TsHKT1;2, a HKT1 Homolog from the Extremophile Arabidopsis Relative Thellungiella salsuginea, Shows K+ Specificity in the Presence of NaCl

Abstract

Cellular Na⁺/K⁺ ratio is a crucial parameter determining plant salinity stress resistance. We tested the function of plasma membrane Na⁺/K⁺ cotransporters in the High-affinity K⁺ Transporter (HKT) family from the halophytic Arabidopsis (Arabidopsis thaliana) relative Thellungiella salsuginea. T. salsuginea contains at least two HKT genes. TsHKT1;1 is expressed at very low levels, while the abundant TsHKT1;2 is transcriptionally strongly up-regulated by salt stress. TsHKT-based RNA interference in T. salsuginea resulted in Na⁺ sensitivity and K⁺ deficiency. The athkt1 mutant lines overexpressing TsHKT1;2 proved less sensitive to Na⁺ and showed less K⁺ deficiency than lines overexpressing AtHKT1. TsHKT1;2 ectopically expressed in yeast mutants lacking Na⁺ or K⁺ transporters revealed strong K⁺ transporter activity and selectivity for K⁺ over Na⁺. Altering two amino acid residues in TsHKT1;2 to mimic the AtHKT1 sequence resulted in enhanced sodium uptake and loss of the TsHKT1;2 intrinsic K⁺ transporter activity. We consider the maintenance of K⁺ uptake through TsHKT1;2 under salt stress an important component supporting the halophytic lifestyle of T. salsuginea.