Interactions of melatonin, reactive oxygen species, and nitric oxide during fruit ripening: an update and prospective view

Abstract

Fruit ripening is a physiological process that involves a complex network of signaling molecules that act as switches to activate or deactivate certain metabolic pathways at different levels, not only by regulating gene and protein expression but also through post-translational modifications of the involved proteins. Ethylene is the distinctive molecule that regulates the ripening of fruits, which can be classified as climacteric or non-climacteric according to whether or not, respectively, they are dependent on this phytohormone. However, in recent years it has been found that other molecules with signaling potential also exert regulatory roles, not only individually but also as a result of interactions among them. These observations imply the existence of mutual and hierarchical regulations that sometimes make it difficult to identify the initial triggering event. Among these 'new' molecules, hydrogen peroxide, nitric oxide, and melatonin have been highlighted as prominent. This review provides a comprehensive outline of the relevance of these molecules in the fruit ripening process and the complex network of the known interactions among them.

Keywords: Hydrogen peroxide; melatonin; nitric oxide; nitrosomelatonin; postharvest; ripening.

Fig. 1.

Venn diagram analysis of the number of publications on the different signal molecules, namely nitric oxide (NO), hydrogen peroxide (H₂O₂), hydrogen sulfide (H₂S), and melatonin (MEL), related to fruits, found in the PubMed database in the period 1980–2022.

Fig. 2.

Melatonin-derived metabolites resulting from the interaction of melatonin with ROS and RNS. The reactions involve the addition of a hydroxyl group (-OH) in position 2, 4, or 6; the addition of NO (1-nitrosomelatonin); or the addition of a nitro group (NO₂) in position 1, 4, or 6.

Fig. 3.

Simple model of melatonin (Mel) nitrosation, S-nitrosation of glutathione (GSH), cysteine (Cys), or protein thiol (P-SH), and trans-nitrosation. Nitric oxide (NO) interacts with Mel, GSH, Cys, and P-SH to generate nitrosomelatonin (NOMel), S-nitrosoglutathione (GSNO), S-nitrosocysteine (CysNO), or nitrosated protein (P-SNO), respectively, which can undergo trans-nitrosation processes.

Fig. 4.

Overview of the cascade of signals triggered by the application of exogenous melatonin (Mel), NO, or H₂O₂ to modulate fruit ripening and quality.