Salt tolerance of barley: Relations between expression of isoforms of vacuolar Na+/H+-antiporter and 22Na+ accumulation

Two barley cultivars (Hordeum vulgare L., cvs. Elo and Belogorskii) differing in salt tolerance were used to study Na-22(+) uptake, expression of three isoforms of the Na+/H+ antiporter HvNHX1-3, and the cellular localization of these isoforms in the elongation zone of seedling roots. During short (1 h) incubation, seedling roots of both cultivars accumulated approximately equal quantities of Na-22(+). However, after 24-h incubation the content of Na-22(+) in roots of a salt-tolerant variety Elo was 40% lower than in roots of the susceptible variety Belogorskii. The content of Na-22(+) accumulated in shoots of cv. Elo after 24-h incubation was 6.5 times lower than in shoots of cv. Belogorskii and it was 4 times lower after the salt stress treatment. The cytochemical examination revealed that three proteins HvNHX1-3 are co-localized in the same cells of almost all root tissues; these proteins were present in the tonoplast and prevacuolar vesicles. Western blot analysis of HvNHX1-3 has shown that the content of isoforms in vacuolar membranes increased in response to salt stress in seedling roots and shoots of both cultivars, although the increase was more pronounced in the tolerant cultivar. The content of HvNHX1 in the seedlings increased in parallel with the enhanced expression of HvNHX1, whereas the increase in HvNHX2 and HvNHX3 protein content was accompanied by only slight changes in expression of respective genes. The results provide evidence that salt tolerance of barley depends on plant ability to restrict Na+ transport from the root to the shoot and relies on regulatory pathways of HvNHX1-3 expression in roots and shoots during salt stress.