Exposure of plants to MF intensities higher than the geomagnetic filed

A consistent number of papers described the effect of MF intensities higher than the GMF levels. In general, intensities higher than GMF relate to values higher than 100 μT. As summarized in Table ​ Table1, 1 , experimental values can reach very high MF levels, ranging from 500 μT up to 15 T. Most of the attention has been focused on seed germination of important crops like wheat, rice and legumes. However, many other physiological effects on plants of high MF described plant responses in terms of growth, development, photosynthesis, and redox status.

Table 1

Summary of magnetic field (MF) effects on plants.

Plant speciesPlant organEffectMF intensityReferences
EXPOSURE TO MF VALUES LOWER THAN THOSE OF THE GMF
Actinidia deliciosaPollenRelease of internal Ca 2+ 10 μTBetti et al., 2011
Allium cepaRoot and shootDecrease in the cell number with enhanced DNA content<GMFNanushyan and Murashov, 2001 Belyavskaya, 2004 and references cited therein
Arabidopsis thaliana Delayed flowering Reproductive growthNear nullXu et al., 2012, 2013
Glycine maxProtoplasts SeedsIncreased protoplasts fusion<GMFNedukha et al., 2007
Seed germination1500 nTRadhakrishnan and Kumari, 2013
Helianthus annuusSeedlingsIncreases in fresh weight20 μTFischer et al., 2004
Hordeum volgareSeedlingsDecrease in fresh weight10 nTLebedev et al., 1977
Lepidium sativumRootsNegative gravitropism<GMFKordyum et al., 2005
Nicotiana tabacumProtoplastsIncreased protoplasts fusion<GMFNedukha et al., 2007
Pisum sativumEpicotylPromotion of cell elongation ultrastructural peculiarities increase in the [Ca 2+ ]cyt level<GMFNegishi et al., 1999 Belyavskaya, 2001 Yamashita et al., 2004
Solanum spp.In vitro culturesStimulation/inhibition of growth<GMFRakosy-Tican et al., 2005
Triticum aestivumSeeds and seedlingsActivation of esterases reduction of growthfrom 20 nT to 0.1 mTBogatina et al., 1978 Aksenov et al., 2000
Vicia fabaRoot tipsAlter membrane transport processes10 and 100 μTStange et al., 2002
EXPOSURE TO MF VALUES HIGHER THAN THOSE OF THE GMF
Abelmoschus esculentusSeedPromotion of germination99 mTNaz et al., 2012
Allium ascalonicumSeedlingsIncreased lipid peroxidation and H2O2 levels7 mTCakmak et al., 2012
Arabidopsis thalianaSeedlingsEnhanced blue light-dependent phosphorylations of CRY1 and CRY2 hypocotyl growth500 μTHarris et al., 2009 Xu et al., 2014
Callus culture15 TWeise et al., 2000
Amyloplast displacementManzano et al., 2013
Diamagnetic levitationHerranz et al., 2013
Proteomic alterationsPaul et al., 2006
Induced expression of the Adh/GUS transgene in the roots and leaves
Beta vulgarisSeedlingsIncreased root and leaf yield5 mTRochalska, 2008 Rochalska, 2005
Increased chlorophyll contentRochalska, 2005
Carica papayaPollenIncreased pollen germination>GMFAlexander and Ganeshan, 1990
Catharanthus roseusProtoplastEffect on cell wall302 mTHaneda et al., 2006
Cicer arietinumSeedPromotion of germination0� mTVashisth and Nagarajan, 2008
RootIncrease in root length, surface area and volume
Coffea arabicaSeedlingsDecrease of SOD, CAT, and APX activities2 mTAleman et al., 2014
Cryptotaenia japonicaSeedPromotion of germination500, 750 μTKobayashi et al., 2004
Cucumis sativusSeedlingsIncrease in superoxide radicals and H2O2100� mTBhardwaj et al., 2012
Desmodium gyransLeafReduced rhythmic leaflet movements50 mTSharma et al., 2000
Dioscorea oppositaSeedlingIncreased root length and number2× GMFLi, 2000
Fragaria vescaPlantletsIncreased fruit yield per plant0.096, 0.192 and 0.384 TEsitken and Turan, 2004
Glycine maxSeedlingsReduction of O2-radical level150,200 mTBaby et al., 2011 Radhakrishnan and Kumari, 2012, 2013 Shine et al., 2012
Reactive oxygen species production
Increased RubiscoShine et al., 2011
Helianthus annuusSeedlingsIncreased seedling dry weight, root length, root surface area and root volume50, 200 mTVashisth and Nagarajan, 2010
Increased activities of α-amylase, dehydrogenase and protease
Helianthus annuusSeedlingsIncreased chlorophyll concentration>GMFTurker et al., 2007
Helix aspesaSeedlingsOxidative burst50-HzRegoli et al., 2005
Hordeum vulgareSeedlingsIncreases in length and weight125 mTMartinez et al., 2000
Leymus chinensisSeedlingsIncreased peroxidase activity200,300 mTXia and Guo, 2000
Oryza sativaSeedReduction of germination125,250 mTFlorez et al., 2004
Paulownia fortuneiTissue culturesIncreased regeneration capability2.9𠄴.8 mTYaycili and Alikamanoglu, 2005
Paulownia tomentosaTissue culturesIncreased regeneration capability2.9𠄴.8 mTYaycili and Alikamanoglu, 2005
Petroselinum crispumCellsEffects on CAT and APX activity30 mTRajabbeigi et al., 2013
Phaseolus vulgarisSeedsPromotion of germination2 or 7 mTSakhnini, 2007 Cakmak et al., 2010
Increased chlorophyll emission fluorescence3 100,160 mTJovanic and Sarvan, 2004
Pisum sativumSeedPromotion of germination60,120,180 mTIqbal et al., 2012
SeedlingsIncreased length and weight125, 250 mTCarbonell et al., 2011
Induction of SOD activityPolovinkina et al., 2011
Raphanus sativusSeedlingsSuppression of SOD and CAT activities185� μTSerdyukov and Novitskii, 2013
Reduced CO2 uptake500 μTYano et al., 2004
Stimulation of lipid synthesisNovitskaya et al., 2010 Novitskii et al., 2014
Solanum lycopersicumSeedPromotion of germination160� mTDe Souza et al., 2010 Poinapen et al., 2013a
Solanum lycopersicumSeedPromotion of germination160� mTDe Souza et al., 2010 Poinapen et al., 2013a
ShootsEffect on gravitropismo
Magnetophoretic curvatureHasenstein and Kuznetsov, 1999
Increased mean fruit weight, yield per plant and per areaDe Souza et al., 2006
Geminivirus and early blight and a reduced infection rate
Solanum tuberosumSeedlingsAmyloplast displacement4 mTHasenstein et al., 2013
PlantletsGrowth promotion and enhancement of CO2 uptake enhanced lipid orderIimoto et al., 1998
Poinapen et al., 2013b
Taxus chinensisSuspension culturePromotion of taxol production3.5 mTShang et al., 2004
Tradescantia spp.InflorescencePink mutations in stamen hair cells0.16, 0.76, 0.78 TBaum and Nauman, 1984
Triticum aestivumSeedPromotion of germination4 or 7 mT 30-mTCakmak et al., 2010
SeedlingsAmyloplast displacement increased catalase but reduced peroxidase activity30-mTHasenstein et al., 2013
Payez et al., 2013
Vicia fabaPlantletsAccumulation of ROS15 mTJouni et al., 2012
Modification of catalase and MAPK accumulation of H2O230 mTHaghighat et al., 2014
Vigna radiataSeedPromotion of germination87 to 226 mTMahajan and Pandey, 2014
SeedlingsDecrease of malondialdehyde, H2O2 and O − 2, and increase of NO and NOS activity600 mTChen et al., 2011
Zea maysSeedPromotion of germination Bilalis et al., 2012
SeedlingsIncrease of fresh weight125,250 mTFlorez et al., 2007
Amyloplast displacementHasenstein et al., 2013
Decreased levels of hydrogen peroxide and antioxidant defense system enzymes100,200 mTAnand et al., 2012
Shine and Guruprasad, 2012
Reduction of antioxidant enzymesTurker et al., 2007 Javed et al., 2011 Anand et al., 2012
Increased stomatal conductance and chlorophyll content100,200 mT
100,200 mT

Effects on germination

A MF applied to dormant seeds was found to increase the rate of subsequent seedling growth of barley, corn (Zea mays), beans, wheat, certain tree fruits, and other tree species. Moreover, a low frequency MF (16 Hz) can be used as a method of post-harvest seed improvement for different plant species, especially for seeds of temperature sensitive species germinating at low temperatures (Rochalska and Orzeszko-Rywka, 2005).

Seeds of hornwort (Cryptotaenia japonica) exposed to sinusoidally time-varying extremely low frequency (ELF) MFs (AC fields) in combination with the local GMF showed a promoted activity of cells and enzymes in germination stage of the seed. This suggests that an optimum ELF MF might exist for the germination of hornwort seeds under the local GMF (Kobayashi et al., 2004). The application of AC field also promoted the germination of bean (Phaseolus vulgaris) seeds (Sakhnini, 2007).

In seeds of mung bean (Vigna radiata), exposed in batches to static MFs of 87 to 226 mT intensity for 100 min, a linear increase in germination magnetic constant with increasing intensity of MF was found. Calculated values of mean germination time, mean germination rate, germination rate coefficient, germination magnetic constant, transition time, water uptake, indicate that the impact of applied static MF improves the germination of mung beans seeds even in off-season (Mahajan and Pandey, 2014).

The seeds of pea exposed to full-wave rectified sumusoidal non-uniform MF of strength 60, 120, and 180 mT for 5, 10, and 15 min prior to sowing showed significant increase in germination. The emergence index, final emergence index and vigor index increased by 86, 13, and 205%, respectively. Furthermore, it was found that exposure of 5 min for MF strengths of 60 and 180 mT significantly enhanced the germination parameters of the pea and these treatments could be used practically to accelerate the germination in pea (Iqbal et al., 2012).

MF application with a strength from 0 to 250 mT in steps of 50 mT for 1𠄴 h significantly enhanced speed of germination, seedling length and seedling dry weight compared to unexposed control in chickpea (Cicer arietinum). It was also found that magnetically treated chickpea seeds may perform better under rainfed (un-irrigated) conditions where there was a restrictive soil moisture regime (Vashisth and Nagarajan, 2008).

Different intensities of static MF (4 or 7 mT) were tested on seed germination and seedling growth of bean or wheat seeds in different media having 0, 2, 6, and 10 atmosphere (atm) osmotic pressure prepared with sucrose or salt. The application of both MFs promoted the germination ratios, regardless of increasing osmotic pressure of sucrose or salt. The greatest germination and growth rates in both plants were from the test groups exposed to 7 mT (Cakmak et al., 2010).

Seeds of wheat were imbibed in water overnight and then treated with or without a 30 mT static magnetic field (SMF) and a 10 kHz EMF for 4 days, each 5 h. Exposure to both MF increased the speed of germination, compared to the control group, suggesting promotional effects of EMFs on membrane integrity and growth characteristics of wheat seedlings (Payez et al., 2013).

