NHX1, HKT, and monovalent cation transporters regulate K+ and Na+ transport during abiotic stress

Abiotic stresses like high temperature, ultraviolet light, drought, salt stress, etc., are the most harmful and unavoidable factors on Earth that affect crop yield. Among these, salinity stress perturbs the homeostasis of the K+/Na+ ratio. Different plant species survive under salinity stress via effector ion transport channels that regulate the excess concentration of Na+ ions, so as to avoid its drastic effect on vital metabolic pathways. Ion channels play a wider role to minimize and retranslocate the Na+ from shoot to root. Na+ generally translocates from the apoplastic and symplastic routes through the root system. Three kinds of defense lines mainly play a wider role to avoid excess Na+ loading. The first line of defense channels is reported as plasma membrane (PM) H+-ATPase. Pyrophosphatase acts as a primary transporter and generates the proton motive force (H+) as a proton gradient for facilitating the ion exchange activity of SOS1 antiporter. The second line of defense channels is localized in the vacuolar membrane, V-H+-ATPase, vacuolar pyrophosphatase as a primary transporter generating the same proton motive force in vacuolar sap that helps to sequester the Na+ in vacuole through antiporter channels (NHXs) by its ion exchange activity. The third line of defense system has been reported as high-affinity potassium transporter (HKT) channels that retranslocate the Na+ from xylem to phloem to avoid the Na+ toxicity in the shoot. Such defense mechanisms are very important for maintaining the optimum ratio of K+/Na+. Recent research works emphasized that most of these ion channels are regulated through a phosphorylation mechanism in its regulatory domain for functional activity, e.g., PM H+-ATPase and SOS1. The present chapter provides an overview of the major transporters regulating K+/Na+ homeostasis during abiotic stress.