Phytomelatonin: An Emerging Regulator of Plant Biotic Stress Resistance

Melatonin enhances the biotic stress tolerance against pathogen attack. Melatonin modulates melatonin receptor–mitogen-activated protein kinase cascades, typical phytohormone signaling pathways to contribute to the direct defense response. Melatonin has diverse functions in plant development and stress tolerance, with recent evidence showing a beneficial role in plant biotic stress tolerance. It has been hypothesized that pathogenic invasion causes the immediate generation of melatonin, reactive oxygen species (ROS), and reactive nitrogen species (RNS), with these being mutually dependent, forming the integrative melatonin–ROS–RNS feedforward loop. Here we discuss how the loop, possibly located in the mitochondria and chloroplasts, maximizes disease resistance in the early pathogen ingress stage, providing on-site protection. We also review how melatonin interacts with phytohormone signaling pathways to mediate defense responses and discuss the evolutionary context from the beginnings of the melatonin receptor–mitogen-activated protein kinase (MAPK) cascade in unicellular green algae, followed by the occurrence of phytohormone pathways in land plants. Melatonin has diverse functions in plant development and stress tolerance, with recent evidence showing a beneficial role in plant biotic stress tolerance. It has been hypothesized that pathogenic invasion causes the immediate generation of melatonin, reactive oxygen species (ROS), and reactive nitrogen species (RNS), with these being mutually dependent, forming the integrative melatonin–ROS–RNS feedforward loop. Here we discuss how the loop, possibly located in the mitochondria and chloroplasts, maximizes disease resistance in the early pathogen ingress stage, providing on-site protection. We also review how melatonin interacts with phytohormone signaling pathways to mediate defense responses and discuss the evolutionary context from the beginnings of the melatonin receptor–mitogen-activated protein kinase (MAPK) cascade in unicellular green algae, followed by the occurrence of phytohormone pathways in land plants. the direct movement of genetic information between different species without reproduction, a process between prokaryotes and eukaryotes, and between the DNA-containing organelles including the nucleus, mitochondrion, and chloroplast of eukaryotes. rapid and localized cell death evoked via pathogen attack in higher plants. It is not a disease syndrome but an efficient, host-regulated defense response conferring the neutralization of intruding pathogens. under biotic stress conditions, melatonin, ROS, and RNS promote the production of each other, forming the melatonin–ROS–RNS feedforward loop. a broad range of highly conserved serine-threonine kinases regulating various cellular roles in the regulation of gene expression, cellular growth, and stress responses. a type of normal cell suicide via specialized and genetically regulated cellular machinery to kill itself. several NO-derived compounds from NO· and O2·-. the derivatives of molecular oxygen produced as an attribute of aerobic life. the dynamic equilibrium between ROS and RNS formation and removal to maintain the optimum physiological redox steady state. It is essential for the response to stressful challenges. a distinct signal transduction pathway that plays an important role in long-lasting protection against a broad spectrum of microorganisms. SAR is closely related to the accumulation of pathogenesis-related proteins, contributing to plant resistance.