ERECTA family signaling constrainsCLAVATA3andWUSCHELto the center of the shoot apical meristem

The shoot apical meristem (SAM) is a reservoir of stem cells that gives rise to all post-embryonic aboveground plant organs. The size of the SAM remains stable over time due to a precise balance of stem cell replenishment versus cell incorporation into organ primordia. The WUSCHEL (WUS)/CLAVATA (CLV) negative feedback loop is central to SAM size regulation. Its correct function depends on accurate spatial expression of WUS and CLV3 . A signaling pathway, consisting of ERECTA family (ERf) receptors and EPIDERMAL PATTERNING FACTOR LIKE (EPFL) ligands, restricts SAM width and promotes leaf initiation. While ERf receptors are expressed throughout the SAM, EPFL ligands are expressed in its periphery. Our genetic analysis demonstrated that ERfs and CLV3 synergistically regulate the size of the SAM, and wus is epistatic to erfs . Furthermore, activation of ERf signaling with exogenous EPFLs resulted in a rapid decrease of CLV3 and WUS expression. ERf-EPFL signaling inhibits expression of WUS and CLV3 in the periphery of the SAM, confining them to the center. These findings establish the molecular mechanism for stem cell positioning along the radial axis. Summary statement ERf signaling restricts the width of the shoot apical meristem, a structure which generates aboveground plant organs, by inhibiting expression of two principal regulators, CLV3 and WUS , at its periphery.