Characterization and expression of a vacuolar Na+/H+ antiporter gene from the monocot halophyte Aeluropus littoralis

Plant vacuolar Na+/H+ antiporter plays an important role in salt tolerance. In order to understand the molecular basis of vacuolar Na+/H+ antiporter responded to salinity and reveal a possible role of salt tolerance in monocots, a vacuolar Na+/H+ antiporter gene (AlNHX) was isolated by reverse transcription-PCR and RNA ligase mediated rapid amplification of cDNA ends (RLM-RACE) based on the homology from Aeluropus littoralis (Gouan) Parl, a graminaceous halophyte. The AlNHX sequence contained 2706 bp with an open read frame of 1623 bp and the deduced transcripts encoding 540 amino acids shared a high homology with those putative vacuolar Na+/H+ antiporters of higher plants. AlNHX was predicted containing ten putative hydrophobic regions, which was different with AtNHX1 and OsNHX1. DNA gel blot analysis indicated that there were two or three copies of AlNHX in the A. littoralis genome. The increased transcript levels of AlNHX were much higher in roots than that in shoots under salt stress. In addition, overexpression of AlNHX in tobacco conferred high salt tolerance to the transgenic plants. The analysis of ion contents indicated that under high salt stress for one month, the transgenic plants compartmentalized more Na+ in the roots and kept a relative high K+/Na+ ratio in the leaves compared with wild-type plants.