CLE40 signalling regulates the fate of root stem cells in Arabidopsis

The quiescent center (QC) of the Arabidopsis root meristem acts as an organiser that promotes stem cell fate in adjacent cells and patterns the stem cell niche. This stem cell niche is covered by cells of the columella, which are continuously replaced during root growth by a layer of columella stem cells (CSCs) that are maintained in an undifferentiated state by the QC-expressed transcription factor WOX5. The differentiated columella cells provide a feedback signal via secretion of the peptide CLE40, which acts through the plasmodesmata-localised receptor kinases ACR4 and CLV1 to control WOX5 expression. Previously, WOX5 protein movement from the QC into CSCs was proposed to be required for CSC maintenance, and the CLE40/CLV1/ACR4 signalling module was suggested to restrict WOX5 mobility or function. Here, these assumptions were tested, and the function of CLE40/CLV1/ACR4 in CSC maintenance was investigated. However, no role for CLE40/CLV1/ACR4 in constricting mobility of WOX5 or other fluorescent test proteins was found. Furthermore, in contrast to previous observations, WOX5 mobility was not required to inhibit CSC differentiation. Instead, we propose that WOX5 acts mainly in the QC, where other short-range signals are generated that not only inhibit differentiation but also promote stem cell division in adjacent cells. Therefore, the main function of columella-derived CLE40 signals is to position the QC at a defined distance from the root tip by repressing QC-specific gene expression via the ACR4 and CLV1 receptors in the distal domain and by promoting WOX5 expression via the CLV2 receptor in the proximal meristem.