Leaf adaxial-abaxial polarity specification and lamina outgrowth: evolution and development

Model for the role of auxin polar transport in patterning of the leaf adaxial-abaxial axis

Leaf adaxial-abaxial polarity refers to the two leaf faces, which have different types of cells performing distinct biological functions. In 1951, Ian Sussex reported that when an incipient leaf primordium was surgically isolated by an incision across the vegetative shoot apical meristem (SAM), a radialized structure without an adaxial domain would form. This led to the proposal that a signal, now called the Sussex signal, is transported from the SAM to emerging primordia to direct leaf adaxial-abaxial patterning. It was recently proposed that instead of the Sussex signal, polar transport of the plant hormone auxin is critical in leaf polarity formation. However, how auxin polar transport functions in the process is unknown. Through live imaging, we established a profile of auxin polar transport in and around young leaf primordia. Here we show that auxin polar transport in lateral regions of an incipient primordium forms auxin convergence points. We demonstrated that blocking auxin polar transport in the lateral regions of the incipient primordium by incisions abolished the auxin convergence points and caused abaxialized leaves to form. The lateral incisions also blocked the formation of leaf middle domain and margins and disrupted expression of the middle domain/margin-associated marker gene WUSCHEL-RELATED HOMEOBOX 1 (SlWOX1). Based on these results we propose that the auxin convergence points are required for the formation of leaf middle domain and margins, and the functional middle domain and margins ensure leaf adaxial-abaxial polarity. How middle domain and margins function in the process is discussed.

Comparative transcriptomics provide insight into the morphogenesis and evolution of fistular leaves in Allium

BACKGROUND

Fistular leaves frequently appear in Allium species, and previous developmental studies have proposed that the process of fistular leaf formation involves programmed cell death. However, molecular evidence for the role of programmed cell death in the formation of fistular leaf cavities has yet to be reported.

RESULTS

In this study, we characterized the leaf transcriptomes of nine Allium species, including six fistular- and three solid-leaved species. In addition, we identified orthologous genes and estimated their Ka and Ks values, in order to ascertain their selective pattern. Phylogenetic analysis based on the transcriptomes revealed that A. tuberosum was the most ancestral among the nine species, and analysis of orthologous genes between A. tuberosum and the other eight species indicated that 149 genes were subject to positive selection; whereas >3000 had undergone purifying selection in each species.

CONCLUSIONS

We found that many genes that are potentially related to programmed cell death either exhibited rapid diversification in fistular-leaved species, or were conserved in solid-leaved species in evolutionary history. These genes potentially involved in programmed cell death might play important roles in the formation of fistular leaf cavities in Allium, and the differing selection patterns in fistular- and solid-leaved species may be responsible for the evolution of fistular leaves.

AUXIN RESPONSE FACTOR3 Regulates Compound Leaf Patterning by Directly Repressing PALMATE-LIKE PENTAFOLIATA1 Expression in Medicago truncatula

Diverse leaf forms can be seen in nature. In Medicago truncatula, PALM1 encoding a Cys(2)His(2) transcription factor is a key regulator of compound leaf patterning. PALM1 negatively regulates expression of SGL1, a key regulator of lateral leaflet initiation. However, how PALM1 itself is regulated is not yet known. To answer this question, we used promoter sequence analysis, yeast one-hybrid tests, quantitative transcription activity assays, ChIP-PCR analysis, and phenotypic analyses of overexpression lines and mutant plants. The results show that M. truncatula AUXIN RESPONSE FACTOR3 (MtARF3) functions as a direct transcriptional repressor of PALM1. MtARF3 physically binds to the PALM1 promoter sequence in yeast cells. MtARF3 selectively interacts with specific auxin response elements (AuxREs) in the PALM1 promoter to repress reporter gene expression in tobacco leaves and binds to specific sequences in the PALM1 promoter in vivo. Upregulation of MtARF3 or removal of both PHANTASTICA (PHAN) and ARGONAUTE7 (AGO7) pathways resulted in compound leaves with five narrow leaflets arranged in a palmate-like configuration. These results support that MtARF3, in addition as an adaxial-abaxial polarity regulator, functions to restrict spatiotemporal expression of PALM1, linking auxin signaling to compound leaf patterning in the legume plant M. truncatula.

