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Leaf development and evolution
Abstract
The number of publications on "leaf morphogenesis" has increased annually since the early 1990s, when the Arabidopsis 'model plant' concept began being applied to studies of mechanisms of leaf morphogenesis. Nearly 20 years have passed since then, and a great leap in the understanding of leaf organogenesis has been made. As a result, so-called evo/devo studies on leaf development have joined the trend in leaf morphogenesis/development studies in the latter part of the present decade. This article is a brief overview of the progress made in the research to date and an introduction to the articles that appear in this special issue.
... At the beginning in this issue, No. 1, Vol. 123, we see the first JPR Symposium: ''Leaf development and evolution.'' As mentioned in the preface (Tsukaya 2010), understanding of the mechanisms of leaf development has greatly expanded in the years after model plants were adopted for use in this kind of research. As a result, molecular genetics studies on leaf development have become more and more common now. ...
... The results are seen in the JPR Symposium ''Leaf development and evolution.'' Again, please see the preface (Tsukaya 2010) for details. There is not only this JPR Symposium; other JPR Symposia are being planned for 2010. ...
... For example, the flower and fruit development genetics and epigenetics are more studied in comparation with leaf development 11 . This is, among other causes, due to the difficulty in differentiating and describing individual processes in leaf growth because multiple gene activation and cell differentiation events happen sequentially or almost at the same time in the leaf primordia or meristem 12 . Nevertheless, technological and computational advances in GWAS and phenomics have made significant strides. ...
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.
... Evo-devo studies of leaf evolution have only recently started [149,150] and so far have largely focused on leaves with widely disparate origins, for instance comparing lycophyll, fern frond and Arabidopsis leaf development pathways rstb.royalsocietypublishing.org Phil. Trans. ...
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’.
... provide insights into the dynamics of cellular events that underlie the development from 88 primordia to the final flat and polar organ (Tsukaya, 2010Tsukaya, , 2013Vanhaeren et al., 2015). Growth 89 and development are controlled by complex molecular networks that integrate internal and 90 external signals (Cho et al., 2007;Wolters and Jürgens, 2009). ...
Pavement cells (PCs) are the most frequently occurring cell type in the leaf epidermis and play important roles in leaf growth and function. In many plant species, PCs form highly complex jigsaw puzzle shaped cells with interlocking lobes. Understanding of their development is of high interest for plant science research because of their importance for leaf growth and hence for plant fitness and crop yield. Studies of PC development, however, are limited because robust methods are lacking that enable automatic segmentation and quantification of PC shape parameters suitable to reflect their cellular complexity. Here, we present our new ImageJ-based tool, PaCeQuant, which provides a fully automatic image analysis workflow for PC shape quantification. PaCeQuant automatically detects cell boundaries of PCs from confocal input images, and enables manual correction of automatic segmentation results or direct import of manually segmented cells. PaCeQuant simultaneously extracts 27 shape features that include global, contour-based, skeleton-based and PC-specific object descriptors. In addition, we included a method for classification and analysis of lobes at two-cell-junctions and three-cell-junctions, respectively. We provide an R script for graphical visualization and statistical analysis. We validated PaCeQuant by extensive comparative analysis to manual segmentation and existing quantification tools, and demonstrated its usability to analyze PC shape characteristics during development and between different genotypes. PaCeQuant thus provides a platform for robust, efficient and reproducible quantitative analysis of PC shape characteristics that can easily be applied to study PC development in large data sets.
... Considerable amounts of information are available on the development of leaves of model organisms (Tsukaya, 2010). However, the only tendrilled species in which leaf development has been studied in detail is pea. ...
Leaves have undergone structural modifications over evolutionary time, and presently exist in many forms. For instance, in F abaceae and B ignoniaceae, 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 B ignonieae: A mphilophium buccinatorium , D olichandra unguis‐cati , and B ignonia 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 B ignonieae, 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 B ignonieae. Moreover, PHAN is probably involved in tendril diversification in B ignonieae, as it has distinct expression patterns in different leaf types.
