Differential recruitment of WOX transcription factors for lateral development and organ fusion in Petunia and Arabidopsis

LATHYROIDES, encoding a WUSCHEL-related Homeobox1 transcription factor, controls organ lateral growth, and regulates tendril and dorsal petal identities in garden pea (Pisum sativum L.)

During organ development, many key regulators have been identified in plant genomes, which play a conserved role among plant species to control the organ identities and/or determine the organ size and shape. It is intriguing whether these key regulators can acquire diverse function and be integrated into different molecular pathways among different species, giving rise to the immense diversity of organ forms in nature. In this study, we have characterized and cloned LATHYROIDES (LATH), a classical locus in pea, whose mutation displays pleiotropic alteration of lateral growth of organs and predominant effects on tendril and dorsal petal development. LATH encodes a WUSCHEL-related homeobox1 (WOX1) transcription factor, which has a conserved function in determining organ lateral growth among different plant species. Furthermore, we showed that LATH regulated the expression level of TENDRIL-LESS (TL), a key factor in the control of tendril development in compound leaf, and LATH genetically interacted with LOBED STANDARD (LST), a floral dorsal factor, to affect the dorsal petal identity. Thus, LATH plays multiple roles during organ development in pea: it maintains a conserved function controlling organ lateral outgrowth, and modulates organ identities in compound leaf and zygomorphic flower development, respectively. Our data indicated that a key regulator can play important roles in different aspects of organ development and dedicate to the complexity of the molecular mechanism in the control of organ development so as to create distinct organ forms in different species.

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

A key innovation in leaf evolution is the acquisition of a flat lamina with adaxial-abaxial polarity, which optimizes the primary function of photosynthesis. The developmental mechanism behind leaf adaxial-abaxial polarity specification and flat lamina formation has long been of interest to biologists. Surgical and genetic studies proposed a conceptual model wherein a signal derived from the shoot apical meristem is necessary for adaxial-abaxial polarity specification, and subsequent lamina outgrowth is promoted at the juxtaposition of adaxial and abaxial identities. Several distinct regulators involved in leaf adaxial-abaxial polarity specification and lamina outgrowth have been identified. Analyses of these genes demonstrated that the mutual antagonistic interactions between adaxial and abaxial determinants establish polarity and define the boundary between two domains, along which lamina outgrowth regulators function. Evolutionary developmental studies on diverse leaf forms of angiosperms proposed that alteration to the adaxial-abaxial patterning system can be a major driving force in the generation of diverse leaf forms, as represented by 'unifacial leaves', in which leaf blades have only the abaxial identity. Interestingly, unifacial leaf blades become flattened, in spite of the lack of adaxial-abaxial juxtaposition. Modification of the adaxial-abaxial patterning system is also utilized to generate complex organ morphologies, such as stamens. In this review, we summarize recent advances in the genetic mechanisms underlying leaf adaxial-abaxial polarity specification and lamina outgrowth, with emphasis on the genetic basis of the evolution and diversification of leaves.

Redefining C and D in the petunia ABC

According to the ABC(DE) model for flower development, C-genes are required for stamen and carpel development and floral determinacy, and D-genes were proposed to play a unique role in ovule development. Both C- and D-genes belong to the AGAMOUS (AG) subfamily of MADS box transcription factors. We show that the petunia (Petunia hybrida) C-clade genes PETUNIA MADS BOX GENE3 and FLORAL BINDING PROTEIN6 (FBP6) largely overlap in function, both in floral organ identity specification and floral determinacy, unlike the pronounced subfunctionalization observed in Arabidopsis thaliana and snapdragon (Antirrhinum majus). Some specialization has also evolved, since FBP6 plays a unique role in the development of the style and stigma. Furthermore, we show that the D-genes FBP7 and FBP11 are not essential to confer ovule identity. Instead, this function is redundantly shared among all AG members. In turn, the D-genes also participate in floral determinacy. Gain-of-function analyses suggest the presence of a posttranscriptional C-repression mechanism in petunia, most likely not existing in Arabidopsis. Finally, we show that expression maintenance of the paleoAPETALA3-type B-gene TOMATO MADS BOX GENE6 depends on the activity of C-genes. Taken together, this demonstrates considerable variation in the molecular control of floral development between eudicot species.

