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Diverse branching forms have evolved multiple times across the tree of life to facilitate resource acquisition and exchange with the environment. In land plants, sporophyte branching enabled the diversification of the dominant vascular plant clade and arose in their last shared common ancestor; the bryophyte sisters to vascular plants are unbranched. Mechanisms for sporophyte branching are well known in Arabidopsis, where branch initiation and plastic branch outgrowth require directional auxin transport by PIN proteins. However, no broadly applicable genetic mechanisms for branching in vascular plants are known. We have used a combination of surgical and pharmacological treatments and PIN expression analyses in the lycophyte Selaginella kraussiana to identify PIN-mediated auxin transport as the ancestral mechanism for branching within vascular plants. We show that shortrange auxin transport out of the shoot tips promotes branching, and that branch dominance is coordinated by long-range auxin transport throughout the shoot system. Moreover, the plastic outgrowth of a branch from a unique organ system innovated in lycophytes (the rhizophore) is regulated by long-range auxin transport and associated with a transitory drop in PIN expression. We conclude that an ancestral mechanism for branching was independently recruited into plastic branch outgrowth in lycophytes and seed plants. Considered in conjunction with data from other species, our results highlight a pivotal role for the co-option of PINs into the evolution of branching in diverse plant forms.

The CLAVATA pathway is a key regulator of stem cell function in the multicellular shoot tips of Arabidopsis, where it acts via the WUSCHEL transcription factor to modulate hormone homeostasis. Broad scale evolutionary comparisons have shown that CLAVATA is a conserved regulator of land plant stem cell function, but CLAVATA acts independently of WUSCHEL-like (WOX) proteins in bryophytes. The relationship between CLAVATA, hormone homeostasis and the evolution of land plant stem cell functions is unknown.Here we show that in the moss, Physcomitrella (Physcomitrium patens), CLAVATA affects stem cell activity by modulating hormone homeostasis. CLAVATA pathway genes are expressed in the tip cells of filamentous tissues, regulating cell identity, filament branching, plant spread and auxin synthesis. The receptor-like kinase PpRPK2 plays the major role, and Pprpk2 mutants have abnormal responses to cytokinin, auxin, and auxin transport inhibition, and show reduced expression of PIN auxin transporters. We propose a model whereby PpRPK2 modulates auxin gradients in filaments to determine stem cell identity and overall plant form. Our data indicate that CLAVATA-mediated auxin homeostasis is a fundamental property of plant stem cell function, likely exhibited by the last shared common ancestor of land plants.

Summary There can be no doubt that early land plant evolution transformed the planet but, until recently, how and when this was achieved was unclear. Coincidence in the first appearance of land plant fossils and formative shifts in atmospheric oxygen and CO2 are an artefact of the paucity of earlier terrestrial rocks. Disentangling the timing of land plant bodyplan assembly and its impact on global biogeochemical cycles has been precluded by uncertainty concerning the relationships of bryophytes to one another and to the tracheophytes, as well as the timescale over which these events unfolded. New genome and transcriptome sequencing projects, combined with the application of sophisticated phylogenomic modelling methods, have yielded increasing support for the Setaphyta clade of liverworts and mosses, within monophyletic bryophytes. We consider the evolution of anatomy, genes, genomes and of development within this phylogenetic context, concluding that many vascular plant (tracheophytes) novelties were already present in a comparatively complex last common ancestor of living land plants (embryophytes). Molecular clock analyses indicate that embryophytes emerged in a mid-Cambrian to early Ordovician interval, compatible with hypotheses on their role as geoengineers, precipitating early Palaeozoic glaciations.