... Accumulating knowledge on molecular genetic mechanisms of leaf morphogenesis is leading to an increasingly evo-devo approach to studies of leaf diversity (Tsukaya 2010); this progress is aided by establishing new model species (e.g. Cardamine hirsuta and the rush; Canales et al. 2010;. ...
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.
... network structure. For example, the fascinating enigma of plant evolutionary novelties such as leaf morphogenesis [69], the development of drought resistance [70] and the establishment of floral symmetry [71] might be solved. ...
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.
The variability in leaf form in nature is immense. Leaf patterning occurs by differential growth that occurs during a limited window of morphogenetic activity at the leaf marginal meristem. While many regulators have been implicated in the designation of the morphogenetic window and in leaf patterning, how these effectors interact to generate a particular form is still not well understood.
We addressed the interaction among different effectors of tomato compound leaf development, using genetic and molecular analyses.
Mutations in the tomato auxin response factor SlARF5/SlMP, which promotes leaflet formation, suppressed the increased leaf complexity of mutants with extended morphogenetic window. Impaired activity of the NAC/CUC transcription factor GOBLET (GOB), which specifies leaflet boundaries, also reduced leaf complexity in these backgrounds. Analysis of genetic interactions showed that the patterning factors SlMP, GO B and the MYB transcription factor LYRATE (LYR) act in parallel to promote leaflet formation.
This work places an array of developmental regulators in a morphogenetic context. It reveals how organ-level differentiation rate and local growth are coordinated to sculpture an organ. These concepts and findings are applicable to other plant species and developmental processes that are regulated by patterning and differentiation.
We performed morphological and anatomical analyses to clarify the correlation of variation between leaves and floral characters of Cymbidium goeringii (Rchb. f.) Rchb. f. We observed that the morphological variation in leaf size was correlated with those of floral characters in this species. Moreover, the anatomical analyses indicated that the size variations in leaves, sepals, and petals were caused by decreased number of cells but those of the lips were caused by decreased cell number as well as decreased cell size. Various genes that do not participate in the formation of petals or sepals are involved in the development of the lip, suggesting that such genes contribute to specific processes related to the size variation of the lip at the cellular level.
The comparative morphology and anatomy of the leaves of the rheophytic Rhododendron ripense and the closely related inland species Rhododendron macrosepalum were examined. The leaf of R. ripense is thinner than that of R. macrosepalum, with leaf length to width ratios (leaf index) of 2.92 and 1.91, respectively. Moreover, the leaf of R. ripense consists of fewer cells than the leaf of R. macrosepalum, suggesting stenophyllization of R. ripense caused by the decreased number of cells. In addition, leaf thickness and the number of stomata per leaf of R. ripense were significantly greater than those of R. macrosepalum, but the density of the short glandular pilose hairs on the leaf of R. ripense was lower. The observed morphological differences between the two species may be explained by certain aspects of the riparian environment, such as high irradiation and frequent flooding after heavy rainfall, to which R. ripense is exposed.
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 leguminous flora of Delhi comprises 78 Papilionoideae, 24 Caesalpinioideae and 24 Mimosoideae species; 80 of them are perennials. Five types of imparipinnate and two types of paripinnate compound leaves was observed on the species. The paripinnate leaves are bipinnate in 25 species (mostly mimosoid) and bifoliate in two species. The imparipinnate leaves were trifoliate or multifoliate in 59 papilionoid species and multifoliate in 16 caesalpinioid species; four of the papilionoid species produced leafletted and tendrilled unipinnate leaves. Leaves were bifacially simple in 22 species, simple with ectopic terminal growth in one species and simple tendril in one species. Twenty-one species (mostly mimosoid) were devoid of stipules. In 82 species stipules were small and free. Stipules were large and lobed in 17 species and large and adnate in four species. Two species of Caesalpinioideae produce compound leaf-like stipules. All four stipule phenotypes of 126 species corresponded with stipular phenotypes observed in wild type, coch, st and coch st genotypes of the model legume P. sativum. The seven leaf phenotypes observed in 126 species corresponded with phenotypes expected among combinations of uni (uni-tac), af, ins, mfp and tl mutants of P. sativum and sgl1, cfl1, slm1 and palm1 mutants of M. truncatula, also an IRL model legume. All the variation in leaf and stipule morphologies observed in the leguminous flora of Delhi could be explained in terms of the gene regulatory networks already revealed in P. sativum and M. truncatula. It is hypothesized that the ancestral gene regulatory networks for leaves and stipules produced in Leguminosae were like that prevalent in P. sativum.