Pre-sowing treatment of corn seeds with pulsed EMFs for 0, 15, 30, and 45 min improved germination percentage, vigor, chlorophyll content, leaf area, plant fresh and dry weight, and finally yields. Seeds that have been exposed to MF for 30 and 45 min have been found to perform the best results with economic impact on producer's income in a context of a modern, organic, and sustainable agriculture (Bilalis et al., 2012).

Various combinations of MF strength and exposure time significantly improved tomato (Solanum lycopersicum) cv. Lignon seed performance in terms of reduction of time required for the first seeds to complete germination, time to reach 50% germination, time between 10 and 90% germination with increasing germination rate, and increased germination percentage at 4 and 7 days, seedling shoot and root length compared to the untreated control seeds. The combinations of 160 mT for 1 min and 200 mT for 1 min gave the best results (De Souza et al., 2010). Higher germination (about 11%) was observed in magnetically-exposed tomato var. MST/32 seed than in non-exposed ones, suggesting a significant effect of non-uniform MFs on seed performance with respect to RH (Poinapen et al., 2013a).

The effect of pre-sowing magnetic treatments was investigated on germination, growth, and yield of okra (Abelmoschus esculentus cv. Sapz paid) with an average MF exposure of 99 mT for 3 and 11 min. A significant increase (P < 0.05) was observed in germination percentage, number of flowers per plant, leaf area, plant height at maturity, number of fruits per plant, pod mass per plant, and number of seeds per plant. The 99 mT for 11 min exposure showed better results as compared to control (Naz et al., 2012).

However, contrasting results have also been reported. For instance, the mean germination time of rice (Oryza sativa) seeds exposed to one of two MF strengths (125 or 250 mT) for different times (1 min, 10 min, 20 min, 1 h, 24 h, or chronic exposure) was significantly reduced compared to controls, indicating that this type of magnetic treatment clearly affects germination and the first stages of growth of rice plants (Florez et al., 2004).

Effects on cryptochrome

The blue light receptor cryptochrome can form radical pairs after exposure to blue light and has been suggested to be a potential magnetoreceptor based on the proposition that radical pairs are involved in magnetoreception. Nevertheless, the effects of MF on the function of cryptochrome are poorly understood. When Arabidopsis seedlings were grown in a 500 μT MF and a near-null MF it was found that the 500 μT MF enhanced the blue light-dependent phosphorylations of CRY1 and CRY2, whereas the near-null MF weakened the blue light-dependent phosphorylation of CRY2 but not CRY1. Dephosphorylations of CRY1 and CRY2 in the darkness were slowed down in the 500 μT MF, whereas dephosphorylations of CRY1 and CRY2 were accelerated in the near-null MF. These results suggest that MF with strength higher or weaker than the local GMF affects the activated states of cryptochromes, which thus modifies the functions of cryptochromes (Xu et al., 2014). Moreover, the magnitude of the hyperfine coupling constants (A (iso) max = 17.5 G) suggests that artificial MFs (1𠄵 G) involved in experiments with Arabidopsis can affect the signal transduction rate. On the other hand, hyperfine interactions in the FADH ▪ -Trp ▪+ biradicals are much stronger than the Zeeman interaction with the MF of the Earth (𢒀.5 G). Therefore, an alternative mechanism for the bird avian compass has been proposed very recently. This mechanism involves radicals with weaker hyperfine interactions (O ▪− 2 and FADH ▪ ), and thus, it could be more plausible for explaining incredible sensitivity of some living species to even tiny changes in the MF (Izmaylov et al., 2009).

However, contrasting results were obtained when the intensity of the ambient MF was varied from 33� to 500 μT. According to Ahmad et al. (2007) there was an enhanced growth inhibition in Arabidopsis under blue light, when cryptochromes are the mediating photoreceptor, but not under red light when the mediating receptors are phytochromes, or in total darkness. Hypocotyl growth of Arabidopsis mutants lacking cryptochromes was unaffected by the increase in magnetic intensity. Additional cryptochrome-dependent responses, such as blue-light-dependent anthocyanin accumulation and blue-light-dependent degradation of CRY2 protein, were also enhanced at the higher magnetic intensity. On the contrary, Harris et al. (2009) by using the experimental conditions chosen to match those of the Ahmad study, found that in no case consistent, statistically significant MF responses were detected. For a more comprehensive discussion on cryptochromes see below.

Effects on roots and shoots

Increased growth rates have been observed in different species when seeds where treated with increased MF. Treated corn plants grew higher and heavier than control, corresponding with increase of the total fresh weight. The greatest increases were obtained for plants continuously exposed to 125 or 250 mT (Florez et al., 2007). A stimulating effect on the first stages of growth of barley seeds was found for all exposure times studied. When germinating barley seeds were subjected to a MF of 125 mT for different times (1, 10, 20, and 60 min, 24 h, and chronic exposure), increases in length and weight were observed (Martinez et al., 2000). Pants of pea exposed to 125 or 250 mT stationary MF generated by magnets under laboratory conditions for 1, 10, and 20 min, 1 and 24 h and continuous exposure were longer and heavier than the corresponding controls at each time of evaluation. The major increases occurred when seeds were continuously exposed to the MF (Carbonell et al., 2011).

By treating with twice gradient MF Dioscorea opposita it was found that they could grow best in the seedling stage. Compared with the control, the rate of emergence increased by 39%, root number increased by 8%, and the average root length increased by 2.62 cm (Li, 2000). The 16 Hz frequency and 5 mT MF as well as alternating MF influence increased sugar beet (Beta vulgaris var saccharifera) root and leaf yield (Rochalska, 2008) while a dramatic increase in root length, root surface area and root volume was observed in chickpea exposed in batches to static MF of strength from 0 to 250 mT in steps of 50 mT for 1𠄴 h (Vashisth and Nagarajan, 2008). In the same conditions, seedlings of sunflower showed higher seedling dry weight, root length, root surface area and root volume. Moreover, in germinating seeds, enzyme activities of α-amylase, dehydrogenase and protease were significantly higher in treated seeds than controls (Vashisth and Nagarajan, 2010).

Effects on gravitropic responses

The growth response that is required to maintain the spatial orientation is called gravitropism and consists of three phases: reception of a gravitational signal, its transduction to a biochemical signal that is transported to the responsive cells and finally the growth response, or bending of root, or shoot. Primary roots exhibit positive gravitropism, i.e., they grow in the direction of a gravitational vector. Shoots respond negatively to gravity and grow upright opposite to the gravitational vector. However, lateral roots and shoots branches are characterized by intermediate set-point angles and grow at a particular angle that can change over time (Firn and Digby, 1997). Gravitropism typically is generated by dense particles that respond to gravity. Experimental stimulation by high-gradient MF provide a new approach to selectively manipulate the gravisensing system.

High-gradient MF has been used to induce intracellular magnetophoresis of amyloplasts and the obtained data indicate that a magnetic force can be used to study the gravisensing and response system of roots (Kuznetsov and Hasenstein, 1996). The data reported strongly support the amyloplast-based gravity-sensing system in higher plants and the usefulness of high MF to substitute gravity in shoots (Kuznetsov and Hasenstein, 1997 Kuznetsov et al., 1999). For example, in shoots of the lazy-2 mutant of tomato that exhibit negative gravitropism in the dark, but respond positively gravitropically in (red) light, induced magnetophoretic curvature showed that lazy-2 mutants perceive the displacement of amyloplasts in a similar manner than wt and that the high MF does not affect the graviresponse mechanism (Hasenstein and Kuznetsov, 1999). Arabidopsis stems positioned in a high MF on a rotating clinostat demonstrate that the lack of apical curvature after basal amyloplast displacement indicates that gravity perception in the base is not transmitted to the apex (Weise et al., 2000). The movement of corn, wheat, and potato (Solanum tuberosum) starch grains in suspension was examined with videomicroscopy during parabolic flights that generated 20� s of weightlessness. During weightlessness, a magnetic gradient was generated by inserting a wedge into a uniform, external MF that caused repulsion of starch grains. Magnetic gradients were able to move diamagnetic compounds under weightless or microgravity conditions and serve as directional stimulus during seed germination in low-gravity environments (Hasenstein et al., 2013). The response of transgenic seedlings of Arabidopsis, containing either the CycB1-GUS proliferation marker or the DR5-GUS auxin-mediated growth marker, to diamagnetic levitation in the bore of a superconducting solenoid magnet was evaluated. Diamagnetic levitation led to changes that are very similar to those caused by real- [e.g., on board the International Space Station (ISS)] or mechanically-simulated microgravity [e.g., using a Random Positioning Machine (RPM)]. These changes decoupled meristematic cell proliferation from ribosome biogenesis, and altered auxin polar transport (Manzano et al., 2013). Arabidopsis in vitro callus cultures were also exposed to environments with different levels of effective gravity and MF strengths simultaneously. The MF itself produced a low number of proteomic alterations, but the combination of gravitational alteration and MF exposure produced synergistic effects on the proteome of plants (Herranz et al., 2013). However, MF leads to redistribution of the cellular activities and this is why application of the proteomic analysis to the whole organs/plants is not so informative.

Effects on redox status

Effects of MFs have been related to uncoupling of free radical processes in membranes and enhanced ROS generation. It has been experimentally proven that MF can change activities of some scavenging enzymes such as catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR), glutathione transferase (GT), peroxidase (POD), ascobtate peroxidase (APX), and polyphenoloxidase (POP). Experiments have been performed on several plant species, including pea, land snail (Helix aspesa), radish (Raphanus sativus), Leymus chinensis, soybean, cucumber (Cucumis stivus), broad bean, corn, parsley (Petroselinum crispum), and wheat (Xia and Guo, 2000 Regoli et al., 2005 Baby et al., 2011 Polovinkina et al., 2011 Anand et al., 2012 Bhardwaj et al., 2012 Jouni et al., 2012 Radhakrishnan and Kumari, 2012, 2013 Shine and Guruprasad, 2012 Shine et al., 2012 Payez et al., 2013 Rajabbeigi et al., 2013 Serdyukov and Novitskii, 2013 Aleman et al., 2014 Haghighat et al., 2014). The results suggest that exposure to increased MF causes accumulation of reactive oxygen species and alteration of enzyme activities. The effects of continuous, low-intensity static MF (7 mT) and EF (20 kV/m) on antioxidant status of shallot (Allium ascalonicum) leaves, increased lipid peroxidation and H2O2 levels in EF applied leaves. These results suggested that apoplastic constituents may work as potentially important redox regulators sensing and signaling MF changes. Static continuous MF and EF at low intensities have distinct impacts on the antioxidant system in plant leaves, and weak MF is involved in antioxidant-mediated reactions in the apoplast, resulting in overcoming a possible redox imbalance (Cakmak et al., 2012). In mung bean seedlings treated with 600 mT MF followed by cadmium stress the concentration of malondialdehyde, H2O2 and O − 2, and the conductivity of electrolyte leakage decreased, while the NO concentration and NOS activity increased compared to cadmium stress alone, showing that MF compensates for the toxicological effects of cadmium exposure are related to NO signal (Chen et al., 2011).

