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Is the Arabidopsis root niche protected by sequestration of the CLE40 signal by its putative receptor ACR4?

A tight but also dynamic regulation is necessary to control the size of stem cell populations in response to internal and external cues. The stem cells of the Arabidopsis shoot and root meristems are governed by the niche cells of the organizing centre (OC) and the quiescent centre (QC), respectively. The well characterized CLV3/WUS negative feedback loop adjusts homeostasis of the stem cell population in the shoot. Here, the CLAVATA3 (CLV3) dodecapeptide, expressed by the stem cells, signals to repress WUSCHEL (WUS), which is expressed in the subjacent OC cells, and in turn activates CLV3 expression non-cell autonomously. However, a similar signaling module controlling the root stem cell population was as yet unknown. In the June issue of Current Biology we report on such a signaling module comprising CLE40 (a CLV3 homologue) that acts via the receptor kinase Arabidopsis Crinkly4 (ACR4) to repress the WUS homologue WOX5 which maintains distal root stem cells. Furthermore, we showed that CLE40 peptide (CLE40p) treatment upregulates ACR4 expression. In this Addendum, we are further elaborating our hypothesis in which the upregulation of ACR4 as a consequence of ectopic CLE40p builds a protective barrier for the QC niche cells.