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.
The last decade of the twentieth century has witnessed a period of renewed interest, redefinition of questions, and some dramatic advances toward resolving some of the long-standing issues related to the developmental regulation of leaf morphogenesis. New interest has been sparked by the application of developmental genetics, molecular biology, and mosaic analysis to the study of genetic model species. The integration of knowledge gained from these newer approaches with that derived from more than a century of comparative developmental morphology is crucial for advancing understanding of leaf morphogenesis. This link is particularly important for the interpretation of mutant phenotypes and gene expression patterns. In this brief review article, we provide a general framework for the study of leaf morphogenesis and identify areas where we believe that important issues remain unresolved.
In leaves, cells get larger as cell division decreases or the ploidy increases. This might seem a logical response, but the controls are more complicated.
The number of cells in an organ is a major factor that specifies its size. However, the genetic basis of cell number determination is not well understood. To obtain insight into this genetic basis, three grandifolia-D (gra-D) mutants of Arabidopsis thaliana were characterized that developed huge leaves with two to three times more cells than the wild-type. Genetic and microarray analyses showed that a large segmental duplication had occurred in all the gra-D mutants, consisting of the lower part of chromosome 4. In the duplications, genes were found that encode AINTEGUMENTA (ANT), a factor that extends the duration of cell proliferation, and CYCD3;1, a G1/S cyclin. The expression levels of both genes increased and the duration of cell proliferation in the leaf primordia was extended in the gra-D mutants. Data obtained by RNAi-mediated knockdown of ANT expression suggested that ANT contributed to the huge-leaf phenotype, but that it was not the sole factor. Introduction of an extra genomic copy of CYCD3;1 into the wild-type partially mimicked the gra-D phenotype. Furthermore, combined elevated expression of ANT and CYCD3;1 enhanced cell proliferation in a cumulative fashion. These results indicate that the duration of cell proliferation in leaves is determined in part by the interaction of ANT and CYCD3;1, and also demonstrate the potential usefulness of duplication mutants in the elucidation of genetic relationships that are difficult to uncover by standard single-gene mutations or gain-of-function analysis. We also discuss the potential effect of chromosomal duplication on evolution of organ size.
Leaf size and shape define the photosynthetic capability of a plant and have a significant impact on important agronomic traits,
such as yield, quality, disease resistance and stress responses. Cultivated varieties of many plant species show remarkable
variations in leaf morphology. Such variation usually exhibits a continuous phenotypic distribution and is controlled by the
interaction of multiple genes known as quantitative trait loci (QTL). Here, we review several studies that evaluate natural
variations in the leaf morphologies of crop species, as well as in the model plant Arabidopsis thaliana. The use of high-throughput, genome-wide approaches such as transcriptomics and metabolomics is helping to identify the nucleotide
polymorphism(s) responsible for the phenotypic differences attributed to some of these QTL.
A full understanding of the leaf is essential for a full understanding of plant morphology. However, leaf morphogenesis is
still poorly understood, in particular in dicotyledonous plants, because of the complex nature of the development of leaves.
Mutational analysis seems to be the most suitable strategy for investigations of such processes, and should allow us to dissect
the developmental pathways into genetically programmed unit processes. The techniques of developmental genetics have been
applied to the study of leaf morphogenesis in model plants, such asArabidopsis thaliana, and several key processes in leaf morphogenesis have been identified. The fundamental processes in leaf morphogenesis include
the identification of leaf organs, determination of leaf primordia (occurrence of marginal meristem), and the polar or non-polar
elongation of leaf cells. This review will focus on the genes that are essential for these processes and have been identified
in mutational analyses. Mutational analyses of the photomorphogenesis is also briefly summarized from the perspective of the
plasticity of leaf morphogenesis.
