Melatonin as a modulator of MAPK cascade and ROS-RNS feedforward loop during plant pathogen interaction

Sustainable agricultural practices encounter formidable obstacles posed by a diverse array of biotic stressors, including fungi, bacteria, viruses, viroids, phytoplasma, and nematodes, which are widespread across the globe. The severity of these stressors is shaped by variables such as weather patterns, cropping techniques, cultivation methodologies, crop varieties, and their resistance capabilities. Melatonin, a multifunctional compound present in various organisms, plays vital roles, especially in enhancing plant resilience to environmental challenges. Its use can alleviate the negative effects of abiotic factors on plants. Recent research indicates its positive influence on plant defense against biotic stresses. In this review, we discuss the contributions of melatonin in enhancing plant resilience against pathogenic attacks by initiating early defense responses, regulating reactive oxygen species (ROS) and reactive nitrogen species (RNS) levels, and interacting with signaling pathways involved in plant defense mechanisms. Following pathogenic attacks, ROS and RNS are rapidly generated, forming an interconnected loop with melatonin, termed the melatonin-ROS-RNS feedforward loop. We discuss how the loop, which may be present in the mitochondria and chloroplasts, increases disease resistance at the earliest possible stage of pathogen entry while providing on-site defense. We also consider the development of the melatonin receptor-mitogen-activated protein kinase (MAPK) cascade and the emergence of phytohormone pathways in plants, and how melatonin interacts with these signaling pathways to drive defense responses. This multifunctional compound holds promise for sustainable agricultural practices by potentially mitigating the negative impacts of various biotic stressors on crop yield and quality.