Introduction.Aphanorrhegma (Physcomitrella) patens(Hedw.) Lindb. is an attractive model system for comparative analyses of leaf development because it evolved leaves (phyllids) independently toflowering plants, yet its genome contains homologues of many gene families that regulate angiosperm leaf development. In addition,A. patensphyllids are primarily a single cell layer thick, making it simple to identify the cellular basis of defects that perturb shape. Identification of gene functions in shape determination depends on like-for- like comparison of mutant versus wild-type plants.Key results.Here we show that, if heteroblasty is not perturbed, such comparisons should use phyllid L13 or above in the heteroblastic series, and fully expanded phyllids above P7 in the developmental series. Using a quantitative approach, we show that heteroblastic size variation reflects differences in cell proliferation rather than cell size and shape. A comparison of control topinA pinBmutant phyllid development verifies that PIN proteins promote cell proliferation and suppress expansion to determine phyllid shape.Conclusion.The results and approach that we have generated will be applicable to any study of A. patensphyllid development to reveal the cellular basis of phyllid size and shape variations.

Successful collaborative research is dependent on excellent ideas and innovative experimental approaches, as well as the provision of appropriate support networks. Collaboration requires venues, infrastructures, training facilities, and, perhaps most importantly, a sustained commitment to work together as a community. These activities do not occur without significant effort, yet can be facilitated and overseen by the leadership of a research network that has a clearly defined role to help build resources for their community. Over the past 20 years, this is a role that the UKRI-BBSRC-funded GARNet network has played in the support of the UK curiosity-driven, discovery-led plant science research community. This article reviews the lessons learnt by GARNet in the hope that they can inform the practical implementation of current and future research networks., Lessons learnt from the activities of the GARNet research network in their support of the global plant science community are discussed.

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•Broad scale evolutionary comparisons have shown that branching forms arose by convergence in vascular plants and bryophytes, but the trajectory of branching form diversification in bryophytes is unclear. Mosses are the most species rich bryophyte lineage and two sub-groups are circumscribed by alternative reproductive organ placements. In one, reproductive organs form apically, terminating growth of the primary shoot (gametophore) axis. In the other, reproductive organs develop on very short lateral branches. A switch from apical to lateral reproductive organ development is proposed to have primed branching form diversification. •Moss gametophores have modular development and each module develops from a single apical cell. Here we define the architectures of 175 mosses by the number of module classes, branching patterns and the pattern in which similar modules repeat. Using ancestral character state reconstruction we identify two stages of architectural diversification. •During a first stage there were sequential changes in the module repetition pattern, reproductive organ position, branching pattern and number of module classes. During a second stage, vegetative changes occurred independently of reproductive fate. •The results pinpoint the nature of developmental change priming branching form diversification in mosses and provide a framework for mechanistic studies of architectural diversification.

International audience; The diverse forms of today's dominant vascular plant flora are generated by the sustained proliferative activity of sporophyte meristems at plants' shoot and root tips, a trait known as indeterminacy [1]. Bryophyte sister lineages to the vascular plants lack such indeterminate meristems and have an overall sporophyte form comprising a single small axis that ceases growth in the formation of a reproductive sporangium [1]. Genetic mechanisms regulating indeterminacy are well characterized in flowering plants, involving a feedback loop between class I KNOX genes and cytokinin [2, 3], and class I KNOX expression is a conserved feature of vascular plant meristems [4]. The transition from determinate growth to indeterminacy during evolution was a pre-requisite to vascular plant diversification, but mechanisms enabling the innovation of indeterminacy are unknown [5]. Here, we show that class I KNOX gene activity is necessary and sufficient for axis extension from an intercalary region of determinate moss shoots. As in Arabidopsis, class I KNOX activity can promote cytokinin biosynthesis by an ISOPENTENYL TRANSFERASE gene, PpIPT3. PpIPT3 promotes axis extension, and PpIPT3 and exogenously applied cytokinin can partially compensate for loss of class I KNOX function. By outgroup comparison, the results suggest that a pre-existing KNOX-cytokinin regulatory module was recruited into vascular plant shoot meristems during evolution to promote indeterminacy, thereby enabling the radiation of vascular plant shoot forms.

Contents 545 I. 545 II. 546 III. 546 IV. 548 V. 548 VI. 549 VII. 549 Acknowledgements 549 References 549 SUMMARY: Branching is one of the most striking aspects of land plant architecture, affecting resource acquisition and yield. Polar auxin transport by PIN proteins is a primary determinant of flowering plant branching patterns regulating both branch initiation and branch outgrowth. Several lines of experimental evidence suggest that PIN-mediated polar auxin transport is a conserved regulator of branching in vascular plant sporophytes. However, the mechanisms of branching and auxin transport and relationships between the two are not well known outside the flowering plants, and the paradigm for PIN-regulated branching in flowering plants does not fit bryophyte gametophytes. The evidence reviewed here suggests that divergent auxin transport routes contributed to the diversification of branching forms in distinct land plant lineages.

