Melatonin-Producing Microorganisms: A Rising Research Interest in Their Melatonin Biosynthesis and Effects on Crops.

Melatonin is imperative in animals and plants, contributing to multiple physiological roles, and its microbial production could offer an eco-friendly alternative to synthetic melatonin. However, detecting and characterizing it in microorganisms remains ongoing, and the biosynthesis pathways are still poorly explored. We noted that not all microorganisms possess similar enzymes and substrates for melatonin production. Its biosynthesis pathway is well-characterized in yeast, potentiating its importance in agricultural practices in a melatonin-dependent manner. Intercellular melatonin production in algae and fungi boosts their resilience to oxidative cell death by activating the antioxidant defenses. Few studies on the use of Bacillus sp., Pseudomonas sp., and Enterobacter sp. have shown that these bacteria increase their endogenous melatonin contents, which may exchange with their host plants; thereby, mitigating abiotic stresses by modulating cellular damages, ion exchanges, hormonal levels, and related transcript expressions. Though plant-growth-promoting microbes show promise to enhance crop production, melatonin-producing microorganisms (M-PMs) are limited in identification, and their ecological and biological applications are still underutilized in agriculture. With the compounded benefits from M-PMs, it could be an untapped tool for rhizospheric bioengineering. Therefore, this review delivers comprehensive insights into M-PMs for practicing sustainable agriculture under increased climatic changes.