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

摘要

Excessive sodium (Na super(+)) accumulation in plants due to soil salinity is toxic to most higher plants including crop plants. Many genes encoding Na super(+) 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 super(+) permeable membrane proteins in plants. Interestingly, most Na super(+) permeable proteins including HKT-type transporters/channels in plants were shown or deduced to play a protective role against salinity stress. A Na super(+) selective transporter/channel in rice (Oryza sativa), OsHKT2; 1, however, has recently been proven to function in 'nutritional Na super(+) absorption' in K super(+)-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 super(+)/Na super(+) 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 super(+) concentrations during salinity stress by removing Na super(+) from the xylem sap.