The origin of land plants was accompanied by new adaptations to life on land, including the evolution of stomata-pores on the surface of plants that regulate gas exchange. The genes that underpin the development and function of stomata have been extensively studied in model angiosperms, such as Arabidopsis. However, little is known about stomata in bryophytes, and their evolutionary origins and ancestral function remain poorly understood. Here, we resolve the position of bryophytes in the land plant tree and investigate the evolutionary origins of genes that specify stomatal development and function. Our analyses recover bryophyte monophyly and demonstrate that the guard cell toolkit is more ancient than has been appreciated previously. We show that a range of core guard cell genes, including SPCH/MUTE, SMF, and FAMA, map back to the common ancestor of embryophytes or even earlier. These analyses suggest that the first embryophytes possessed stomata that were more sophisticated than previously envisioned and that the stomata of bryophytes have undergone reductive evolution, including their complete loss from liverworts.

International audience; Strigolactones (SLs) are key hormonal regulators of flowering plant development and are widely distributed amongst streptophytes. In Arabidopsis, SLs signal via the F-box protein MORE AXILLARY GROWTH2 (MAX2), affecting multiple aspects of development including shoot branching, root architecture and drought tolerance. Previous characterization of a Physcomitrella patens moss mutant with defective SL synthesis supports an ancient role for SLs in land plants, but the origin and evolution of signalling pathway components are unknown. Here we investigate the function of a moss homologue of MAX2, PpMAX2, and characterize its role in SL signalling pathway evolution by genetic analysis. We report that the moss Ppmax2 mutant shows very distinct phenotypes from the moss SL-deficient mutant. In addition, the Ppmax2 mutant remains sensitive to SLs, showing a clear transcriptional SL response in dark conditions, and the response to red light is also altered. These data suggest divergent evolutionary trajectories for SL signalling pathway evolution in mosses and vascular plants. In P.patens, the primary roles for MAX2 are in photomorphogenesis and moss early development rather than in SL response, which may require other, as yet unidentified, factors.

International audience; Forward genetics is now straightforward in the moss Physcomitrella patens, and large mutant populations can be screened relatively easily. However, perturbation of development before the formation of gametes currently leaves no route to gene discovery. Somatic hybridization has previously been used to rescue sterile mutants and to assign P.patens mutations to complementation groups, but the cellular basis of the fusion process could not be monitored, and there was no tractable way to identify causative mutations. Here we use fluorescently tagged lines to generate somatic hybrids between Gransden (Gd) and Villersexel (Vx) strains of P.patens, and show that hybridization produces fertile diploid gametophytes that form phenotypically normal tetraploid sporophytes. Quantification of genetic variation between the two parental strains reveals single nucleotide polymorphisms at a frequency of 1/286bp. Given that the genetic distinction between Gd and Vx strains exceeds that found between pairs of strains that are commonly used for genetic mapping in other plant species, the spore populations derived from hybrid sporophytes provide suitable material for bulk segregant analysis and gene identification by genome sequencing.

Plant evo-devo research aims to identify the nature of genetic change underpinning the evolution of diverse plant forms. A transcriptomic study comparing gene expression profiles in the meristematic shoot tips of three distantly related vascular plants suggests that different genes were recruited to regulate similar meristematic processes during evolution.

The colonization of land by plants shaped the terrestrial biosphere, the geosphere and global climates. The nature of morphological and molecular innovation driving land plant evolution has been an enigma for over 200 years. Recent phylogenetic and palaeobotanical advances jointly demonstrate that land plants evolved from freshwater algae and pinpoint key morphological innovations in plant evolution. In the haploid gametophyte phase of the plant life cycle, these include the innovation of mulitcellular forms with apical growth and multiple growth axes. In the diploid phase of the life cycle, multicellular axial sporophytes were an early innovation priming subsequent diversification of indeterminate branched forms with leaves and roots. Reverse and forward genetic approaches in newly emerging model systems are starting to identify the genetic basis of such innovations. The data place plant evo-devo research at the cusp of discovering the developmental and genetic changes driving the radiation of land plant body plans. This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.

