Figure 4.

Signalling pathways responsible for sodium extrusion in Arabidopsis under salt stress. Excess Na⁺ and high osmolarity are separately perceived by yet unidentified sensors at the plasma membrane level, which then induce an increase in cytosolic Ca²⁺ concentration. This increase is then sensed by SOS3 which activates SOS2. The activated SOS3-SOS2 protein complex phosphorylates SOS1, the plasma membrane Na⁺/H⁺ antiporter, resulting in the efflux of Na⁺ ions.¹⁰⁷ SOS2 has also been shown to regulate NHX1 antiport activity¹⁰¹ and V-H⁺-ATPase activity⁵⁹ in a SOS3-independent manner, possibly by SOS3-like Ca²⁺-binding proteins (SCaBP) that target it to the tonoplast. Salt stress can also induce the accumulation of ABA, which, by means of ABI1 and ABI2, can negatively regulate SOS2 or SOS1 and NHX1.¹¹⁵