Changes in Expression Level ofOsHKT1;5Alters Activity of Membrane Transporters Involved in K+and Ca2+Acquisition and Homeostasis in Salinized Rice Roots

In rice, theOsHKT1;5gene has been reported to be a critical determinant of salt tolerance. This gene is harbored by theSKC1locus, and its role was attributed to Na(+)unloading from the xylem. No direct evidence, however, was provided in previous studies. Also, the reported function ofSKC1on 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(SKC1) plants. Our data showed that Na(+)reabsorption was observed in WT, but not NIL(SKC1) plants, thus questioning the functional role ofHKT1;5as a transporter operating in the direct Na(+)removal from the xylem. Instead, changes in the expression level ofHKT1;5altered the activity of membrane transporters involved in K(+)and Ca(2+)acquisition and homeostasis in the rice epidermis and stele, explaining the observed phenotype. We conclude that the role ofHKT1;5in 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.