NARROW AND ROLLED LEAF 2 regulates leaf shape, male fertility, and seed size in rice

Grain yield in rice (Oryza sativa L.) is closely related to leaf and flower development. Coordinative regulation of leaf, pollen, and seed development in rice as a critical biological and agricultural question should be addressed. Here we identified two allelic rice mutants with narrow and semi-rolled leaves, named narrow and rolled leaf 2-1 (nrl2-1) and nrl2-2. Map-based molecular cloning revealed that NRL2 encodes a novel protein with unknown biochemical function. The mutation of NRL2 caused pleiotropic effects, including a reduction in the number of longitudinal veins, defective abaxial sclerenchymatous cell differentiation, abnormal tapetum degeneration and microspore development, and the formation of more slender seeds compared with the wild type (WT). The NRL2 protein interacted with Rolling-leaf (RL14), causing the leaves of the nrl2 mutants to have a higher cellulose content and lower lignin content than the WT, which may have been related to sclerenchymatous cell differentiation and tapetum degeneration. Thus, this gene is an essential developmental regulator controlling fundamental cellular and developmental processes, serving as a potential breeding target for high-yielding rice cultivars.

Ectopic expression of SlAGO7 alters leaf pattern and inflorescence architecture and increases fruit yield in tomato

ARGONAUTE7 (AGO7), a key regulator of the trans-acting small interfering RNAs (ta-siRNA) pathway, plays a conserved role in controlling leaf pattern among species. However, little is known about the ta-siRNA pathway in regulating inflorescence architecture and fruit yield. In this study, we characterized the expression pattern, subcellular localization and developmental functions of SlAGO7 in tomato (Solanum lycopersicum). Overexpressing SlAGO7 in tomato exhibited pleiotropic phenotypes, including improved axillary bud formation, altered leaf morphology and inflorescence architecture, and increased fruit yield. Cross-sectioning of leaves showed that the number of vascular bundles was significantly increased in 35:SlAGO7 lines. Overexpression of SlAGO7 increased the production of ta-siRNA, and repressed the expression ta-siRNA-targeted genes (SlARF2a, SlARF2b, SlARF3 and SlARF4). Further analysis showed that overexpression of SlAGO7 alters the expression of key genes implicated in leaf morphology, inflorescence architecture, auxin transport and signaling. In addition, the altered auxin response of 35:SlAGO7 lines were also investigated. These results suggested that SlAGO7 plays a positive role in determining inflorescence architecture and fruit yield though the ta-siRNA pathway. Therefore, SlAGO7 represents a useful gene that can be incorporated in tomato breeding programs for developing cultivars with yield potential.

Developmental mechanisms underlying variable, invariant and plastic phenotypes

BACKGROUND

Discussions of phenotypic robustness often consider scenarios where invariant phenotypes are optimal and assume that developmental mechanisms have evolved to buffer the phenotypes of specific traits against stochastic and environmental perturbations. However, plastic plant phenotypes that vary between environments or variable phenotypes that vary stochastically within an environment may also be advantageous in some scenarios.

SCOPE

Here the conditions under which invariant, plastic and variable phenotypes of specific traits may confer a selective advantage in plants are examined. Drawing on work from microbes and multicellular organisms, the mechanisms that may give rise to each type of phenotype are discussed.

CONCLUSION

In contrast to the view of robustness as being the ability of a genotype to produce a single, invariant phenotype, changes in a phenotype in response to the environment, or phenotypic variability within an environment, may also be delivered consistently (i.e. robustly). Thus, for some plant traits, mechanisms have probably evolved to produce plasticity or variability in a reliable manner.

