C. Jill Harrison

Independent recruitment of a conserved developmental mechanism during leaf evolution

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.

Local Cues and Asymmetric Cell Divisions Underpin Body Plan Transitions in the Moss Physcomitrella patens

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.

Plasma Membrane-Targeted PIN Proteins Drive Shoot Development in a Moss

Summary Background Plant body plans arise by the activity of meristematic growing tips during development and radiated independently in the gametophyte (n) and sporophyte (2n) stages of the life cycle during evolution. Although auxin and its intercellular transport by PIN family efflux carriers are primary regulators of sporophytic shoot development in flowering plants, the extent of conservation in PIN function within the land plants and the mechanisms regulating bryophyte gametophytic shoot development are largely unknown. Results We have found that treating gametophytic shoots of the moss Physcomitrella patens with exogenous auxins and auxin transport inhibitors disrupts apical function and leaf development. Two plasma membrane-targeted PIN proteins are expressed in leafy shoots, and pin mutants resemble plants treated with auxins or auxin transport inhibitors. PIN-mediated auxin transport regulates apical cell function, leaf initiation, leaf shape, and shoot tropisms in moss gametophytes. pin mutant sporophytes are sometimes branched, reproducing a phenotype only previously seen in the fossil record and in rare natural moss variants. Conclusions Our results show that PIN-mediated auxin transport is an ancient, conserved regulator of shoot development.

Growth from two transient apical initials in the meristem of Selaginella kraussiana

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.

Paralogous Radiations of PIN Proteins with Multiple Origins of Noncanonical PIN Structure

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.

A conserved mechanism for nitrile metabolism in bacteria and plants

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.

Meiosis in flowering plants and other green organisms

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.

Developmental and genetic innovations in the evolution of plant body plans

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’.

Anisocotyly and meristem initiation in an unorthodox plant, Streptocarpus rexii (Gesneriaceae)

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.

Regional Growth Rate Differences Specified by Apical Notch Activities Regulate Liverwort Thallus Shape

Summary 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.