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Early Evolution of the Vascular Plant Body Plan — the Missing Mechanisms

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Early Evolution of the Vascular Plant Body Plan — the Missing Mechanisms Available online at www.sciencedirect.com ScienceDirect Early evolution of the vascular plant body plan — the missing mechanisms 1 2 3 Alexandru MF Tomescu , Sarah E Wyatt , Mitsuyasu Hasebe and 2,4 Gar W Rothwell The complex body plan of modern vascular plants evolved by tracheophytes are polysporangiophytes, i.e. plants with modification of simple systems of branching axes which branched sporophytes, but the polysporangiophytes also originated from the determinate vegetative axis of a bryophyte- include extinct lineages that did not produce vascular grade ancestor. Understanding body plan evolution and tissue (i.e. xylem with tracheids as conducting elements homologies has implications for land plant phylogeny and and phloem) [2]. Early polysporangiophytes had simple requires resolution of the specific developmental changes and sporophytes consisting of undifferentiated, dichotomously their evolutionary sequence. The branched sporophyte may branching axes [1,3]. Nevertheless, the evolution of the have evolved from a sterilized bryophyte sporangium, but branched sporophyte paved the way to indeterminate prolongation of embryonic vegetative growth is a more modular growth which, in concert with the evolution of parsimonious explanation. Research in the bryophyte model highly specialized conductive tissues (xylem and phloem), system Physcomitrella points to mechanisms regulating led to nutritional independence of the diploid phase. sporophyte meristem maintenance, indeterminacy, branching Combined with subsequent evolution of specialized vege- and the transition to reproductive development. These results tative organs (stems, leaves, roots) these led to the plant can form the basis for hypotheses to identify and refine the sporophyte-sustained ecosystems that are ubiquitous on nature and sequence of changes in development that occurred land today. during the evolution of the indeterminate branched sporophyte from an unbranched bryophyte-grade sporophyte. Currently, there is intense interest in the origins of Addresses polysporangiophytes and vascular plants, which has 1 Department of Biological Sciences, Humboldt State University, Arcata, generated several recent reviews of the topic CA 95521, USA 2 [4,5 ,6 ,7 ,8 ]. Polysporangiophytes, along with the Department of Environmental and Plant Biology, Ohio University, three bryophyte lineages — liverworts, hornworts and Athens, OH 45701, USA 3 mosses — form the embryophyte clade [1,8 ,9 ]. National Institute for Basic Biology and Department of Basic Biology, School of Life Science, The Graduate School for Advanced Studies, Although there is wide agreement that polysporangio- Okazaki 444-8585, Japan phytes (including vascular plants) evolved from bryophy- 4 Department of Botany and Plant Pathology, Oregon State University, tic grade embryophytes [1,9 ] somewhere between the Corvallis, OR 97331, USA mid-Ordovician and the mid-Silurian, 450–430 Ma ago Corresponding author: Tomescu, Alexandru MF ([email protected]) [10–13], several aspects of this process remain obscure or are in dispute: the immediate ancestor or sister group of polysporangiophytes; the specific changes that initiated Current Opinion in Plant Biology 2014, 17:126–136 polysporangiophyte origins; the homologies of ancestral This review comes from a themed issue on Growth and development polysporangiophyte vegetative organs; the exact evol- Edited by David R Smyth and Jo Ann Banks utionary sequence of changes leading to modern plant structure; and of fundamental importance, the genetic regulatory mechanisms that underlie the changes. S1369-5266/$ – see front matter, # 2013 Elsevier Ltd. All rights Attempts to answer these questions using a phylogenetic reserved. approach — that is, resolve phylogeny and use it to infer http://dx.doi.org/10.1016/j.pbi.2013.11.016 steps and mechanisms of evolutionary change, and hom- ologies — have thus far not borne fruit. Despite a plethora of studies employing molecular markers, the phyloge- Polysporangiophyte origins — the interplay of netic relationships between the basal embryophytes phylogeny and development lineages (liverworts, hornworts, mosses, and polysporan- The origin of vascular plants is arguably the most important giophytes) remain largely unresolved [14,15]. Because of evolutionary event in the development of the terrestrial the depth of phylogenetic divergences associated with biosphere. In modern terrestrial biomes, vascular plants are the early stages of embryophyte and polysporangiophyte the dominant source of primary productivity, providing the evolution, as well as the taxon-sampling limitations foundation for virtually all terrestrial ecosystems. Current