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 Ser²⁹⁴, 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 Ca²⁺, and that Ca²⁺ 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.