Evolutionary developmental biology (evo-devo) is the study of developmental programs and patterns from an evolutionary perspective.^[1] It seeks to understand the various influences shaping the form and nature of life on the planet.Evo-devo arose as a separate branch of science rather recently. An early sign of this occurred in 1999.^[2]

Most of the synthesis in evo-devo has been in the field ofanimal evolution, one reason being the presence ofmodel systems likeDrosophila melanogaster,C. elegans,zebrafish andXenopus laevis. However, since 1980, a wealth of information onplant morphology, coupled with modern molecular techniques has helped shed light on the conserved and unique developmental patterns in theplant kingdom also.^[3][4]

The origin of the term "morphology" is generally attributed toJohann Wolfgang von Goethe (1749–1832). He was of the opinion that there is an underlying fundamental organisation (Bauplan) in the diversity offlowering plants. In his bookThe Metamorphosis of Plants, he proposed that theBauplan enabled us to predict the forms of plants that had not yet been discovered.^[5] Goethe was the first to make the perceptive suggestion thatflowers consist of modifiedleaves. He also entertained different complementary interpretations.^[6][7]

In the middle centuries, several basic foundations of our current understanding of plant morphology were laid down.Nehemiah Grew,Marcello Malpighi,Robert Hooke,Antonie van Leeuwenhoek,Wilhelm von Nageli were just some of the people who helped build knowledge onplant morphology at various levels of organisation. It was the taxonomical classification ofCarl Linnaeus in the eighteenth century though, that generated a firm base for the knowledge to stand on and expand.^[8] The introduction of the concept ofDarwinism in contemporary scientific discourse also had had an effect on the thinking on plant forms and their evolution.^{[citation needed]}

Wilhelm Hofmeister, one of the most brilliant botanists of his times, was the one to diverge away from the idealist way of pursuing botany. Over the course of his life, he brought aninterdisciplinary outlook into botanical thinking. He came up with biophysical explanations on phenomena likephototaxis andgeotaxis, and also discovered thealternation of generations in the plant life cycle.^[5]

The past century witnessed a rapid progress in the study ofplant anatomy. The focus shifted from thepopulation level to morereductionist levels. While the first half of the century saw expansion in developmental knowledge at thetissue and theorgan level, in the latter half, especially since the 1990s, there has also been a strong impetus on gaining molecular information.^{[citation needed]}

Edward Charles Jeffrey was one of the earlyevo-devo researchers of the 20th century. He performed a comparative analyses of the vasculatures of living andfossilgymnosperms and came to the conclusion that the storageparenchyma has been derived fromtracheids.^[9] His research^[10] focussed primarily onplant anatomy in the context ofphylogeny. This tradition of evolutionary analyses of plant architectures was further advanced byKatherine Esau, best known for her bookThe Plant Anatomy. Her work focussed on the origin and development of various tissues in different plants. Working withVernon Cheadle,.^[11] She also explained the evolutionary specialization of thephloem tissue with respect to its function.^{[citation needed]}

In 1959 Walter Zimmermann published a revised edition ofDie Phylogenie der Planzen.^[12] This very comprehensive work, which has not been translated into English, has no equal in the literature. It presents plant evolution as the evolution of plant development (hologeny). In this sense it is plant evolutionary developmental biology (plant evo-devo). According to Zimmermann, diversity in plant evolution occurs though various developmental processes. Three very basic processes areheterochrony (changes in the timing of developmental processes), heterotopy (changes in the relative positioning of processes), and heteromorphy (changes in form processes).^[13]

In the meantime, by the beginning of the latter half of the 1900s,Arabidopsis thaliana had begun to be used in some developmental studies. The first collection ofArabidopsis thaliana mutants were made around 1945.^[14] However it formally became established as amodel organism only in 1998.^[15]

Wikispecies has information related toArabidopsis thaliana.

The recent spurt in information on various plant-related processes has largely been a result of the revolution inmolecular biology. Powerful techniques likemutagenesis andcomplementation were made possible inArabidopsis thaliana via generation ofT-DNA containing mutant lines, recombinantplasmids, techniques liketransposon tagging etc. Availability of complete physical and genetic maps,^[16]RNAi vectors, and rapidtransformation protocols are some of the technologies that have significantly altered the scope of the field.^[15] Recently, there has also been a massive increase in thegenome andEST sequences^[17] of various non-model species, which, coupled with thebioinformatics tools existing today, generate opportunities in the field of plant evo-devo research.

