Expression level of Na+ homeostasis-related genes and salt-tolerant abilities in backcross introgression lines of rice crop under salt stress at reproductive stage

Salt stress in the rice field is one of the most common abiotic stresses, reducing crop productivity, especially at reproductive stage, which is very sensitive to salt stress. The aim of this investigation was to study mRNA-related Na+ uptake/translocation and Na+ enrichment in the cellular level, leading to physiological changes, growth characteristics, and yield attributes in FL530 [salt-tolerant genotype; carrying SKC1 (in relation to high-affinity potassium transporters controlling Na+ and K+ translocation) and qSt1b (linking to salt injury score) QTLs] and KDML105 (salt-sensitive cultivar; lacking both QTLs) parental lines and 221-48 (carrying SKC1 and qSt1b QTLs) derived from BILs (backcross introgression lines) at 50% flowering of rice, under 150-mM NaCl until harvesting process. The upregulation of OsHKT1;5 (mediating Na+ exclusion into xylem parenchyma cells) and OsNHX1 (Na+/H+ exchanger to secrete Na+ into vacuole) and downregulation of OsHKT2;1 and OsHKT2;2 (mediating Na+ restriction in the roots, leaf sheath and older leaves) in cvs. FL530 and 221-48 (+SKC1; + qSt1b) under salt stress were observed. It restricted Na+ level in flag leaf, thereby preventing salt toxicity, as indicated bymaintenance of photon yield of PSII (Phi(PSII)), net photosynthetic rate (P-n), transpiration rate (E) and overall growth performances. In contrast, Na+ enrichment in flag leaf of cv. KDML105 (-SKC1;-qSt1b) caused the reduction in Phi(PSII) by 30.5% over the control, leading to the reduction in P-n by 62.3%, in seed sterility by 88.2%, and yield loss by 85.1%. Moreover, the negative relationships between Na+ enrichment in flag leaf, physiological changes, and yield traits in rice crop grown under salt stress were demonstrated. Based on this investigation, rice genotype 221-48 was found to possess salt-tolerant traits at reproductive stage and thus could prove to be a potential candidate for future breeding programs.