MdCCX2 of apple functions positively in modulation of salt tolerance

Abstract High salinity is a major environmental constraint that limits global agricultural productivity. The CCX (cation/calcium exchanger) proteins are involved in ion transport and homeostasis and salt stress response. However, the mechanism by which CCX proteins mediate plant salt tolerance is largely unknown, and no detailed functional studies of apple CCX proteins have been reported. Here, we cloned a CCX family gene MdCCX2 (MD12G1011800) from apple (Malus domestica). Its expression was strongly induced by NaCl treatment. As a plasma membrane-localized protein, MdCCX2-overexpression suppressed the sensitivity of Na+-sensitive yeast mutant to NaCl and reduce the Na+ content in yeast cells under NaCl treatment. To further explore its role in salt stress, we transformed MdCCX2 into Arabidopsis, apple calli, and apple plants. The results of growth phenotype comparison and stress-related physiological index determination under NaCl treatment showed that MdCCX2-overexpression improved the salt tolerance of these transgenic materials. The significantly reduced Na+ content and increased activity of antioxidant enzymes suggested that MdCCX2 could enhance plant salt tolerance by inhibiting sodium accumulation and promoting ROS scavenging. These results provide a solid basis for further study of the mechanisms by which CCX proteins regulate apple salt tolerance.