Gérard Cusset provided a detailed in-depth analysis of the history of plant morphology, including plant development and evolution, from its beginnings to the end of the 20th century.^[18]Rolf Sattler discussed fundamental principles of plant morphology^[19][7] and plant evo-devo.^[13][20][21]Rolf Rutishauser surveyed the past and future of plant evo-devo with regard to continuum and process morphology.^[22] Two voluminous, richly illustrated volumes have become milestones for plant morphology and plant evo-devo in the 21st century: Kaplan, D. R, edited by Specht, C.D. (2022)Principles of Plant Morphology^[23][24] and Claβen-Bockhoff, R. (2024)Die Pflanze: Morphologie, Entwickung und Evolution von Vielfalt (The Plant: Morphology, Development and Evolution of Diversity)^[25][26] Whereas Kaplan's work remained firmly within the framework of classical mainstream morphology,^[27] Claβen-Bockhoff introduced conceptual innovations that open up new avenues for evo-devo, especially morpho evo-devo.^[28] The most recent innovation isArticulation Morphology.^[29] Articulation Morphology, grounded in the open growth of plants, focuses on ramification as the key principle of plant morphology. This principle entails articulation: the formation of articles. Unlike organs, which are defined in terms of a morphological theory such as the root-stem-leaf model, articles, which have been almost completely overlooked, are directly observable, that is, they are factual and objective. As such, articulation morphology based on articles offers a common foundation for all morphologists, regardless of their theoretical preferences. While so far plant morphology has been centered around homology, articulation morphology shifts the focus to the transformation of ramification and articulation during development and evolution. This shift has far-reaching consequences for plant morphology and plant evo-devo.^[29]

The most importantmodel systems in plant development have beenarabidopsis andmaize. Maize has traditionally been the favorite of plant geneticists, while extensive resources in almost every area ofplant physiology and development are available forArabidopsis thaliana. Apart from these,rice,Antirrhinum majus,Brassica, andtomato are also being used in a variety of studies. The genomes ofArabidopsis thaliana and rice have been completely sequenced, while the others are in process.^[30] It must be emphasized here that the information from these "model" organisms form the basis of our developmental knowledge. WhileBrassica has been used primarily because of its convenient location in thephylogenetic tree in themustard family,Antirrhinum majus is a convenient system for studyingleaf architecture.Rice has been traditionally used for studying responses tohormones likeabscissic acid andgibberelin as well as responses tostress. However, recently, not just thedomesticated rice strain, but also thewild strains have been studied for their underlying genetic architectures.^[31]

Some people have objected against extending the results ofmodel organisms to theplant world. One argument is that the effect ofgene knockouts in lab conditions wouldn't truly reflect even the same plant's response in thenatural world. Also, these supposedlycrucial genes might not be responsible for the evolutionary origin of that character. For these reasons, a comparative study of planttraits has been proposed as the way to go now.^[32]

Since the past few years, researchers have indeed begun looking at non-model, "non-conventional" organisms using modern genetic tools. One example of this is theFloral Genome Project, which envisages to study the evolution of the current patterns in the genetic architecture of the flower through comparative genetic analyses, with a focus on EST sequences.^[33] Like the FGP, there are several such ongoing projects that aim to find out conserved and diverse patterns in evolution of the plant shape.Expressed sequence tag (EST) sequences of quite a few non-model plants likesugarcane,apple,barley,cycas,coffee, to name a few, are available freely online.^[34] The Cycad Genomics Project,^[35] for example, aims to understand the differences in structure and function ofgenes betweengymnosperms andangiosperms through sampling in theorderCycadales. In the process, it intends to make available information for the study ofevolution of seeds,cones and evolution of life cycle patterns. Presently the most important sequenced genomes from anevo-devo point of view include those ofA. thaliana (a flowering plant),poplar (a woody plant),Physcomitrella patens (a bryophyte),Maize (extensive genetic information), andChlamydomonas reinhardtii (a green alga). The impact of such a vast amount of information on understanding common underlying developmental mechanisms can easily be realised.

