Figure 1.

An overview of the mechanisms of Na⁺ influx, compartmentalization of Na⁺ in vacuoles, and Na⁺-sensing followed by Na⁺ efflux in response to salt stress. Na⁺ enters the cell via channels such as NSCCs and transporters such as HAK5 and HKTs. Na⁺ is sensed by an unidentified sensory mechanism, and Ca²⁺ signaling cascade is activated. The salt stress-induced increase in cytosolic concentration of Ca²⁺ is sensed by SOS3 (a myristoylated Ca²⁺ sensor protein). SOS3 interacts with SOS2 and activates its kinase activity. The SOS2–SOS3 complex becomes localized to the plasma membrane. SOS2 then phosphorylates SOS1 and activates its Na⁺/H⁺ antiporter activity facilitating Na⁺ efflux from the cell. Additionally, Na⁺ is effluxed through a novel efflux mechanism involving a putative ouabain (OU)-sensitive ATPase. Na⁺ is also compartmentalized inside vacuoles by NHX (a Na⁺/H⁺ exchanger) as a response to salt stress. Abbreviations: NSCCs, non-selective cation channels; HAK5, high-affinity potassium transporter 5; HKTs, high-affinity potassium transporters; SOS, salt overly sensitive; NHX, Na⁺/H⁺ exchanger.