Effects on photosynthesis

Photosynthesis, stomatal conductance and chlorophyll content increased in corn plants exposed to static MFs of 100 and 200 mT, compared to control under irrigated and mild stress condition (Anand et al., 2012). Pre-seed electromagnetic treatments has been used to minimize the drought-induced adverse effects on different crop plants. Pretreatment of seeds of two corn cultivars with different magnetic treatments significantly alleviated the drought-induced adverse effects on growth by improving chlorophyll a and photochemical quenching and non-photochemical quenching. Of all magnetic treatments, 100 and 150 mT for 10 min were most effective in alleviating the drought-induced adverse effects (Javed et al., 2011). Polyphasic chlorophyll a fluorescence transients from magnetically treated soybean plants gave a higher fluorescence yield. The total soluble proteins of leaves showed increased intensities of the bands corresponding to a larger subunit (53 KDa) and smaller subunit (14 KDa) of Rubisco in the treated plants. Therefore, pre-sowing magnetic treatment was found to improve biomass accumulation in soybean (Shine et al., 2011). Other general effects on MF application on chlorophyll content have been documented for several plant species (Voznyak et al., 1980 Rochalska, 2005 Turker et al., 2007 Radhakrishnan and Kumari, 2013).

The CO2 uptake rate of MF exposed radish seedlings was lower than that of the control seedlings. The dry weight and the cotyledon area of MF exposed seedlings were also significantly lower than those of the control seedlings (Yano et al., 2004). A MF of around 4 mT had beneficial effects, regardless of the direction of MF, on the growth promotion and enhancement of CO2 uptake of potato plantlets in vitro. However, the direction of MF at the MF tested had no effects on the growth and CO2 exchange rate (Iimoto et al., 1998).

A permanent MF induces significant changes in bean leaf fluorescence spectra and temperature. The fluorescence intensity ratio (FIR) and change of leaf temperature ΔT increase with the increase of MF intensity. The increase of ΔT due to MFs is explained in bean with a simple ion velocity model. Reasonable agreement between calculated ΔT, based on the model, and measured ΔT was obtained (Jovanic and Sarvan, 2004).

Effects on lipid composition

In radish seedlings grown in lowlight and darkness in an ELF MF characterized by 50 Hz frequency and approximate to 500 μT flux density, MF exposure increased the production of polar lipids by threefold specifically, glycolipids content increased fourfold and phospholipids content rose 2.5 times, compared to seeds. MF stimulated lipid synthesis in chloroplast, mitochondrial, and other cell membranes (Novitskii et al., 2014). Furthermore, among fatty acids, MF exerted the strongest effect on the content of erucic acid: it increased in the light and in darkness approximately by 25% and decreased in the light by 13%. Therefore, MF behaved as a correction factor affecting lipid metabolism on the background of light and temperature action (Novitskaya et al., 2010).

Plasma membranes of seeds of tomato plants were purified, extracted, and applied to a silicon substrate in a buffer suspension and their molecular structure was studied using X-ray diffraction. While MFs had no observable effect on protein structure, enhanced lipid order was observed, leading to an increase in the gel components and a decrease in the fluid component of the lipids (Poinapen et al., 2013b).

Other effects

Inflorescences from Tradescantia clones subjected to high MF showed pink mutations in stamen hair cells (Baum and Nauman, 1984). Pollen grains of papaya (Carica papaya) exposed to MF germinated faster and produced longer pollen tubes than the controls (Alexander and Ganeshan, 1990). In kiwifruit (Actinidia deliciosa) MF treatment partially removed the inhibitory effect caused by the lack of Ca 2+ in the pollen culture medium, inducing a release of internal Ca 2+ stored in the secretory vesicles of pollen plasma membrane (Betti et al., 2011). Short day strawberry (Fragaria vesca) plants treated with MF strengths of 0.096, 0.192, and 0.384 Tesla (T) in heated greenhouse conditions showed increased fruit yield per plant (208.50 and 246.07 g, respectively) and fruit number per plant (25.9 and 27.6, respectively), but higher MF strengths than 0.096 T reduced fruit yield and fruit number. Increasing MF strength from control to 0.384 T also increased contents of N, K, Ca, Mg, Cu, Fe, Mn, Na, and Zn, but reduced P and S content (Esitken and Turan, 2004). The effects of pre-sowing magnetic treatments on growth and yield of tomato increased significantly (P < 0.05) the mean fruit weight, the fruit yield per plant, the fruit yield per area, and the equatorial diameter of fruits in comparison with the controls. Total dry matter was also significantly higher for plants from magnetically treated seeds than controls (De Souza et al., 2006).

In the presence of a static MF, the rhythmic leaflet movements of the plant Desmodium gyrans tended to slowdown. Leaflets moving up and down in a MF of approximately 50 mT flux density increased the period by about 10% due to a slower motion in the “up” position. Since during this position a rapid change of the extracellular potentials of the pulvinus occurs, it was proposed that the effects could be mediated via the electric processes in the pulvinus tissue (Sharma et al., 2000). Electric process imply ion flux variations. The influence of a high-gradient MF on spatial distribution of ion fluxes along the roots, cytoplasmic streaming, and the processes of plant cell growth connected with intracellular mass and charge transfer was demonstrated (Kondrachuk and Belyavskaya, 2001).

In tomato, a significant delay in the appearance of first symptoms of geminivirus and early blight and a reduced infection rate of early blight were observed in the plants from exposed seeds to increased MFs (De Souza et al., 2006).

Single suspension-cultured plant cells of the Madagascar rosy periwinkle (Catharanthus roseus) and their protoplasts were anchored to a glass plate and exposed to a MF of 302 ± 8 mT for several hours. Analysis suggested that exposure to the MF roughly tripled the Young's modulus of the newly synthesized cell wall without any lag (Haneda et al., 2006). In vitro tissue cultures of Paulownia tomentosa and Paulownia fortunei exposed to a magnetic flow density of 2.9𠄴.8 mT and 1 m s 𢄡 flow rate for a period of 0, 2.2, 6.6, and 19.8 s showed increased regeneration capability of Paulownia cultures and a shortening of the regeneration time. When the cultures were exposed to a MF with strength of 2.9𠄴.8 mT for 19.8 s, the regenerated P. tomentosa and P. fortunei plants dominated the control plants (Yaycili and Alikamanoglu, 2005).

Increase in MF conditions may also affect secondary plant metabolism. The growth of suspension cultures of Taxus chinensis var. mairei and Taxol production were promoted both by a sinusoidal alternating current MF (50 Hz, 3.5 mT) and by a direct current MF (3.5 mT). Taxol production increased rapidly from the 4th day with the direct current MF but most slowly with the alternating current MF. The maximal yield of Taxol was 490 μg 1 𢄡 with the direct current MF and 425 μg 1 𢄡 with the alternating current MF after 8 d of culture, which were, respectively, 1.4-fold and 1.2-fold of that without exposure to a MF (Shang et al., 2004).

The biological impact of MF strengths up to 30 Tesla on transgenic Arabidopsis plants engineered with a stress response gene consisting of the alcohol dehydrogenase (Adh) gene promoter driving the β-glucuronidase (GUS) gene reporter. Field strengths in excess of about 15 Tesla induce expression of the Adh/GUS transgene in the roots and leaves. From the microarray analyses that surveyed 8000 genes, 114 genes were differentially expressed to a degree greater than 2.5 fold over the control. The data suggest that MF in excess of 15 Tesla have far-reaching effect on the genome. The wide-spread induction of stress-related genes and transcription factors, and a depression of genes associated with cell wall metabolism, are prominent examples. The roles of MF orientation of macromolecules and magnetophoretic effects are possible factors that contribute to the mounting of this response (Paul et al., 2006).

Table ​ Table1 1 summarizes the effects of low and high intensity MF on plants.


New data on the evolution of plants and origin of species

There are over 500,000 plant species in the world today. They all evolved from a common ancestor. How this leap in biodiversity happened is still unclear. In the upcoming issue of Nature, an international team of researchers, including scientists from Martin Luther University Halle-Wittenberg, presents the results of a unique project on the evolution of plants. Using genetic data from 1,147 species the team created the most comprehensive evolutionary tree for green plants to date.

The history and evolution of plants can be traced back by about one billion years. Algae were the first organisms to harness solar energy with the help of chloroplasts. In other words, they were the first plant organisms to perform photosynthesis. Today, there are over 500,000 plant species, including both aquatic and terrestrial plants. The aim of the new study in Nature was to unravel the genetic foundations for this development. "Some species began to emerge and evolve several hundreds of millions of years ago. However, today we have the tools to look back and see what happened at that time," explains plant physiologist Professor Marcel Quint from the Institute of Agricultural and Nutritional Sciences at MLU.

Quint is leading a sub-project with bioinformatician Professor Ivo Grosse, also from MLU, as part of the "One Thousand Plant Transcriptomes Initiative," a global network of about 200 researchers. The team collected samples of 1,147 land plant and algae species to analyse each organism's genome-wide gene expression patterns (transcriptome). Using these data, the researchers reconstructed the evolutionary development of plants and the emergence of individual species. Their focus was on plant species that, as of yet, have not been studied on this level, including numerous algae, moss and also flowering plants.

"This was a very special project because we did not just analyse individual components, but complete transcriptomes, of over one thousand plants, providing a much broader foundation for our findings," explains Ivo Grosse. The sub-project led by MLU scientists looked at the development and expansion of large gene families in plants. "Some of these gene families have duplicated over the course of millions of years. This process might have been a catalyst for the evolution of plants: Having significantly more genetic material might unleash new capacities and completely new characteristics," says Marcel Quint. One of the main objectives of the project was to identify a potential connection between genetic duplications and key innovations in the plant kingdom, such as the development of flowers and seeds. Quint and Grosse carried out their research in collaboration with scientists from the universities in Marburg, Jena, and Cologne, and the Max Planck Institute for Evolutionary Biology in Plön. The majority of the analyses was conducted by Martin Porsch, a PhD student in the lab of Ivo Grosse.

The researchers used the comprehensive dataset to create a new evolutionary tree for plants. It shows that one of our previous assumptions on the evolution of plants was inaccurate: "We used to think that the greatest genetic expansion had occurred during the transition to flowering plants. After all, this group contains the majority of existing plant species today," says Martin Porsch. However, the new data reveal that the genetic foundations for this expansion in biodiversity had been laid much earlier. "The transition from aquatic to terrestrial plants was the starting point for all further genetic developments. This development was the greatest challenge for plants, and so they needed more genetic innovations than ever before," continues Porsch. "We found an enormous increase in genetic diversity at the time of this transition, after that it reached a plateau. From this time on, almost all of the genetic material was available to drive evolutionary progress and generate the biodiversity we see today," concludes Ivo Grosse. According to the researchers, the major expansion of flowering plants only started many millions of years later because, among other things, there was a lack of suitable environmental conditions for a long time. Furthermore, as evolution is not a planned process, certain genetic potentials only manifested themselves much later -- or not at all.


