The role of YABBY genes in shoot phyllotaxis

Knock out mutations of YABBY family genes, filamentous flower (fil) and yabby-3 (yab3), can alter the correct development, position and number of lateral organs. The YABBY genes mRNA is restricted to the abaxial side of all established organ primordia. Thus, while YABBY genes are expressed in a distance of 3-5 cells from the incipient primordia and central AM domain they are important for correct phyllotaxis establishment occurred there, presumably by a signaling process. A possible in planta mechanism for transmission of non-cell autonomous signals is via cell-to-cell protein or RNA trafficking. To investigate the basis of YABBY-mediated signaling, functional YABBY-GFP marker expression was monitored. However, no evidence for YABBY proteins or RNA mobility beyond their expression domain was found. For additional evaluation of YABBY activity, mis-expression of YABBY genes under the control of different vegetative and flower specific promoters was pursued. In all cases, expansion of YABBY expression into the inter-organ meristem periphery resulted in induction of phyllotactic aberrations in subsequently formed domains, and severity of the phenotypes was dependent on the levels of ectopic- YABBY expression. Analysis of central-meristem specific markers, showed dramatic changes in their expression domains in plants with altered YABBY expression. All together these results support YABBY's role as short range signaling molecules. However inability of YABBY RNA or protein to traffic into adjacent cells, imply that YABBY morphogenic activity is achieved through secondary messengers termed "YABBY-born signals".;In order to identify the YABBY-born signaling network, different approaches were pursued. Expression analysis of plants with altered YABBY levels revealed high numbers of changing genes, making this approach too difficult to pursue. Alternative approach was EMS mutagenesis of AP1::YAB3 hemizygous plants and screen for suppressors of the ectopic YABBY induced phenotypes. Mutations in four complementation groups were identified in this screen and non of the mutants resembled YABBY loss of function plants. However, double and triple mutant combinations of the newly identified mutations exhibited the phyllotaxis and organ morphology defects found in YABBY loss of function plants. These mutants were subjected to allelism tests, genetic mapping, and when necessary, to map based cloning. Of these, GEVA (GEV) and LUNING (LUG ) share the same expression pattern throughout plant development, while YABBY expression overlaps only in young primordia during a short developmental window. Yeast two hybrid assays indicate that YABBY and LUG may interact directly, while GEV does not interact with neither YABBY nor LUG.;Another suppressor of the AP1::YAB3 plants was found to be allelic with the previously described LATERAL SUPRESURE (LAS) gene. In contrast to GEV and LUG, LAS does not share the same expression domain with YABBY genes and is expressed exclusively in the meristem-organ boundary domain. LAS belongs to the GRAS gene family, which consist of another gene -- SHORT ROOT (SHR) that was shown to act as a short range signal during root development. Other members of the GRAS gene family (GAI/RGI/HAM/SCR) were found to act as mediators of hormonal and developmental signals. Our results suggest that LAS acts as a mediator of YABBY born signals and that LAS expression is tightly regulated by YABBY in a non autonomous manner.;Through analyses of over expression, LAS is implicated in modulation of Brasinosteroid (BR) hormonal signaling in the meristem-organ boundary cells. This suggests a previously unidentified role for BR in phyllotaxis control, and implies that YABBY affects phyllotaxis by modulating the expression domain of LAS. However inactive LAS gene can't be complemented by artificially reduced sensitivity to BR signal in the meristem-organ boundary. This suggests that LAS gene acts as a mediator and regulator of the several different signaling pathways.;In conclusion, YABBY born signals define the organ primordia domain in an autonomous manner whereas the specification of the meristem-organ boundary and the central meristem domains are regulated in a non- autonomous manner. While, the molecular nature of the YABBY born signals remains unclear, the present study uncovered component of this complex signaling network, and provided novel genetic and biochemical tools. Future understanding of this signaling cascade will facilitate better understanding of the continuous plant development process, and contribute to the understanding of self organizing biological processes. (Abstract shortened by UMI.)...