Transcriptional expression of Na+ homeostasis-related genes and physiological responses of rice seedlings under salt stress

Being a salt-sensitive crop, the growth, development and productivity of rice is severely affected by saline soil. In the present study, Na+ homeostasis, physiological and morphological responses in both the root and leaf tissues of rice (Oryza sativa) seedlings of backcross introgression lines of KDML105 (KD; −/−) × FL530 (+/+) with or without SKC1 and qSt1b QTLs grown under 200 mM NaCl were investigated. Expression levels of OsHKT1;5 in FL530 (+/+) were very low, causing to low Na+ in both the root and leaf tissues, whereas it was up-regulated in KD (−/−), relating to Na+ enrichment. Expression levels of OsHKT2;1 in rice lines, 221-44 (+/+) and 221-48 (+/+) were down-regulated, thus limiting Na+ xylem upload, and store Na+ in the root tissues. OsSOS1 expression level in the leaves of 221-44 (+/+) and 221-58 (+/−) was up-regulated in relation to Na+ efflux. Na+ secretion into vacuoles via OsNHX1 in the leaf tissues was evidently demonstrated in 221-48 (+/+) as a major defense mechanism adopted by the plants to maintain photosynthetic abilities and growth performances, when plants were subjected to salt stress. In addition, the high Fv/Fm ratio in the leaves of 221-58 (+/−) was associated with increased expression level of OsNHX3, in comparison to KD (−/−) seedlings grown under salt stress (diminution by 22.8% over control). Moreover, reduction on growth characters and physiological parameters, viz., shoot fresh weight, shoot height, number of leaves, leaf area, Pn, and accumulation of Na+ in the roots under salt stress were applied as effective indices; thereby classifying FL530 (+/+) and 221-48 (+/+) as salt-tolerant, and 221-44 (+/+), 221-58 (+/−), KD (−/−), 221-3 (+/−), and 221-54 (−/−) as salt-susceptible.