Melatonin and Phytomelatonin: Chemistry, Biosynthesis, Metabolism, Distribution and Bioactivity in Plants and Animals-An Overview

Abstract

Melatonin is a ubiquitous indolamine, largely investigated for its key role in the regulation of several physiological processes in both animals and plants. In the last century, it was reported that this molecule may be produced in high concentrations by several species belonging to the plant kingdom and stored in specialized tissues. In this review, the main information related to the chemistry of melatonin and its metabolism has been summarized. Furthermore, the biosynthetic pathway characteristics of animal and plant cells have been compared, and the main differences between the two systems highlighted. Additionally, in order to investigate the distribution of this indolamine in the plant kingdom, distribution cluster analysis was performed using a database composed by 47 previously published articles reporting the content of melatonin in different plant families, species and tissues. Finally, the potential pharmacological and biostimulant benefits derived from the administration of exogenous melatonin on animals or plants via the intake of dietary supplements or the application of biostimulant formulation have been largely discussed.

Keywords: N-acetyl-5-methoxytriptamine; biostimulant; cluster analysis; dietary supplements; indolamine.

Figure 1

Chemical structure of melatonin.

Figure 2

Melatonin metabolism and its related metabolites. ONOO⁻ = peroxynitrite; ROS = reactive oxygen species; RNS = reactive nitrogen species; O₂⁻: superoxide anion; OH⁻: hydroxyl radical; AMFK: N1-acetyl-N2-formyl-5-methoxykynuramine; AMK: N1-acetyl-5-methoxykynuramine; AMMC: 3-acetamidomethyl-6-methoxycinnolinone; AMNK: N1-acetyl-5-methoxy-3 nitrokynura-mine.

Figure 3

Biosynthetic pathway involved in the synthesis of tryptophan, the key compound for the formation of melatonin in plants. PEP: 2-phosphoenolpyruvate; DAHP: 3-deoxy-D-arabinoheptulosonate 7-phosphate; DHQ: 3-dehydroquinic acid; DHS: 3-dehydroshikimate; PEP: 2-phosphoenolpyruvate; EPSP: 5-enolpyruvylshikimate-3-phosphate; PRA: Phosporibosyl antranilate; PRAI: PRA isomerase; PRPP: phosphoribosylpyrophosphate; IGP: indole-3-glycerol phosphate; EC: enzyme commission number).

Figure 4

First two reactions of the melatonin biosynthetic pathway leading to the formation of the essential intermediate serotonin. TDC: L-tryptophan decarboxylase; TPH: tryptophan hydroxylase; T5H: tryptamine 5-hydroxylase; EC: enzyme commission number).

Figure 5

The last two potential reactions leading to the formation of melatonin. SNATs: serotonin N-acetyltransferase; ASMTs: acetylserotonin O-methyltransferase; COMT: caffeic acid 3-O-methyltransferase; EC: enzyme commission number.

Figure 6

Subcellular localization of melatonin intermediates and enzymes involved in the transformation of tryptophan into melatonin in the different melatonin biosynthetic routes. TDC: L-tryptophan decarboxylase; TPH: tryptophan hydroxylase; T5H: tryptamine 5-hydroxylase; SNATs: serotonin N-acetyltransferase; ASMTs: acetylserotonin O-methyltransferase; COMT: caffeic acid 3-O-methyltransferase.

Figure 7

The classic melatonin synthetic pathway in animals. TPH: tryptophan hydroxylase; AADC: aromatic amino acid decarboxylase; AANAT: aralkylamine N-acetyltransferase; ASMTs: acetylserotonin O-methyltransferase.

Figure 8

The melatonin biosynthetic pathway in mitochondria (orange broken arrows) and in chloroplasts (solid green arrows). In plants both pathways are probably present: when the main chloroplast pathway is interrupted (like in Sekiguchi mutant rice) the mitochondrial pathway takes over to compensate for the lack [85,86]. TDC: L-tryptophan decarboxylase; TPH: tryptophan hydroxylase; T5H: tryptamine 5-hydroxylase; SNATs: serotonin N-acetyltransferase; ASMTs: acetylserotonin O-methyltransferase; COMT: caffeic acid 3-O-methyltransferase.

Figure 9

Synthetic capacity of serotonin from tryptophan.

Figure 10

Cluster distribution of melatonin within plant species, according to previously published data [38,115,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173]. Data are expressed as ng melatonin per g of FW Euclidean distances, calculated with centroid method. Statistical analysis and graphical representation were made using SPSS v. 24 software.

Figure 11

Cluster distribution of melatonin within plant families, according to previously published data [38,115,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173]. Euclidean distances were calculated with centroid method. Data were expressed as ng melatonin per g of FW. Statistical analysis and graphical representation were made using SPSS v. 24 software.

Figure 12

Cluster distribution of melatonin in plant organs, according to previously published data [38,115,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173]. Euclidean distances were calculated with centroid method. Statistical analysis and graphical representation were made using SPSS v. 24 software.