Nitric oxide interplay with hydrogen sulfide modulates gene expression and photosystem II function through enhanced...

Drought is one of the most detrimental abiotic stresses, severely constraining crop productivity on a global scale. Prolonged water deficits associated with climate change impair plant growth and contribute to substantial agricultural yield losses each year. In this study, we investigated the roles of nitric oxide (NO) and hydrogen sulfide (H₂S) in mediating stress tolerance in common bean, integrating physiological, biochemical, and molecular analyses to elucidate their regulatory functions under simulated water-deficit conditions. PEG-induced osmotic stress significantly reduced plant growth, which was associated with impaired nutrient uptake, enhanced oxidative stress, decreased PSII efficiency, and reduced photosynthetic performance. However, PEG-induced osmotic stress also stimulated the antioxidant defense system, increased proline accumulation, and elevated endogenous NO and H₂S levels, contributing to a partial mitigation of oxidative damage (O₂•^-, H₂O₂, and TBARS). Furthermore, PEG-induced osmotic stress upregulated the expression of antioxidant-related genes while downregulating photosynthesis-related genes. Exogenous application of sodium hydrosulfide (200 μM NaHS; H₂S donor) and sodium nitroprusside (100 μM SNP; NO donor) effectively alleviated stress-induced damage by maintaining photosynthetic capacity through enhanced expression of PSII-associated genes, improved stomatal regulation, and preservation of chloroplast ultrastructure. In addition, NaHS and SNP treatments promoted endogenous NO and H₂S production and reduced oxidative damage by activating antioxidant defense mechanisms. Notably, the combined application of SNP and NaHS produced the strongest protective effect under PEG-induced osmotic stress conditions. Pharmacological approaches using the NO scavenger cPTIO and the H₂S scavenger hypotaurine revealed that the protective effects of both donors were significantly attenuated upon scavenging either signaling molecule, indicating a strong functional interdependence between NO and H₂S pathways. Overall, our findings demonstrate that NO and H₂S operate through a coordinated signaling network to regulate plant tolerance under PEG-induced osmotic stress, providing a potential strategy for improving stress resilience in legume crops.