Apart fromEST andgenome sequences, several other tools likePCR,yeast two-hybrid system,microarrays,RNA Interference,SAGE,QTL mapping etc. permit the rapid study of plant developmental patterns. Recently, cross-species hybridization has begun to be employed on microarray chips, to study the conservation and divergence inmRNA expression patterns between closely relatedspecies.^[36] Techniques for analyzing this kind of data have also progressed over the past decade. We now have better models formolecular evolution, more refined analysisalgorithms and bettercomputing power as a result of advances incomputer sciences.^{[citation needed]}

Evidence suggests that an algal scum formed on the land1,200 million years ago, but it was not until the Ordovician period, around500 million years ago, that land plants appeared. These began to diversify in the late Silurian period, around420 million years ago, and the fruits of their diversification are displayed in remarkable detail in an earlyDevonian fossil assemblage known as theRhynie chert. This chert preserved early plants in cellular detail, petrified in volcanic springs. By the middle of the Devonian period most of the features recognised in plants today are present, including roots and leaves. By the late Devonian, plants had reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued after the Devonian period. Most plant groups were relatively unscathed by thePermo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the evolution of flowering plants in the Triassic (~200 million years ago), which exploded the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from around40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO₂ and warm, dry conditions of the tropics over the last10 million years. Although animals and plants evolved theirbodyplan independently, they both express a developmental constraint during mid-embryogenesis that limits their morphological diversification.^{[37][38][39][40][41]}

Themeristem architectures differ betweenangiosperms,gymnosperms andpteridophytes. Thegymnosperm vegetative meristem lacks organization into distinct tunica and corpus layers. They possess large cells called central mother cells. Inangiosperms, the outermost layer of cells divides anticlinally to generate the new cells, while in gymnosperms, the plane of division in the meristem differs for different cells. However, the apical cells do contain organelles like largevacuoles andstarch grains, like the angiosperm meristematic cells.

Pteridophytes, likefern, on the other hand, do not possess a multicellular apical meristem. They possess atetrahedral apical cell, which goes on to form the plant body. Anysomatic mutation in this cell can lead to hereditary transmission of thatmutation.^[42] The earliest meristem-like organization is seen in analgal organism from groupCharales that has a single dividing cell at the tip, much like the pteridophytes, yet simpler.One can thus see a clear pattern in evolution of the meristematic tissue, from pteridophytes to angiosperms: Pteridophytes, with a single meristematic cell; gymnosperms with a multicellular, but less definedorganization; and finally,angiosperms, with the highest degree of organization.

Transcription factors and transcriptional regulatory networks play key roles in plant development and stress responses, as well as their evolution. During plant landing, many novel transcription factor families emerged and are preferentially wired into the networks of multicellular development, reproduction, and organ development, contributing to more complex morphogenesis of land plants.^[43]

Leaves are the primaryphotosynthetic organs of a plant. Based on their structure, they are classified into two types -microphylls, that lack complex venation patterns andmegaphylls, that are large and with a complexvenation. It has been proposed that these structures arose independently.^[44] Megaphylls, according to thetelome theory, have evolved from plants that showed a three-dimensional branching architecture, through three transformations:planation, which involved formation of aplanar architecture,webbing, or formation of the outgrowths between the planar branches andfusion, where these webbed outgrowths fused to form a properleaf lamina.Studies have revealed that these three steps happened multiple times in the evolution of today's leaves.^[45]

Contrary to the telome theory, developmental studies of compound leaves have shown that, unlike simple leaves, compound leaves branch in three dimensions.^[46][47] Consequently, they appear partially homologous with shoots as postulated byAgnes Arber in her partial-shoot theory of the leaf.^[48] They appear to be part of a continuum between morphological categories, especially those of leaf and shoot.^[49][50] Molecular genetics confirmed these conclusions (see below).