Plant microbial fuel cells are a recently developed technology that exploits photosynthesis in vascular plants by harnessing solar energy and generating electrical power. In this study, the model moss species Physcomitrella patens, and other environmental samples of mosses, have been used to develop a non-vascular bryophyte microbial fuel cell (bryoMFC). A novel three-dimensional anodic matrix was successfully created and characterized and was further tested in a bryoMFC to determine the capacity of mosses to generate electrical power. The importance of anodophilic microorganisms in the bryoMFC was also determined. It was found that the non-sterile bryoMFCs operated with P. patens delivered over an order of magnitude higher peak power output (2.6 ± 0.6 µW m-2) than bryoMFCs kept in near-sterile conditions (0.2 ± 0.1 µW m-2). These results confirm the importance of the microbial populations for delivering electrons to the anode in a bryoMFC. When the bryoMFCs were operated with environmental samples of moss (non-sterile) the peak power output reached 6.7 ± 0.6 mW m-2. The bryoMFCs operated with environmental samples of moss were able to power a commercial radio receiver or an environmental sensor (LCD desktop weather station).

Plants have undergone 470 million years of evolution on land and different groups have distinct body shapes. Liverworts are the most ancient land plant lineage and have a flattened, creeping body (the thallus), which grows from apical cells in an invaginated "notch." The genetic mechanisms regulating liverwort shape are almost totally unknown, yet they provide a blueprint for the radiation of land plant forms. We have used a combination of live imaging, growth analyses, and computational modeling to determine what regulates liverwort thallus shape in Marchantia polymorpha. We find that the thallus undergoes a stereotypical sequence of shape transitions during the first 2 weeks of growth and that key aspects of global shape depend on regional growth rate differences generated by the coordinated activities of the apical notches. A "notch-drives-growth" model, in which a diffusible morphogen produced at each notch promotes specified isotropic growth, can reproduce the growth rate distributions that generate thallus shape given growth suppression at the apex. However, in surgical experiments, tissue growth persists following notch excision, showing that this model is insufficient to explain thallus growth. In an alternative "notch-pre-patterns-growth" model, a persistently acting growth regulator whose distribution is pre-patterned by the notches can account for the discrepancies between growth dynamics in the notch-drives-growth model and real plants following excision. Our work shows that growth rate heterogeneity is the primary shape determinant in Marchantia polymorpha and suggests that the thallus is likely to have zones with specialized functions.

In common with most Old World Gesneriaceae; Streptocarpus Lindl. shows anisocotylous growth, i.e., the continuous growth of one cotyledon after germination. Linked to this phenomenon is an unorthodox behaviour of the shoot apical meristem (SAM) that determines the growth pattern of acaulescent species (subgenus Streptocarpus). In contrast caulescent species develop a conventional central post-embryonic SAM (mainly subgenus Streptocarpella). We used S. rexii Lindl. as a model to investigate anisocotyly and meristem initiation in Streptocarpus by using histological techniques and analyses of the expression pattern of the meristematic marker SrSTM1 during ontogeny. In contrast to Arabidopsis thaliana (L.) Heynh., S. rexii does not establish a SAM during embryogenesis, and the first evidence of a SAM-like structure occurs during post-embryonic development on the axis (the petiolode) between the two cotyledons. The expression pattern of SrSTM1 suggests a function in maintaining cell division activity in the cotyledons before becoming localized in the basal meristem, initially at the proximal ends of both cotyledons, later at the base of the continuously growing macrocotyledon, and the groove meristem on the petiolode. The latter is equivalent to a displaced SAM seemingly originating de novo under the influence of endogenous factors. Applied cytokinin retains SrSTM1expression in the small cotyledon, thus promoting isocotyly and re-establishment of a central post-embryonic SAM. Hormone-dependent delocalization of the process of meristem development could underlie anisocotyly and the unorthodox SAM formation in Streptocarpus.