inherent to molecular phylogenetics [16,17], the phylo- understanding of phylogeny indicates that vascular plants genetic approach may never bring full resolution to the (tracheophytes) form a monophyletic group [1]. All extant basal nodes of embryophyte phylogeny. However, an Current Opinion in Plant Biology 2014, 17:126–136 www.sciencedirect.com Early evolution of the vascular plant body plan Tomescu et al. 127 evolutionary-developmental (evo-devo) approach that from a bryophytic sporophyte by intercalation of a novel first, recognizes what structural changes result from the vegetative axial organ between the foot and the sporan- functional changes of specific genes, second, records the gium. The hypothesis, as formulated, has a set of corre- sequence in which comparable structural changes have lates that are mutually inconsistent. On the one hand, the occurred in the fossil record, and third, screens the hypothesis proposes that intercalation of the vegetative genomes of living plants with similar structural features organ occurs early in embryogeny, before sporangial for the presence of orthologues, can provide a framework differentiation. On the other hand, the bryophyte spor- for formulating testable hypotheses of plant body plan ophyte is seen as a footed sporangium (termed a spor- evolution. Such an approach may be better suited for ogonium). The seta, when present, is interpreted as an understanding the developmental changes that led to the extension and integral part of the sporangium, which evolution of polysporangiophytes and, thus, illuminate implies that the bryophyte embryo expresses only two the homologies of the polysporangiophyte sporophyte. types of developmental programs — vegetative in the foot and reproductive in the sporangium. Given these Hypotheses for the origin of constraints (and because the foot has no role in this polysporangiophytes hypothesis), intercalation of a vegetative organ before Currently, there are three competing hypotheses for the sporangial differentiation is not possible, except through origin of polysporangiophytes (including vascular plants). repression of the reproductive developmental program in Two of these hypotheses are explicit [6 ,7 ,18 ] and a the sporangial primordium. Therefore, the vegetative third is implicit from traditional paleobotanical inference axial organ proposed by this hypothesis is not novel; it and systematic analyses (e.g. [1,3,9 ]). All three assert that is a sterilized sporangial primordium (implicit in Fig. 3 of polysporangiophytes evolved from bryophyte-grade [18 ]). The intercalation hypothesis is, thus, essentially a plants and that the ancestral polysporangiophyte had a sterilization hypothesis (see below). branching sporophyte which bore terminal sporangia and lacked vascular tissue. However, the three hypotheses The interpolation hypothesis only implies sporangial differ in the evolutionary mechanisms they predict and sterilization and does not propose specific changes in the homologies they propose for the axes of the ancestral development with genetic/regulatory origins, thus it is polysporangiophyte. not testable. By proposing that the vegetative organ of the polysporangiophyte sporophyte is a (leafless) stem, this Traditional (implicit) hypothesis — branching of a hypothesis suggests homology of the early polysporan- bryophyte-grade sporophyte giophyte sporophyte with a modern tracheophyte stem. The idea that the vascular plant sporophyte originated by This view corresponds to a downward outlook on evol- increase in length and branching of a bryophyte-grade ution (i.e. interpreting the structures of extinct plants by sporophyte has a long history [19]. This hypothesis, for- reference to living plants [21,23 ]) that goes against the mulated by Campbell [20] and implicit in Bower’s [21] large body of evidence generated by paleobotanical stu- discussions of ‘‘the origin of the polysporangiate state’’, is dies over the last two centuries, which demonstrates that reflected in more recent applications, such as models of the sporophytic axes of early polysporangiophytes dif- growth [22] or the coding of morphological characters for fered dramatically from the stems of modern plants in phylogenetic analyses (e.g. [1]). However, this ‘traditional’ morphology (e.g. absence of leaves and phytomeres), hypothesis groups together ideas that are rather diverse and anatomy (e.g. absence of tracheids as water conducting lack specificity. Whereas most of these ideas imply modi- cells, stelar organization, sterome), and branching (apical fication of a moss-like sporophyte (e.g. [1,9 ,22]), some isotomous) (e.g. [1,3]). involve modification of a hornwort sporophyte (e.g. [19]). None of the authors proposes a complete, internally con- Sterilization hypothesis sistent suite of specific changes in development, nor expli- Another recent hypothesis
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