It has been proposed that the before the evolution ofleaves, plants had thephotosynthetic apparatus on the stems. Today's megaphyll leaves probably became commonplace some 360mya, about 40 my after the simple leafless plants had colonized the land in theearly Devonian period. This spread has been linked to the fall in the atmosphericcarbon dioxide concentrations in the latePaleozoic era associated with a rise in density ofstomata on leaf surface. This must have allowed for bettertranspiration rates and gas exchange. Large leaves with less stomata would have heated up in the sun's rays, but an increased stomatal density allowed for a better-cooled leaf, thus making its spread feasible.^[51][52]

Various physical and physiological forces likelight intensity,humidity,temperature,wind speeds etc. are thought to have influenced evolution of leaf shape and size. It is observed that high trees rarely have large leaves, owing to the obstruction they generate for winds. This obstruction can eventually lead to the tearing of leaves, if they are large. Similarly, trees that grow intemperate ortaiga regions have pointed leaves, presumably to preventnucleation of ice onto the leaf surface and reduce water loss due to transpiration.Herbivory, not only by large mammals, but also smallinsects has been implicated as a driving force in leaf evolution, an example being plants of the genusAciphylla, that are commonly found inNew Zealand. The now-extinctmoas (birds) fed upon these plants, and the spines on the leaves probably discouraged the moas from feeding on them. Other members ofAciphylla that did not co-exist with the moas were spineless.^[53]

At the genetic level, developmental studies have shown that repression of the KNOX genes is required for initiation of theleafprimordium. This is brought about byARP genes, which encodetranscription factors. Genes of this type have been found in many plants studied till now, and the mechanism i.e. repression of KNOX genes in leaf primordia, seems to be quite conserved. Expression of KNOX genes in leaves produces complex leaves. It is speculated that theARP function arose quite early invascular plant evolution, because members of the primitive grouplycophytes also have a functionally similar gene^[54] Other players that have a conserved role in defining leaf primordia are the phytohormoneauxin,gibberelin andcytokinin.^{[citation needed]}

One feature of a plant is itsphyllotaxy. The arrangement of leaves on the plant body is such that the plant can maximally harvest light under the given constraints, and hence, one might expect the trait to be geneticallyrobust. However, it may not be so. Inmaize, a mutation in only one gene calledabphyl (abnormal phyllotaxy) was enough to change the phyllotaxy of the leaves. It implies that sometimes, mutational tweaking of a single locus on thegenome is enough to generate diversity. Theabphyl gene was later on shown to encode acytokinin response regulator protein.^[55]

Once the leaf primordial cells are established from the SAM cells, the newaxes for leaf growth are defined, one important (and more studied) among them being the abaxial-adaxial (lower-upper surface) axis. The genes involved in defining this, and the other axes seem to be more or less conserved among higher plants. Proteins of theHD-ZIPIII family have been implicated in defining the adaxial identity. These proteins deviate some cells in the leafprimordium from the defaultabaxial state, and make themadaxial. It is believed that in early plants with leaves, the leaves just had one type of surface - the abaxial one. This is the underside of today's leaves. The definition of the adaxial identity occurred some 200 million years after the abaxial identity was established.^[32] One can thus imagine the early leaves as an intermediate stage in evolution of today's leaves, having just arisen from spiny stem-like outgrowths of their leafless ancestors, covered withstomata all over, and not optimized as much forlight harvesting.^{[citation needed]}

How the infinite variety of plant leaves is generated is a subject of intense research. Some common themes have emerged. One of the most significant is the involvement of KNOX genes in generatingcompound leaves, as intomato(see above). But this again is not universal. For example,pea uses a different mechanism for doing the same thing.^[56][57] Mutations in genes affecting leafcurvature can also change leaf form, by changing the leaf from flat, to a crinkly shape,^[58] like the shape ofcabbage leaves. There also exist differentmorphogen gradients in a developing leaf which define the leaf's axis. Changes in these morphogen gradients may also affect the leaf form. Another very important class of regulators of leaf development are themicroRNAs, whose role in this process has just begun to be documented. The coming years should see a rapid development in comparative studies on leaf development, with manyEST sequences involved in the process coming online.^{[citation needed]}

Molecular genetics has also shed light on the relation between radial symmetry (characteristic of stems) and dorsiventral symmetry (typical for leaves). James (2009) stated that "it is now widely accepted that... radiality [characteristic of most shoots] and dorsiventrality [characteristic of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!"^[59] In fact there is evidence for this continuum already at the beginning of land plant evolution.^[60] Furthermore, studies in molecular genetics confirmed that compound leaves are intermediate between simple leaves and shoots, that is, they are partially homologous with simple leaves and shoots, since "it is now generally accepted that compound leaves express both leaf and shoot properties".^[61] This conclusion was reached by several authors on purely morphological grounds.^[46][47]

Flower-like structures first appear in thefossil records some ~130 mya, in theCretaceous era.^[62]