Genetic Variation and Divergence of Genes Involved in Leaf Adaxial-Abaxial Polarity Establishment in Brassica rapa

Alterations in leaf adaxial-abaxial (ad-ab) polarity are one of the main factors that influence leaf curvature. In Chinese cabbage, leaf incurvature is an essential prerequisite to the formation of a leafy head. Identifying ad-ab patterning genes and investigating their genetic variation may facilitate elucidation of the mechanisms underlying leaf incurvature during head formation. Comparative genomic analysis of 45 leaf ad-ab patterning genes in Brassica rapa based on 26 homologs of Arabidopsis thaliana indicated that these genes underwent expansion and were retained after whole genome triplication (WGT). We also assessed the nucleotide diversity and selection footprints of these 45 genes in a collection of 94 Brassica rapa accessions that were composed of heading and non-heading morphotypes. Six of the 45 genes showed significant negative Tajima's D indices and nucleotide diversity reduction in heading accessions compared to those in non-heading accessions, indicating that they underwent purifying selection. Further testing of the BrARF3.1 gene, which was one of the selection signals from a larger collection, confirmed that purifying selection did occur. Our results provide genetic evidence that ad-ab patterning genes are involved in leaf incurvature, which is associated with formation of a leafy head, as well as promote an understanding of the genetic mechanism underlying leafy head formation in Chinese cabbage.

Loss of YABBY2-Like Gene Expression May Underlie the Evolution of the Laminar Style in Canna and Contribute to Floral Morphological Diversity in the Zingiberales

The Zingiberales is an order of tropical monocots that exhibits diverse floral morphologies. The evolution of petaloid, laminar stamens, staminodes, and styles contributes to this diversity. The laminar style is a derived trait in the family Cannaceae and plays an important role in pollination as its surface is used for secondary pollen presentation. Previous work in the Zingiberales has implicated YABBY2-like genes, which function in promoting laminar outgrowth, in the evolution of stamen morphology. Here, we investigate the evolution and expression of Zingiberales YABBY2-like genes in order to understand the evolution of the laminar style in Canna. Phylogenetic analyses show that multiple duplication events have occurred in this gene lineage prior to the diversification of the Zingiberales. Reverse transcription-PCR in Canna, Costus, and Musa reveals differential expression across floral organs, taxa, and gene copies, and a role for YABBY2-like genes in the evolution of the laminar style is proposed. Selection tests indicate that almost all sites in conserved domains are under purifying selection, consistent with their functional relevance, and a motif unique to monocot YABBY2-like genes is identified. These results contribute to our understanding of the molecular mechanisms underlying the evolution of floral morphologies.

Transition from two to one integument in Prunus species: expression pattern of INNER NO OUTER (INO), ABERRANT TESTA SHAPE (ATS) and ETTIN (ETT)

While gymnosperm ovules have one integument, in most angiosperms two integuments surround the ovules. Unitegmic ovules have arisen independently several times during the evolution of angiosperms, but the ultimate genetic cause of the presence of a single integument remains elusive. We compared species of the genus Prunus that have different numbers of integuments: bitegmic species, such as Prunus armeniaca (apricot) and Prunus persica (peach), and unitegmic species, such as Prunus incisa, analyzing the expression pattern of genes that are involved in integument development in Arabidopsis thaliana: INNER NO OUTER (INO), ABERRANT TESTA SHAPE (ATS) and ETTIN (ETT). Bitegmic and unitegmic species showed similar INO expression patterns, indicative of the conservation of an outer integument. However, expression of ETT, which occurs in the boundary of the outer and inner integuments, was altered in unitegmic ovules, which showed lack of ETT expression. These results strongly suggest that the presence of a single integument could be attributable to the amalgamation of two integuments and support the role of ETT in the fusion of the outer and inner integuments in unitegmic ovules, a situation that could be widespread in other unitegmic species of angiosperms.