*Summary The aim of this paper is to enable those who have never reconstructed a phylogeny to do so from scratch. The paper does not attempt to be a comprehensive theoretical guide, but describes one rigorous way of obtaining phylogenetic trees. Those who follow the methods outlined should be able to understand the basic ideas behind the steps taken, the meaning of the phylogenetic trees obtained and the scope of questions that can be answered with phylogenetic methods. The protocols have been successfully tested by volunteers with no phylogenetic experience.

Shoot branching is a primary contributor to plant architecture, evolving independently in flowering plant sporophytes and moss gametophytes. Mechanistic understanding of branching is largely limited to flowering plants such as Arabidopsis, which have a recent evolutionary origin. We show that in gametophytic shoots of Physcomitrella, lateral branches arise by re-specification of epidermal cells into branch initials. A simple model co-ordinating the activity of leafy shoot tips can account for branching patterns, and three known and ancient hormonal regulators of sporophytic branching interact to generate the branching pattern- auxin, cytokinin and strigolactone. The mode of auxin transport required in branch patterning is a key divergence point from known sporophytic pathways. Although PIN-mediated basipetal auxin transport regulates branching patterns in flowering plants, this is not so in Physcomitrella, where bi-directional transport is required to generate realistic branching patterns. Experiments with callose synthesis inhibitors suggest plasmodesmal connectivity as a potential mechanism for transport. DOI: http://dx.doi.org/10.7554/eLife.06808.001, eLife digest Most land plants have shoots that form branches and plants can regulate when and where they grow these branches to best exploit their environment. Plants with flowers and the more ancient mosses both have branching shoots, but these two groups of plants evolved to grow in this way independently of each other. Most studies on shoot branching have focused on flowering plants and so it is less clear how branching works in mosses. Three plant hormones—called auxin, cytokinin and strigolactone—control shoot branching in flowering plants. Auxin moves down the main shoot of the plant to prevent new branches from forming. This movement is controlled by the PIN proteins and several other families of proteins. On the other hand, cytokinin promotes the growth of new branches; and strigolactone can either promote or inhibit shoot branching depending on how the auxin is travelling around the plant. Coudert, Palubicki et al. studied shoot branching in a species of moss called Physcomitrella patens. The experiments show that cells on the outer surface of the main shoot are essentially reprogrammed to become so-called ‘branch initials’, which will then develop into new branches. Next, Coudert, Palubicki et al. made a computational model that was able to simulate the pattern of shoot branching in the moss. Further experiments supported the predictions made by the model. Coudert, Palubicki et al. found that, as in flowering plants, auxin from the tip of the main shoot suppresses branching in the moss, and cytokinin promotes branching. The experiments also showed that strigolactone inhibits shoot branching, but its role is restricted to the base of the shoots. The model predicts that, unlike in flowering plants, auxin must flow in both directions in moss shoots to produce the observed patterns of shoot branching. Also, the experiments suggest that the PIN proteins and another group of proteins that control the movement of auxin do not regulate shoot branching in moss. Instead, it appears that auxin may move through microscopic channels that link one moss cell to the next. Coudert, Palubicki et al.'s findings suggest that both flowering plants and mosses have evolved to use the same three hormones to control shoot branching, but that these hormones interact in different ways. One key next step will be to find out how auxin is transported during shoot branching in moss by manipulating the opening of the channels between the cells. A further challenge will be to find out the precise details of how the hormones control the activity of the branch initial cells. DOI: http://dx.doi.org/10.7554/eLife.06808.002

The morphology of plant fossils from the Rhynie chert has generated longstanding questions about vascular plant shoot and leaf evolution, for instance, which morphologies were ancestral within land plants, when did vascular plants first arise and did leaves have multiple evolutionary origins? Recent advances combining insights from molecular phylogeny, palaeobotany and evo–devo research address these questions and suggest the sequence of morphological innovation during vascular plant shoot and leaf evolution. The evidence pinpoints testable developmental and genetic hypotheses relating to the origin of branching and indeterminate shoot architectures prior to the evolution of leaves, and demonstrates underestimation of polyphyly in the evolution of leaves from branching forms in ‘telome theory’ hypotheses of leaf evolution. This review discusses fossil, developmental and genetic evidence relating to the evolution of vascular plant shoots and leaves in a phylogenetic framework.This article is part of a discussion meeting issue ‘The Rhynie cherts: our earliest terrestrial ecosystem revisited’.

