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

• Melatonin's role in plants : this review examines the versatile role of melatonin in plants, emphasizing its biosynthesis, production levels, and signaling mechanisms. • Tryptamine 5-Hydroxylase (T5H) : the enzyme T5H is identified as a key player in converting tryptamine into serotonin, with implications for creating melatonin knockout mutants to explore lesion-mimicking phenotypes. • Biosynthesis and regulation : melatonin is synthesized from tryptophan through multiple enzymatic steps, regulated by environmental cues and endogenous factors, critical for plant response to abiotic stresses. • Abiotic stress protection : melatonin helps protect plants from abiotic stresses (drought, salinity, temperature extremes, heavy metals) by regulating antioxidant systems, scavenging reactive oxygen species, altering ion homeostasis, and upregulating stress-responsive genes. • Signaling pathways : both receptor-mediated and receptor-independent melatonin signaling pathways are involved in stress responses, with potential cross-talk with other signaling molecules. The MAPK cascade and the debated role of Cand2 as a receptor are highlighted. • Promoting growth and yield : melatonin enhances plant growth, development, and yield in poor environmental conditions by exerting strong antioxidant effects and activating stress-responsive pathways. • Historical and current research : the review covers the qualitative development of melatonin research from 1995 to 2023, highlighting the growth in scholarly publications and the expanding understanding of melatonin's benefits in plant physiology and stress tolerance. • Antioxidant mechanisms : melatonin's superior antioxidant properties, compared to ascorbates and tocopherols, are emphasized, playing a crucial role in regulating stress, protecting photosynthetic machinery, and promoting tissue proliferation. • Future research directions : the review suggests future research directions to further elucidate the complex regulatory networks of plant melatonin-mediated stress responses. 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.