Changes in Expression Level of OsHKT1;5 Alters Activity of Membrane Transporters Involved in K+ and Ca2+ Acquisition and Homeostasis in Salinized Rice Roots

Abstract

In rice, the <i>OsHKT1</i>;<i>5</i> gene has been reported to be a critical determinant of salt tolerance. This gene is harbored by the <i>SKC1</i> locus, and its role was attributed to Na⁺ unloading from the xylem. No direct evidence, however, was provided in previous studies. Also, the reported function of <i>SKC1</i> on the loading and delivery of K⁺ to the shoot remains to be explained. In this work, we used an electrophysiological approach to compare the kinetics of Na⁺ uptake by root xylem parenchyma cells using wild type (WT) and NIL(<i>SKC1</i>) plants. Our data showed that Na⁺ reabsorption was observed in WT, but not NIL(<i>SKC1</i>) plants, thus questioning the functional role of HKT1;5 as a transporter operating in the direct Na⁺ removal from the xylem. Instead, changes in the expression level of <i>HKT1</i>;<i>5</i> altered the activity of membrane transporters involved in K⁺ and Ca²⁺ acquisition and homeostasis in the rice epidermis and stele, explaining the observed phenotype. We conclude that the role of HKT1;5 in plant salinity tolerance cannot be attributed to merely reducing Na⁺ concentration in the xylem sap but triggers a complex feedback regulation of activities of other transporters involved in the maintenance of plant ionic homeostasis and signaling under stress conditions.