Many domesticated plants arose through the meeting of multiple genomes through hybridization and genome doubling, known as polyploidy. Chalhoub et al. sequenced the polyploid genome of Brassica napus, which originated from a recent combination of two distinct genomes approximately 7500 years ago and gave rise to the crops of rape oilseed (canola), kale, and rutabaga. B. napus has undergone multiple events affecting differently sized genetic regions where a gene from one progenitor species has been converted to the copy from a second progenitor species. Some of these gene conversion events appear to have been selected by humans as part of the process of domestication and crop improvement.

Oilseed rape (Brassica napus L.) was formed

7500 years ago by hybridization between B. rapa and B. oleracea, followed by chromosome doubling, a process known as allopolyploidy. Together with more ancient polyploidizations, this conferred an aggregate 72× genome multiplication since the origin of angiosperms and high gene content. We examined the B. napus genome and the consequences of its recent duplication. The constituent An and Cn subgenomes are engaged in subtle structural, functional, and epigenetic cross-talk, with abundant homeologous exchanges. Incipient gene loss and expression divergence have begun. Selection in B. napus oilseed types has accelerated the loss of glucosinolate genes, while preserving expansion of oil biosynthesis genes. These processes provide insights into allopolyploid evolution and its relationship with crop domestication and improvement.

The Brassicaceae are a large eudicot family (1) and include the model plant Arabidopsis thaliana. Brassicas have a propensity for genome duplications (Fig. 1) and genome mergers (2). They are major contributors to the human diet and were among the earliest cultigens (3).

Genomic alignments between the basal angiosperm Amborella trichopoda (24), the basal eudicot Vitis vinifera (25), and the model crucifer A. thaliana, as well as B. rapa (9), B. oleracea (10, 11), and B. napus, are shown. A typical ancestral region in Amborella is expected to match up to 72 regions in B. napus (69 were detected for this specific region). Gray wedges in the background highlight conserved synteny blocks with more than 10 gene pairs.

B. napus (genome AnAnCnCn) was formed by recent allopolyploidy between ancestors of B. oleracea (Mediterranean cabbage, genome CoCo) and B. rapa (Asian cabbage or turnip, genome ArAr) and is polyphyletic (2, 4), with spontaneous formation regarded by Darwin as an example of unconscious selection (5). Cultivation began in Europe during the Middle Ages and spread worldwide. Diversifying selection gave rise to oilseed rape (canola), rutabaga, fodder rape, and kale morphotypes grown for oil, fodder, and food (4, 6).

The homozygous B. napus genome of European winter oilseed cultivar ‘Darmor-bzh’ was assembled with long-read [>700 base pairs (bp)] 454 GS-FLX+ Titanium (Roche, Basel, Switzerland) and Sanger sequence (tables S1 to S5 and figs. S1 to S3) (7). Correction and gap filling used 79 Gb of Illumina (San Diego, CA) HiSeq sequence. A final assembly of 849.7 Mb was obtained with SOAP (8) and Newbler (Roche), with 89% nongapped sequence (tables S2 and S3). Unique mapping of

5× nonassembled 454 sequences from B. rapa (‘Chiifu’) or B. oleracea (‘TO1000’) assigned most of the 20,702 B. napus scaffolds to either the An (8294) or the Cn (9984) subgenomes (tables S4 and S5 and fig. S3). The assembly covers

79% of the 1130-Mb genome and includes 95.6% of Brassica expressed sequence tags (ESTs) (7). A single-nucleotide polymorphism (SNP) map (tables S6 to S9 and figs. S4 to S8) genetically anchored 712.3 Mb (84%) of the genome assembly, yielding pseudomolecules for the 19 chromosomes (table S10).

The assembled Cn subgenome (525.8 Mb) is larger than the An subgenome (314.2 Mb), consistent with the relative sizes of the assembled Co genome of B. oleracea (540 Mb, 85% of the

630-Mb genome) and the Ar genome of B. rapa (312 Mb, 59% of the

530-Mb genome) (911). The B. napus assembly contains 34.8% transposable elements (TEs), less than the 40% estimated from raw reads (tables S11 to S14) (7), with asymmetric distribution in the An and Cn subgenomes (table S12) as in the progenitor genomes (911). A small TE fraction has proliferated since B. napus separated from its progenitors (7), at lower rates in the B. napus subgenomes than the corresponding progenitor genomes (table S14 and figs. S9 and S10).

The B. napus genome contains 101,040 gene models estimated from 35.5 Gb of RNA sequencing (RNA-seq) data (tables S15 and S16) in combination with ab initio gene prediction, protein and EST alignments, and transposon masking (7). Of these, 91,167 were confirmed by matches with B. rapa and/or B. oleracea predicted proteomes. Genes are abundant in distal euchromatin but sparse near centromeric heterochromatin (Fig. 2). RNA-seq data revealed alternative splicing in 48% of genes, with frequent intron retention (62%) and rare exon skipping (3%) (tables S17 and S18 and fig. S11).

The genome comprises 9 chromosomes belonging to the Cn subgenome and 10 to the An subgenome, scaled on the basis of their assembled lengths. Tracks displayed are (A) gene density (nonoverlapping, window size = 100 kb for all tracks). Positions showing loss of one or more consecutive genes are displayed (triangles) along with homeologous exchanges, detected as missing genomic segments that have been replaced by duplicates of corresponding homeologous segments (red rectangles). (B and C) Transcription states estimated by RNA-seq in leaves (B) and roots (C) (in nonoverlapping 100-kb windows). (D) DNA transposon density. (E) Retrotransposon density. (F) CpG methylation in leaves (green) and roots (brown) both curves are overlapping. (G) Centromeric repeats (densities exaggerated for visual clarity). Homeologous relationships between An and Cn chromosomes are displayed with connecting lines colored according to the Cn chromosomes.

The B. napus An and Cn subgenomes are largely colinear to the corresponding diploid Ar and Co genomes, with asymmetric gene distribution (42,320 and 48,847, respectively) and 93% of the diploid gene space in orthologous blocks (fig. S12) (7). We identified 34,255 and 38,661 orthologous gene pairs between the An and Cn subgenomes and their respective progenitor genomes (fig. S13). Comparison of An-Ar and Cn-Co orthologous gene pairs suggested a divergence 7500 to 12,500 years ago (fig. S14), indicating formation of B. napus after this date. Synteny with Arabidopsis (table S19) confirmed the triplicated mesoploid structure (911) of the An and Cn subgenomes, with the recent allopolyploidy conferring on B. napus an aggregate 72× genome multiplication since the origin of angiosperms (Fig. 1) (7).

Most orthologous gene pairs in B. rapa and B. oleracea remain as homeologous pairs in B. napus (tables S19 to S25 and figs. S12 to S17) (7). DNA sequence analysis (7) confirmed the loss of 112 An and 91 Cn genes in B. napus ‘Darmor-bzh’ (tables S21 to S26),

2.6 times higher than the 41 and 37 genes lost in B. rapa ‘Chiifu’ and B. oleracea ‘TO1000’ respectively (tables S26 and S27 χ 2 test P = 5.3 × 10 –14 ). Further analyses of a Brassica diversity set showed that

47% of Darmor-bzh An and 31% of Cn deleted genes were also deleted in at least one additional progenitor genotype (tables S28 and S29), indicating that their deletion probably predated allopolyploidization of B. napus (7). A high proportion (27% to 54%) of the remaining Darmor-bzh deleted genes were also deleted from diverse B. napus genotypes (tables S28 and S29).

Homeologous exchanges (HEs), including crossovers and noncrossovers, are frequent between B. napus subgenomes and range in size from large segments to single SNPs (7) (Fig. 3, figs. S17 to S24, and tables S30 to S39).

(A) Coverage depth obtained along the An2 chromosome after mapping Illumina sequence reads from seven natural and one resynthesized B. napus genotypes (named on the right) to the reference genome of B. napus ‘Darmor-bzh.’ (B and C) Coverage depth obtained for Ar2 and Co2 chromosomes, respectively, after mapping >21 genome-equivalents of Illumina sequence reads from B. napus Darmor-bzh on the B. rapa and B. oleracea genome assemblies concatenated together. (D) Similar to (A), where the Cn2 chromosome of Darmor-bzh is displayed. Segmental HEs are revealed based on sequence read coverage analysis, where a duplication (red) is revealed by significantly greater coverage for a given segment than the rest of the genome (black) and a deletion (blue) by little or no coverage for the corresponding homeologous segment. Sizes of chromosomes are indicated in Mb. An-to-Cn converted genes (at 60% or more conversion sites) are plotted as blue dots on Ar2 (B) and red dots on Co2 (C). Cn-to-An converted genes are plotted as blue dots on Co2 (C) and red dots on Ar2 (B). Open circles denote entirely converted genes using the same color code. Light gray lines connecting (A), (B), (C), and (D) indicate orthology relationships, and dark gray lines highlight segmental HEs in Darmor-bzh (names and descriptions detailed in table S31). Further HEs occurring between other homeologous chromosomes are shown in fig. S19. Black arrows in (A) indicate HEs involving GSL and FLC genes.

At the chromosome segment level, HEs are characterized by replacement of a chromosomal region with a duplicated copy from the corresponding homeologous subgenome (7). We identified 17 HEs, 14 Cn to An and 3 An to Cn (Fig. 3, fig. S19, and tables S30 and S31). Sequences from seven diverse B. napus genotypes revealed both shared and specific segmental HEs. These are of varying sizes and are most frequent between chromosomes An1-Cn1, An2-Cn2, and An9-Cn9 (table S32, Fig. 3, and fig. S19). Larger HEs found in the synthetic B. napus H165 affect, for example, most of chromosomes An1-Cn1 and An2-Cn2 (Fig. 3 and fig. S19). Functional annotation of genes within HEs suggests some have experienced selection, contributing to the diversification of winter, spring, and Asian types of oilseed rape, rutabaga, and kale vegetables (Fig. 3B, fig. S19, and table S33).

We also identified 37 Cn to An and 56 An to Cn whole-gene conversions (12) (table S34).

At the single-nucleotide scale, exchanges between homeologous subgenomes account for

86% of allelic differences between B. napus and its progenitors, with nearly

1.3 times more conversions from the An to the Cn subgenome than the reverse (χ 2 , P < 1.6 × 10 −16 ) (tables S35 and S36). A total of 16,938 An and 13,429 Cn genes (with 10,258 from homeologous pairs) had at least two conversion sites (table S37) 842 An and 579 Cn genes were highly converted with 60 to 90% conversion sites (table S37).

Transcript abundance indicated that 96% of genes are expressed in leaves, roots, or both (7) (figs. S25 to S29 and tables S40 to S42). Subgenome and tissue effects and tissue-by-subgenome interactions were statistically significant (χ 2 , P < 0.01), with 45 expression patterns (fig. S26) grouping into nine clusters (table S41).

An and Cn homeologs contributed similarly to gene expression for 17,326 (58.3%) gene pairs (χ 2 , P < 0.01) (fig. S27 and table S41). Both tissues showed higher expression for 4665 (15.7%) An homeologs and 5437 (17.3%) Cn homeologs (fig. S28 and table S41). There were 1062 gene pairs (3.7%) with higher expression of the An homeolog over the Cn homeolog in leaves, whereas the reverse was true in roots (fig. S28 and table S41). Conversly, for 966 gene pairs (3.3%), An homeologs had lower expression than Cn homeologs in leaves, with the pattern inverted in roots (fig. S28 and table S41).

