Insight into melatonin-mediated response and signaling in the regulation of plant defense under biotic stress

Melatonin plays a crucial role in the mitigation of plant biotic stress through induced defense responses and pathogen attenuation. Utilizing the current knowledge of signaling and associated mechanism of this phytoprotectant will be invaluable in sustainable plant disease management. Biotic stress in plants involves complex regulatory networks of various sensory and signaling molecules. In this context, the polyfunctional, ubiquitous-signaling molecule melatonin has shown a regulatory role in biotic stress mitigation in plants. The present review conceptualized the current knowledge concerning the melatonin-mediated activation of the defense signaling network that leads to the resistant or tolerant phenotype of the infected plants. Fundamentals of signaling networks involved in melatonin-induced reactive oxygen species (ROS) or reactive nitrogen species (RNS) scavenging through enzymatic and non-enzymatic antioxidants have also been discussed. Increasing evidence has suggested that melatonin acts upstream of mitogen-activated proteinase kinases in activation of defense-related genes and heat shock proteins that provide immunity against pathogen attack. Besides, the direct application of melatonin on virulent fungi and bacteria showed disrupted spore morphology, destabilization of cell ultrastructure, reduced biofilm formation, and enhanced mortality that led to attenuate disease symptoms on melatonin-treated plants. The transcriptome analysis has revealed the down-regulation of pathogenicity genes, metabolism-related genes, and up-regulation of fungicide susceptibility genes in melatonin-treated pathogens. The activation of melatonin-mediated systemic acquired resistance (SAR) through cross-talk with salicylic acid (SA), jasmonic acid (JA) has been essential for viral disease management. The high endogenous melatonin concentration has also been correlated with the up-regulation of genes involved in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). The present review highlights the versatile functions of melatonin towards direct inhibition of pathogen propagule along with active participation in mediating oxidative burst and simulating PTI, ETI and SAR responses. The hormonal cross-talk involving melatonin mediated biotic stress tolerance through defense signaling network suggests its suitability in a sustainable plant protection system.