The CLAVATA signaling pathway mediating stem cell fate in shoot meristems requires Ca 2+ as a secondary cytosolic messenger

Summary CLAVATA 1 ( CLV 1) is a receptor protein expressed in the shoot apical meristem ( SAM ) that translates perception of a non‐cell‐autonomous CLAVATA 3 ( CLV 3) peptide signal into altered stem cell fate. CLV 3 reduces expression of WUSCHEL ( WUS ) and FANTASTIC FOUR  2 ( FAF 2 ) in the SAM . Expression of WUS and FAF 2 leads to maintenance of undifferentiated stem cells in the SAM . CLV 3 binding to CLV 1 inhibits expression of these genes and controls stem cell fate in the SAM through an unidentified signaling pathway. Cytosolic Ca 2+ elevations, cyclic nucleotide ( cGMP )‐activated Ca 2+ channels, and cGMP have been linked to signaling downstream of receptors similar to CLV 1. Hence, we hypothesized that cytosolic Ca 2+ elevation mediates the CLV 3 ligand/ CLV 1 receptor signaling that controls meristem stem cell fate. CLV 3 application to Arabidopsis seedlings results in elevation of cytosolic Ca 2+ and cGMP . CLV 3 control of WUS was prevented in a genotype lacking a functional cGMP ‐activated Ca 2+ channel. In wild‐type plants, CLV 3 inhibition of WUS and FAF 2 expression was impaired by treatment with either a Ca 2+ channel blocker or a guanylyl cyclase inhibitor. When CLV 3‐dependent repression of WUS is blocked, altered control of stem cell fate leads to an increase in SAM size; we observed a larger SAM size in seedlings treated with the Ca 2+ channel blocker. These results suggest that the CLV 3 ligand/ CLV 1 receptor system initiates a signaling cascade that elevates cytosolic Ca 2+ , and that this cytosolic secondary messenger is involved in the signal transduction cascade linking CLV 3/ CLV 1 to control of gene expression and stem cell fate in the SAM.