Mechanistic review of melatonin metabolism and signaling pathways in plants: Biosynthesis, regulation, and roles under abiotic stress

Abstract

Melatonin, a multifunctional biomolecule found in animals and plants, is critical in regulating various physiological processes. In plants, melatonin biosynthesis primarily originates from tryptophan through multiple enzymatic steps, finely controlled by different environmental and endogenous factors. This review synthesizes current knowledge on melatonin biosynthesis, production levels, and signaling mechanisms, focusing on its role in plant stress responses. Research shows that melatonin mitigates abiotic stresses such as drought, salinity, and temperature extremes by enhancing antioxidant systems, scavenging reactive oxygen species, and modulating ion homeostasis. Additionally, melatonin signaling, which may involve the mitogen-activated protein kinase (MAPK) cascade and potentially the Cand2 receptor (though its role remains debated), is essential for promoting plant growth and enhancing stress tolerance. This review also examines the potential of targeting specific enzymes, such as tryptamine 5-hydroxylase (T5H), to create melatonin knockout mutants, offering new insights into the regulatory networks of melatonin-mediated stress responses. The review concludes by identifying future research directions, particularly in understanding the complex cross-talk within melatonin signaling pathways and the debated role of Cand2 in plant resilience. © 2024 The Author(s)