An SOS gene has been isolated through PCR cloning from Thellungiella halophila and is designated TSOS1. The polynucleotide encodes a transmembrane protein with similarities to plasma membrane Na.sup.+/H.sup.+ antiporters from bacteria and fungi. The invention encompasses the TSOS1 gene, the corresponding protein, closely related polypeptides that confer salt tolerance, and related polynucleotides that encode a polypeptide that has Na+/H+ antiporter activity.