**2. The biosynthetic pathway of melatonin in plants**

Melatonin is a small molecule, which can shuttle freely in and between cells due to its hydrophilic and lipophilic molecular structure [3]. Using St. John's wort (*Hypericum perforatum* L. cv. Anthos) seedlings as the plant material, Murch et al. [4] exogenously supplied 14C labeled tryptophan and found the presence of radioactive tryptamine, 5-hydroxytryptophan, serotonin, indoleacetic acid and melatonin via adopted isotope tracer approach. Among them, tryptophan and 5-hydroxytryptophan are the synthetic precursors of melatonin in animals. The study indicated that the biosynthetic pathway of melatonin was similar and conserved from animals to plants. Nowadays, the biosynthesis of melatonin using l-tryptophan as substrate has been evidenced in plants with the following steps [5]: l-tryptophan is decarboxylated under the catalysis of l-tryptophan decarboxylase (TDC) to form tryptamine; tryptophan reacts further with tryptophan 5-hydroxylase (T5H) to produce serotonin; serotonin-*N*-acetyltransferase (SNAT) catalyzes the production of *N*-acetylserotonin; and *N-*acetylserotonin can be further catalyzed by *N*-acetylserotonin *O*-methyltransferase (ASMT) or caffeic acid *O*-methyltransferase (CAMT) to produce melatonin (**Figure 1**).

At present, key genes of melatonin synthesis, *TrpDC*, *T5H*, *AcSNMT* and *HIOMT*, have been successfully cloned from plants and their expression patterns have also been carried out [6]. In recent years, plant chloroplasts and mitochondria are considered as the key organelles of the melatonin synthesis [7]. The hypothesis to establish the basis for melatonin is an important biological antioxidant, and chloroplasts and mitochondria are main reactive oxygen species (ROS) sources; these two organelles generate large amounts of melatonin, which is used to remove

#### **Figure 1.**

*A comparison of the biosynthetic pathways of melatonin in animals and in plants. Black occurs only in animals; green occurs only in plants; and red occurs in both animals and plants.*

**109**

*Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

**influential factors**

tic roles in plants [18].

cherry (13.46 ng g<sup>−</sup><sup>1</sup>

to 114.5 ng g<sup>−</sup><sup>1</sup>

from 0.5 pg g<sup>−</sup><sup>1</sup>

excess ROS to reduce the oxidation of cells. However, a recent study in rice (*Oryza sativa* L.) has shown that the key melatonin synthesis gene *COMT* is located in plant chloroplasts and has demonstrated that melatonin synthesis is increased through the 5-MT pathway [8]. CAND2/PMTR1, a hypothetic plant melatonin receptor, recently has been identified in *Arabidopsis thaliana* [9]. It is located in the plasma membrane with a receptor-like topology and interacts with the G-protein subunit (GPA1), while expression in different tissues is induced by melatonin. The binding of plant melatonin receptors triggers Gγb and Gα activation of NADPH oxidasedependent H2O2 production (RBOH), increasing Ca2+ inflow, promoting K+

and ultimately leading to stomatal closure [10]. The cloning of these melatoninmetabolizing genes and the discovery of the receptors can lay a certain foundation

Although policies regarding the free sale, universal use and food supplement of melatonin are still controversial, most developed countries have classified melatonin as an over-the-counter drug and allowed it to be sold freely in pharmacy stores. Currently, melatonin is regarded as an over-the-counter medicine and health care product to relieve sub-health and improve sleep [11]. Some studies indicated that regular consumption of melatonin-rich foods can significantly improve human health [12–14]. In view of the health benefits of melatonin, more and more nutritionists begin to pay attention to the amount of melatonin in food, hoping that

Two research groups identified melatonin in some edible plants in 1995. Vantassel et al. [15] have identified the presence of plant-derived melatonin in higher plants of morning glory (*Ipomoea nil* L.) and tomato (*Solanum lycopersicum* L.) by radio immunoprecipitation and gas chromatography-mass spectrometry analysis; this was also the first report of plant-derived melatonin in horticultural crops. Since then, melatonin has been isolated and identified in numbers of horticultural crops (**Table 1**). Melatonin levels are usually high in seeds and low in fruits in these edible organs of horticultural crops. According to the statistics, melatonin concentrations in various tissues of plants generally conform to the following rules from high to low: seeds, leaves, roots, flowers and fruits [16]. Moreover, the melatonin concentrations are not evenly distributed in the same tissue. Using white lupine (*Lupinus micranthus* Guss.) leaves as an example, the highest concentrations of melatonin are in the leaf tip, then in the middle leaf, and the least in the leaf base [17]. Through comparative analysis, the distributed gradient of melatonin is similar to the distribution of auxin, which indicates that these two indoleamine compounds may play similar or synergis-