Gene expression is generally inversely related to CpG, CHG, and CHH cytosine DNA methylation levels (p is phosphate, implying a C is directly followed by a G, and H is A, C, or T) (7). Methyl bisulfite sequencing in Darmor-bzh (figs. S30 to S32 and tables S43 to S45) showed 4 to 8% higher methylation in Cn genes than in their homoelogous An genes (table S44), possibly because of greater transposon density in the Cn subgenome (Fig. 2F). Of the

3100 gene pairs with differential gene body and/or untranslated region methylation between An and Cn homeologs in both roots and leaves, 51% were equally expressed. Only

34% showed higher expression for the less-methylated homeologs, and the remaining

15% showed the opposite pattern (table S45).

It is interesting that partitioning of homeolog gene expression is largely established in B. napus with patterns of both genome dominance and genome equivalence. The absence of significant bias toward either subgenome of the recent B. napus allopolyploid contrasts with many old and recent polyploids (1317) but concurs with other old polyploids (18).

Oilseed B. napus has undergone intensive breeding to optimize seed oil content and lipid composition, decrease nutritionally undesirable erucic acid and glucosinolates (GSLs), optimize flowering behavior, and improve pathogen resistance.

The expansion of B. napus lipid biosynthesis genes exceeds that known in other oilseed plants, with 1097 and 1132 genes annotated in the An and Cn subgenomes, respectively (7) (tables S46 to S48). Most lipid biosynthesis genes identified in the progenitor genomes are conserved in B. napus. For 18 acyl lipid orthologs, 3 and 2 genes appeared to be deleted from An and Cn subgenomes, respectively. Another 13 have been converted by HEs, nine from An to Cn and four from Cn to An (tables S47 and S48) (7).

Genetic variation for reduced seed GSLs also appears to be under breeding-directed selection. GSLs are sulfur-rich secondary metabolites important for plant defense and human health (19) however, high levels in seeds form toxic breakdown products in animal feeds (20). All 22 GSL catabolism genes identified in B. rapa and B. oleracea (10) are conserved in B. napus (7), and orthologs of only three Co and one Ar GSL biosynthesis genes are missing (table S49). One deleted homeologous pair, corresponding to orthologs of B. oleracea Bo2g161590 and B. rapa Bra02931 , colocates with two quantitative trait loci (QTLs) for total aliphatic GSL content (21) and corresponds to a HE in which a segment of An2, with a missing GSL gene, has replaced the Cn2 homeolog (Fig. 3). Two additional QTLs for aliphatic GSL content (21) colocalize with a deletion of the B. rapa Bra035929 ortholog on An9 and its nondeleted homeolog on Cn9 (BnaC09g05300D, fig. S17).

We identified 425 nucleotide binding site leucine-rich repeat (NBS-LRR) sequences encoding resistance gene homologs (245 on Cn and 180 on An). Of these, 75% (153 An and 224 Cn) are syntenic to Ar and Co progenitors (7) (table S50 and figs. S33 and S34). We confirmed the absence of five NBS-LRR genes from the An subgenome, three from the Cn subgenome, and three from B. rapa (Ar), with none absent from B. oleracea Co. This variation may reflect differential selection for resistance to diseases.

B. napus morphotypes show broad adaptation to different climatic zones and latitudes. A key adaptive gene controlling vernalization and photoperiod responses, FLOWERING LOCUS C (FLC) is expanded from one copy in A. thaliana to four in B. rapa and B. oleracea and nine or more in B. napus (7) (table S51). Different FLC homologs lie within HEs, from Cn2 to An2 in the Asian semiwinter oilseed forms Yudal and Aburamasari (Fig. 3) and Cn9 to An10 in late-flowering swedes (fig. S19 and table S51). These loci correspond to important QTLs for vernalization requirement and flowering time (22).

Human cultivation and breeding of B. napus morphotypes may have selected favorable HEs, causing subgenome restructuring of regions containing genes controlling valuable agronomic traits such as those shown here for oil biosynthesis, seed GSL content, disease resistance, and flowering. Because B. napus is a young allopolyploid beginning gene loss and genome reorganization, further partitioning of expression may become a key determinant for the long-term preservation of its duplicated genes (23). The integrative genomic resources that we report provide unique perspectives on the early evolution of a domesticated polyploid and will facilitate the manipulation of useful variation, contributing to sustainable increases in oilseed crop production to meet growing demands for both edible and biofuel oils.


Evolution of Land Plants

No discussion of the evolution of plants on land can be undertaken without a brief review of the timeline of the geological eras. The early era, known as the Paleozoic, is divided into six periods. It starts with the Cambrian period, followed by the Ordovician, Silurian, Devonian, Carboniferous, and Permian. The major event to mark the Ordovician, more than 500 million years ago, was the colonization of land by the ancestors of modern land plants. Fossilized cells, cuticles, and spores of early land plants have been dated as far back as the Ordovician period in the early Paleozoic era. The oldest-known vascular plants have been identified in deposits from the Devonian. One of the richest sources of information is the Rhynie chert, a sedimentary rock deposit found in Rhynie, Scotland (Figure 4), where embedded fossils of some of the earliest vascular plants have been identified.

Figure 4. This Rhynie chert contains fossilized material from vascular plants. The area inside the circle contains bulbous underground stems called corms, and root-like structures called rhizoids. (credit b: modification of work by Peter Coxhead based on original image by “Smith609”/Wikimedia Commons scale-bar data from Matt Russell)

Paleobotanists distinguish between extinct species, as fossils, and extant species, which are still living. The extinct vascular plants, classified as zosterophylls and trimerophytes, most probably lacked true leaves and roots and formed low vegetation mats similar in size to modern-day mosses, although some trimetophytes could reach one meter in height. The later genus Cooksonia, which flourished during the Silurian, has been extensively studied from well-preserved examples. Imprints of Cooksonia show slender branching stems ending in what appear to be sporangia. From the recovered specimens, it is not possible to establish for certain whether Cooksonia possessed vascular tissues. Fossils indicate that by the end of the Devonian period, ferns, horsetails, and seed plants populated the landscape, giving rising to trees and forests. This luxuriant vegetation helped enrich the atmosphere in oxygen, making it easier for air-breathing animals to colonize dry land. Plants also established early symbiotic relationships with fungi, creating mycorrhizae: a relationship in which the fungal network of filaments increases the efficiency of the plant root system, and the plants provide the fungi with byproducts of photosynthesis.

Paleobotanist

How organisms acquired traits that allow them to colonize new environments—and how the contemporary ecosystem is shaped—are fundamental questions of evolution. Paleobotany (the study of extinct plants) addresses these questions through the analysis of fossilized specimens retrieved from field studies, reconstituting the morphology of organisms that disappeared long ago. Paleobotanists trace the evolution of plants by following the modifications in plant morphology: shedding light on the connection between existing plants by identifying common ancestors that display the same traits. This field seeks to find transitional species that bridge gaps in the path to the development of modern organisms. Fossils are formed when organisms are trapped in sediments or environments where their shapes are preserved. Paleobotanists collect fossil specimens in the field and place them in the context of the geological sediments and other fossilized organisms surrounding them. The activity requires great care to preserve the integrity of the delicate fossils and the layers of rock in which they are found.

One of the most exciting recent developments in paleobotany is the use of analytical chemistry and molecular biology to study fossils. Preservation of molecular structures requires an environment free of oxygen, since oxidation and degradation of material through the activity of microorganisms depend on its presence. One example of the use of analytical chemistry and molecular biology is the identification of oleanane, a compound that deters pests. Up to this point, oleanane appeared to be unique to flowering plants however, it has now been recovered from sediments dating from the Permian, much earlier than the current dates given for the appearance of the first flowering plants. Paleobotanists can also study fossil DNA, which can yield a large amount of information, by analyzing and comparing the DNA sequences of extinct plants with those of living and related organisms. Through this analysis, evolutionary relationships can be built for plant lineages.

Some paleobotanists are skeptical of the conclusions drawn from the analysis of molecular fossils. For example, the chemical materials of interest degrade rapidly when exposed to air during their initial isolation, as well as in further manipulations. There is always a high risk of contaminating the specimens with extraneous material, mostly from microorganisms. Nevertheless, as technology is refined, the analysis of DNA from fossilized plants will provide invaluable information on the evolution of plants and their adaptation to an ever-changing environment.


Angiosperms: Evolution, Concept and Life Cycle | Flowering Plants

Let us make an in-depth study of Angiosperms. After reading this article you will learn about: 1. Evolution of Angiosperms 2. Concept of Angiosperms 3. Objectives 4. Angiosperms or Flowering Plants 5. Alternation of Generations: Life Cycle of Angiosperms 6. Origin of Angiosperms 7. Groups of Angiosperms 8. Principles of Taxonomy and Phylogeny 9. Oldest Angiosperms 10. Ancestry of Angiosperms.

Evolution of Angiosperms:

(i) Diversified habit and vegetative forms

(ii) Higher degree of perfection of the vascular system the xylem in addition to the tracheids, contains wood vessels, and the phloem possesses companion cells

(iii) Successful invasion of all habitats

(iv) Adaptation of flower to insect pollination

(v) Bisexual flower, the bisexuality ensures self-pollination if cross-pollination fails, hence the seeds are formed in any case

(vi) Development of ovules within the ovary which ensures proper protection to the developing ovules and seeds

(vii) Efficient and effective dispersal by insects, birds, other animals, wind, water and by several other methods

(viii) Efficient and varied means of vegetative propagation, which result in rapid multiplication

(ix) Their utmost economic importance, and

(x) Besides the above listed reasons, there must be some hereditary causes such as better equipment of gene potential and useful gene mutations, which enable them to encounter variations in temperature and other environmental changes and led to their conquest over other plant groups.

Concept of Angiosperms:

What is Systematic Botany?

Systematic botany is the science which deals with the classification and naming of the plants. The science of classifying the plants is said to be plant taxonomy and lays emphasis upon phylogenetic relationships. The naming of the plants is known as nomenclature and provides each plant with a name. This way, the systematic botany consists of classifying and naming of the plants.

The important task included in the field of systematic botany is the collection of the plant specimens from all over the world and building the great herbaria. The plants are being classified chiefly on the basis of the study of their comparative morphology.

In modern days the study of systematic botany needs a good background in general botany, cytology, genetics, ecology, plant geography and paleobotany otherwise, the origin of the plants cannot be traced out.

Nomenclature makes the important part of systematic botany. It deals with names which may or may not indicate relationships. The science of plant taxonomy classifies the plants on the basis of similarities and differences, which are now known as “phylogenetic relationships”. Without naming the plants, they cannot be classified and therefore, nomenclature makes a very important part of systematic botany.

Ecology and systematic botany are interrelated to each other. Certain species are definitely hydrophytic, xerophytic, mesophytic or halophytic. The soil characters are being indicated by the vegetation growing upon. The character of the soil may be indicated by its vegetation.