Roles of the middle domain-specific WUSCHEL-RELATED HOMEOBOX genes in early development of leaves in Arabidopsis

During leaf development in flowering plants, adaxial (upper) and abaxial (lower) side-specific genes are responsible for blade outgrowth, which takes places predominantly in the lateral direction, and for margin development as well as differentiation of adaxial and abaxial tissues. However, the underlying mechanisms are poorly understood. Here, we show that two WUSCHEL-RELATED HOMEOBOX (WOX) genes, PRESSED FLOWER (PRS)/WOX3 and WOX1, encoding homeobox transcription factors, act in blade outgrowth and margin development downstream of adaxial/abaxial polarity establishment. The expression of PRS and WOX1 defines a hitherto undescribed middle domain, including two middle mesophyll layers and the margin, as a center that organizes the outgrowth of leaf blades. The expression of PRS and WOX1 is repressed in the abaxial leaf domain by the abaxial-specific transcription factor KANADI. Furthermore, PRS and WOX1 coordinate adaxial/abaxial patterning together with adaxial- and abaxial-specific genes. Our data suggest a model of blade outgrowth and adaxial/abaxial patterning via the middle domain-specific WOX genes in Arabidopsis thaliana leaves.

Control of dicot leaf blade expansion by a WOX gene, STF

WUSCHEL-RELATED HOMEOBOX (WOX) genes are plant specific transcription factors that serve as master switches controlling key developmental programs from embryo apical-basal asymmetric patterning to organizing stem cells and development of lateral organs. Recently, we reported the requirement of a WOX1/MAW-like gene, STENOFOLIA (STF), for blade outgrowth and leaf vascular patterning in Medicago truncatula and Nicotiana sylvestris. The stf mutant in Medicago produces narrow leaves where mediolateral outgrowth of the blade is severely curtailed while proximodistal growth and trifoliate identity remain unaffected. The lam1 mutant in N. sylvestris produces leaves devoid of blade tissue with just 1-2 layers of rudimentary strips and lacks stem elongation. stf and lam1 mutants have narrow petals and are female sterile due to defective ovule development. Morphological analysis of mutants and STF expression patterns suggest that STF regulates blade outgrowth mainly by controlling cell division in the margins of leaf primordium. Both the blade and flower phenotypes of lam1 can be complemented with WUS expressed under the STF promoter suggesting a conserved mechanism in stem cell maintenance and lateral organ development.

Over-expression of WOX1 leads to defects in meristem development and polyamine homeostasis in Arabidopsis

In plants, the meristem has to maintain a separate population of pluripotent cells that serve two main tasks, i.e., self-maintenance and organ initiation, which are separated spatially in meristem. Prior to our study, WUS and WUS-like WOX genes had been reported as essential for the development of the SAM. In this study, the consequences of gain of WOX1 function are described. Here we report the identification of an Arabidopsis gain-of-function mutant wox1-D, in which the expression level of the WOX1 (WUSCHEL HOMEOBOX 1) was elevated and subtle defects in meristem development were observed. The wox1-D mutant phenotype is dwarfed and slightly bushy, with a smaller shoot apex. The wox1-D mutant also produced small and dark green leaves, and exhibited a failure in anther dehiscence and male sterility. Molecular evidences showed that the transcription of the stem cell marker gene CLV3 was down-regulated in the meristem of wox1-D but accumulated in the other regions, i.e., in the root-hypocotyl junction and at the sites for lateral root initiation. The fact that the organ size and cell size in leaves of wox1-D are smaller than those in wild type suggests that cell expansion is possibly affected in order to have partially retarded the development of lateral organs, possibly through alteration of CLV3 expression pattern in the meristem. An S-adenosylmethionine decarboxylase (SAMDC) protein, SAMDC1, was found able to interact with WOX1 by yeast two-hybrid and pull-down assays in vitro. HPLC analysis revealed a significant reduction of polyamine content in wox1-D. Our results suggest that WOX1 plays an important role in meristem development in Arabidopsis, possibly via regulation of SAMDC activity and polyamine homeostasis, and/or by regulating CLV3 expression.