Organogenesis in plants: initiation and elaboration of leaves

Plant organs initiate from meristems and grow into diverse forms. After initiation, organs enter a morphological phase where they develop their shape, followed by differentiation into mature tissue. Investigations into these processes have revealed numerous factors necessary for proper development, including transcription factors such as the KNOTTED-LIKE HOMEOBOX (KNOX) genes, the hormone auxin, and miRNAs. Importantly, these factors have been shown to play a role in organogenesis in various diverse model species, revealing both deep conservation of regulatory strategies and evolutionary novelties that led to new plant forms. We review here recent work in understanding the regulation of organogenesis and in particular leaf formation, highlighting how regulatory modules are often redeployed in different organ types and stages of development to achieve diverse forms through the balance of growth and differentiation.

Role of transcriptional regulation in the evolution of plant phenotype: A dynamic systems approach

A growing body of evidence suggests that alterations in transcriptional regulation of genes involved in modulating development are an important part of phenotypic evolution, and this can be documented among species and within populations. While the effects of differential transcriptional regulation in organismal development have been preferentially studied in animal systems, this phenomenon has also been addressed in plants. In this review, we summarize evidence for cis-regulatory mutations, trans-regulatory changes and epigenetic modifications as molecular events underlying important phenotypic alterations, and thus shaping the evolution of plant development. We postulate that a mechanistic understanding of why such molecular alterations have a key role in development, morphology and evolution will have to rely on dynamic models of complex regulatory networks that consider the concerted action of genetic and nongenetic components, and that also incorporate the restrictions underlying the genotype to phenotype mapping process. Developmental Dynamics 244:1074-1095, 2015. © 2015 Wiley Periodicals, Inc.

Repression of AS2 by WOX family transcription factors is required for leaf development in Medicago and Arabidopsis

WOX transcription factors are key regulators of meristematic activity in plants. The Medicago WOX gene, STF, functions in maintenance of leaf marginal meristem, analogous to the function of WUS in the shoot apical meristem. Both STF and WUS directly repress AS2 expression in their respective domains. Ectopic expression of AS2 with WUS promoter leads to a narrow leaf phenotype and other phenotypes similar to the wus mutant. We also found that a wox1 prs wus triple mutant produces much narrower leaf blades than the wox1 prs double mutant, indicating that WUS genetically interacts with WOX1 and PRS in Arabidopsis leaf blade development. Our data points to a general requirement for AS2 repression in meristematic regions to allow cell proliferation.

What determines a leaf's shape?

The independent origin and evolution of leaves as small, simple microphylls or larger, more complex megaphylls in plants has shaped and influenced the natural composition of the environment. Significant contributions have come from megaphyllous leaves, characterized usually as flat, thin lamina entrenched with photosynthetic organelles and stomata, which serve as the basis of primary productivity. During the course of evolution, the megaphylls have attained complexity not only in size or venation patterns but also in shape. This has fascinated scientists worldwide, and research has progressed tremendously in understanding the concept of leaf shape determination. Here, we review these studies and discuss the various factors that contributed towards shaping the leaf; initiated as a small bulge on the periphery of the shoot apical meristem (SAM) followed by asymmetric outgrowth, expansion and maturation until final shape is achieved. We found that the underlying factors governing these processes are inherently genetic: PIN1 and KNOX1 are indicators of leaf initiation, HD-ZIPIII, KANADI, and YABBY specify leaf outgrowth while ANGUSTIFOLIA3 and GROWTH-REGULATING FACTOR5 control leaf expansion and maturation; besides, recent research has identified new players such as APUM23, known to specify leaf polarity. In addition to genetic control, environmental factors also play an important role during the final adjustment of leaf shape. This immense amount of information available will serve as the basis for studying and understanding innovative leaf morphologies viz. the pitchers of the carnivorous plant Nepenthes which have evolved to provide additional support to the plant survival in its nutrient-deficient habitat. In hindsight, formation of the pitcher tube in Nepenthes might involve the recruitment of similar genetic mechanisms that occur during sympetaly in Petunia.

