Melatonin enhances metal oxide nanoparticles tolerance in rice seedlings through dual regulation of antioxidant defense and photosynthetic efficiency

The growing utilization of metal oxide nanoparticles (MONPs) presents novel and potential hazards to plants. However, the impacts of MONPs on plants and the mechanisms underlying their tolerance to MONPs remain unclear. In this study, we demonstrated that both CuO and ZnO nanoparticles hindered plant growth and triggered oxidative damage in rice seedlings. The role of melatonin in rice tolerance to MONPs was elucidated through a comprehensive analysis of OsCOMT mutant and overexpression plants, which showed melatonin deficiency and sufficiency, respectively. Our results revealed that the melatonin-deficient OsCOMT mutant plants exhibited hypersensitivity to MONPs, while the melatonin-sufficient OsCOMT overexpression plants showed enhanced MONPs tolerance. Physiological assessments further indicated that melatonin counteracted rice oxidative damage triggered by MONPs by increasing the activities of antioxidative enzymes, including superoxide dismutase, peroxidase, catalase, and glutathione reductase. Moreover, melatonin was found to foster rice growth under MONP stress by positively regulating the maximum photochemical efficiency, reducing non-photochemical fluorescence quenching, and promoting the biosynthesis of sucrose and starch. These findings not only highlight the hazardous effects of MONPs on plants, but also underscore the pivotal role of melatonin in bolstering plant resilience against MONPs.