A genetic framework for fruit patterning in Arabidopsis thaliana

In the model plant Arabidopsis thaliana, the establishment of organ polarity leads to the expression of FILAMENTOUS FLOWER (FIL) and YABBY3 (YAB3) on one side of an organ. One important question that has remained unanswered is how does this positional information lead to the correct spatial activation of genes controlling tissue identity? We provide the first functional link between polarity establishment and the regulation of tissue identity by showing that FIL and YAB3 control the non-overlapping expression patterns of FRUITFULL (FUL) and SHATTERPROOF (SHP), genes necessary to form stripes of valve margin tissue that allow the fruit to shatter along two defined borders and disperse the seeds. FIL and YAB3 activate FUL and SHP redundantly with JAGGED (JAG), a gene that also promotes growth in organs, indicating that several pathways converge to regulate these genes. These activities are negatively regulated by REPLUMLESS (RPL), which divides FIL/JAG activity, creating two distinct stripes of valve margin.     

The Polycomb group protein MEDEA controls cell proliferation and embryonic patterning in Arabidopsis

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Evolving role of Antennapedia protein in arthropod limb patterning

Evolutional changes in homeotic gene functions have contributed to segmental diversification of arthropodan limbs, but crucial molecular changes have not been identified to date. The first leg of the crustacean Daphnia lacks a prominent ventral branch found in the second to fourth legs. We show here that this phenotype correlates with the loss of Distal-less and concomitant expression of Antennapedia in the limb primordium. Unlike its Drosophila counterpart, Daphnia Antennapedia represses Distal-less in Drosophila assays, and the protein region conferring this activity was mapped to the N terminal region of the protein. The results imply that Dapnia Antennapedia specifies leg morphology by repressing Distal-less, and this activity was acquired through a change in protein structure after separation of crustaceans and insects.     

Common regulatory networks in leaf and fruit patterning revealed by mutations in the Arabidopsis ASYMMETRIC LEAVES1 gene

Carpels and leaves are evolutionarily related organs, as the former are thought to be modified leaves. Therefore, developmental pathways that play crucial roles in patterning both organs are presumably conserved. In leaf primordia of Arabidopsis thaliana, the ASYMMETRIC LEAVES1 (AS1) gene interacts with AS2 to repress the class I KNOTTED1-like homeobox (KNOX) genes BREVIPEDICELLUS (BP), KNAT2 and KNAT6, restricting the expression of these genes to the meristem. In this report, we describe how AS1, presumably in collaboration with AS2, patterns the Arabidopsis gynoecium by repressing BP, which is expressed in the replum and valve margin, interacts in the replum with REPLUMLESS (RPL), an essential gene for replum development, and positively regulates the expression of this gene. Misexpression of BP in the gynoecium causes an increase in replum size, while the valve width is slightly reduced, and enhances the effect of mutations in FRUITFULL (FUL), a gene with an important function in valve development. Altogether, these findings strongly suggest that BP plays a crucial role in replum development. We propose a model for pattern formation along the mediolateral axis of the ovary, whereby three domains (replum, valve margin and valve) are specified by the opposing gradients of two antagonistic factors, valve factors and replum factors, the class I KNOX genes working as the latter.     

Generation of a spacing pattern: the role of triptychon in trichome patterning in Arabidopsis.

Trichomes in Arabidopsis are single-celled hairs that exhibit a regular spacing pattern. Here, the role of TRIPTYCHON (TRY) in the generation of this spacing pattern is studied. By using genetic mosaics, we demonstrate that the formation of trichome clusters in try mutants is not correlated with cell lineage, indicating that TRY is required to single out trichome cells in a process involving cellular interactions. The genetic interactions of TRY, GLABRA1 (GL1), and TRANSPARENT TESTA GLABRA (T TG) in trichome patterning are assessed by determining the cluster frequency in various genetic combinations. It is shown that TRY acts as a negative regulator of GL1- and TTG-dependent pathways. Furthermore, it is demonstrated that trichome initiation in ttg-1, a strong ttg allele, is rescued almost to wild-type levels in a try background in which GL1 is expressed under the control of the cauliflower mosaic virus 35S promoter, indicating that T TG acts upstream of GL1 and TRY. These findings are incorporated into a model to explain the generation of a trichome spacing pattern from a homogeneous population of epidermal cells.     

