The Role of Phyto-Melatonin and Related Metabolites in Response to Stress

Abstract

Plant hormone candidate melatonin has been widely studied in plants under various stress conditions, such as heat, cold, salt, drought, heavy metal, and pathogen attack. Under stress, melatonin usually accumulates sharply by modulating its biosynthesis and metabolic pathways. Beginning from the precursor tryptophan, four consecutive enzymes mediate the biosynthesis of tryptamine or 5-hydroxytryptophan, serotonin, N-acetylserotonin or 5-methoxytryptamine, and melatonin. Then, the compound is catabolized into 2-hydroxymelatonin, cyclic-3-hydroxymelatonin, and N¹-acetyl-N²-formyl-5-methoxyknuramine through 2-oxoglutarate-dependent dioxygenase catalysis or reaction with reactive oxygen species. As an ancient and powerful antioxidant, melatonin directly scavenges ROS induced by various stress conditions. Furthermore, it confreres stress tolerance by activating the plant's antioxidant system, alleviating photosynthesis inhibition, modulating transcription factors that are involved with stress resisting, and chelating and promoting the transport of heavy metals. Melatonin is even proven to defense against pathogen attacks for the plant by activating other stress-relevant hormones, like salicylic acid, ethylene, and jasmonic acid. Intriguingly, other precursors and metabolite molecules involved with melatonin also can increase stress tolerance for plant except for unconfirmed 5-methoxytryptamine, cyclic-3-hydroxymelatonin, and N¹-acetyl-N²-formyl-5-methoxyknuramine. Therefore, the precursors and metabolites locating at the whole biosynthesis and catabolism pathway of melatonin could contribute to plant stress resistance, thus providing a new perspective for promoting plant stress tolerance.

Keywords: biosynthesis; catabolism; melatonin; reactive oxygen species; stress resistance.

Figure 1

Melatonin biosynthesis and catabolism pathways in plants. Abbreviation: TDC, tryptophan decarboxylase; TPH, tryptophan hydroxylase; T5H, tryptamine 5-hydroxylase; SNAT, serotonin N-acetyltransferase; ASMT, N-acetylserotonin methyltransferase; COMT, caffeic acid O-methyltransferase; AFMK, N¹-acetyl-N²-formyl-5-methoxyknuramine; AMK, N-acetyl-5-methoxyknuramine; M2H, melatonin 2-hydroxylase; M3H, melatonin 3-hydroxylase; IDO, indoleamine 2,3-dioxygenase; 2-ODD, 2-oxoglutarate-dependent dioxygenase; ROS, reactive oxygen species. Dotted arrows represent the hypothetical steps.

Figure 2

Melatonin-mediated abiotic stress response in plants. Abbreviation: ROS, reactive oxygen species; GSH, glutathione; PCs, phytochelatins; Cd, cadmium; SOD, superoxide dismutase; APX, ascorbate peroxidase; CAT, catalase; GPX, glutathione peroxidase; ASA, antioxidants ascorbic acid; GSH, glutathione. Dotted arrows represent the hypothetical pathway.

Figure 3

Melatonin-mediated biotic stress response in plants. Abbreviation: ROS, reactive oxygen species; PAMPs, pathogen-associated molecular patterns; MAMPs, microbe-associated molecular patterns; MAPK, Mitogen-activated protein kinase; NO, nitric oxide; SA, salicylic acid; JA, jasmonic acid; ET, ethylene. Dotted arrows represent the hypothetical pathway.