The ZxNHX gene encoding tonoplast Na+/H+ antiporter from the xerophyte Zygophyllum xanthoxylum plays important roles in response to salt and drought

Sodium (Na+) has been found to play important roles in the adaptation of xerophytic species to drought conditions. The tonoplast Na+/H+ antiporter (NHX) proved to be involved in the compartmentalization of Na+ into vacuoles from the cytosol. In this study, a gene (ZxNHX) encoding tonoplast Na+/H+ antiporter was isolated and characterized in Zygophyllum xanthoxylum, a succulent xerophyte growing in desert areas of northwest China. The results revealed that ZxNHX consisted of 532 amino acid residues with a conserved binding domain (78LFFIYLLPPI87) for amiloride and shared high similarity (73–81%) with the identified tonoplast Na+/H+ antiporters in other plant species. Semi-quantitative RT-PCR analysis showed that the mRNA level of ZxNHX was significantly higher in the leaf than in stem or root. The transcript abundance of ZxNHX in Z. xanthoxylum subjected to salt (5–150 mM NaCl) or drought (50–15% of field water capacity (FWC)) was 1.4–8.4 times or 2.3–4.4 times that of plants grown in the absence of NaCl or 70% of FWC, respectively. Leaf Na+ concentration in plants exposed to salt or drought was 1.7–5.2 times or 1.5–2.2 times that of corresponding control plants, respectively. It is clear that there is a positive correlation between up-regulation of ZxNHX and accumulation of Na+ in Z. xanthoxylum exposed to salt or drought. Furthermore, Z. xanthoxylum accumulated larger amounts of Na+ than K+ in the leaf under drought conditions, even in low salt soil. In summary, our results suggest that ZxNHX encodes a tonoplast Na+/H+ antiporter and plays important roles in Na+ accumulation and homeostasis of Z. xanthoxylum under salt and drought conditions.