Leaf development and evolution

Natural variation in Arabidopsis thaliana rosette area unveils new genes involved in plant development

Growth is a complex trait influenced by multiple genes that act at different moments during the development of an organism. This makes it difficult to spot its underlying genetic mechanisms. Since plant growth is intimately related to the effective leaf surface area (ELSA), identifying genes controlling this trait will shed light on our understanding of plant growth. To find new genes with a significant contribution to plant growth, here we used the natural variation in Arabidopsis thaliana to perform a genome-wide association study of ELSA. To do this, the projected rosette area of 710 worldwide distributed natural accessions was measured and analyzed using the genome-wide efficient mixed model association algorithm. From this analysis, ten genes were identified having SNPs with a significant association with ELSA. To validate the implication of these genes into A. thaliana growth, six of them were further studied by phenotyping knock-out mutant plants. It was observed that rem1.2, orc1a, ppd1, and mcm4 mutants showed different degrees of reduction in rosette size, thus confirming the role of these genes in plant growth. Our study identified genes already known to be involved in plant growth but also assigned this role, for the first time, to other genes.

The origin and early evolution of vascular plant shoots and leaves

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'.

Acquisition and diversification of tendrilled leaves in Bignonieae (Bignoniaceae) involved changes in expression patterns of SHOOTMERISTEMLESS (STM), LEAFY/FLORICAULA (LFY/FLO), and PHANTASTICA (PHAN)

Leaves have undergone structural modifications over evolutionary time, and presently exist in many forms. For instance, in Fabaceae and Bignoniaceae, leaf parts can be modified into tendrils. Currently, no data are available on genic control of tendrilled leaf development outside Fabaceae. Here, we conducted a detailed study of three representatives of Bignonieae: Amphilophium buccinatorium, Dolichandra unguis-cati, and Bignonia callistegioides, bearing multifid, trifid, and simple-tendrilled leaves, respectively. We investigated the structure of their petioles, petiolules, leaflets, and tendrils through histological analyses. Additionally, the expression of SHOOTMERISTEMLESS (STM), PHANTASTICA (PHAN), and LEAFY/FLORICAULA (LFY/FLO) during leaf development was analyzed by in situ hybridizations. Tendrils share some anatomical similarities with leaflets, but not with other leaf parts. Transcripts of both STM and LFY/FLO were detected in leaf primordia, associated with regions from which leaflets and tendril branches originate. PHAN expression was found to be polarized in branched tendrils, but not in simple tendrils. In Bignonieae, tendrils are modified leaflets that, as a result of premature completion of development, become bladeless organs. Bignonieae leaves develop differently from those of peas, as both LFY/FLO and STM are expressed in developing leaves of Bignonieae. Moreover, PHAN is probably involved in tendril diversification in Bignonieae, as it has distinct expression patterns in different leaf types.

Pisum sativum wild-type and mutant stipules and those induced by an auxin transport inhibitor demonstrate the entire diversity of laminated stipules observed in angiosperms

About a quarter of angiosperm species are stipulate. They produce stipule pairs at stem nodes in association with leaves. Stipule morphology is treated as a species-specific characteristic. Many species bear stipules as laminated organs in a variety of configurations, including laterally free large foliaceous, small, or wholly leaf-like stipules, and as fused intrapetiolar, opposite, ochreate or interpetiolar stipules. In Pisum sativum, the wild-type and stipule-reduced and cochleata mutants are known to form free large, small, and leaf-like stipules, respectively. Auxin controls initiation and development of plant organs and perturbations in its availability and distribution in the meristems, caused by auxin transport inhibitor(s) (ATIs), lead to aberrations in leaf development. The effect(s) of ATI(s) on stipule development are unexplored. To study the effect of the ATI 1-N-naphthylphthalamic acid (NPA) on stipule morphogenesis, P. sativum explants were grown in vitro in presence of a sublethal concentration of NPA. The NPA-treated shoots produced fused stipules of all the different types described in angiosperms. The observations indicate that (a) the gene sets for stipule differentiation may be common in angiosperms and (b) the interspecies stipule architectural differences are due to mutations, affecting gene expression or activity that got selected in the course of evolution.

Leaf development

Leaves are the most important organs for plants. Without leaves, plants cannot capture light energy or synthesize organic compounds via photosynthesis. Without leaves, plants would be unable perceive diverse environmental conditions, particularly those relating to light quality/quantity. Without leaves, plants would not be able to flower because all floral organs are modified leaves. Arabidopsis thaliana is a good model system for analyzing mechanisms of eudicotyledonous, simple-leaf development. The first section of this review provides a brief history of studies on development in Arabidopsis leaves. This history largely coincides with a general history of advancement in understanding of the genetic mechanisms operating during simple-leaf development in angiosperms. In the second section, I outline events in Arabidopsis leaf development, with emphasis on genetic controls. Current knowledge of six important components in these developmental events is summarized in detail, followed by concluding remarks and perspectives.

The evolution and functional significance of leaf shape in the angiosperms

Angiosperm leaves manifest a remarkable diversity of shapes that range from developmental sequences within a shoot and within crown response to microenvironment to variation among species within and between communities and among orders or families. It is generally assumed that because photosynthetic leaves are critical to plant growth and survival, variation in their shape reflects natural selection operating on function. Several non-mutually exclusive theories have been proposed to explain leaf shape diversity. These include: thermoregulation of leaves especially in arid and hot environments, hydraulic constraints, patterns of leaf expansion in deciduous species, biomechanical constraints, adaptations to avoid herbivory, adaptations to optimise light interception and even that leaf shape variation is a response to selection on flower form. However, the relative importance, or likelihood, of each of these factors is unclear. Here we review the evolutionary context of leaf shape diversification, discuss the proximal mechanisms that generate the diversity in extant systems, and consider the evidence for each the above hypotheses in the context of the functional significance of leaf shape. The synthesis of these broad ranging areas helps to identify points of conceptual convergence for ongoing discussion and integrated directions for future research.

Promoting the promoter

Recent evolutionary studies clearly indicate that evolution is mainly driven by changes in the complex mechanisms of gene regulation and not solely by polymorphism in protein-encoding genes themselves. After a short description of the cis-regulatory mechanism, we intend in this review to argue that by applying newly available technologies and by merging research areas such as evolutionary and developmental biology, population genetics, ecology and molecular cell biology it is now possible to study evolution in an integrative way. We contend that, by analysing the effects of promoter sequence variation on phenotypic diversity in natural populations, we will soon be able to break the barrier between the study of extant genetic variability and the study of major developmental changes. This will lead to an integrative view of evolution at different scales. Because of their sessile nature and their continuous development, plants must permanently regulate their gene expression to react to their environment, and can, therefore, be considered as a remarkable model for these types of studies.