Cardamine hirsuta, a small crucifer closely related to the model organism Arabidopsis thaliana, offers high genetic tractability and has emerged as a powerful system for studying the genetic basis for diversification of plant form. Contrary to A. thaliana, which has simple leaves, C. hirsuta produces dissected leaves divided into individual units called leaflets. Leaflet formation requires activity of Class I KNOTTED1-like homeodomain (KNOX) proteins, which also promote function of the shoot apical meristem (SAM). In C. hirsuta, KNOX genes are expressed in the leaves whereas in A. thaliana their expression is confined to the SAM, and differences in expression arise through cis-regulatory divergence of KNOX regulation. KNOX activity in C. hirsuta leaves delays the transition from proliferative growth to differentiation thus facilitating the generation of lateral growth axes that give rise to leaflets. These axes reflect the sequential generation of cell division foci across the leaf proximodistal axis in response to auxin activity maxima, which are generated by the PINFORMED1 (PIN1) auxin efflux carriers in a process that resembles organogenesis at the SAM. Delimitation of C. hirsuta leaflets also requires the activity of CUP
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COTYLEDON (CUC) genes, which direct formation of organ boundaries at the SAM. These observations show how species-specific deployment of fundamental shoot development networks may have sculpted simple versus dissected leaf forms. These studies also illustrate how extending developmental genetic studies to morphologically divergent relatives of model organisms can greatly help elucidate the mechanisms underlying the evolution of form.
The fossil record reveals that seed plant leaves evolved from ancestral lateral branch systems. Over time, the lateral branch systems evolved to become determinate, planar and eventually laminar. Considering their evolutionary histories, it is instructive to compare the developmental genetics of shoot apical meristems (SAMs) and leaves in extant seed plants. Genetic experiments in model angiosperm species have assigned functions of meristem maintenance, specification of stem cell identity, boundary formation, polarity establishment and primordium initiation to specific genes. Investigation of roles of the same or homologous genes during leaf development has revealed strikingly similar functions in leaves compared to SAMs. Specifically, the marginal blastozone that characterizes many angiosperm leaves appears to function in a manner mechanistically similar to the SAM. We argue here that the similarities may be homologous due to descent from ancestral roles in an ancestral shoot system. Molecular aspects of SAM and leaf development in gymnosperms is largely neglected and could provide insight into seed plant leaf evolution.
An important objective in evolutionary developmental biology is to understand the molecular genetic mechanisms that have given rise to morphological diversity. Leaves in angiosperms generally develop as a flattened structure with clear adaxial-abaxial polarity. In monocots, however, a unifacial leaf has evolved in a number of divergent species, in which leaf blades consist of only the abaxial identity. The mechanism of unifacial leaf development has long been a matter of debate for comparative morphologists. However, the underlying molecular genetic mechanism remains unknown. Unifacial leaves would be useful materials for developmental studies of leaf-polarity specification. Moreover, these leaves offer unique opportunities to investigate important phenomena in evolutionary biology, such as repeated evolution or convergent evolution of similar morphological traits. Here we describe the potential of unifacial leaves for evolutionary developmental studies and present our recent approaches to understanding the mechanisms of unifacial leaf development and evolution using Juncus as a model system.
