Melatonin (N-acetyl-5-methoxytryptamine) is an animal hormone but was discovered in plants in 1995 (Arnao and Hernandez-Ruiz 2006). Melatonin is derived from cyanobacteria (endosymbiont) which was introduced to animal and plant through evolution (Tan et al., 2012). In plants melatonin is present in leaves, stems, roots, fruits and seeds. Phytomelatonin is a protective biomolecule that activates tolerance and resistance response in plant against bacteria, viruses, fungi, insect, parasitic nematodes and weeds (Hernandez-Ruiz et al., 2023). Melatonin influences plant innate immunity through mitogen-activated protein kinase (MAPK) cascade-mediated signaling (Lee and Back 2017). It may act as a defense signaling molecule triggering defense responses against pathogen attack in Arabidopsis and tobacco plants (Lee et al., 2014). Melatonin can directly inhibit pathogen’s growth, also improving the defensive capacity of the host plant upregulating defense genes involving reactive oxygen species (ROS) scavenging and nitric oxide (NO) production (Hernandez-Ruiz et al., 2023). At lower concentrations melatonin displayed antibacterial effect against Gram-positive and Gram-negative bacteria (Tekbas et al., 2008). Melatonin functions as an antioxidant in a receptor independent manner as in animals (Arnao and Hernandez-Ruiz 2013; Zhao et al., 2019). Moreover, melatonin can exert effect through specific cellular receptors on the plasma membrane, like other hormones or through receptor independent mechanisms that involve complex molecular cross talk with other players (Tarocco et al., 2019). Melatonin’s other function including its multiple receptors developed later in evolution (Zhao et al., 2019).
Melatonin is amphiphilic in nature and can penetrate the cell membrane and distribute in all the cells compartments the mitochondria, cytosol and the nucleus (Acuna-Castroviejo et al., 2001, Ali et al., 2021). Mitochondria and chloroplast are a source of free radical generation in living organisms. Melatonin is a pleiotropic molecule with diverse action in plants. It is an antioxidant that controls ROS and reactive nitrogen species, involved in multiple physiological actions and protects against biotic or abiotic stresses (Arnao and Hernandez-Ruiz 2019). Mitochondria and chloroplast must be protected from free radical and associated oxidative stress. The high-level production of melatonin by mitochondrial and chloroplast protect these cellular organisms against oxidative stress (Tan et al., 2012). Melatonin-ROS-NO play a role in the control of phytohormonal network as an intermediary in the expression of pathogenesis-related (PR) genes such as PR1 and PR5 to improve resistance to a virus (Hernandez-Ruiz et al., 2023). Melatonin-induced nitric oxide production is responsible for innate immunity response of Arabidopsis against Pseudomonas syringae pv. tomato DC3000 infection (Shi et al., 2015). The induction of melatonin upon infection with the avirulent pathogen Pseudomonas syringae DC3000 was independent of hydrogen peroxide (H2O2) and NO individually but dependent on the combination of H2O2 and NO (Lee and Back 2017). Melatonin efficiently enabled plant to maintain intracellular H2O2 concentration in steady-state levels and enhanced the activities of antioxidant enzymes which may improve disease resistance (Ali et al., 2021).
Melatonin modulates salicylic acid (SA) and jasmonic acid (JA) signaling cascades to enhance plant innate immunity against infection by various pathogens (Yang et al., 2022). Further melatonin caused translocation of NPR1 (nonexpresser of PR1) protein from the cytoplasm into the nucleus indicating the melatonin-elicited pathogen resistance in response to avirulent pathogen attack is salicylic acid-dependent in Arabidopsis (Lee et al., 2015). The regulation of stress specific gene and activation of PR protein and antioxidant enzymes under biotic and abiotic stress makes it a more versatile molecule (Sharif et al., 2018). Melatonin influences gene expression and synergistically work with other plant hormones to confer plant resistance (Zeng et al., 2022; Khan et al., 2023).
In plant-pathogen interaction melatonin plays a role in plant resistance to Botrytis cinerea (Moustafa-Farag et al., 2020). Increased melatonin levels in watermelon can lead to resistance against diverse pathogens (Mandal et al., 2018). Stomatal closure is a plant immune response to restrict pathogens invasion (Melotto et al., 2006). Melatonin-mediated stomatal closure depends on the activity of NADPH oxidase (nicotinamide adenine dinucleotide phosphate oxidase), an enzyme involved in production of ROS that functions as a secondary messenger to induce stomatal closure (Kwak et al., 2003; Moreno and Campos 2022). Melatonin can be used in biocontrol treatment as an elicitor molecule (Hernandez-Ruiz et al., 2023).
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