M Vedhachalam, A Gurusamy, E Subramanian, P. Arunachalam, K. G. Sabarinathan
Discussed melatonin as a plant hormone with roles in seed germination, growth, photosynthesis, and stress responses, produced mainly within chloroplasts and mitochondria. Explained melatonin's protective mechanisms through direct scavenging of reactive species and boosting antioxidant enzymes. Melatonin coordinates with other plant hormones to balance growth and defense under environmental challenges.
Melatonin, a naturally occurring compound derived from the amino acid tryptophan, was first discovered in animals but has recently attracted significant interest as an important signalling molecule in plants. Plant cells produce melatonin mainly within their chloroplasts and mitochondria using a series of specialised enzymes. The recent discovery of melatonin receptors in plants, combined with growing evidence of its wide-ranging effects, suggests that melatonin functions as a true plant hormone rather than just a simple metabolite. In plants, melatonin influences many essential processes including seed germination, root and shoot growth, photosynthesis, leaf aging and stress responses. One of its most remarkable abilities is its capacity to coordinate with other major plant hormones such as abscisic acid, auxins, gibberellins, salicylic acid, jasmonic acid, brassinosteroids and ethylene. This coordination allows plants to maintain an optimal balance between growing and defending themselves when faced with environmental challenges. Melatonin protects plants through two complementary pathways. It directly scavenges harmful reactive oxygen and nitrogen species using its chemically reactive structure, while simultaneously strengthening the plant's natural defence mechanisms by boosting antioxidant enzymes like superoxide dismutase, catalase and ascorbate peroxidase. When plants encounter stressful conditions including drought, salinity, temperature extremes, waterlogging, or heavy metal contamination, melatonin helps maintain stable photosynthesis, regulate water loss through stomata, preserve chlorophyll content, balance mineral absorption and prevent membrane damage. Overall, melatonin acts as a master regulator that integrates stress tolerance with normal growth processes, making it a promising candidate for developing climate-resilient crops in modern agriculture.