Transcriptome profiling reveals roles of meristem regulators and polarity genes during fruit trichome development in cucumber (Cucumis sativus L.)

Trichomes are epidermal hair-like structures that function in plant defence against biotic and abiotic stresses. Extensive studies have been performed on foliar trichomes development in Arabidopsis and tomato, but the molecular mechanism of fruit trichome formation remains elusive. Cucumber fruit is covered with trichomes (spines) that directly affect the appearance and quality of cucumber products. Here, we characterized the fruit spine development in wild-type (WT) cucumber and a spontaneous mutant, tiny branched hair (tbh). Our data showed that the cucumber trichome was multicellular and non-glandular, with malformed organelles and no endoreduplication. Fruit spine development was generally homogenous and marked by a rapid base expansion stage. Trichomes in the tbh mutant were tiny and branched, with increased density and aberrant cell shape. Transcriptome profiling indicated that meristem-related genes were highly enriched in the upregulated genes in the tbh versus the WT, as well as in WT spines after versus before base expansion, and that polarity regulators were greatly induced during spine base expansion. Quantitative reverse transcription PCR and in situ hybridization confirmed the differential expression of CUP-SHAPED COTYLEDON3 (CUC3) and SHOOT MERISTEMLESS (STM) during spine development. Therefore, cucumber trichomes are morphologically different from those of Arabidopsis and tomato, and their development may be regulated by a distinct pathway involving meristem genes and polarity regulators.

BLADE-ON-PETIOLE genes: setting boundaries in development and defense

BLADE-ON-PETIOLE (BOP) genes encode an ancient and conserved subclade of BTB-ankryin transcriptional co-activators, divergent in the NPR1 family of plant defense regulators. Arabidopsis BOP1/2 were originally characterized as regulators of leaf and floral patterning. Recent investigation of BOP activity in a variety of land plants provides a more complete picture of their conserved functions at lateral organ boundaries in the determination of leaf, flower, inflorescence, and root nodule architecture. BOPs exert their function in part through promotion of lateral organ boundary genes including ASYMMETRIC LEAVES2, KNOTTED1-LIKE FROM ARABIDOPSIS6, and ARABIDOPSIS THALIANA HOMEOBOX GENE1 whose products restrict growth, promote differentiation, and antagonize meristem activity in various developmental contexts. Mutually antagonistic interactions between BOP and meristem factors are important in maintaining a border between meristem-organ compartments and in controlling irreversible transitions in cell fate associated with differentiation. We also examine intriguing new evidence for BOP function in plant defense. Comparisons to NPR1 highlight previously unexplored mechanisms for co-ordination of development and defense in land plants.

The trans-acting short interfering RNA3 pathway and no apical meristem antagonistically regulate leaf margin development and lateral organ separation, as revealed by analysis of an argonaute7/lobed leaflet1 mutant in Medicago truncatula

Leaf shape elaboration and organ separation are critical for plant morphogenesis. We characterized the developmental roles of lobed leaflet1 by analyzing a recessive mutant in the model legume Medicago truncatula. An ortholog of Arabidopsis thaliana argonaute7 (AGO7), Mt-AGO7/lobed leaflet1, is required for the biogenesis of a trans-acting short interfering RNA (ta-siRNA) to negatively regulate the expression of auxin response factors in M. truncatula. Loss of function in AGO7 results in pleiotropic phenotypes in different organs. The prominent phenotype of the ago7 mutant is lobed leaf margins and more widely spaced lateral organs, suggesting that the trans-acting siRNA3 (TAS3) pathway negatively regulates the formation of boundaries and the separation of lateral organs in M. truncatula. Genetic interaction analysis with the smooth leaf margin1 (slm1) mutant revealed that leaf margin formation is cooperatively regulated by the auxin/SLM1 (ortholog of Arabidopsis PIN-formed1) module, which influences the initiation of leaf margin teeth, and the TAS3 ta-siRNA pathway, which determines the degree of margin indentation. Further investigations showed that the TAS3 ta-siRNA pathway and no apical meristem (ortholog of Arabidopsis cup-shaped cotyledon) antagonistically regulate both leaf margin development and lateral organ separation, and the regulation is partially dependent on the auxin/SLM1 module.

