Functional analysis of ion transport properties and salt tolerance mechanisms of RtHKT1 from the recretohalophyte Reaumuria trigyna.

Reaumuria trigyna is an endangered recretohalophyte and a small archaic feral shrub that is endemic to arid and semiarid plateau regions of Inner Mongolia, China. Based on transcriptomic data, we isolated a high-affinity potassium transporter gene (RtHKT1) from R. trigyna, which encoded a plasma membrane-localised protein. RtHKT1 was rapidly upregulated by high Na+ or low K+ and exhibited different tissue-specific expression patterns before and after stress treatment. Transgenic yeast showed tolerance to high Na+ or low K+, while transgenic Arabidopsis exhibited tolerance to high Na+ and sensitivity to high K+, or high Na+-low K+, confirming that Na+ tolerance in transgenic Arabidopsis depends on a sufficient external K+ concentration. Under external high Na+, high K+, and low K+ conditions, transgenic yeast accumulated more Na+-K+, Na+, and K+; while transgenic Arabidopsis accumulated less Na+-more K+, more Na+, and more Na+-K+, respectively; indicating that the ion transport properties of RtHKT1 depend on the external Na+-K+ environment. Salt stress induced upregulation of some ion-transporter genes (AtSOS1/AtHAK5/AtKUP5-6), as well as downregulation of some genes (AtNHX1/AtAVP1/AtKUP9-12), revealing that multi-ion-transporter synergism maintains Na+/K+ homeostasis under salt stress in transgenic Arabidopsis. Overexpression of RtHKT1 enhanced K+ accumulation and prevented Na+ transport from roots to shoots, improved biomass accumulation and chlorophyll content in salt-stressed transgenic Arabidopsis. The proline content and RWC increased significantly, and some proline biosynthesis genes (AtP5CS1-2) were also upregulated in salt-stressed transgenic plants. These results suggest that RtHKT1 confers salt tolerance on transgenic Arabidopsis by maintenancing Na+/K+ homeostasis and osmotic homeostasis.