The plant hormone auxin is a conserved regulator of development which has been implicated in the generation of morphological novelty. PIN-FORMED1 (PIN) auxin efflux carriers are central to auxin function by regulating its distribution. PIN family members have divergent structures and cellular localizations, but the origin and evolutionary significance of this variation is unresolved. To characterize PIN family evolution, we have undertaken phylogenetic and structural analyses with a massive increase in taxon sampling over previous studies. Our phylogeny shows that following the divergence of the bryophyte and lycophyte lineages, two deep duplication events gave rise to three distinct lineages of PIN proteins in euphyllophytes. Subsequent independent radiations within each of these lineages were taxonomically asymmetric, giving rise to at least 21 clades of PIN proteins, of which 15 are revealed here for the first time. Although most PIN protein clades share a conserved canonical structure with a modular central loop domain, a small number of noncanonical clades dispersed across the phylogeny have highly divergent protein structure. We propose that PIN proteins underwent sub- and neofunctionalization with substantial modification to protein structure throughout plant evolution. Our results have important implications for plant evolution as they suggest that structurally divergent PIN proteins that arose in paralogous radiations contributed to the convergent evolution of organ systems in different land plant lineages.

Floral diversity in Streptocarpus and Saintpaulia can be classified into six distinct types on the basis of quantitative variation. Species are differentiated by overall flower size and by corolla lobe size in relation to corolla tube opening size, characteristics implicated in pollination ecology. Saintpaulia has a very short corolla tube and yellow protruding anthers, strikingly different from Streptocarpus , and probably associated with buzz pollination. Some species of Streptocarpus and all species of Saintpaulia are enantiostylous, a feature often linked to buzz pollination. Mapping of these floral characters onto a molecular phylogeny based on ITS sequence data showed that inStreptocarpus flower size evolved from small to large, with reversals in four species. The putatively bee pollinated ‘open-tubed’ type had two independent origins, and the putatively lepidopteran-pollinated ‘keyhole’ type had four separate origins. Enantiostyly had one or two origins. The degree of zygomorphy varies withinStreptocarpus/Saintpaulia. The most extreme difference is between Saintpaulia ionantha and aSaintpaulia peloric cultivar. The developmental basis of floral variation was compared using SEM in three very different types: Streptocarpus primulifolius, Saintpaulia ionantha and a Saintpaulia peloric cultivar. Differences in corolla tube length and zygomorphy are established very early in flower development. Small, early changes in flower development establish major changes in mature floral morphology.

