The WOX13 homeobox gene promotes replum formation in the Arabidopsis thaliana fruit

The Arabidopsis fruit forms a seedpod that develops from the fertilized gynoecium. It is mainly comprised of an ovary in which three distinct tissues can be differentiated: the valves, the valve margins and the replum. Separation of cells at the valve margin allows for the valves to detach from the replum and thus dispersal of the seeds. Valves and valve margins are located in lateral positions whereas the replum is positioned medially and retains meristematic properties resembling the shoot apical meristem (SAM). Members of the WUSCHEL-related homeobox family have been involved in stem cell maintenance in the SAM, and within this family, we found that WOX13 is expressed mainly in meristematic tissues including the replum. We also show that wox13 loss-of-function mutations reduce replum size and enhance the phenotypes of mutants affected in the replum identity gene RPL. Conversely, misexpression of WOX13 produces, independently from BP and RPL, an oversized replum and valve defects that closely resemble those of mutants in JAG/FIL activity genes. Our results suggest that WOX13 promotes replum development by likely preventing the activity of the JAG/FIL genes in medial tissues. This regulation seems to play a role in establishing the gradient of JAG/FIL activity along the medio-lateral axis of the fruit critical for proper patterning. Our data have allowed us to incorporate the role of WOX13 into the regulatory network that orchestrates fruit patterning.