Proposed model of melatonin accumulation and its regulatory role in fruit ripening during different developmental stages in climacteric and nonclimacteric fruits. It is suggested that melatonin promotes fruit growth by itself or by a possible crosstalk with IAA in both climacteric and nonclimacteric fruits. In nonclimacteric fruits, melatonin may act in the advanced developmental stages, interacting antagonistically with ABA through the downregulation of its biosynthesis-related genes such as NCED. By contrast, melatonin might have a positive crosstalk with ethylene, positively influencing its biosynthesis and signaling in climacteric fruits. ACC, 1-aminocyclopropane-1-carboxylic acid; IAA, indole-3-acetic acid; NCED, 9-cis-epoxycarotenoid dioxygenase; SAM, S-adenosyl-methionine.