Figure 1.
The SOS signaling pathway for the regulation of Na+ homeostasis and salt tolerance in Arabidopsis. High Na+ stress triggers a calcium signal that activates the SOS3-SOS2 protein kinase complex, which then stimulates the Na+/H+ exchange activity of SOS1 at the plasma membrane. SOS2 also activates Na+/H+ (AtNHX) and Ca2+/H+ (CAX1) exchangers on the vacuolar membrane. The protein phosphatase ABI2 has been shown to physically interact with SOS2 and is proposed to inactivate SOS2. The SOS pathway may down-regulate the activity of Na+ influx transporters (AtHKT1 and NCS). SOS1 in gray indicates that this transporter may also have a sensory role. Dotted lines indicate possible regulation.