The evolution, morphology, and development of fern leaves

Leaves are lateral determinate structures formed in a predictable sequence (phyllotaxy) on the flanks of an indeterminate shoot apical meristem. The origin and evolution of leaves in vascular plants has been widely debated. Being the main conspicuous organ of nearly all vascular plants and often easy to recognize as such, it seems surprising that leaves have had multiple origins. For decades, morphologists, anatomists, paleobotanists, and systematists have contributed data to this debate. More recently, molecular genetic studies have provided insight into leaf evolution and development mainly within angiosperms and, to a lesser extent, lycophytes. There has been recent interest in extending leaf evolutionary developmental studies to other species and lineages, particularly in lycophytes and ferns. Therefore, a review of fern leaf morphology, evolution and development is timely. Here we discuss the theories of leaf evolution in ferns, morphology, and diversity of fern leaves, and experimental results of fern leaf development. We summarize what is known about the molecular genetics of fern leaf development and what future studies might tell us about the evolution of fern leaf development.

Pattern dynamics in adaxial-abaxial specific gene expression are modulated by a plastid retrograde signal during Arabidopsis thaliana leaf development

The maintenance and reformation of gene expression domains are the basis for the morphogenic processes of multicellular systems. In a leaf primordium of Arabidopsis thaliana, the expression of FILAMENTOUS FLOWER (FIL) and the activity of the microRNA miR165/166 are specific to the abaxial side. This miR165/166 activity restricts the target gene expression to the adaxial side. The adaxial and abaxial specific gene expressions are crucial for the wide expansion of leaf lamina. The FIL-expression and the miR165/166-free domains are almost mutually exclusive, and they have been considered to be maintained during leaf development. However, we found here that the position of the boundary between the two domains gradually shifts from the adaxial side to the abaxial side. The cell lineage analysis revealed that this boundary shifting was associated with a sequential gene expression switch from the FIL-expressing (miR165/166 active) to the miR165/166-free (non-FIL-expressing) states. Our genetic analyses using the enlarged fil expression domain2 (enf2) mutant and chemical treatment experiments revealed that impairment in the plastid (chloroplast) gene expression machinery retards this boundary shifting and inhibits the lamina expansion. Furthermore, these developmental effects caused by the abnormal plastids were not observed in the genomes uncoupled1 (gun1) mutant background. This study characterizes the dynamic nature of the adaxial-abaxial specification process in leaf primordia and reveals that the dynamic process is affected by the GUN1-dependent retrograde signal in response to the failure of plastid gene expression. These findings advance our understanding on the molecular mechanism linking the plastid function to the leaf morphogenic processes.

Modification and co-option of leaf developmental programs for the acquisition of flat structures in monocots: unifacial leaves in Juncus and cladodes in Asparagus

It has been suggested that modification and co-option of existing gene regulatory networks (GRNs) play an important role in the morphological diversity. In plants, leaf development is one of active research areas, and the basic GRN for leaf development is beginning to be understood. Moreover, leaves show wide variation in their form, and some of this variation is thought to be the result of adaptation. Thus, leaves and leaf-like organs are an emerging and interesting model to reveal how existing GRNs give rise to novel forms and architectures during evolution. In this review, we highlight recent findings in evo-devo studies, especially on Juncus unifacial leaves, which are composed of lamina with abaxialized identities, and Asparagus cladodes, which are leaf-like organs at the axils of scale leaves. Based on these studies, we discuss how flat structures have evolved and morphologically diversified in shoot systems of monocot species, focusing on the modification and co-option of GRN for leaf development.