STENOFOLIA regulates blade outgrowth and leaf vascular patterning in Medicago truncatula and Nicotiana sylvestris

Dicot leaf primordia initiate at the flanks of the shoot apical meristem and extend laterally by cell division and cell expansion to form the flat lamina, but the molecular mechanism of lamina outgrowth remains unclear. Here, we report the identification of STENOFOLIA (STF), a WUSCHEL-like homeobox transcriptional regulator, in Medicago truncatula, which is required for blade outgrowth and leaf vascular patterning. STF belongs to the MAEWEST clade and its inactivation by the transposable element of Nicotiana tabacum cell type1 (Tnt1) retrotransposon insertion leads to abortion of blade expansion in the mediolateral axis and disruption of vein patterning. We also show that the classical lam1 mutant of Nicotiana sylvestris, which is blocked in lamina formation and stem elongation, is caused by deletion of the STF ortholog. STF is expressed at the adaxial-abaxial boundary layer of leaf primordia and governs organization and outgrowth of lamina, conferring morphogenetic competence. STF does not affect formation of lateral leaflets but is critical to their ability to generate a leaf blade. Our data suggest that STF functions by modulating phytohormone homeostasis and crosstalk directly linked to sugar metabolism, highlighting the importance of coordinating metabolic and developmental signals for leaf elaboration.

Characterization of expression dynamics of WOX homeodomain transcription factors during somatic embryogenesis in Vitis vinifera

Different cultivars of Vitis vinifera vary in their potential to form embryogenic tissues. The WUSCHEL (WUS)-related homeobox (WOX) genes have been shown to play an important role in coordinating the gene transcription involved in the early phases of embryogenesis. The expression dynamics of 12 VvWOX genes present in the V. vinifera genome in embryogenic and other tissues of 'Chardonnay' were analysed. In order to understand the influence of WOX genes on the somatic embryogenic process, their expression profiles were compared in two cultivars of V. vinifera ('Chardonnay' and 'Cabernet Sauvignon') that show different aptitudes for embryogenesis. The expression of all VvWOX genes was influenced by culture conditions. VvWOX2 and VvWOX9 were the principal WOX genes expressed during the somatic embryogenesis process, and the low aptitude for embryogenesis of 'Cabernet Sauvignon' was generally correlated with the low expression levels of these VvWOX genes. VvWOX3 and VvWOX11 were strongly activated in correspondence to torpedo and cotyledonary stages of somatic embryos, with low expression in the earlier developmental stages (pre-embryogenic masses and globular embryos) and during embryo germination. VvWOX genes appeared to be key regulators of somatic embryogenesis in grapevine, and the regulation of these genes during early phases of somatic embryogenesis differed between the two cultivars of the same species.

Comparative expression pattern analysis of WUSCHEL-related homeobox 2 (WOX2) and WOX8/9 in developing seeds and somatic embryos of the gymnosperm Picea abies

• In seed plants, current knowledge concerning embryonic pattern formation by polar auxin transport (PAT) and WUSCHEL-related homeobox (WOX) gene activity is primarily derived from studies on angiosperms, while less is known about these processes in gymnosperms. In view of the differences in their embryogeny, and the fact that somatic embryogenesis is used for mass propagation of conifers, a better understanding of embryo development is vital. • The expression patterns of PaWOX2 and PaWOX8/9 were followed with quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and in situ hybridization (ISH) during seed and somatic embryo development in Norway spruce (Picea abies), and in somatic embryos treated with the PAT inhibitor N-1-naphthylphthalamic acid (NPA). • Both PaWOX2 and PaWOX8/9 were highly expressed at the early growth stages of zygotic and somatic embryos, and shared a similar expression pattern over the entire embryo. At later embryo stages, high expression of PaWOX8/9 became restricted to cotyledon primordia, epidermis, procambium and root apical meristem (RAM), which became most evident in NPA-treated somatic embryos, while expression of PaWOX2 was much lower. • Our results suggest an ancestral role of WOX in seed plant embryo development, and strengthen the proposed connection between PAT, PIN-FORMED (PIN) and WOX in the regulation of embryo patterning in seed plants.

