# Biosynthetic Pathways of Hormones in Plants
## 6 Melatonin  
Melatonin, which is formally known as \(N\)-acetyl-5-methoxytripatamine, is an indolamine plant hormone that plays a key role in controlling growth and various physiological responses. Affected functions include root structure and development, flowering, leaf aging, fruit maturation, and determining the levels of chlorophyll, proline, and carbohydrates. It also functions as a signaling molecule during non-biological stress situations, like cold, drought, heavy metal exposure, UV radiation, and salinity, and it helps to direct plant defense responses against pathogen invasions [80; 81]. The highest levels of melatonin in plants are found in the mitochondria, endoplasmic reticulum, and chloroplasts, suggesting that these organelles are the primary sites involved in its biosynthesis. The varied locations of melatonin within the plant could potentially influence its mode of action in different ways [82]. The biosynthesis of melatonin begins with an essential aromatic amino acid tryptophan, which is synthesized de novo via the shikimate pathway (Figure 6). The pathway involved in the synthesis of this phytohormone occurs through various four-step reactions catalyzed by six enzymes: tryptophan decarboxylase (TDC), tryptophan hydroxylase (TPH), tryptamine 5-hydroxylase (T5H), serotonin \(N\)-acetyltransferase (SNAT), \(N\)-acetylserotonin \(O\)-methyltransferase (ASMT), and caffe acid \(O\)-methyltransferase (COMT) [83]. These enzymes have different cellular localization. Chloroplasts and mitochondria contain SNAT. TPH, ASMT/COMT, and TDC are present in the cytoplasm. T5H can be only found in the endoplasmic reticulum [84]. Recent research suggests that the synthesis of melatonin in plants primarily takes place in chloroplasts and mitochondria. These organelles have inherited the ability to produce melatonin from their bacterial forebears. Under normal circumstances, melatonin production predominantly occurs in the chloroplasts. However, if the pathway in the chloroplasts is obstructed, the production of melatonin shifts to the mitochondria. Therefore, under stress conditions, melatonin is chiefly synthesized in the mitochondria [85; 86]. The first step of melatonin biosynthesis is the formation of serotonin from Trp. Two different pathways involved in Trp and serotonin synthesis were identified in vascular plants. The first pathway involves the decarboxylation of tryptophan into tryptamine via the reaction catalyzed by the enzyme TDC. Next, tryptamine is hydroxylated into serotonin via the action of an enzyme called T5H. Another possibility is that the hydroxylation of Trp into 5-hydroxytyptophan via the action of enzyme TPH occurs. Next, the decarboxylation of 5-hydrotryptophan into serotonin by TDC takes place. The enzyme TDC is localized in the cytoplasm and characterized by a good affinity for two substrates: tryptophan and 5-hydroxytriptophan; thus, both pathways can be present in plants at levels of equal importance. The activity of TDC is considered to be the rate-limiting step in melatonin biosynthesis [80; 87; 88; 89]. Thereafter, serotonin is transformed into melatonin via a two-step reaction. The roles of three different enzymes--SNAT, ASMT, and COMT--in this reaction were confirmed. These enzymes may possess various biochemical isoforms. SNAT is responsible for N-acetylation of serotonin, which is further methylated by the methyltransferases ASMT or/and COMT, leading to production of melatonin [90]. In addition, SNAT can convert serotonin directly into \(N\)-acetylserotonin through alternative minor pathways. The enzymes ASMT and COMT catalyze the conversion of serotonin into 5-methoxyryptamine. This compound is then changed into melatonin by the enzyme SNAT. These three enzymes are characterized by an affinity for substrates such as serotonin, \(N\)-acetylserotonin, and 5-methoxyryptamine;  
Figure 6: The biosynthetic pathways of melatonin (phytohormone is green). therefore, the order in which the different enzymes act differs [81].