‘The evolution of plant form’ is volume 45 of Wiley-Blackwell's ‘Annual Plant Reviews’, and it provides an interdisciplinary analysis of this field, ranging from paleobotany to genomics. The first half of the book addresses morphological innovations in plant evolution by clade in an up-to-date phylogenetic context. An introductory chapter presents an overview of the characteristics of each major land plant group mapped onto a recent phylogenetic framework. It includes a detailed discussion of a selection of fossils in relation to characteristics not shown by any living groups, although the affinities allocated differ from treatments of other authors (e.g. Boyce, 2008; Harrison and Langdale 2008). The following five chapters present accounts of the morphology of each major land plant group, including bryophytes, lycophytes, ferns, gymnosperms and angiosperms. The chapter on bryophytes provides a good level of detail about liverworts, mosses and hornworts in the context of recent group-specific phylogenies. This is accompanied by some interesting speculation about character transitions within the bryophytes, and between the bryophytes and the vascular plants, with hypotheses about how they could be tested. The chapter on lycophytes presents an informative overview of morphological distinctions between the three major extant lycophyte lineages, whilst the one on ferns discusses the labile modularity of fern architecture in an explicit homology framework. Particular attention is paid to the evolution of embryos, shoots, leaves, and sporangium bearing structures. The chapter on gymnosperms provides an overview of the main organ systems in gymnosperms and discusses divergences in leaf and seed form between groups. The clonal analyses of Korn (2001, 2002) would have added to the discussion of shoot and leaf development here. The chapter on angiosperms addresses hypotheses relating to the origin of flowers from reproductive shoot systems, the innovation of the carpel, and the elaboration of floral symmetry in an integrated morphological and molecular context. The second half of the book moves into a consideration of genetic mechanisms which could have underpinned the radiation of land plant phyla. Chapter 7 presents an accessible and up-to-date explanation of QTL approaches which assesses the genetic basis of morphological diversity. It goes on to discuss genetic signatures of selection and how they can be statistically evaluated. A set of minimum requirements for determining a causal link between genes and character evolution is clearly identified, and some case studies from flowering plants are discussed in more detail. Chapter 8 gives an overview of plant genome evolution in relation to the generation of genetic diversity. It considers genome size within the land plants in relation to gene density and factors that influence genome evolution. The attributes of genomes sampled across the plant tree of life are discussed in relation to the amplification of gene families that may have been important in plant evolution. Chapter 9 presents three specific areas where evo-devo studies using reverse genetics have recently made an impact, namely the evolution of reproductive shoots, floral whorls and leaves. The discussion is informative and accessible. Chapter 10 considers the interplay between environment and development and its significance in plant evolution, and a final chapter pulls together the main themes addressed in the book, pointing out some areas for further future research. Strengths of this book are that the progression of topics covered by chapter is clear and logical, and the information covered is up-to-date and well integrated. I particularly liked the way that a discussion of fossil taxa is intercalated with considerations of living taxa throughout the book. I found some of the hypotheses of evolution interesting and innovative. In my view the book is more firmly rooted in a morphological and phylogenetic context than a developmental and genetic context, although the latter contributions were also clear and accessible. I thought that recent advances in developmental genetics could have allowed the authors to postulate specific testable hypotheses about some of the particular character changes distinguishing groups. In particular I felt that a consideration of mechanisms underpinning the innovation of the embryo, a vegetative sporophyte shoot, and branching would have added something, and these areas have been covered by recent reviews in the developmental literature (e.g. Harrison et al., 2010; Bosca et al., 2011; Domagalska and Leyser, 2011). Overall I felt that this book will be a great first port of call for those interested in morphological evolution and it will be useful at all levels from undergraduate onwards. It will be particularly useful for molecular and developmental biologists wishing to move into a comparative approach.

Sexual eukaryotes generate gametes using a specialized cell division called meiosis that serves both to halve the number of chromosomes and to reshuffle genetic variation present in the parent. The nature and mechanism of the meiotic cell division in plants and its effect on genetic variation are reviewed here. As flowers are the site of meiosis and fertilization in angiosperms, meiotic control will be considered within this developmental context. Finally, we review what is known about the control of meiosis in green algae and non-flowering land plants and discuss evolutionary transitions relating to meiosis that have occurred in the lineages giving rise to the angiosperms.

Background: Land plants evolved from aquatic algae more than 450 million years ago. Algal sisters of land plants grow through the activity of apical initial cells that cleave either in one plane to generate filaments or in two planes to generate mats. Acquisition of the capacity for cell cleavage in three planes facilitated the formation of upright bushy body plans and enabled the invasion of land. Evolutionary transitions between filamentous, planar, and bushy growth are mimicked within moss life cycles. Results: We have developed lineage analysis techniques to assess how transitions between growth forms occur in the moss Physcomitrella patens. We show that initial cells giving rise either to new filaments or bushy shoots are frequently juxtaposed on a single parent filament, suggesting a role for short-range cues in specifying differences in cell fate. Shoot initials cleave four times to establish a tetrahedral shape and subsequently cleave in three planes, generating bushy growth. Asymmetric and self-replacing divisions from the tetrahedral initial generate leaf initials that divide asymmetrically to self-replace and to produce daughter cells with restricted fate. The cessation of division in the leaf is distributed unevenly and contributes to final leaf shape. Conclusions: In contrast to flowering plants, changes in body plan in P. patens are regulated by cues acting at the level of single cells and are mediated through asymmetric divisions. Genetic mechanisms regulating shoot and leaf development in P. patens are therefore likely to differ substantially from mechanisms operating in plants with more recent evolutionary origins.