Scope of Systematic Botany:

(1) Plant taxonomy-which establishes the phylogenetic relationships that exist naturally between many groups of plants,

(2) Nomenclature, i.e., giving of names to all kinds of plants. There are millions of plants all over the world, and it is very clear, that no two individuals are exactly alike.

Most of the individuals are so different from each other that hardly they show any bond of relationship. On the basis of the comparative study of their morphology, the taxonomists have arranged the individual plants in a fairly orderly system. Thousands of specimens have been provided with their names and descriptions.

The group relationships, distribution, properties, etc., of the plants have been studied. This way, systematic botany prepares foundation for all sciences concerning plants.

The systematic botany is of great value in forestry. Every type of tree of the forest must be named and classified. At the same time, the characteristic features, distribution and abundance of the plants must be learned.

The study of systematic botany is equally essential in agriculture and horticulture. To get resistant varieties, etc., the crossing is needed, which is furnished by foreign seeds and plants. The foreign seeds and plants are introduced by the men who have sufficient knowledge of systematic botany.

The work of soil conservation needs the good study of systematic botany. The ecologists who are well trained in systematic botany choose the native grasses and plants which act as sand soil binders.

To study the plant ecology, the knowledge of systematic botany becomes essential. The plant ecologist must be knowing the names of the plants and their relationships to habitat and environment. The character of the soil of a land area may easily be judged by the vegetation growing there.

The study of systematic botany is of great use in tracing the origin of plants. The fossils, scattered here and there in fragments are found to be very useful in tracing out the characters of the ancestral forms of the plants. The study of these fossils requires the sound knowledge of systematic botany.

Objectives of Systematic Botany:

1. The first objective of systematic botany is to know the various kinds of plants on the surface of earth with their names, affinities, geographical distribution, habit characteristics and their economic importance.

2. The second objective of this science is to have a reference system for plants, where the scientists can work with named entities such as species, genus and family.

3. The third objective is to demonstrate the manifold diversities of plant kingdom and their relation to evolution of plants. A systematic reconstruction of plant kingdom can be made only after the complete knowledge of the individual plants. After making an inventory of the components of plant kingdom, the different facts of evolutionary knowledge with an accurate phylogenetic scheme can be obtained.

4. The fourth objective is to ascertain nomenclature. To every plant a binomial name is given, e.g., Solanum tuberosum Linn. The first name refers to its genus, the second to its species and Linn., or L. for Linnaeus, who first observed and reported the plant.

Angiosperms or Flowering Plants:

Angiosperms or flowering plants form the largest group of plant kingdom, including about 300 families (411 families, Hutchinson), 8,000 genera and 300,000 species. They are considered to be highest evolved plants on the surface of the earth. From Cretaceous age, the angiosperms eclipsed all other vegetation and now they are dominant. They are found almost everywhere in each possible type of habitat and climate.

They occur in deep lakes, deserts, in beds of seas and even on high peaks of mountains. The species of Opuntia (Cactaceae) can survive without water in acute desert conditions, whereas on the other hand the species of Hydrilla (aquatic plant) are extremely sensitive to drought conditions.

Some species are found on rocks, some in waterfalls and also some are marine. The species of Rhizophora, popularly known as ‘mangrove vegetation’ are found near the water of the sea. The epiphytes, parasites, saprophytes, symbionts and even insectivorous plants are also not uncommon.

They may be annual, biennial or perennial herbs, shrubs, trees, climbers, twiners and lianes. On one hand the angiosperms may be as minute in size as a pin head, e.g., Wolffia microscopica, on the other extremity like eucleptiles of Australia may reach upto 300 feet in height.

Alternation of Generations: Life Cycle of Angiosperms:

In the life-cycle of angiosperms, there is alternation of nutritionally independent and more complex sporophyte with the inconspicuous, reduced and parasitic gametophytes. The sporophyte, which may be a herb, shrub or a tree is differentiated into roots, stem and leaves each with a vascular tissue with the highest degree of perfection.

In addition, it bears floral leaves or sporophylls organised into a structure called the flower. The sporophylls are of two kinds microsporophylls and megasporophylls. Each microsporophyll bears four microsporangia. The latter contains microspores which are differentiated by meiosis from the diploid microspore mother cells.

The megasporophyll in a flower part, which is called the ovary, contains one or more ovules. The ovule consists of one or two integuments enclosing the nucellus or megasporangium.

Towards the micropylar end of the megasporangium is differentiated, by meiosis, a single functional megaspore which is haploid. The microspores and megaspores are the first structures of the gametophyte generation.

The sporophyte generation in the angiosperms, thus consists of the sporophyte plant, flowers, microsporophylls and megasporophylls, microsporangia, megasporangia, microspore mother cells and the megaspore mother cells.

With meiosis the sporophyte phase switches on the gametophyte phase. The first structures of the gametophyte phase are the microspores and the megaspores. On germination, they produce the alternate structures in the life-cycle which are the male and the female gametophytes and not the sporophyte plant. The male and the female gametophytes are extremely reduced and parasitic.

The female gametophyte consists only of six cells and two nuclei. Three cells at the micropylar end form the egg apparatus. The other three at the chalazal end form the antipodal group. There are two polar nuclei in the centre. The male gametophyte consists of a pollen tube containing the tube nucleus and two male nuclei.

The eggs and the male nuclei represent the gametes which are the last structures of the gametophyte phase. With the fusion of one of the male nuclei with the egg, the gametophyte phase ends. Fertilization thus is the second critical point in the life cycle.

With it the gametophyte generation switches on to the sporophyte generation. The fertilized egg or oospore is the first structure of the next sporophyte. By segmentation it produces the embryo (i.e., the baby plant in the seed) the ovule to the seed and the ovary as a whole to the fruit.

The embryo lies dormant in the seed and the latter lies embedded in the fruit. The seed and the fruit give adequate protection to the embryo, store up food material for it, and are often well adapted for dispersal. Sooner or later as the seed germinates the embryo grows into a seedling which gradually grows into a mature plant.

Taxonomic group of any rank or unit, viz., family, class, order, genus, species, etc., is called taxon (plural: taxa).

For example, the genus Solanum (the name of the genus begins always with a capital letter) consists of many species (the name of the species begins always with a small letter) such as tuberosum, melongena, nigrum, etc., but certain common characters of the genus Solanum can separate this genus from all other genera of the family Solanaceae.

Higher to genus is the family, where one or many genera are correlated between greater number of morphological characters is grouped together. The name of the family ends in aceae, e.g., Brassicaceae, Poaceae, etc.

The family represents a more natural taxon or unit than any of the higher categories, e.g., Brassicaceae, Palmaceae, Solanaceae, Rosaceae are identified as natural taxa by certain characters peculiar to them. The angiospermic families may be separated from one another by floral characters, inflorescence, type, nature of perianth, ovary position, number of carpels, placentation, etc.

The families which are allied to one another are grouped into an order which makes a natural taxon. For example, the order Ranales comprises of Ranunculaceae, Nymphaeceae, etc. Here the families are related to each other in general floral constitution.

What is a Species?

According to new definition the species are biological units that have evolved from a series of often identifiable ancestors, by which the smaller taxa of plants (genera, species, and their subdivisions) may be circumscribed by a combination of genetical, morphological and ecological criteria. According to this view, a species is considered as an objective definitive unit.

Taxonomy and Its Significance:

The study of taxonomy has among its objectives the learning of the kinds of plants on earth and their names, of their distinctions and their affinities, their distributions and characteristics of habitats, and the correlation of these facets of knowledge with pertinent scientific data. A secondary objective of taxonomy is the assemblage of knowledge gained. Floras are published to account for the plants of a given area.

All the products of taxonomic research add to the resources available to the scientist. They are essential to any study of the natural resources of an area, to studies of land potentials, to evaluation of resources of raw materials possible suited to man’s needs (for example, forest products, medicines, food, agricultural crops, ornamentals and industry).

A third important and scientific’ objective is the demonstration of the tremendous diversity of the plants and their relation to man’s understanding of evolution.

Interrelationships with allied sciences:

Taxonomy is dependent on many other sciences and they in turn are equally dependent on it. One who studies taxonomy must have a knowledge of morphology, embryology, floral anatomy, ontogenetical development and teratological variations of the plants with which he works. Modem taxonomists place considerable value on the importance of cytogenetic findings as criteria in delimiting the species and its elements.

In addition to an appreciation and understanding of the contributory value of morphological, anatomical and cytogenetical findings, modern taxonomic studies reflect the significance of distributional patterns and of more detailed data of the extent of normal variation and its causes.

All these viewpoints demonstrate the increasing dependence of taxonomy on the findings of related sciences the product of modern taxonomic research is rapidly becoming one of synthesis rather than of individual conclusions.

During the eighteenth and early nineteenth centuries the study of taxonomy dominated the field of botanical activity. Today the taxonomist is interested in the problems associated with the distribution of the plants. Knowledge of plant distribution is relevant to the determination of geographic areas of origins of species, of genera, and often of families.

These studies in distribution and geography bring taxonomy into the field of phytogeography, the inquiry into why a group occupies that area, how long it has been there, how rapidly it is migrating, and what evolutionary trends it is showing. Studies with this wider viewpoint represent a synthesis of ecologic, genetic and taxonomic aspects leading to a better understanding of a series of common problems.

Origin of Angiosperms:

The angiosperms appeared suddenly in Cretaceous age about 65 million years back. Charles Darwin described this sudden appearance of angiosperms in lower or upper Cretaceous as an ‘abominable mystery. When angiosperms appeared for the first time in lower or upper Cretaceous, they were full fledged like the trees and the herbs of today.

In support of this view Prof. Knowlton advocates in his ‘Plant of the past’, “from the time of their appearance they did not progress at all due to their full-fledged appearance in the Cretaceous”.

The fossil records of the angiosperms also support their appearance full-fledged in lower or upper Cretaceous. The fossils of that age are so characteristic and modem in appearance that most of them can be referred unmistakably to living families, general and even to some species.

Prof. Knowlton stated in his book, ‘Plant of the past’ “if a student of present day trees and shrubs, could have wandered over the hills and vales in those days, he would have found himself quite at home among the trees and shrubs growing there”.

The forms of cycads and conifers, which long dominated the universe were already pushed background and the earth had become infact the earth of flowering plants. Charles Darwin has called this sudden appearance of angiosperms as an “abominable mystery”.

However, some workers do not agree with the doctrine of ‘abominable mystery’. According to H.H. Thomas (1936), the angiosperms of the past replaced many of older gymnosperms in asturine and marshy waters. Graud Eury (1906) believes that the angiosperms came into existence through mutation. Guppy (1919) however, supported the view of mutation.

Prof. Bertrand is of opinion that all the great groups of vascular plants (Pteridophyta, Gymnospermae and Angiosperms) not only arose quite independently of each other but also they originated simultaneously as far back in the Archian period (2000 million years old-oldest).

There is a very considerable but scattered literature on the origin and phylogeny of angiosperms. The palaeobotanical evidence shows that there seems three possibilities as regards the origin of angiosperms.

