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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 ((LFFIYLLPPI87)-L-78) 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 1 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. (C) 2010 Elsevier GmbH. All rights reserved.