A Ca-sensor switch for tolerance to elevated salt stress in Arabidopsis.

Excessive Na in soils inhibits plant growth. Here, we report that Na stress triggers primary calcium signals specifically in a cell group within the root differentiation zone, thus forming a "sodium-sensing niche" in Arabidopsis. The amplitude of this primary calcium signal and the speed of the resulting Ca wave dose-dependently increase with rising Na concentrations, thus providing quantitative information about the stress intensity encountered. We also delineate a Ca-sensing mechanism that measures the stress intensity in order to mount appropriate salt detoxification responses. This is mediated by a Ca-sensor-switch mechanism, in which the sensors SOS3/CBL4 and CBL8 are activated by distinct Ca-signal amplitudes. Although the SOS3/CBL4-SOS2/CIPK24-SOS1 axis confers basal salt tolerance, the CBL8-SOS2/CIPK24-SOS1 module becomes additionally activated only in response to severe salt stress. Thus, Ca-mediated translation of Na stress intensity into SOS1 Na/H antiporter activity facilitates fine tuning of the sodium extrusion capacity for optimized salt-stress tolerance.