The flowering plants have long been assumed to have evolved from within thegymnosperms; according to the traditional morphological view, they are closely allied to thegnetales. However, recent molecular evidence is at odds to this hypothesis,^[63][64] and further suggests that gnetales are more closely related to some gymnosperm groups than angiosperms,^[65] and that gymnosperms form a distinct clade to the angiosperms,.^[63][64][65]Molecular clock analysis predicts the divergence offlowering plants (anthophytes) andgymnosperms to ~300 mya^[66]

The main function of a flower isreproduction, which, before the evolution of the flower andangiosperms, was the job of microsporophylls and megasporophylls. A flower can be considered a powerful evolutionaryinnovation, because its presence allowed the plant world to access new means and mechanisms for reproduction.^{[citation needed]}

It seems that on the level of the organ, theleaf may be the ancestor of the flower, or at least some floral organs. When wemutate some crucial genes involved in flower development, we end up with a cluster of leaf-like structures. Thus, sometime in history, the developmental program leading to formation of a leaf must have been altered to generate a flower. There probably also exists an overall robust framework within which the floral diversity has been generated. An example of that is a gene calledLEAFY (LFY), which is involved in flower development inArabidopsis thaliana. Thehomologs of this gene are found inangiosperms as diverse astomato,snapdragon,pea,maize and evengymnosperms. Expression ofArabidopsis thaliana LFY in distant plants likepoplar andcitrus also results in flower-production in these plants. TheLFY gene regulates the expression of some gene belonging to theMADS-box family. These genes, in turn, act as direct controllers of flower development.^{[citation needed]}

The members of theMADS-box family of transcription factors play a very important and evolutionarily conserved role in flower development. According to theABC model of flower development, three zones - A, B and C - are generated within the developing flower primordium, by the action of sometranscription factors, that are members of theMADS-box family. Among these, the functions of the B and C domain genes have been evolutionarily more conserved than the A domain gene. Many of these genes have arisen throughgene duplications of ancestral members of this family. Quite a few of them show redundant functions.^{[citation needed]}

The evolution of theMADS-box family has been extensively studied. These genes are present even inpteridophytes, but the spread and diversity is many times higher inangiosperms.^[68] There appears to be quite a bit of pattern into how this family has evolved. Consider the evolution of the C-region geneAGAMOUS (AG). It is expressed in today's flowers in thestamens, and thecarpel, which are reproductive organs. It's ancestor ingymnosperms also has the same expression pattern. Here, it is expressed in thestrobili, an organ that producespollens or ovules.^[69] Similarly, the B-genes'(AP3 and PI) ancestors are expressed only in the male organs ingymnosperms. Their descendants in the modern angiosperms also are expressed only in thestamens, the male reproductive organ. Thus, the same, then-existing components were used by the plants in a novel manner to generate the first flower. This is a recurring pattern inevolution.^{[citation needed]}

How is the enormous diversity in the shape, color and sizes of flowers established? There is enormous variation in the developmental program in different plants. For example,monocots possess structures likelodicules and palea, that were believed to be analogous to thedicot petals and carpels respectively. It turns out that this is true, and the variation is due to slight changes in the MADS-box genes and their expression pattern in the monocots. Another example is that of the toad-flax,Linaria vulgaris, which has two kinds of flower symmetries:radial andbilateral. These symmetries are due to changes in copy number, timing, and location of expression inCYCLOIDEA, which is related to TCP1 in Arabidopsis.^[62][70]

Arabidopsis thaliana has a gene calledAGAMOUS that plays an important role in defining how manypetals andsepals and other organs are generated. Mutations in this gene give rise to the floralmeristem obtaining an indeterminate fate, and many floral organs keep on getting produced. We have flowers likeroses,carnations andmorning glory, for example, that have very dense floral organs. These flowers have been selected by horticulturists since long for increased number ofpetals. Researchers have found that the morphology of these flowers is because of strongmutations in theAGAMOUS homolog in these plants, which leads to them making a large number of petals and sepals.^[71] Several studies on diverse plants likepetunia,tomato,impatiens,maize etc. have suggested that the enormous diversity of flowers is a result of small changes ingenes controlling their development.^[72]

