Overexpression of Iris lactea tonoplast Na+/H+ antiporter gene IlNHX confers improved salt tolerance in tobacco

Sodium cation compartmentalization into vacuoles is one of the effective strategies for adaptation of halophytes to saline environments. Tonoplast Na+/H+ antiporter (NHX) is involved in Na+ sequestration into vacuoles under salt stress. However, the function of NHX in halophyte Iris lactea is still unclear. In this study, a significant positive correlation was observed between Na+ accumulations and IlNHX expression in tissues under 0 - 200 mM NaCl, indicating IlNHX might be responsible for Na+ accumulation of I. lactea under salt stress. More important, IlNHX was specifically localized to the tonoplast. Transgenic tobacco expressing IlNHX grew better and showed higher tolerance to 200 mM NaCl than respective wild type (WT). Compared to WT, transgenic tobacco accumulated more Na+ and K+ and maintained higher K+/Na+ ratios in tissues, accompanied by the reduction of chlorophyll loss and lipid peroxidation in the presence of NaCl. Moreover, transgenic tobacco exhibited markedly higher vacuolar H+-ATPase (V-ATPase) activity relative to WT when subjected to salt stress. The findings suggest that transgenic plants overexpressing IlNHX could compartmentalize more Na+ into vacuoles in tobacco via enhanced V-ATPase activity, which further contributes to maintaining K+ and Na+ homeostasis, to improved photosynthesis, and to protection of cell membrane integrity under salt stress.