Signals derived from YABBY gene activities in organ primordia regulate growth and partitioning of Arabidopsis shoot apical meristems

Shoot apical meristems (SAMs) are self-sustaining groups of cells responsible for the ordered initiation of all aerial plant tissues, including stems and lateral organs. The precise coordination of these processes argues for crosstalk between the different SAM domains. The products of YABBY (YAB) genes are limited to the organ primordium domains, which are situated at the periphery of all SAMs and which are separated by a margin of three to seven cells from the central meristem zone marked by WUSCHEL and CLAVATA3 expression. Mutations in the two related YAB1 genes, FILAMENTOUS FLOWER and YABBY3 (YAB3), cause an array of defects, including aberrant phyllotaxis. We show that peripheral YAB1 activity non-autonomously and sequentially affects the phyllotaxis and growth of subsequent primordia and coordinates the expression of SAM central zone markers. These effects support a role for YAB1 genes in short-range signaling. However, no evidence was found that YAB1 gene products are themselves mobile. A screen for suppression of a floral YAB1 overexpression phenotype revealed that the YAB1-born signals are mediated in part by the activity of LATERAL SUPPRESSOR. This GRAS protein is expressed at the boundary of organ primordia and the SAM central zone, distinct from the YAB1 expression domain. Together, these results suggest that YAB1 activity stimulates signals from the organs to the meristem via a secondary message or signal cascade, a process essential for organized growth of the SAM.