Some of these changes also cause changes in expression patterns of the developmental genes, resulting in differentphenotypes. TheFloral Genome Project looked at theEST data from various tissues of many flowering plants. The researchers confirmed that theABC Model of flower development is not conserved across allangiosperms. Sometimes expression domains change, as in the case of manymonocots, and also in some basal angiosperms likeAmborella. Different models of flower development like thefading boundaries model, or theoverlapping-boundaries model which propose non-rigid domains of expression, may explain these architectures.^[73] There is a possibility that from the basal to the modern angiosperms, the domains of floral architecture have gotten more and more fixed through evolution.^{[citation needed]}

Another floral feature that has been a subject ofnatural selection is flowering time. Some plants flower early in their life cycle, others require a period ofvernalization before flowering. This decision is based on factors liketemperature,light intensity, presence ofpollinators and other environmental signals. InArabidopsis thaliana it is known that genes likeCONSTANS (CO),FRIGIDA,Flowering Locus C (FLC) andFLOWERING LOCUS T (FT) integrate the environmental signals and initiate the flower development pathway. Allelic variation in these loci have been associated with flowering time variations between plants. For example,Arabidopsis thaliana ecotypes that grow in the coldtemperate regions require prolonged vernalization before they flower, while thetropical varieties and common lab strains, do not. Much of this variation is due to mutations in theFLC andFRIGIDA genes, rendering them non-functional.^[74]

Many genes in the flowering time pathway are conserved across all plants studied to date. However, this does not mean that the mechanism of action is similarly conserved. For example, the monocotrice accelerates its flowering in short-day conditions, whileArabidopsis thaliana, a eudicot, responds to long-day conditions. In both plants, the proteinsCO andFT are present but inArabidopsis thalianaCO enhancesFT production, while in rice theCO homolog repressesFT production, resulting in completely opposite downstream effects.^[75]

There are many theories that propose how flowers evolved. Some of them are described below.

TheAnthophyte Theory was based on the observation that a gymnospermic familyGnetaceae has a flower-likeovule. It has partially developedvessels as found in theangiosperms, and themegasporangium is covered by three envelopes, like theovary structure of angiosperm flowers. However, many other lines of evidence show that gnetophytes are not related to angiosperms.^[67]

TheMostly Male Theory has a more genetic basis. Proponents of this theory point out that the gymnosperms have two very similar copies of the geneLFY while angiosperms only have one.Molecular clock analysis has shown that the otherLFY paralog was lost in angiosperms around the same time as flower fossils become abundant, suggesting that this event might have led to floral evolution.^[76] According to this theory, loss of one of theLFYparalog led to flowers that were more male, with theovules being expressed ectopically. These ovules initially performed the function of attractingpollinators, but sometime later, may have been integrated into the core flower.^{[citation needed]}

In 1878 Charles Darwin published a book "The Effects of Cross and Self-Fertilization in the Vegetable Kingdom"^[77] and in the initial paragraph of chapter XII noted "The first and most important of the conclusions which may be drawn from the observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented."Flowers likely emerged in plant evolution as an adaptation to facilitate cross-fertilisation (outcrossing), a process that allows the masking of recessive deleteriousmutations in thegenome of progeny. This masking effect is referred to asgenetic complementation.^[78] This beneficial effect of cross-fertilisation on progeny is also considered to be the basis ofhybrid vigor orheterosis. Once flowers became established in a lineage with the adaptive function of promoting cross-fertilization, subsequent switching to inbreeding usually then becomes disadvantageous, in large part because it allows expression of the previously masked deleterious recessive mutations, i.e.inbreeding depression. Also,meiosis, the process in flowering plants by which seed progeny are produced, provides a direct mechanism forrepairing germ-line DNA through genetic recombination.^[79] Thus, in flowering plants, the two fundamental aspects of sexual reproduction are cross-fertilization (outcrossing) and meiosis and these appear to be maintained respectively by the advantages of genetic complementation and recombinational repair of germline DNA.^[78]

