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 semi-arid 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-localized protein. RtHKT1 was rapidly up-regulated 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 up-regulation of some ion transporter genes (AtSOS1/AtHAK5/AtKUP5-6), as well as down-regulation 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 Chl content in salt-stressed transgenic Arabidopsis. The proline content and relative water content increased significantly, and some proline biosynthesis genes (AtP5CS1 and AtP5CS2) were also up-regulated in salt-stressed transgenic plants. These results suggest that RtHKT1 confers salt tolerance on transgenic Arabidopsis by maintaining Na⁺/K⁺ homeostasis and osmotic homeostasis.