A salt-responsive gene WRSI5 was characterized from salt-tolerant cultivar Shanrong No. 3 (SR3), an introgression line via asymmetric somatic hybrid between Triticum aestivum L. cv. Jinan177 (JN177) and Thinopyrum ponticum Podp. The peptide encoded by WRSI5 contains a Bowman-Birk domain sharing a high level of sequence identity to monocotyledonous protease inhibitors. When expressed in vitro, the WRSI5 gene product exhibited trypsin, but not chymotrypsin inhibition. The expression level of WRSI5 was increased in SR3 roots exposed to salt, drought or oxidative stress. In situ hybridization showed that it is induced in the endodermal cells of the mature region of the SR3 root tip, with no signal detectable in the corresponding region of the salt-susceptible cultivar JN177. SR3 has a higher selectivity for K(+) over Na(+), and therefore limits the transport of Na(+) from the root to the shoot. When overexpressed in Arabidopsis thaliana, WRSI5 improves the ability of seedlings to grow on a medium containing 150 mM NaCl. We suggest that WRSI5 plays an important role in regulating the plant growth rate or long-distance Na(+) transport in SR3 plants exposed to salt stress.