Plantsecondary metabolites are lowmolecular weight compounds, sometimes with complex structures that have no essential role inprimary metabolism. They function in processes such as anti-herbivory,pollinator attraction,communication between plants,allelopathy, maintenance ofsymbiotic associations withsoil flora and enhancing the rate offertilization^[how?]. Secondary metabolites have great structural and functional diversity and many thousands of enzymes may be involved in their synthesis, coded for by as much as 15–25% of the genome.^[80] Many plant secondary metabolites such as the colour and flavor components ofsaffron and the chemotherapeutic drugtaxol are of culinary and medical significance to humans and are therefore of commercial importance. In plants they seem to have diversified using mechanisms such as gene duplications, evolution of novel genes and the development of novel biosynthetic pathways. Studies have shown that diversity in some of these compounds may be positively selected for.^{[citation needed]}Cyanogenic glycosides may have been proposed to have evolved multiple times in different plant lineages, and there are several other instances ofconvergent evolution. For example, the enzymes for synthesis oflimonene – aterpene – are more similar between angiosperms and gymnosperms than to their own terpene synthesis enzymes. This suggests independent evolution of the limonene biosynthetic pathway in these two lineages.^[81]

While environmental factors are significantly responsible for evolutionary change, they act merely as agents fornatural selection. Some of the changes develop through interactions withpathogens. Change is inherently brought about via phenomena at the genetic level –mutations, chromosomal rearrangements andepigenetic changes. While the general types of mutations hold true across the living world, in plants, some other mechanisms have been implicated as highly significant.^{[citation needed]}

Polyploidy is a very common feature in plants. It is believed that at least half plants are or have been polyploids. Polyploidy leads togenome doubling, thus generating functional redundancy in most genes. The duplicated genes may attain new function, either by changes in expression pattern or changes in activity. Polyploidy andgene duplication are believed to be among the most powerful forces in evolution of plant form. It is not known though, why genome doubling is such a frequent process in plants. One possible reason is the production of large amounts ofsecondary metabolites in plant cells. Some of them might interfere in the normal process ofchromosomal segregation, leading topolypoidy.^{[citation needed]}

In recent times, plants have been shown to possess significantmicroRNA families, which are conserved across many plant lineages. In comparison toanimals, while the number of plant miRNA families is less, the size of each family is much larger. The miRNA genes are also much more spread out in the genome than those in animals, where they are found clustered. It has been proposed that these miRNA families have expanded by duplications of chromosomal regions.^[82] Many miRNA genes involved in regulation ofplant development have been found to be quite conserved between plants studied.^{[citation needed]}

Domestication of plants such asmaize,rice,barley,wheat etc. has also been a significant driving force in their evolution. Some studies^{[clarification needed]} have looked at the origins of the maize plant and found that maize is a domesticated derivative of a wild plant fromMexico calledteosinte. Teosinte belongs to thegenusZea, just as maize, but bears very smallinflorescence, 5–10 hard cobs, and a highly branched and spread-out stem.^{[citation needed]}

Crosses between a particular teosinte variety and maize yield fertile offspring that are intermediate inphenotype between maize and teosinte.QTL analysis has also revealed some loci that when mutated in maize yield a teosinte-like stem or teosinte-like cobs.Molecular clock analysis of these genes estimates their origins to some 9000 years ago, well in accordance with other records of maize domestication. It is believed that a small group of farmers must have selected some maize-like natural mutant of teosinte some 9000 years ago in Mexico, and subjected it to continuous selection to yield the maize plant as known today.^[83]

Another case is that ofcauliflower. The edible cauliflower is a domesticated version of the wild plantBrassica oleracea, which does not possess the dense undifferentiated inflorescence, called the curd, that cauliflower possesses.^{[citation needed]}

Wikispecies has information related toBrassicaceae.

Cauliflower possesses a single mutation in a gene calledCAL, controllingmeristem differentiation into inflorescence. This causes the cells at the floral meristem to gain an undifferentiated identity, and instead of growing into aflower, they grow into a lump of undifferentiated cells.^[84] This mutation has been selected through domestication at least since theGreek empire.^{[citation needed]}

• Plant morphology
• Comparative phylogenetics
• Plant evolution
• Evolutionary history of plants

• The Genetics of plant morphological evolution
• Plant evolution and development in a post-genomic context
• Evolution of leaf developmental mechanisms
• Developmental genetics and plant evolutionISBN 0-415-25791-3

• Plant development
• Evolutionary biology
• Evolution of plants

• Webarchive template wayback links
• Articles with short description
• Short description matches Wikidata
• Articles containing German-language text
• All articles with unsourced statements
• Articles with unsourced statements from June 2025
• Wikipedia articles needing clarification from April 2013
• Articles with unsourced statements from April 2013
• Wikipedia articles needing clarification from June 2013