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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. [ABSTRACT FROM AUTHOR]

Summary: Melatonin, a natural phytohormone in plants, plays multiple critical roles in plant growth and stress responses. Although melatonin biosynthesis‐related genes have been suggested to possess diverse biological functions, their roles and functional mechanisms in regulating rice grain yield remain largely unexplored. Here, we uncovered the roles of a caffeic acid O‐methyltransferase (OsCOMT) gene in mediating rice grain yield through dual regulation of leaf senescence and vascular development. In vitro and in vivo evidence revealed that OsCOMT is involved in melatonin biosynthesis. Transgenic assays suggested that OsCOMT significantly delays leaf senescence at the grain filling stage by inhibiting degradation of chlorophyll and chloroplast, which, in turn, improves photosynthesis efficiency. In addition, the number and size of vascular bundles in the culms and leaves were significantly increased in the OsCOMT‐overexpressing plants, while decreased in the knockout plants, suggesting that OsCOMT plays a positive role in vascular development of rice. Further evidence indicated that OsCOMT‐mediated vascular development might owe to the crosstalk between melatonin and cytokinin. More importantly, we found that OsCOMT is a positive regulator of grain yield, and overexpression of OsCOMT increase grain yield per plant even in a high‐yield variety background, suggesting that OsCOMT can be used as an important target for enhancing rice yield. Our findings shed novel insights into melatonin‐mediated leaf senescence and vascular development and provide a possible strategy for genetic improvement of rice grain yield. [ABSTRACT FROM AUTHOR]