From night hormone to green signal: The journey of melatonin.

Melatonin (MT), once considered exclusive to animals, is now recognized as a universal and multifunctional molecule in plants, playing pivotal roles in growth regulation, stress tolerance, and antioxidant defense. Since its discovery in plants in the mid-1990s, the presence of MT has been reported to be prevalent across diverse plant taxa. MT modulates plant responses to abiotic stresses such as heat, cold, salinity, drought, heavy metals, and UV radiation, as well as biotic stresses including pathogen attack and herbivory, primarily by regulating antioxidant enzymes, stress-responsive genes, hormone signaling pathways, and production of secondary metabolites. Advanced analytical techniques such as HPLC, LC-MS, and NMR have facilitated sensitive detection and structural characterization of MT in various plant species. Molecular studies have identified receptors of MT and its roles in regulating different processes, highlighting its integration with other phytohormones and signaling molecules. Genetic engineering tools have been proven effective in raising the level of endogenous MT levels, which has immense potential for enhancing crop resilience. Emerging evidence points to the role of MT in modulating the plant microbiome and systemic signaling, offering new avenues for research. Despite significant advances, key challenges remain in understanding the structure of receptors, signaling crosstalk, and the regulation of its biosynthesis under stress. This review discusses the methods for detecting MT in plants and evaluates its physiological, molecular, and functional aspects. Tracing the evolution of MT research in plants, from its initial discovery to the present day, key milestones, cross-talk, and signaling pathways of MT in plants have been discussed.