The melatonin concentrations in horticultural crops are closely influenced by species, varieties, growing environment, cultivated methods, harvesting time and processing methods. As shown in **Table 1**, melatonin concentration in sweet

In different varieties of tomato, the melatonin concentrations fluctuated greatly

type of the variety itself. Climate and environmental factors in different years have significant effects on the melatonin concentrations of horticultural crops. For example, the melatonin concentrations of 'Marbone' tomato harvested in 2010 were six times higher than those in 2009, while the melatonin concentrations of 'Festival' strawberries harvested in 2010 were three times lower than those

) is threefold higher than that in tomato. (4.1 ng g<sup>−</sup><sup>1</sup>

. This difference is largely influenced by the geno-

**3. Concentrations of melatonin in horticultural crops and its** 

for the study of melatonin-related functions in the future.

people can get more natural melatonin from daily food.

outflow,

) [19].

*Review of Melatonin in Horticultural Crops DOI: http://dx.doi.org/10.5772/intechopen.90935*

*Melatonin - The Hormone of Darkness and Its Therapeutic Potential and Perspectives*

for the application of melatonin in horticultural crop production.

**2. The biosynthetic pathway of melatonin in plants**

(CAMT) to produce melatonin (**Figure 1**).

crops, and looked forward to the future research to provide some theoretical basis

Melatonin is a small molecule, which can shuttle freely in and between cells due to its hydrophilic and lipophilic molecular structure [3]. Using St. John's wort (*Hypericum perforatum* L. cv. Anthos) seedlings as the plant material, Murch et al. [4] exogenously supplied 14C labeled tryptophan and found the presence of radioactive tryptamine, 5-hydroxytryptophan, serotonin, indoleacetic acid and melatonin via adopted isotope tracer approach. Among them, tryptophan and 5-hydroxytryptophan are the synthetic precursors of melatonin in animals. The study indicated that the biosynthetic pathway of melatonin was similar and conserved from animals to plants. Nowadays, the biosynthesis of melatonin using l-tryptophan as substrate has been evidenced in plants with the following steps [5]: l-tryptophan is decarboxylated under the catalysis of l-tryptophan decarboxylase (TDC) to form tryptamine; tryptophan reacts further with tryptophan 5-hydroxylase (T5H) to produce serotonin; serotonin-*N*-acetyltransferase (SNAT) catalyzes the production of *N*-acetylserotonin; and *N-*acetylserotonin can be further catalyzed by *N*-acetylserotonin *O*-methyltransferase (ASMT) or caffeic acid *O*-methyltransferase

At present, key genes of melatonin synthesis, *TrpDC*, *T5H*, *AcSNMT* and *HIOMT*, have been successfully cloned from plants and their expression patterns have also been carried out [6]. In recent years, plant chloroplasts and mitochondria are considered as the key organelles of the melatonin synthesis [7]. The hypothesis to establish the basis for melatonin is an important biological antioxidant, and chloroplasts and mitochondria are main reactive oxygen species (ROS) sources; these two organelles generate large amounts of melatonin, which is used to remove

*A comparison of the biosynthetic pathways of melatonin in animals and in plants. Black occurs only in* 

*animals; green occurs only in plants; and red occurs in both animals and plants.*

**108**

**Figure 1.**

excess ROS to reduce the oxidation of cells. However, a recent study in rice (*Oryza sativa* L.) has shown that the key melatonin synthesis gene *COMT* is located in plant chloroplasts and has demonstrated that melatonin synthesis is increased through the 5-MT pathway [8]. CAND2/PMTR1, a hypothetic plant melatonin receptor, recently has been identified in *Arabidopsis thaliana* [9]. It is located in the plasma membrane with a receptor-like topology and interacts with the G-protein subunit (GPA1), while expression in different tissues is induced by melatonin. The binding of plant melatonin receptors triggers Gγb and Gα activation of NADPH oxidasedependent H2O2 production (RBOH), increasing Ca2+ inflow, promoting K+ outflow, and ultimately leading to stomatal closure [10]. The cloning of these melatoninmetabolizing genes and the discovery of the receptors can lay a certain foundation for the study of melatonin-related functions in the future.
