Dynamic changes of phosphatidylinositol and phosphatidylinositol 4-phosphate levels modulate H+-ATPase and Na+/H+ antiporter activities to maintain ion homeostasis in Arabidopsis under salt stress

Plant metabolites are dynamically modified and distributed in response to environmental changes. However, it is poorly understood how metabolic change functions in plant stress responses. Maintaining ion homeostasis under salt stress requires coordinated activation of two types of central regulators: plasma membrane (PM) H+-ATPase and Na+/H+ antiporter. In this study, we used a bioassay-guided isolation approach to identify endogenous small molecules that affect PM H+-ATPase and Na+/H+ antiporter activities and identified phosphatidylinositol (PI), which inhibits PM H+-ATPase activity under non-stress conditions in Arabidopsis by directly binding to the C terminus of the PM H+-ATPase AHA2. Under salt stress, the phosphatidylinositol 4-phosphate-to-phosphatidylinositol (PI4P-to-PI) ratio increased, and PI4P bound and activated the PM Na+/H+ antiporter. PI prefers binding to the inactive form of PM H+-ATPase, while PI4P tends to bind to the active form of the Na+/H+ antiporter. Consistent with this, pis1 mutants, with reduced levels of PI, displayed increased PM H+-ATPase activity and salt stress tolerance, while the pi4kβ1 mutant, with reduced levels of PI4P, displayed reduced PM Na+/H+ antiporter activity and salt stress tolerance. Collectively, our results reveal that the dynamic change between PI and PI4P in response to salt stress in Arabidopsis is crucial for maintaining ion homeostasis to protect plants from unfavorable environmental conditions.