Melatonin biosynthesis in plants: multiple pathways catalyze tryptophan to melatonin in the cytoplasm or chloroplasts

Abstract

Melatonin is an animal hormone as well as a signaling molecule in plants. It Depending on the pathways, the final subcellular sites of melatonin synthesis vary at either the cytoplasm or chloroplasts, which may differentially affect the mode of action of melatonin in plants.

Overexpression of rice serotonin N‐acetyltransferase 1 in transgenic rice plants confers resistance to cadmium and senescence and increases grain yield

“…The main factor involved in the yield penalty in ovine SNAT transgenic rice was the marked decrease in spikelet number per panicle, which was 45% less than that in wild‐type controls, whereas the decrease in spikelet number per panicle was less than 10% in OsSNAT1 transgenic rice. The precise reasons for the obvious phenotypic differences between ovine SNAT and OsSNAT1 transgenic rice plants are not yet clear, but are likely due to differences in enzyme kinetics, gene regulation, biosynthetic pathways, and subcellular localization between animal and plant SNATs . In fact, the two proteins are proposed to have evolved from different endosymbiotic events, where the plant SNAT is derived from anaerobic cyanobacteria while animal SNAT evolved from nonpurple sulfur bacteria …”

Melatonin induction and its role in high light stress tolerance in Arabidopsis thaliana

“…Since the identification of melatonin in plants in 1995, most plants, including crop plants, have been shown to synthesize melatonin in concentrations varying from picogram to microgram per gram tissue weight . In addition, all plant genes responsible for melatonin biosynthesis have been successfully cloned from a number of plant species, beginning with the cloning of tryptophan decarboxylase, the first committed step for melatonin biosynthesis . In contrast to animals, plants are enzymatically capable of metabolizing melatonin into 2‐hydroxymelatonin and cyclic 3‐hydroxymelatonin by the enzymes melatonin 2‐hydroxylase and melatonin 3‐hydroxylase, respectively .…”

“…In support of previous reports, the penultimate step for plant melatonin biosynthesis occurs in chloroplasts, where serotonin is converted into N ‐acetylserotonin by SNAT. Furthermore, N ‐acetylserotonin O ‐methyltransferase (ASMT), the final enzyme catalyzing N ‐acetylserotonin into melatonin, is reportedly expressed in chloroplasts, although major groups of ASMT genes, including caffeic acid O ‐methyltransferase (COMT), are expressed in the cytoplasm . These results indicate that melatonin production or melatonin enrichment in chloroplasts may have a pivotal antioxidative role in preventing chloroplast damage from ROS generated by various stresses, especially HL stress.…”

Differential roles of melatonin in plant‐host resistance and pathogen suppression in cucurbits

“…Recent research on melatonin has been focused on its biosynthetic pathway, its physiological role in plant growth and development, its importance as a phytoremediative molecule, and its role as a powerful antioxidant capable of scavenging reactive oxygen species (ROS) and reactive nitrogen species (RNS) . In plants, the melatonin biosynthesis takes place through the shikimate pathway with tryptophan as the precursor amino acid . In contrast to vertebrates, there are 6 genes (tryptophan decarboxylase, TDC; tryptamine 5‐hydroxylase, T5H, tryptophan 5‐hydroxylase, TPH; serotonin N‐acetyltransferase, SNAT; acetylserotonin methyltransferase, ASMT; and caffeic acid O ‐methyltransferase, COMT) involved in the melatonin biosynthetic process in plants.…”