Exogenous silicon effectively enhances salt stress resistance of rice by upregulating antioxidant enzymes activities and expression of genes related to Na/K homeostasis

Objectives Salt stress is one of the global abiotic obstacles severely limiting crop growth. Exogenous silicon (Si) has been proven to effectively enhance salt stress resistance in many plant species. To investigate the mechanisms of Si-induced salt stress resistance, we assessed the effects of Si on the activities of antioxidant enzymes and the expression of Na/K homeostasis related genes in rice.

Methods Rice (Oryza sativa L. cv. Nipponbare) was grown hydroponically with both NaCl (0, 50 and 100 mmol/L) and Na₂SiO₃ levels (0, 0.5, 1.5 mmol/L). After five days of treatment, plant length and biomass, photosynthesis and transpiration rates, chlorophyll and malondialdehyde concentrations, and Na/K concentrations were measured. Then, the activities of antioxidant enzymes and the expression of Na/K homeostasis related genes in rice grown with 0 and 100 mmol/L NaCl with or without the addition of 1.5 mmol/L Si were measured.

Results Salt stress significantly inhibited the growth of both shoot and root, while Si addition enhanced the length and dry weight of shoot but not those of root in rice under salt stress. In addition, exogenous Si addition alleviated salt stress-induced decline in photosynthesis rate and chlorophyll concentration and accumulation of MDA, with no significant impact on transpiration rate. As for Na/K homeostasis, salt stress caused an increment in Na concentration and a decrement in K concentration in both shoot and root. Si decreased Na concentration in shoot but not in root. The concentration of K was not affected by Si in both shoot and root. In general, treatment with salt at a higher level (100 mmol/L NaCl) affected rice growth, photosynthesis system, membrane stabilization and Na/K homeostasis more negatively than at a lower level (50 mmol/L NaCl), while salt stress was more effectively alleviated by Si added at a higher level of Si (1.5 mmol/L Si) than at a lower level (0.5 mmol/L Si). Under 100 mmol/L NaCl salt stress, addition of 1.5 mmol/L Si improved SOD, CAT and APX activity but not POD activity. The expression of K uptake genes (OsHAK1, OsHAK7, OsHAK11 and OsHAK12), Na exclusion gene (OsSOS1) and Na compartmentation genes (OsNHX1, OsNHX3 and OsNHX5) was improved by the addition of 1.5 mmol/L Si under 100 mmol/L NaCl.

Conclusions Si can regulate antioxidant enzymes activities and Na/K homeostasis related genes expression, thereby alleviating salt stress in rice. 1.5 mmol/L of Si is more effective than 0.5 mmol/L in alleviating salt stress in rice.