Functions of HKT transporters in sodium transport in roots and in protecting leaves from salinity stress

Abstract

Excessive sodium (Na+) accumulation in plants due to soil salinity is toxic to most higher plants including crop plants. Many genes encoding Na+ permeable transporters/channels have been identified for the last 15 years, based on genetic approaches, genome-sequencing projects and functional complementation screening using yeast mutants. The HKT-type transporter/channel class is one of the best characterized Na+ permeable membrane proteins in plants. Interestingly, most Na+ permeable proteins including HKT-type transporters/channels in plants were shown or deduced to play a protective role against salinity stress. A Na+ selective transporter/channel in rice (Oryza sativa), OsHKT2;1, however, has recently been proven to function in "nutritional Na+ absorption" in K+-starved roots rather than functioning in a protective role under salinity stress. Here we review findings on the HKT-type transporters/channels, mainly focusing on the function of OsHKT2;1 that is tightly regulated by K+/Na+ homeostatic mechanisms of rice plants. We also discuss functions of Arabidopsis thaliana AtHKT1;1 and rice OsHKT1;5 in protecting plant leaves from over-accumulating toxic Na+ concentrations during salinity stress by removing Na+ from the xylem sap.