Hydrogen sulfide and ethylene regulate photosynthesis, sugar metabolism, and tolerance to heat stress in the presence of sulfur in rice

Heat stress impacts photosynthesis and carbohydrate metabolism, challenging food security. To comprehend the mechanisms of thermotolerance, we examined the role of ethylene (ET) and hydrogen sulfide (H₂S) with or without sulfur (S) in rice (Oryza sativa L.). Both ET and H₂S promoted heat stress tolerance more conspicuously in the presence of S, restoring the balance between carbon assimilation and utilization. The enhanced photosynthesis in ET and H₂S-treated plants under heat stress was linked with increased relative expression of Rubisco subunits rbcS and rbcL and carbohydrate metabolizing, including Sucrose Synthase 2 (SuSy2) and Sucrose transport 1 (SUT1). Notably, the H₂S application showed the highest increase of 2.3, 3.2, 3.0, and 2.4-fold expression of the rbcS, rbcL, SuSy2, and SUT1, respectively, compared to the heat stress alone. The application of H₂S with S more prominently increased starch content, total soluble sugar, and soluble invertase activity by 59.3%, 35.7%, and 25.9%, and also activity of soluble starch synthase and granule-bound starch synthase by 47.2% and 32.8%, respectively, compared to heat-stressed plants. The treatment (H₂S plus S) elevated cysteine and GSH content and the activity of the antioxidant enzymes to maintain cellular redox potential under heat stress. These observed tolerance responses were less pronounced in plants treated with hypotaurine (HT; H₂S scavenger) than those treated with norbornadiene (NBD; ET inhibitor), underscoring the superior role of H₂S over ET in mitigating heat stress. The present study's findings explain that H₂S is crucial for the ET-mediated response in augmenting photosynthesis and heat stress tolerance in rice.