Role of Melatonin and Plant-Growth-Promoting Rhizobacteria in the Growth and Development of Plants

Melatonin is a tryptophan-based indole molecule found in primitive photosynthetic bacteria to mammals. It performs different functions in plants like rhizogenesis, enhancing plant growth, seed germination, plant yield, biomass production, photosynthesis, circadian rhythm, and fruit ripening. In addition, one of the most significant attributes of melatonin is its antioxidant activity. Moreover, it works as an anti-stress agent against different biotic and abiotic stresses like drought, salinity, potentially toxic metals, and pathogens. Melatonin forms antioxidant cascade reaction by scavenging free radicals that enhances its antioxidant capacity. In response to different stress conditions, expression of genes involved in melatonin synthesis is increased. In the same way, plant-growth-promoting rhizobacteria colonize plant roots and enhance plant growth by a number of mechanisms like phosphate solubilization, nitrogen fixation, siderophore production, production of phytohormones, phytoremediation, disease suppression, and production of 1-aminocyclopropane- 1-carboxylate deaminase. Thus, melatonin and plant-growth-promoting rhizobacteria are involved in enhancing plant growth under abiotic and biotic stress but the mechanisms of action of both are different. Therefore, in this study, the data on the impact of melatonin and plant-growth-promoting rhizobacteria on plants are combined for the first time and how these could be useful in enhancing the plant growth is examined. In addition, the research gaps are identified in melatonin and plant-growth-promoting rhizobacteria research already conducted from the last few decades that will help the scientific community in further research.