TDIF peptide signaling regulates vascular stem cell proliferation via the WOX4 homeobox gene in Arabidopsis

The indeterminate nature of plant growth and development depends on the stem cell system found in meristems. The Arabidopsis thaliana vascular meristem includes procambium and cambium. In these tissues, cell-cell signaling, mediated by a ligand-receptor pair made of the TDIF (for tracheary element differentiation inhibitory factor) peptide and the TDR/PXY (for TDIF RECEPTOR/ PHLOEM INTERCALATED WITH XYLEM) membrane protein kinase, promotes proliferation of procambial cells and suppresses their xylem differentiation. Here, we report that a WUSCHEL-related HOMEOBOX gene, WOX4, is a key target of the TDIF signaling pathway. WOX4 is expressed preferentially in the procambium and cambium, and its expression level was upregulated upon application of TDIF in a TDR-dependent manner. Genetic analyses showed that WOX4 is required for promoting the proliferation of procambial/cambial stem cells but not for repressing their commitment to xylem differentiation in response to the TDIF signal. Thus, at least two intracellular signaling pathways that diverge after TDIF recognition by TDR might regulate independently the behavior of vascular stem cells. Detailed observations in loss-of-function mutants revealed that TDIF-TDR-WOX4 signaling plays a crucial role in the maintenance of the vascular meristem organization during secondary growth.

Analyses of WOX4 transgenics provide further evidence for the evolution of the WOX gene family during the regulation of diverse stem cell functions

The WOX (WUSCHEL-related homeobox) gene family of Arabidopsis comprises fifteen plant-specific transcriptional factors that play important development roles. Genetic, phylogenetic, and genomic analyses suggest that WOX genes generally act non-autonomously to organize stem-cell and initial-cell populations within plant meristems and organ anlagen. Previous cross-complementation analyses indicate that the functional diversification of distinct WOX paralogs may be explained largely by promoter evolution, although paralog-specific protein::protein interactions are also implicated. A recent report described WOX4 function during development of the procambium, which comprises the meristematic tissues of the plant vasculature. Here we show that WOX4 fails to complement PRS1/WOX3 function, when driven from the PRS1/WOX3 native promoter. These data suggest that WOX4 identifies different DNA targets and/or interacting proteins during development of the vasculature procambium than does PRS1/WOX3 during the specification of lateral organ initial cells. The identification of super-compound leaf phenotypes induced by overexpression of the SlWOX4 ortholog in tomato suggests a functional link between vascular patterning and leaf complexity.

Differentiating Arabidopsis shoots from leaves by combined YABBY activities

In seed plants, leaves are born on radial shoots, but unlike shoots, they are determinate dorsiventral organs made of flat lamina. YABBY genes are found only in seed plants and in all cases studied are expressed primarily in lateral organs and in a polar manner. Despite their simple expression, Arabidopsis thaliana plants lacking all YABBY gene activities have a wide range of morphological defects in all lateral organs as well as the shoot apical meristem (SAM). Here, we show that leaves lacking all YABBY activities are initiated as dorsiventral appendages but fail to properly activate lamina programs. In particular, the activation of most CINCINNATA-class TCP genes does not commence, SAM-specific programs are reactivated, and a marginal leaf domain is not established. Altered distribution of auxin signaling and the auxin efflux carrier PIN1, highly reduced venation, initiation of multiple cotyledons, and gradual loss of the SAM accompany these defects. We suggest that YABBY functions were recruited to mold modified shoot systems into flat plant appendages by translating organ polarity into lamina-specific programs that include marginal auxin flow and activation of a maturation schedule directing determinate growth.

Genetic framework for flattened leaf blade formation in unifacial leaves of Juncus prismatocarpus

