HKT1;1 modulated by vacuolar Na+ compartmentation functions in maintaining the salt-accumulating trait in a xerophyte

Abstract

The xerophyte Zygophyllum xanthoxylum can accumulate large amounts of Na⁺ in leaves for osmotic adjustment. HKT I is crucial for withdrawing Na⁺ from root xylem in salt-excluding species, however, its function in maintaining the characteristics of salt accumulation in Z. xanthoxylum remains unclear. Here, we found that ZxHKT1;1, a HKT I homolog in Z. xanthoxylum, is localized to the plasma membrane and functions as a Na⁺-selective transporter based on the heterologous expression analyses conducted in yeast and Xenopus laevis oocytes. The results of in situ PCR showed that ZxHKT1;1 was specifically expressed in the root stele. The over-expression of ZxHKT1;1 under the control of AtHKT1;1 native promoter significantly enhanced the retrieval of Na⁺ from root xylem and loading of K⁺ into xylem, thereby reducing Na⁺ accumulation and increasing K⁺ accumulation in shoots, and consequently, improving the salt tolerance of wild-type Arabidopsis or athkt1;1 mutant. Interestingly, the expression of ZxHKT1;1 was significantly down-regulated in roots of Z. xanthoxylum while up-regulated in roots of the ZxNHX1-silenced line under 50 mM NaCl, a salt condition that stimulates growth of Z. xanthoxylum. These results demonstrated that ZxHKT1;1 functions in maintaining the characteristics of salt accumulation in Z. xanthoxylum by modulating the retrieval of Na⁺ from root xylem, and this regulation is determined by its distinct expression patterns relying on the capacity of vacuolar Na⁺ compartmentation mediated by ZxNHX1 in leaves. Meanwhile, ZxHKT1;1 is involved in regulating K⁺ transport from roots to shoots in Z. xanthoxylum.