Na+ exclusion and selective transport of K+ over Na+ provided salt-adaptive mechanism in introgression lines of rice 'RD6'

Salinity stress immensely inhibits rice(Oryza sativa L.) growth, development, and productivity. Hence, two rice introgression lines, Morkho60-2 and BC4F4132-12-61, were previously developed from 'RD6'rice via marker-assisted backcrossing and their adaptive salt-tolerant mechanisms were characterized in this study. Their seedling physiological and transcriptional responses to salinity stress were observed in comparison to the parental 'RD6'and 'Pokkali'. The salt stress responses were assessed under 150 mM NaCl treatment for 9 d. Interestingly, like 'Pokkali', salinity did not affect the growth parameters of the new rice genotypes.'Pokkali', Morkho60-2, and BC4F4132-12-61 also showed lower shoot Na(+ )content(0.91-1.05 vs. 2.83 mgg-1DW), and higher selective transport of K(+ )over Na+(i.e., ST value of 6.57-10.98 vs.2.65) than RD6 genotype under salinity stress. Accordingly, the partial least squares-discriminant analysis of all physiological parameters suggested shoot Na(+ )accumulation and ST value as the key discriminating parameters between 'RD6'and the new genotypes. Transcriptional responses of Na(+ )homeostasis-related genes further supported the findings. In comparison to the salt susceptible 'RD6'rice, Na(+ )vacuolar compartmentalization was suggested in Morkho60-2rootsdue to the higher abundance of OsNHX1(i.e., relative expression of 2.65 vs. 1.59) and OsNHX2(2.42 vs.1.29)transcripts.Morkho60-2also displayed a significant upregulation of root OsSOS1(2.02 vs. 1.18), which may contribute to root Na(+ )exclusion. Therefore, we suggest that selective transport of K(+ )over Na(+ )and Na(+ )exclusion enhanced ion homeostasis in the newly improved rice genotypes, allowing the seedlings to adapt to the saline condition.