Angiosperm leaves generally develop as bifacial structures with distinct adaxial and abaxial identities. However, several monocot species, such as iris and leek, develop unifacial leaves, in which leaf blades have only abaxial identity. In bifacial leaves, adaxial-abaxial polarity is required for leaf blade flattening, whereas many unifacial leaves become flattened despite their leaf blades being abaxialized. Here, we investigate the mechanisms underlying the development and evolution of flattened leaf blades in unifacial leaves. We demonstrate that the unifacial leaf blade is abaxialized at the gene expression level and that an ortholog of the DROOPING LEAF (DL) gene may promote flattening of the unifacial leaf blade. In two closely related Juncus species, Juncus prismatocarpus, which has flattened unifacial leaves, and Juncus wallichianus, which has cylindrical unifacial leaves, DL expression levels and patterns correlate with the degree of laminar outgrowth. Genetic and expression studies using interspecific hybrids of the two species reveal that the DL locus from J. prismatocarpus flattens the unifacial leaf blade and expresses higher amounts of DL transcript than does that from J. wallichianus. We also show that leaf blade flattening is a trigger for central-marginal leaf polarity differentiation. We suggest that flattened unifacial leaf blades may have evolved via the recruitment of DL function, which plays a similar cellular but distinct phenotypic role in monocot bifacial leaves.

WOX4 promotes procambial development

Plant shoot organs arise from initial cells that are recruited from meristematic tissues. Previous studies have shown that members of the WUSCHEL-related HOMEOBOX (WOX) gene family function to organize various initial cell populations during plant development. The function of the WOX4 gene is previously undescribed in any plant species. Comparative analyses of WOX4 transcription and function are presented in Arabidopsis (Arabidopsis thaliana), a simple-leafed plant with collateral vasculature, and in tomato (Solanum lycopersicum), a dissected-leafed species with bicollateral venation. WOX4 is transcribed in the developing vascular bundles of root and shoot lateral organs in both Arabidopsis and tomato. RNA interference-induced down-regulation of WOX4 in Arabidopsis generated small plants whose vascular bundles accumulated undifferentiated ground tissue and exhibited severe reductions in differentiated xylem and phloem. In situ hybridization analyses of Atwox4-RNA interference plants revealed delayed and reduced expression of both the phloem developmental marker ALTERED PHLOEM1 and HOMEOBOX GENE8, a marker of the vascular procambium. Overexpression of SlWOX4 correlated with overproliferation of xylem and phloem in transgenic tomato seedlings. The cumulative data suggest that the conserved WOX4 function is to promote differentiation and/or maintenance of the vascular procambium, the initial cells of the developing vasculature.

Signaling sides adaxial-abaxial patterning in leaves

Most leaves are dorsiventrally flattened and develop clearly defined upper and lower surfaces. Light capturing is the specialization of the adaxial or upper surface and the abaxial or lower surface is specialized for gas exchange (Fig. 5.1). This division into adaxial and abaxial domains is also key for the outgrowth of the leaf blade or lamina, which occurs along the boundary between the upper and lower sides. How this polarity is set up is not clear but genetic analysis in a range of species suggests that several highly conserved interlocking pathways are involved. Positional information from the meristem is reinforced by signaling through the epidermal layer as the meristem grows away from the leaf primordium. Opposing ta-siRNA and miRNA gradients help refine distinct adaxial and abaxial sides, and mutual inhibition between the genes expressed on each side stabilizes the boundary. In this review we consider how recent work in a range of species is clarifying our understanding of these processes.

The WUS homeobox-containing (WOX) protein family

The plant-specific WOX family of homeobox proteins have key functions in plant development.

The WOX genes form a plant-specific subclade of the eukaryotic homeobox transcription factor superfamily, which is characterized by the presence of a conserved DNA-binding homeodomain. The analysis of WOX gene expression and function shows that WOX family members fulfill specialized functions in key developmental processes in plants, such as embryonic patterning, stem-cell maintenance and organ formation. These functions can be related to either promotion of cell division activity and/or prevention of premature cell differentiation. The phylogenetic tree of the plant WOX proteins can be divided into three clades, termed the WUS, intermediate and ancient clade. WOX proteins of the WUS clade appear to some extent able to functionally complement other members. The specific function of individual WOX-family proteins is most probably determined by their spatiotemporal expression pattern and probably also by their interaction with other proteins, which may repress their transcriptional activity. The prototypic WOX-family member WUS has recently been shown to act as a bifunctional transcription factor, functioning as repressor in stem-cell regulation and as activator in floral patterning. Past research has mainly focused on part of the WOX protein family in some model flowering plants, such as Arabidopsis thaliana (thale cress) or Oryza sativa (rice). Future research, including so-far neglected clades and non-flowering plants, is expected to reveal how these master switches of plant differentiation and embryonic patterning evolved and how they fulfill their function.