Endogenous melatonin mediated by OsCOMT enhances grain yield, nitrogen use efficiency, and grain quality in rice.

Excessive nitrogen (N) application in rice often leads to diminishing yield returns and compromises grain quality, presenting a key challenge for sustainable rice production. Here, we revealed that OsCOMT, a key regulator that modulates endogenous melatonin levels, simultaneously enhances grain yield, nitrogen use efficiency (NUE), and grain quality under varying N conditions. Using two-year field trials conducted under low, normal, and high N conditions, we found that high-melatonin (OsCOMT-OE) plants consistently produced higher grain yield compared to wild type (WT) across all N regimes, whereas low-melatonin (OsCOMT-Cas9) plants showed significant yield reductions. Notably, WT exhibited minimal yield gains under high N relative to normal N, whereas high-melatonin plants sustained a pronounced increase. This yield advantage was underpinned by melatonin-mediated enhancement of N uptake and utilization efficiency. Mechanistically, elevated melatonin promoted N retention in vegetative tissues during grain filling, thereby delaying leaf senescence and extending photosynthetic activity. This N partitioning strategy also limited excessive N transport to grains, thereby lowering grain protein content and chalkiness while improving eating quality. Collectively, our findings establish endogenous melatonin as a key regulator that coordinates high yield, improved NUE, and superior grain quality under high N conditions, and highlight OsCOMT as a promising genetic target for sustainable rice production.