Molecular Characterization of a Tonoplast Na+/H+ Antiporter from Iris Lactea
Na+ compartmentalization into vacuoles is one of the effective strategies for adaptation of halophytes to saline environments. The tonoplast Na+/H+ antiporter (NHX) has been proved to be involved in the compartmentalization of Na+ into vacuoles to alleviate Na+ toxicity in cytoplasm. However, the function of NHX in halophyte Iris lactea is still unclear under salt stress. In this study, a significant positive correlation was observed between Na+ accumulations and IlNHX expression levels in shoots and roots under different concentrations of NaCl (0-200 mM), indicating IlNHX might be involved in Na+ accumulation of I. lactea in response to salt stress. More important, IlNHX was specifically localized to the tonoplast. Transgenic tobacco plants expressing IlNHX grew better and showed higher salinity tolerance under salt (200 mM NaCl) stress than those of wild type (WT) plants. Compared to WT plants, transgenic tobacco plants accumulated more Na+ and K+ and maintained higher K+/Na+ ratios in tissues by salt stress, accompanied by the reduction of chlorophyll loss and lipid peroxidation in the presence of salt. Interestingly, we found that transgenic tobacco plants exhibited markedly higher tonoplast H+-ATPase activity relative to WT plants subjected to salt. Overall, overexpression of IlNHX in tobacco could compartmentalize excessive Na+ into vacuoles to keep the cytosolic K+/Na+ balance by enhanced tonoplast proton pumps activity, which would be contributed to maintain K+ and Na+ homeostasis, to improve photosynthesis efficiency and to protect cell membrane integrity under salt stress.