These possibilities are:

1. That the angiosperms are monophyletic in their origin but have had a very much longer history than at present known, perhaps stretching back into Palaeozoic times and with a whole series of missing links

2. That the angiosperms are monophyletic but that the first and at present unknown group diverged quickly in terms of geological time, into a considerable number of different groups

3. That the angiosperms are polyphyletic.

According to Campbell, “both comparative morphology and the geological record indicate that the existing angiosperms represent a number of distinct phyla which cannot be traced back to a single ancestral type”. This statement shows that he does not believe in monophyletic origin of angiosperms.

According to Thomas, the evolutionary tendencies detected in the three groups, i.e., Caytoniales, Bennettitales and Pteriodsperms furnish reasonable grounds for the idea that the angiosperms were derived from various pteridosperms early in the Mesozoic period.

Parkin also argues for the monophyletic origin of the angiosperms.

In conclusion we can say, that the history of the angiosperms is still almost as great a mystery as it was in the time of Darwin. We do not know, when, where or from what the presumably most recent and now dominant large group of existing terrestrial plants arose.

Lotsy says, that hybridization is the key to evolution of angiosperms. This view has been supported by Anderson. He has suggested on the basis of cytological investigations that the angiosperms may have arisen as a result of hybridization between two gymnosperms.

According to Hagerup, the origin of some angiosperms took place from the Coniferales through Gnetales.

According to Eames, Sinnott and Bailey the more primitive angiosperms were arboreal in habit and the herbaceous angiosperms have been evolved from them.

According to Hutchinson, in certain groups, trees and shrubs are probably more primitive than herbs.

Evidently views in this respect are divergent and speculative, the available data being meagre, fragmentary and isolated.

Some of the theories proposed from time to time in this connection, are as follows:

1. Bennettitales-Ranales Theory:

(a) Hallier’s view (1906) regarding the origin of angiosperms is that Ranales, (e.g., Magnolia) seems to be related to Bennettitales and may have been derived from Cycadeoidea, and that the monocotyledons are an offshoot of the dicotyledons.

The elongated floral axis of Ranales with spirally arranged male and female sporophylls and the cone of Cycadeoidea are definitely alike. Both the groups were also abundant in the Cretaceous.

Ranales is regarded as the earliest stock from which the polycarpic families of dicotyledons have arisen, and also the monocotyledons as an offshoot. But the differences in the anatomical structure of the wood, types of sporophylls, nature and position of ovules, etc., in the two groups, i.e., Bennettitales and Ranales, have led to difficulties in accepting this view. It is more probable that both the groups have evolved from a common ancestry and developed in unrelated parallel lines.

(b) Arber and Parkin (1907), proposed the existence of an imaginary group (Hemiangiospermae) having cycadeoid type of flowers linking the above two groups. From this imaginary group the Ranalian type of flowers might have originated, and from this Ranalian type all other angiosperms had sprung up.

They were of the opinion that the earliest monocotyledons were more primitive than the dicotyledons. They also supported the Hallier’s view.

(c) Hutchinson (1925) considered the origin of angiosperms as monophyletic, and supported the views of Hallier (1906) and Arber and Parkin (1907). He believed in the Bennettitalean origin of angiosperms and stressed on two parallel evolutionary lines for the primitive dicotyledons a woody (arborescent) line, called Lignosae, starting from Magnoliales, and a herbaceous line, called Herbaceae starting from Ranales.

He further held the view that the monocotyledons were derived from a primitive dicotyledonous order. The Ranales.

2. Coniferae-Amentiferae Theory:

Engler and Prantl (1924) rejected the Cycadeoidean origin of angiosperms, as proposed by Hallier (1906) earlier. They held the view that dicotyledons and monocotyledons had arisen independently from a hypothetical group of extinct gymnosperms (allied coniferae) with unisexual strobilus which developed in the Mesozoic.

Thus, according to them, the angiosperms had a polyphyletic origin, and evolution took place on several parallel lines from the beginning. They considered the monocotyledons to be more primitive than the dicotyledons.

The unisexual naked (without perianth) condition of the angiospermic flowers, such as Pandanales (monocotyledons) and Amentiferae (catkin-bearing dicotyledons, e.g., Casuarina. Salix, Betula, etc.) was most primitive. But according to the modem classification these orders are regarded more advanced.

3. Gnetales-Casuarinales Theory:

Wettstein (1935) held the view that angiosperms of Casuarina type evolved from Gnetales, (particularly Ephedra), a highly advanced group of gymnosperms, which branched off from the main gymnospermic line. However, there are some angiospermic features in Gnetales but this group has meagre fossil records, and not gone earlier than Tertiary.

Fagerland (1947) was of the opinion that both Gnetales and Proangiosperms had a common ancestor and the modern angiosperms evolved from the Proangiosperms in Polyphyletic lines.

4. Caytoniales-Angiosperm Theory:

Thomas (1925) suggested that Caytoniales, a small group of angiosperm-like plants, discovered in the Jurassic rocks of Yorkshire (England), might be the ancestor of angiosperms. However, Harris (1932-33) opposed this view.

Knowlton (1927) expressed the view in his book Plants of the Past that both Caytoniales and angiosperms evolved from the large extinct Palaeozoic group of Pteridosperms. Arnold (1948), expressed the view that Caytoniales were allied to the Pteridosperms rather than to the angiosperms.

5. Pteridosperm-Angiosperm Theory:

Andrews (1947) was of opinion that seed ferns or Pteridosperms, an ancient group of the Palaeozoic, might be the starting point for the angiospermic plants.

There are two large groups of angiospertns:

Groups of Origin of Angiosperms:

Origin of Dicotyledons:

The dicots are more important and they are supposed to have originated before the monocotyledons. It is thought that for the first time dicotyledons appeared in the early Mesozoic era, or perhaps the late Palaeozoic. The oldest known fossils of dicots are from the lower Cretaceous rocks.

It is generally believed and agreed that the dicotyledons have been originated from gymnosperms of a type somewhat different from present day forms of gymnosperms. According to some similarities noted in these two groups, the development seems to be parallel, or in some respects convergent development of the two groups.

Origin of Monocotyledons:

This is the subject of great controversy. For some time this was thought that the monocotyledons were more primitive than the dicotyledons and probably they have given rise to the dicotyledons. Now, this belief has been totally given up. It is now generally thought that the dicotyledons are more primitive and they have given rise to the monocotyledons.

This is also believed, that the monocotyledons were an offshoot of the primitive dicotyledons back in the early Mesozoic era. They are thought to be monophyletic

Principles of Taxonomy and Phylogeny of Angiosperms:

The problem of classifying the angiospermic plants in a systematic way may either be termed as, ‘taxonomy of angiosperitis’ or ‘systematic botany’. This is a functional science and deals with identification, nomenclature and classification of the plants found all over the world.

The angiosperms are widely distributed with so many morphological variations that sometimes it seems almost impossible to arrange them in systematic order.

Since the prehistoric times, people were interested in placing the plants in a systematic way and for the first time a few plants were classified according to their medicinal and food value and thus the study of taxonomy of flowering plants began. In nineteenth century, the formation of the principles of taxonomy began and several principles were formed, which are still very helpful in arranging the plants in a systematic order.

Swingle has proposed the principles, which have been uniformly accepted by the botanists. They are as follows:

1. Plant relationships are up and down genetic lines and these must constitute the framework of phylogenetic taxonomy. This will naturally form a branching but not reticulate structure.

2. Some evolutionary processes are progressive while others are regressive.

3. Evolution does not necessarily involve all organs of the plant at one time or in the same direction. One organ may be advancing while another is stationary or retrogressing

4. Evolution has generally been consistent and when a particular progression or regression has set in, it is persisted into the end of the phylum.

5. In any phylum the chlorophyll bearing plants precede the chlorophyll less ones. Saprophytes are derived from independent forms and parasites usually from the saprophytes among the lower plants, and from independent forms among the flowering plants.

6. Usually structures with many similar parts are more primitive, and those with fewer and dissimilar parts are more advanced.

7. Among seed plants the stem structure with collateral bundles arranged in a cylinder is more primitive than that with scattered bundles.

8. In most groups of seed plants woody members have preceded the herbaceous ones.

9. In most groups of seed plants erect members have preceded the vines.

10. Perennials are more primitive than biennials and biennials are usually more primitive than annuals.

11. Scalariform vessels are more primitive than vessels with round pits.

12. The spiral arrangement of the leaves on the stem and of the floral leaves precedes that of opposite and whorled types.

13. Simple leaves are more primitive than compound leaves.

14. Historically leaves were first persistent (evergreen) and later deciduous. This way, the deciduous leaves show advanced character.

15. Among the seed plants the netted venation of leaves is more primitive than the parallel venation.

16. The many-parted flower is the more primitive, the type with few parts being derived from it, and the change is accompanied by a progressive sterilization of sporophylls.

17. A condition in which the perianth consists of like segments is more primitive than one in which sepals and petals are unlike each other.

18. Flowers with petals preceded apetalous ones, the latter being derived by reduction.

19. Polypetalous flowers are more primitive than gamopetalous ones, the latter being derived from the former by symphysis.

20. Regular flowers are more primitive than irregular ones.

21. Spirally imbricate floral parts are more primitive than those that are whorled and valvate.

22. Hypogyny is the primitive structure and from it perigyny and epigyny have been derived.

23. Numerous carpels represent a more primitive condition (than united carpels).

24. Separate carpels represent a more primitive condition than united carpels.

25. Axile placentation preceded parietal and central placentation of ovaries.

26. The presence of numerous stamens indicates a more primitive condition than that of a few stamens.

27. Separate stamens preceded united stamens.

28. Evolution in angiosperms is believed to have proceeded from seeds with two seed coats to those with only one.

29. The primitive seed contains endosperm and a small embryo, the advanced type has little or no endosperm, with the food stored in a large embryo.

30. A straight embryo is usually more primitive than a curved one.

31. The solitary flower is more primitive than the inflorescence.

32. Bisexual flowers preceded unisexual flowers.

33. The monoecious condition is primitive than dioecious.

34. Simple and aggregate fruits preceded multiple fruits.

35. The same evolutionary phenomena have often been repeated as separate occurrences in different parts of the plant kingdom, e.g., loss of chlorophyll, loss of petals, stamens and carpels, acquisition of fleshy texture in fruits and of various types of thorns, change from simple to compound leaves, from erect to prostrate habit and from hypogynous to perigynous or epigynous insertion of floral parts, and lateral union (symphysis) of petals, stamens and carpels.

36. In determining the closeness of relationship between two families or other groups, it is usually best to compare with each other the most primitive, or basal members of each group, rather than those that are simplified by reduction or those that are most highly specialized.

Principles proposed by Swingle:

The following parallel principles of taxonomy proposed by C.E. Bessey (1915) are of equal importance and are used by all modern taxonomists of twentieth century. These are known as Besseyan principles.

