Fig. 1. SOS signaling pathway for ion homeostasis under salt stress in Arabidopsis. Salt stress elicited Ca²⁺ signals are perceived by SOS3, which activates the protein kinase SOS2. Activated SOS2 phosphorylates SOS1, a plasma membrane Na⁺/H⁺ antiporter, which then transports Na⁺ out of the cytosol. The transcript level of SOS1 is regulated by the SOS3-SOS2 kinase complex. SOS2 also activates the tonoplast Na⁺/H⁺ antiporter that sequesters Na⁺ into the vacuole. Na⁺ entry into the cytosol through the Na⁺ transporter HKT1 may also be restricted by SOS2. ABI1 regulates the gene expression of NHX1, while ABI2 interacts with SOS2 and negatively regulates ion homeostasis either by inhibiting SOS2 kinase activity or the activities of SOS2 targets. Double arrow indicates SOS3-independent and SOS2-dependent pathway.