Phosphatidic acid-regulated SOS2 controls Na+/K+ homeostasis in Arabidopsis under salt stress

The maintenance of sodium/potassium (Na+/K+) homeostasis in plant cells is essential for salt tolerance. Plants export excess Na+ out of cells mainly through the Salt Overly Sensitive (SOS) pathway, activated by a calcium signal; however, it is unknown whether other signals regulate the SOS pathway and how K+ uptake is regulated under salt stress. Phosphatidic acid (PA) is emerging as a lipid signaling molecule that modulates responses to both developmental and environmental cues. Here, we show that PA binds to Lys57 of SOS2, a core member of the SOS pathway, under salt stress, and that this binding promotes the activity and plasma membrane localization of SOS2. This, in turn, activates the Na+/H+ antiporter SOS1 to promote Na+ efflux. In addition, we reveal that PA promotes the phosphorylation of SOS3-like calcium binding protein 8 (SCaBP8) by SOS2 under salt stress, which attenuates the SCaBP8-mediated inhibition of Arabidopsis K+ transporter 1, an inward-rectifying K+ channel. These findings suggest that PA regulates the SOS pathway and AKT1 activity under salt stress, promoting Na+ efflux and K+ influx to maintain Na+/K+ homeostasis.