Class I KNOTTED1-LIKE HOMEOBOX (KNOX1) genes are expressed in the shoot apical meristem (SAM) to effect its formation and maintenance. KNOX1 genes are also involved in leaf shape control throughout angiosperm evolution. Leaves can be classified as either simple or compound, and KNOX1 expression patterns in leaf primordia are highly correlated with leaf shape; in most simple-leafed species, KNOX1 genes are expressed only in the SAM but not in leaf primordia, while in compound-leafed species they are expressed both in the SAM and leaf primordia. How can KNOX1 expression be maintained to a high degree in the SAM, but simultaneously be so variable in leaves? This dichotomy suggests that the processes of leaf and SAM development have been compartmentalized during evolution. Here, we introduce our findings regarding the regulation of expression of SHOOT MERISTEMLESS, a KNOX1 gene, together with a brief review of KNOX1 genes from an evolutionary viewpoint. We also present our findings regarding another aspect of KNOX1 regulation via a protein-protein interaction network involved in the natural variation in leaf shape. Both aspects of KNOX1 regulation could be utilized for fine-tuning leaf morphology during evolution without affecting the essential function of KNOX genes in the shoot.
Co-ordination of cell proliferation and cell expansion is a key regulatory process in leaf-size determination, but its molecular details are unknown. In Arabidopsis thaliana, mutations in a positive regulator of cell proliferation often trigger excessive cell enlargement post-mitotically in leaves. This phenomenon, called compensation syndrome, is seen in the mutant angustifolia3 (an3), which is defective in a transcription co-activator. Such compensation, however, does not occur in response to a decrease in cell number in oligocellula (oli). oli2, oli5 and oli7 did not exhibit compensation and the reduction in cell number in these mutants was moderate. However, when an oli mutation was combined with a different oli mutation to create a double mutant, cell number was further reduced and compensation was induced. Similarly, weak suppression of AN3 expression reduced cell number moderately but did not induce compensation compared with an an3 null mutant. Furthermore, double mutants of either oli2, oli5 or oli7 and an3 showed markedly enhanced compensation. These results suggest that compensation is triggered when cell proliferation regulated by OLI2/OLI5/OLI7 and AN3 is compromised in a threshold-dependent manner. OLI2 encodes a Nop2 homolog in Saccharomyces cerevisiae that is involved in ribosome biogenesis, whereas OLI5 and OLI7 encode ribosome proteins RPL5A and RPL5B, respectively. This suggests that a factor involved in the induction of compensation may be under the dual control of AN3 and a ribosome-related process.
Regulation of cell number and cell size is essential for controlling the shape and size of leaves. Here, we report a novel class of Arabidopsis thaliana mutants, more and smaller cells 1 (msc1)-msc3, which have increased cell number and decreased cell size in leaves. msc1 has a miR156-resistant mutation in the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 15 (SPL15) gene. msc2 and msc3 are new alleles of paused and squint mutants, respectively. All msc mutants showed accelerated heteroblasty, a phenomenon in which several morphological traits of leaves change along with phase change. Consistent with this finding, in the wild type, leaves at higher nodes (adult leaves) also have increased cell number and reduced cell size compared with those at lower nodes (juvenile leaves). These facts indicate that precocious acquisition of adult leaf characteristics in the msc mutants may cause alterations in the number and size of cells, and that heteroblasty plays an important role in the regulation of cell number and size. In agreement with this suggestion, such heteroblasty-associated changes in cell number and size are severely inhibited by the constitutive overexpression of miR156 and are promoted by the elevated expression of miR156-insensitive forms of SPL genes. By contrast, rdr6, sgs3, zip, arf3 and arf4 mutations, which affect progression of heteroblasty, had little or no effect on number or size of cells. These results suggest that cell number and size are mainly regulated by an SPL-dependent pathway rather than by a tasiR-ARF-dependent pathway.
Biodiversity of plant shape is mainly attributable to biodiversity of leaf shape and the shape of floral organs, the modified leaves. However, the exact mechanisms of leaf-shape determination remain unclear due to the complexity of flat-structure organogenesis that includes the simultaneous cell cycling and cell enlargement in primordia. Recent studies in developmental and molecular genetics have revealed several important aspects of leaf-shape control mechanisms. For example, understanding of polar control in leaf-blade expansion has advanced greatly. A curious phenomenon called "compensated cell enlargement" found in leaf organogenesis studies should also provide interesting clues regarding the mechanisms of multicellular organ development. This paper reviews recent research findings with a focus on leaf development in Arabidopsis thaliana.