Maintenance of the cytosolic Na(+)/K(+) ratio under saline conditions is crucial for plants. HKT-type Na(+) transporters play a key role in keeping low cytosolic Na(+) concentrations thus retaining a low Na(+)/K(+) ratio, that reduces Na(+) toxicity and causing high salinity stress tolerance. Two HKT-type transporters, AtHKT1 from Arabidopsis and TsHKT1;2 from Thellungiella salsuginea, that share high DNA and protein sequence identities, are distinguished by fundamentally different ion selection and salinity stress behavior. On the level of transcription, TsHKT1;2 is dramatically induced upon salt stress, whereas AtHKT1 is downregulated. TsHKT1;2-RNAi lines show severe potassium deficiency and are also sensitive to high [Na(+)]. We have validated the ability of the TsHKT1;2 protein to act as an efficient K(+) transporter in the presence of high [Na(+)] by expression in yeast cells. K(+) specificity is based on amino acid differences in the pore of the transporter protein relative to AtHKT1.
Keywords: HKT1; Na+/K+ ratio; Thellungiella salsuginea; halophyte; salt stress.