Comparing Essentiality of -Mediated Na Exclusion in Salinity Tolerance between Cultivated and Wild Rice Species.

Soil salinity is a major constraint that affects plant growth and development. Rice is a staple food for more than half of the human population but is extremely sensitive to salinity. Among the several known mechanisms, the ability of the plant to exclude cytosolic Na is strongly correlated with salinity stress tolerance in different plant species. This exclusion is mediated by the plasma membrane (PM) Na/H antiporter encoded by () gene and driven by a PM H-ATPase generated proton gradient. However, it is not clear to what extent this mechanism is operational in wild and cultivated rice species, given the unique rice root anatomy and the existence of the bypass flow for Na. As wild rice species provide a rich source of genetic diversity for possible introgression of abiotic stress tolerance, we investigated physiological and molecular basis of salinity stress tolerance in species by using two contrasting pairs of cultivated () and wild rice species ( and ). Accordingly, dose- and age-dependent Na and H fluxes were measured using a non-invasive ion selective vibrating microelectrode (the MIFE technique) to measure potential activity of -encoded Na/H antiporter genes. Consistent with GUS staining data reported in the literature, rice accessions had (~4-6-fold) greater net Na efflux in the root elongation zone (EZ) compared to the mature root zone (MZ). Pharmacological experiments showed that Na efflux in root EZ is suppressed by more than 90% by amiloride, indicating the possible involvement of Na/H exchanger activity in root EZ. Within each group (cultivated vs. wild) the magnitude of amiloride-sensitive Na efflux was higher in tolerant genotypes; however, the activity of Na/H exchanger was 2-3-fold higher in the cultivated rice compared with their wild counterparts. Gene expression levels of , and were upregulated under 24 h salinity treatment in all the tested genotypes, with the highest level of transcript detected in salt-tolerant wild rice genotype (~5-6-fold increased transcript level) followed by another wild rice, . There was no significant difference in expression observed for cultivated rice (IR1-tolerant and IR29-sensitive) under both 0 and 24 h salinity exposure. Our findings suggest that salt-tolerant cultivated rice relies on the cytosolic Na exclusion mechanism to deal with salt stress to a greater extent than wild rice, but its operation seems to be regulated at a post-translational rather than transcriptional level.