Calcium-activated 14-3-3 proteins as a molecular switch in salt stress tolerance

Calcium is a universal secondary messenger that triggers many cellular responses. However, it is unclear how a calcium signal is coordinately decoded by different calcium sensors, which in turn regulate downstream targets to fulfill a specific physiological function. Here we show that SOS2-LIKE PROTEIN KINASE5 (PKS5) can negatively regulate the Salt-Overly-Sensitive signaling pathway in Arabidopsis. PKS5 can interact with and phosphorylate SOS2 at Ser 294 , promote the interaction between SOS2 and 14-3-3 proteins, and repress SOS2 activity. However, salt stress promotes an interaction between 14-3-3 proteins and PKS5, repressing its kinase activity and releasing inhibition of SOS2. We provide evidence that 14-3-3 proteins bind to Ca 2+ , and that Ca 2+ modulates 14-3-3-dependent regulation of SOS2 and PKS5 kinase activity. Our results suggest that a salt-induced calcium signal is decoded by 14-3-3 and SOS3/SCaBP8 proteins, which selectively activate/inactivate the downstream protein kinases SOS2 and PKS5 to regulate Na + homeostasis by coordinately mediating plasma membrane Na + /H + antiporter and H + -ATPase activity. Salinity triggers an increase in cytosolic Ca 2+ concentration in plants. Here, the authors propose a model whereby 14-3-3 proteins decode this calcium signature and regulate Na + extrusion via Ca 2+ -dependent regulation of PKS5 and SOS2 kinases that in turn regulate the Na + /H + antiporter SOS1 and PM H + -ATPase