The Leaf Adaxial-Abaxial Boundary and Lamina Growth

In multicellular organisms, boundaries have a role in preventing the intermingling of two different cell populations and in organizing the morphogenesis of organs and the entire organism. Plant leaves have two different cell populations, the adaxial (or upper) and abaxial (or lower) cell populations, and the boundary is considered to be important for lamina growth. At the boundary between the adaxial and abaxial epidermis, corresponding to the margin, margin-specific structures are developed and structurally separate the adaxial and abaxial epidermis from each other. The adaxial and abaxial cells are determined by the adaxial and abaxial regulatory genes (including transcription factors and small RNAs), respectively. Among many lamina-growth regulators identified by recent genetic analyses, it has been revealed that the phytohormone, auxin, and the WOX family transcription factors act at the adaxial-abaxial boundary downstream of the adaxial-abaxial pattern. Furthermore, mutant analyses of the WOX genes shed light on the role of the adaxial-abaxial boundary in preventing the mixing of the adaxial and abaxial features during lamina growth. In this review, we highlight the recent studies on the dual role of the adaxial-abaxial boundary.

Meta-analyses of microarrays of Arabidopsis asymmetric leaves1 (as1), as2 and their modifying mutants reveal a critical role for the ETT pathway in stabilization of adaxial-abaxial patterning and cell division during leaf development

It is necessary to use algorithms to analyze gene expression data from DNA microarrays, such as in clustering and machine learning. Previously, we developed the knowledge-based fuzzy adaptive resonance theory (KB-FuzzyART), a clustering algorithm suitable for analyzing gene expression data, to find clues for identifying gene networks. Leaf primordia form around the shoot apical meristem (SAM), which consists of indeterminate stem cells. Upon initiation of leaf development, adaxial-abaxial patterning is crucial for lateral expansion, via cellular proliferation, and the formation of flat symmetric leaves. Many regulatory genes that specify such patterning have been identified. Analysis by the KB-FuzzyART and subsequent molecular and genetic analyses previously showed that ASYMMETRIC LEAVES1 (AS1) and AS2 repress the expression of some abaxial-determinant genes, such as AUXIN RESPONSE FACTOR3 (ARF3)/ETTIN (ETT) and ARF4, which are responsible for defects in leaf adaxial-abaxial polarity in as1 and as2. In the present study, genetic analysis revealed that ARF3/ETT and ARF4 were regulated by modifier genes, BOBBER1 (BOB1) and ELONGATA3 (ELO3), together with AS1-AS2. We analyzed expression arrays with as2 elo3 and as2 bob1, and extracted genes downstream of ARF3/ETT by using KB-FuzzyART and molecular analyses. The results showed that expression of Kip-related protein (KRP) (for inhibitors of cyclin-dependent protein kinases) and Isopentenyltransferase (IPT) (for biosynthesis of cytokinin) genes were controlled by AS1-AS2 through ARF3/ETT and ARF4 functions, which suggests that the AS1-AS2-ETT pathway plays a critical role in controlling the cell division cycle and the biosynthesis of cytokinin around SAM to stabilize leaf development in Arabidopsis thaliana.

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.

Opinion: prospects for improving photosynthesis by altering leaf anatomy

Engineering higher photosynthetic efficiency for greater crop yields has gained significant attention among plant biologists and breeders. To achieve this goal, manipulation of metabolic targets and canopy architectural features has been heavily emphasized. Given the substantial variations in leaf anatomical features among and within plant species, there is large potential to engineer leaf anatomy for improved photosynthetic efficiency. Here we review how different leaf anatomical features influence internal light distribution, delivery of CO(2) to Rubisco and water relations, and accordingly recommend features to engineer for increased leaf photosynthesis under different environments. More research is needed on (a) elucidating the genetic mechanisms controlling leaf anatomy, and (b) the development of a three dimensional biochemical and biophysical model of leaf photosynthesis, which can help pinpoint anatomical features required to gain a higher photosynthesis.