Background: Moderate salt stress, which often occurs in most saline agriculture land, suppresses crop growth and reduces crop yield. Rice, as an important food crop, is sensitive to salt stress and rice genotypes differ in their tolerance to salt stress. Despite extensive studies on salt tolerance of rice, a few studies have specifically investigated the mechanism by which rice plants respond and tolerate to moderate salt stress. Two rice genotypes differing in their tolerance to saline-alkaline stress, Dongdao-4 and Jigeng-88, were used to explore physiological and molecular mechanisms underlying tolerance to moderate salt stress.

Results: Dongdao-4 plants displayed higher biomass, chlorophyll contents, and photosynthetic rates than Jigeng-88 under conditions of salt stress. No differences in K⁺ concentrations, Na⁺ concentrations and Na⁺/K⁺ ratio in shoots between Dongdao-4 and Jigeng-88 plants were detected when challenged by salt stress, suggesting that Na⁺ toxicity may not underpin the greater tolerance of Dongdao-4 to salt stress than that of Jigeng-88. We further demonstrated that Dongdao-4 plants had greater capacity to accumulate soluble sugars and proline (Pro) than Jigeng-88, thus conferring greater tolerance of Dongdao-4 to osmotic stress than Jigeng-88. Moreover, Dongdao-4 suffered from less oxidative stress than Jigeng-88 under salt stress due to higher activities of catalase (CAT) in Dongdao-4 seedlings. Finally, RNA-seq revealed that Dongdao-4 and Jigeng-88 differed in their gene expression in response to salt stress, such that salt stress changed expression of 456 and 740 genes in Dongdao-4 and Jigeng-88, respectively.

Conclusion: Our results revealed that Dongdao-4 plants were capable of tolerating to salt stress by enhanced accumulation of Pro and soluble sugars to tolerate osmotic stress, increasing the activities of CAT to minimize oxidative stress, while Na⁺ toxicity is not involved in the greater tolerance of Dongdao-4 to moderate salt stress.

Keywords: Dongdao-4 Osmotic regulation ROS detoxifying mechanism Jigeng-88 Oryza sativa L. Rice Moderate salt stress