Na+ exclusion mechanism in the roots through the function of OsHKT1;5 confers improved tolerance to salt stress in the salt-tolerant developed rice lines

To explore the mechanisms underlying salt tolerance in rice, the physiological parameters of rice planted for 12 days under salt stress were examined. Under the salt stress and the use of hydroponic cultures and soil-based pot screening methods, the two salt-tolerant developed rice lines, M-1 and M-3, were found to be more salt-tolerant than the Pathum Thani 1 (PTT1) cultivar owing to the lower Na+ accumulation in their leaf blades. Growth and physiological parameters (shoot and root dry weights, electrolyte leakage ratio, leaf water content, and chlorophyll concentration) of both mutant lines were unchanged upon exposure to salt stress, but changes were found in the PTT1. In addition, all the examined tissues under salt stress conditions of the mutant lines showed lower Na+/K+ ratios. In response to salt stress in both screening methods, the OsHKT1;5 expression in the roots of the M-1 and M-3 lines had greatly increased, and the Na+ accumulation in their shoots was decreased. However, the OsNHX1 expression in the leaf sheaths and the roots of PTT1 was highly upregulated by the salt stress compared with the two mutant lines, suggesting that the NHX antiporter of PTT1 did not effectively transport the Na+ into the vacuoles, contributing to a high Na+ accumulation in the leaf blades, which might be related to the repression of the OsHKT1;5 gene in its roots. Molecular analysis suggested that the Na+ retrieval mechanisms via OsHKT1;5 might enhance the salt stress tolerance of the mutant lines by preventing Na+ accumulation in their aerial parts, whereas Na+ exclusion via OsNHX1 may respond to elevated Na+ sequestration in the vacuoles.