Azole Selenourea Carboxylate Enhances Salt Tolerance in Rice through Coordinated Physiological and Molecular Responses

Soil salinity severely limits rice productivity. To address the limitations of inorganic selenium (Se), we developed azole selenourea carboxylates as organic Se-based regulators for enhancing salt tolerance in rice. Among them, compound 11 showed strong radical-scavenging activity and effectively alleviated salt stress during seedling establishment. Physiological, biochemical, and transcriptomic analyses revealed a coordinated defense response, including enhanced antioxidant enzyme activities, reinforcement of the ascorbate–glutathione cycle, modulation of phytohormone signaling, and activation of phenylpropanoid biosynthesis, leading to increased lignin and flavonoid accumulation. Computational analysis suggested a possible association between compound 11 and glutathione peroxidase. Under the tested conditions, no observable acute toxicity was detected in zebrafish or silkworms at 20.0 mg/L. Selenium treatment also improved rice yield and increased grain Se content within the relevant safety range. These results highlight compound 11 as a promising regulator for rice salt tolerance and selenium biofortification.