HAT3.1, a novel Arabidopsis homeodomain protein containing a conserved cysteine-rich region

Homeodomain proteins have been shown to play a major role in the development of various organisms. A novel Arabidopsis homeodomain protein has been isolated based on its capability to interact with a DNA motif derived from the light-induced cab-E promoter of Nicotiana plumbaginifolia . The homeodomain of this protein, designated HAT3.1, differs substantially from those in other plant homeobox proteins identified so far. Furthermore, HAT3.1 is unique among other Arabidopsis proteins in that it does not contain a leucine zipper motif following the homeodomain. HAT3.1 is further characterized by an N-terminal region that shares substantial sequence similarity with the maize homeodomain protein Zmhox1a. Within this conserved region, the presence of eight regularly spaced cysteine/histidine residues was observed reminiscent of other metal-binding domains. Based on the strong evolutionary conservation of this domain, it is proposed that this region represents a novel protein-motif which is denoted PHD-finger ( p lant h omeo- d omain-finger). In vitro DNA binding studies demonstrated that HAT3.1 is capable of interacting with any DNA fragment larger than 100 bp. Interestingly, a deletion of the N-terminal PHD-finger domain completely abolished DNA binding, suggesting that this region may play an important functional role in protein—protein or protein—DNA interaction. HAT3.1 mRNA was primarily detected in root tissue, implying a regulatory function of this protein in root development.     

The role of brinker in eggshell patterning

Drosophila oogenesis provides a useful system to study signal transduction pathways and their interactions. Through clonal analysis, we found that brinker (brk), a repressor of Dpp signaling, plays an important role in the Drosophila ovary, where its function is essential for dorsal appendage formation. In the absence of brk, operculum fates are specified at the expense of dorsal appendage fates. Brk is expressed by most of the oocyte associated follicle cells, starting from stage 8 of oogenesis. Transforming Growth Factor beta (TGFbeta) signaling represses brk expression in both the early stage egg chambers and in the anterior follicle cells. In brk mutant follicle cell clones at the dorsal anterior region, Broad Complex (BR-C) expression is down-regulated in a larger domain than in wild type. We show that BR-C is required for dorsal appendage development. In large anterior BR-C mutant clones, dorsal appendages are absent, and instead, the eggshell has an enlarged operculum like region at the anterior. In addition, we show that the Epidermal Growth Factor (EGF) receptor signaling represses the TGFbeta signaling in oogenesis by up-regulating brk expression. From our results and previously published data, it appears that anterior follicle cells integrate the levels of EGF receptor activation and TGFbeta receptor activation. Operculum fate results when the sum of the level of activation of both pathways reaches a threshold level, and reduction of activity of one pathway can be compensated to some extent by increase in the other pathway.     

The role of GDNF in patterning the excretory system

Mesenchymal-epithelial interactions are an important source of information for pattern formation during organogenesis. In the developing excretory system, one of the secreted mesenchymal factors thought to play a critical role in patterning the growth and branching of the epithelial ureteric bud is GDNF. We have tested the requirement for GDNF as a paracrine chemoattractive factor by altering its site of expression during excretory system development. Normally, GDNF is secreted by the metanephric mesenchyme and acts via receptors on the Wolffian duct and ureteric bud epithelium. Misexpression of GDNF in the Wolffian duct and ureteric buds resulted in formation of multiple, ectopic buds, which branched independently of the metanephric mesenchyme. This confirmed the ability of GDNF to induce ureter outgrowth and epithelial branching in vivo. However, in mutant mice lacking endogenous GDNF, kidney development was rescued to a substantial degree by GDNF supplied only by the Wolffian duct and ureteric bud. These results indicate that mesenchymal GDNF is not required as a chemoattractive factor to pattern the growth of the ureteric bud within the developing kidney, and that any positional information provided by the mesenchymal expression of GDNF may provide for renal branching morphogenesis is redundant with other signals.     

Stomatal development and patterning in Arabidopsis leaves

The functional unit for gas exchange between plants and the atmosphere is the stomatal complex, an epidermal structure composed of two guard cells, which delimit a stomatal pore, and their subsidiary cells. In the present work, we define the basic structural unit formed in Arabidopsis thaliana during leaf development, the anisocytic stomatal complex. We perform a cell lineage analysis by transposon excision founding that at least a small percentage of stomatal complexes are unequivocally non-clonal. We also describe the three-dimensional pattern of stomata in the Arabidopsis leaf. In the epidermal plane, subsidiary cells of most stomatal complexes contact the subsidiary cells of immediately adjacent complexes. This minimal distance between stomatal complexes allows each stoma to be circled by a full complement of subsidiary cells, with which guard cells can exchange water and ions in order to open or to close the pore. In the radial plane, stomata (and their precursors, the meristemoids) are located at the junctions of several mesophyll cells. This meristemoid patterning may be a consequence of signals that operate along the radial axis of the leaf, which establish meristemoid differentiation precisely at these places. Since stomatal development is basipetal, these radially propagated signals may be transmitted in the axial direction, thus guiding stomatal development through the basal end of the leaf.