Control of oriented tissue growth through repression of organ boundary genes promotes stem morphogenesis

The origin of the stem is a major but poorly understood aspect of plant development, partlybecause the stem initiates in a relatively inaccessible region of the shoot apical meristem, calledthe rib zone (RZ). We developed quantitative 3D image analysis and clonal analysis tools, whichrevealed that the Arabidopsis homeodomain protein REPLUMLESS (RPL) establishes distinctpatterns of oriented cell division and growth in the central and peripheral regions of the RZ. Agenome-wide screen for target genes connected RPL directly to many of the key shootdevelopment pathways, including the development of organ boundaries; accordingly, mutationof the organ boundary gene LIGHT-SENSITIVE HYPOCOTYL 4 restored RZ function and stemgrowth in the rpl mutant. Our work opens the way to study a developmental process ofimportance to crop improvement and highlights how apparently simple changes in 3D organgrowth can reflect more complex internal changes in oriented cell activities.