1. There is progressive evolution, i.e., life advances from simplicity to complexity.

2. The existing simpler forms resemble more to their simple ancestors than to the present complex ones.

3. The evolved forms never become like ancestors.

4. The herbaceous habit of plants is more advanced than arboreal habit.

5. Annuals are more advanced than biennials and biennials are more advanced than perennials.

6. Hydrophytes, epiphytes, saprophytes and parasites are more advanced than ordinary terrestrial forms.

7. Compound leaves are more advanced than simple leaves.

8. Bisexual flowers are more primitive than unisexual ones.

9. Dioecious plants are more advanced than monoecious ones.

10. Inflorescence is more advanced than solitary flowers.

11. Gamopetalous flowers are more advanced than polypetalous flowers. The gamopetaly has been derived from polypetaly by the fusion of the petals.

12. Flowers lacking petals (apetaly) are more advanced.

13. Actinomorphic flowers are more primitive than zygomorphic ones.

14. Hypogyny precedes epigyny.

15. Apocarpous condition is more primitive than syncarpous condition.

16. Exalbuminous seeds are more advanced than albuminous seeds.

17. Polyandrous stamens precede jointed condition of stamens.

18. Simple fruits are more primitive than compound fruits.

Turril states, that “the angiosperms were the last of the great groups (Thallophyta, Bryophyta, Pteridophyta, Gymnosperms, Angiosperms) to appear. Hence, changes towards characters peculiar to the angiosperms may with some justification be read as progressive, the more so that they are now the dominant group ecologically over much of the land surface of the globe.”

The ‘conservative’ characters have been supposed by many taxonomists to be most useful and valuable in phylogenetic studies. For plants, the reproductive organs are supposed to be more conservative than the vegetative organs. For example, the morphogenetic evolutionary changes in carpels have been far greater than in any vegetative organ considered throughout Spermophyta.

According to Tuzson, the monocots form an older group than the dicots, because the smaller groups of monocots are separable into series and families which show greater gaps on the whole than those in dicots.

As a Gray is of opinion that a natural system of classification of plants aim to arrange all the known plants of the plant kingdom, in a series of grades according to their resemblances, in all respects, so that each species, genus, tribe, family, order, etc., shall stand next to those which it most resembles in all respects, or rather in the whole plan of structure.

For example, two plants may be very much alike in external appearance, yet very different in their principal structure.

According to Sprague the monophyletic groups are regarded as the only natural ones at and above the family level in existing angiosperms. On the other hand, Hutchinson in his phylogenetic scheme and classification definitely retains groups he considers polyphyletic, e.g., Asterales and Euphorbiales.

Oldest Angiosperms:

In Jurassic rocks have been found the oldest known plants which were angiospermous in the true sense of the term that is, in having seed enclosed in a carpel. These plants represent two closely-related genera of the order Caytoniales.

The carpels were borne on sporophylls. Each sporophyll consisted of a central stalk with pinnately arranged short side branches each of which was terminated by a carpel or fruit. The carpel was completely closed, and the tip of the portion of the pinnule which bent over became the stigma.

Pollen grains have been found attached to the stigmas. The seed were borne within the closed carpel (Fig. 6.3). The integument of the seed is rather strikingly similar to that of certain seed of seed-ferns. The fruits were fleshy, so the seeds may have been scattered by animals that ate the fruit.

The anthers are found on sporophylls similar in general outline to those which bore the carpels. The branches of the sporophylls divided, and the tips of the divisions bore groups of stamens (Fig. 6.2). The anthers were sessile or at the ends of short filaments. They had a four- winged form, and each wing seems to have contained a pollen sac. Thus they had the same general form as the stamen of a modern angiosperm.

The leaves which appear to belong to the known Caytoniales are of a type which was very widespread during the Jurassic period and extended from Triassic to Cretaceous times. The venation was netted, but similar to that of Glossopteris which is generally regarded as a seed-fern. The general character of the leaves, sporophylls, and seed indicate that the Caytoniales were derived from the seed ferns.

In the Caytoniales from the Jurassic the pollen seems to have been caught on a stigma, and the ovules are enclosed in ovaries, but in a form from the upper Triassic pollen grains are found in the micropyles of the seed. In this form there appear to have been canals running through the “stigma,” and through these canals pollen grains reached the micropyles of the seed (Fig. 6.4).

This Triassic Caytonia is yet completely angiospermous. It has been evidently not suggested that the walls of the fruit of the Caytoniales had their origin in the fusion of cupules which surrounded the seed of Cycadofilicales.

Unfortunately we do not know how the sporophylls of the Caytoniales were borne, or what kind of plant bore them, and so the relationship of the Caytoniales to modern angiosperms is obscure.

Ancestry of Angiosperms:

The ancestry of angiosperms has long been a moot question. There is not enough evidence to reach a definite decision, and there is much disagreement as to theory.

Among living gymnosperms the group which is most similar to the angiosperms is the Gnetales. While there is great dissimilarity between the Gnetales and the angiosperms, still there is enough similarity to convince many that either the angiosperms are descended from the Gnetales, perhaps from extinct forms, or else the two groups are closely related and have a common ancestry.

Some regard the Gnetales as being intermediate between the conifers and angiosperms, and so are inclined to a belief in a coniferous ancestry for angiosperms. This belief is based in part on similarity in wood structure and on the fact that in conifers, Gnetales, and angiosperms the fertilization is by male nuclei and not by spermatozoids as in all other living groups of land plants.

A rather complete understanding of the “flower” of Cycadeoidea was followed by an extensive discussion of a cycadophyte ancestry for angiosperms not from Cycadeoidea with its highly specialized stem, rather from one more nearly related to Williamsonia or Williamsoniella, but still more primitive.

It was pointed out that the bracts, stamens, and ovuliferous cone of Cycadeoidea were in the same relative positions as the perianth, stamens, and carpels of the angiosperms. Also, the embryo had two cotyledons, while the seeds were almost enclosed by the inter-seminal bracts.

The discovery of the Caytoniales, first in the Jurrasic and later in the late Triassic, has been taken by some as indicating that the angiosperms, through forms related to the Caytoniales, are descended direct from the seed-ferns.

A recent theory is that the carpels of angiosperms were derived through a modification and fusion of cupules which surrounded the seeds of Cycadofilicales, and that the ovaries of the Caytoniales represent an intermediate condition between the seed-ferns and modern angiosperms.

One feature which is common to practically all theories as to the origin of the angiosperms is that they go far back into early Mesozoic or latter Palaeozoic times also that their ancestors were generalized forms and the not such specialized ones as modem conifers or cycads or the late Mesozoic cycadeoideas.

A conservative suggestion is that they were derived from somewhere in the general gymnospermous complex, from a line in which the marked peculiarities of more modern groups had not become so pronounced as they appear in the well-known specialized types.

Our ignorance as to the ancestry of the angiosperms is not surprising when we remember how scanty our knowledge of plant floras is. If, as seems probable, the angiosperms evolved in Arctic regions, much of the record may be thickly covered with an icy mantle and inaccessible to us.

Also, the record of former vegetation is largely that in and around swamps, about lakes, and in similar situations and any trace of much of that which flourished on higher ground is forever lost.

However, in comparatively recent years a great deal of evidence as to the history and relationships of land plants has been discovered so we may hope that perhaps someday we may have a fairly connected account of the development of the angiosperms.


New take on early evolution of photosynthesis

A team of scientists from Arizona State University's School of Molecular Sciences has begun re-thinking the evolutionary history of photochemical reaction centers (RCs). Their analysis was recently published online in Photosynthesis Research and describes a new pathway that ancient organisms may have taken to evolve the great variety of photosynthetic RCs seen today across bacteria, algae, and plants. The study will go into print later this summer in a special issue dedicated to photochemical reaction centers.

Photosynthesis is the process by which plants, algae and some bacteria use the energy from the sun to power their metabolism. Plants and algae use this light energy to make sugars from water and carbon dioxide, releasing the oxygen that we breathe. But certain bacteria carry out a simpler form of photosynthesis that does not produce oxygen, and is believed to have evolved first.

The molecular engines in all photosynthetic organisms that convert light energy to chemical energy are called photochemical reaction centers. RCs are chlorophyll-containing proteins found in the cell membrane. Their first appearance and subsequent diversification has allowed photosynthesis to power the biosphere for over 3 billion years, in the process supporting the evolution of more complex life forms.

There are two types of RCs that exist today: Type I RCs support metabolism by moving electrons to soluble proteins, while Type II RCs move electrons to membrane-associated molecules. However, evidence has been building in the lab of professor Kevin Redding that the RC from the heliobacteria may be able to perform both of these functions, making it a functional hybrid of the two RC types.

The heliobacterial RC is thought to be one of the simplest RCs still around today. It is homodimeric, meaning that its core is composed of two copies of the same protein. This contrasts with the two RCs from oxygen-producing organisms like plants whose core is heterodimeric, having their core composed of two similar, but not identical, proteins.

It is believed that the RC arose only once during Earth's history, and that all the RCs around today are distantly related to that original ancestor. Over time, these RCs have changed to perform different chemistries. While the amino acid sequences have changed a great deal, astonishingly, the overall structure of RCs has remained similar. The team believes that the ancestral reaction center (ARC) was simpler than the versions that exist today. This ARC was probably homodimeric and interacted with molecules in the membrane, like the modern Type II RCs (and the heliobacterial RC), instead of with soluble proteins.

It is very difficult to reconstruct these evolutionary steps, which took over 3 billion years to occur. One way in which this is generally done is to compare the amino acid sequences of the proteins and note the number of differences between them, assuming that more similarity means that they are more closely related. In their study, however, the team cautions against relying heavily on this method for RCs. The sequence differences are just too numerous and too much time has passed to obtain reliable information from this method.

They instead compared the positions of protein structural elements and chlorophylls within the RCs. In essence, they focused more on the structure and function of the RCs to reconstruct the evolutionary history, starting by making predictions about the ARC's structure and function.

Structural overlay

The team envisions that the ARC, in its simplest form, was probably rather inefficient at its chemistry. Its job was to use the energy of sunlight to provide two electrons to a membrane-associated molecule called a quinone. However, the ARC likely could loosely bind two quinone molecules, one on each side of the core. With two identical-looking quinones, the ARC was not able to prioritize which quinone would get electrons, making it less likely that either would get the two it needed.

This problem was solved in two different ways. In the lineage that led to the modern Type II RCs, the core changed from homodimeric to heterodimeric, which allowed the RC to prioritize which quinone it gave electrons to, accelerating the chemistry. In the lineage that led to the modern Type I RCs, the core remained homodimeric, but a metal cluster was added so that the first electron would end up there, facilitating its delivery to the quinone that received the next electron.

Once the ARC had acquired the metal cluster, thus becoming the ancestor to all modern Type I RCs, more changes occurred to further direct electrons to a soluble acceptor, which resulted in extracting more energy for the cell's metabolism. These included changes in the positions and identities of the chlorophyll cofactors. Much of the later changes in the Type I RCs were driven by the need to deal with the presence of oxygen, as the unstable intermediates within RCs can react with oxygen to generate very damaging molecules. In the opinion of the ASU team, the heliobacterial RC retains clear vestiges of the changes leading to the early Type I RC and that understanding the fine details in how modern RCs work allows for informed hypotheses about how they evolved.


Watch the video: The Evolution of early Plants (February 2023).