Melatonin metabolism, signaling and possible roles in plants

Abstract

Summary Melatonin is a multifunctional biomolecule found in both animals and plants. In this review, the biosynthesis, levels, signaling, and possible roles of melatonin and its metabolites in plants is summarized. Tryptamine 5‐hydroxylase (T5H), which catalyzes the conversion of tryptamine into serotonin, has been proposed as a target to create a melatonin knockout mutant presenting a lesion‐mimic phenotype in rice. With a reduced anabolic capacity for melatonin biosynthesis and an increased catabolic capacity for melatonin metabolism, all plants generally maintain low melatonin levels. Some plants, including Arabidopsis and Nicotiana tabacum (tobacco), do not possess tryptophan decarboxylase (TDC), the first committed step enzyme required for melatonin biosynthesis. Major melatonin metabolites include cyclic 3‐hydroxymelatonin (3‐OHM) and 2‐hydroxymelatonin (2‐OHM). Other melatonin metabolites such as N1‐acetyl‐N2‐formyl‐5‐methoxykynuramine (AFMK), N‐acetyl‐5‐methoxykynuramine (AMK) and 5‐methoxytryptamine (5‐MT) are also produced when melatonin is applied to Oryza sativa (rice). The signaling pathways of melatonin and its metabolites act via the mitogen‐activated protein kinase (MAPK) cascade, possibly with Cand2 acting as a melatonin receptor, although the integrity of Cand2 remains controversial. Melatonin mediates many important functions in growth stimulation and stress tolerance through its potent antioxidant activity and function in activating the MAPK cascade. The concentration distribution of melatonin metabolites appears to be species specific because corresponding enzymes such as M2H, M3H, catalases, indoleamine 2,3‐dioxygenase (IDO) and N‐acetylserotonin deacetylase (ASDAC) are differentially expressed among plant species and even among different tissues within species. Differential levels of melatonin and its metabolites can lead to differential physiological effects among plants when melatonin is either applied exogenously or overproduced through ectopic overexpression.

Targeting Host Defense System and Rescuing Compromised Mitochondria to Increase Tolerance against Pathogens by Melatonin May Impact Outcome of Deadly Virus Infection Pertinent to COVID-19

“…Another strategy taken by microbes including viruses is simply to deplete the precursor of melatonin synthesis, tryptophan, by activating indolamine 2,3-dioxygenase (IDO), which metabolizes tryptophan to kynurenine catabolites [ 148 ]. In mammals and some plants tested, IDO also metabolizes melatonin to AFMK [ 149 , 150 , 151 ]. Considering that mitochondria are major synthetic sites for melatonin production, targeting of mitochondria is a potential mechanism for viruses to lower melatonin production.…”

Melatonin promotes potassium deficiency tolerance by regulating HAK1 transporter and its upstream transcription factor NAC71 in wheat

“…As a signaling molecule or auxin‐like plant regulator, MT plays roles via its receptors and activates mitogen‐activated protein kinase (MAPK) cascades and transcription factors, which are responsible for the induction of the downstream target genes 16 . In higher plant species, some MT‐mediated receptors (CAND2/PMTR1, RLKs, etc), MAPKs (leucine‐rich‐repeat LRR receptor kinase), WRKY transcription factors (WRKY 20/75), and target genes (GST1, PR1, PR5, PEPT1/2 and Glut1 transporters; HBS, tubulin, calreticuline proteins, etc) have been identified and reviewed 16 . The genes encoding the above some proteins (eg, LRR receptor‐like serines, WRKYs, NACs, GSTs, PRs) were also identified in our transcriptomic data (Table ), suggesting that they could be involved in MT‐mediated DK tolerance.…”

“…According to the European Community Legislation, MT has been classified by the European Chemical Agency (ECHA) as one non‐hazardous substance for use as plant protector and/or biostimulant 12 . Combined with its beneficial effects, inexpensive cost, and feasible approaches, MT has become a research hotspot in the plant biology areas because of its diverse functions in some physiological aspects 13‐16 . Significant progresses in exploring its roles, metabolism, and signaling have been achieved in human and other animals, whereas less information has been revealed in plant kingdoms 16 …”

A Beginner’s Guide to Osmoprotection by Biostimulants

“…Serotonin is converted to N-acetyl-serotonin by serotonin/arylalkylamine N-acetyltransferase, and finally into melatonin by N-acetyl-serotonin /hydroxyindole O-methyltransferase [ 146 ]. Melatonin is involved in numerous plant metabolic processes (for an extensive review, see Back [ 147 ]) and has a prolific bibliography, particularly due its involvement in enhanced plant tolerance to abiotic stress.…”