Melatonin metabolism regulates tomato fruit ripening through the ethylene pathway

• Melatonin levels increase during tomato fruit ripening. • COMT1 mutation suppresses melatonin, ethylene, ripening, and fruit quality. • m2h/srg1 mutants accumulate melatonin, accelerate ripening, and boost ethylene. • Melatonin metabolism regulates ripening mainly via the ethylene pathway. Melatonin is essential for plant growth and stress responses, but its role in fruit ripening and its interaction with ethylene remain largely unclear. In this study, we investigated tomato ( Solanum lycopersicum L.) fruit ripening using mutants and overexpression lines of the melatonin biosynthetic gene COMT1 . In wild-type (WT), fruits, melatonin levels increased significantly during ripening. By contrast, comt1 mutants exhibited suppressed melatonin levels, delayed ripening, reduced pigment and sugar accumulation, and markedly suppressed ethylene biosynthesis and signaling. Overexpression of COMT1 elevated melatonin levels but had limited effects on ripening or ethylene production compared with WT. Ripening inhibition by 1-methylcyclopropene (an ethylene perception blocker) and promotion by ethephon (an ethylene releaser) occurred in both WT and comt1 , confirming ethylene’s central role. Evolutionary analysis identified M2H and SRG1 genes, which convert melatonin into 2-hydroxymelatonin and 3-hydroxymelatonin, respectively. The m2h/srg1 double mutants showed elevated melatonin accumulation, accelerated ripening, increased ethylene production, and enhanced expression of ethylene-related genes. Together, these findings demonstrate that melatonin metabolism modulates tomato fruit ripening primarily through the regulating ethylene biosynthesis and signaling.