Pseudomonas fluorescens SBW25 is a plant growth-promoting bacterium that efficiently colonizes the leaf surfaces and rhizosphere of a range of plants. Previous studies have identified a putative plant-induced nitrilase gene (pinA) in P. fluorescens SBW25 that is expressed in the rhizosphere of sugar beet plants. Nitrilase enzymes have been characterised in plants, bacteria and fungi and are thought to be important in detoxification of nitriles, utilisation of nitrogen and synthesis of plant hormones. We reveal that pinA is a NIT4-type nitrilase that catalyses the hydrolysis of beta-cyano-L-alanine, a nitrile common in the plant environment and an intermediate in the cyanide detoxification pathway in plants. In plants cyanide is converted to beta-cyano-L-alanine, which is subsequently detoxified to aspartic acid and ammonia by NIT4. In P. fluorescens SBW25 pinA is induced in the presence of beta-cyano-L-alanine, and the beta-cyano-L-alanine precursors cyanide and cysteine. pinA allows P. fluorescens SBW25 to use beta-cyano-L-alanine as a nitrogen source and to tolerate toxic concentrations of this nitrile. In addition, pinA is shown to complement a NIT4 mutation in Arabidopsis thaliana, enabling plants to grow in concentrations of beta-cyano-L-alanine that would otherwise prove lethal. Interestingly, over-expression of pinA in wild-type A. thaliana not only resulted in increased growth in high concentrations of beta-cyano-L-alanine, but also resulted in increased root elongation in the absence of exogenous beta-cyano-L-alanine, demonstrating that beta-cyano-L-alanine nitrilase activity can have a significant effect on root physiology and root development.

Land plants evolved from aquatic algal ancestors. The algae are a polyphyletic group from which the transition to land, and acquisition of developmental features associated with land plants, have occurred many times (Lewis and McCourt ). Recent phylogenetic evidence points to the charophyte algal lineage as the sister group to the land plants (Figure .). Developmental features shared by charophytes and land

A major transition in land plant evolution was from growth in water to growth on land. This transition necessitated major morphological innovations that were accompanied by the development of three-dimensional apical growth. In extant land plants, shoot growth occurs from groups of cells at the apex known as meristems. In different land plant lineages, meristems function in different ways to produce distinct plant morphologies, yet our understanding of the developmental basis of meristem function is limited to the most recently diverged angiosperms. To redress this balance, we have examined meristem function in the lycophyte Selaginella kraussiana. Using a clonal analysis, we show that S. kraussiana shoots are derived from the activity of two short-lived apical initials that facilitate the formation of four axes of symmetry in the shoot. Leaves are initiated from just two epidermal cells, and the mediolateral leaf axis is the first to be established. This pattern of development differs from that seen in flowering plants. These differences are discussed in the context of the development and evolution of diverse land plant forms.

Vascular plants evolved in the Middle to Late Silurian period, about 420 million years ago. The fossil record indicates that these primitive plants had branched stems with sporangia but no leaves. Leaf-like lateral outgrowths subsequently evolved on at least two independent occasions. In extant plants, these events are represented by microphyllous leaves in lycophytes (clubmosses, spikemosses and quillworts) and megaphyllous leaves in euphyllophytes (ferns, gymnosperms and angiosperms). Our current understanding of how leaves develop is restricted to processes that operate during megaphyll formation. Because microphylls and megaphylls evolved independently, different mechanisms might be required for leaf formation. Here we show that this is not so. Gene expression data from a microphyllous lycophyte, phylogenetic analyses, and a cross-species complementation experiment all show that a common developmental mechanism can underpin both microphyll and megaphyll formation. We propose that this mechanism might have operated originally in the context of primitive plant apices to facilitate bifurcation. Recruitment of this pathway to form leaves occurred independently and in parallel in different plant lineages.