The invention is isolated nucleic acid molecules encoding Na.sup.+/H.sup.+ exchanger polypeptides with at least 95% homology to that of Arabidopsis thaliana for extrusion of monovalent cations from the cytosol of cells to provide the cell with increased salt tolerance. Crop species transformed with the nucleic acid molecule are capable of surviving in soil with high salt levels that would normally inhibit growth of the crop species.