In recent years melatonin has emerged as a research highlight in plant studies. Melatonin has different functions in many aspects of plant growth and development. The most frequently mentioned functions of melatonin are related to abiotic stresses such as drought, radiation, extreme temperature, and chemical stresses. This review mainly focuses on the regulatory effects of melatonin when plants face harsh environmental conditions. Evidence indicates that environmental stress can increase the level of endogenous melatonin in plants. Overexpression of the melatonin biosynthetic genes elevates melatonin levels in transgenic plants. The transgenic plants show enhanced tolerance to abiotic stresses. Exogenously applied melatonin can also improve the ability of plants to tolerate abiotic stresses. The mechanisms by which melatonin alleviates abiotic stresses are discussed.

Roles of melatonin in abiotic stress resistance in plants

Melatonin is a tryptophan-derived molecule with pleiotropic activities. It is present in almost all or all organisms. Its synthetic pathway depends on the species in which it is measured. For example, the tryptophan to melatonin pathway differs in plants and animals. It is speculated that the melatonin synthetic machinery in eukaryotes was inherited from bacteria as a result of endosymbiosis. However, melatonin's synthetic mechanisms in microorganisms are currently unknown. Melatonin metabolism is highly complex with these enzymatic processes having evolved from cytochrome C. In addition to its enzymatic degradation, melatonin is metabolized via pseudoenzymatic and free radical interactive processes. The metabolic products of these processes overlap and it is often difficult to determine which process is dominant. However, under oxidative stress, the free radical interactive pathway may be featured over the others. Because of the complexity of the melatonin degradative processes, it is expected that additional novel melatonin metabolites will be identified in future investigations. The original and primary function of melatonin in early life forms such as in unicellular organisms was as a free radical scavenger and antioxidant. During evolution, melatonin was selected as a signaling molecule to transduce the environmental photoperiodic information into an endocrine message in multicellular organisms and for other purposes as well. As an antioxidant, melatonin exhibits several unique features which differ from the classic antioxidants. These include its cascade reaction with free radicals and its capacity to be induced under moderate oxidative stress. These features make melatonin a potent endogenously-occurring antioxidant that protects organisms from catastrophic oxidative stress.

https://www.mdpi.com/1420-3049/20/10/18886/pdf

Melatonin as a Potent and Inducible Endogenous Antioxidant: Synthesis and Metabolism.

Insomnia is a serious worldwide health threat, affecting nearly one third of the general population. Melatonin has been reported to improve sleep efficiency and it was found that eating melatonin-rich foods could assist sleep. During the last decades, melatonin has been widely identified and qualified in various foods from fungi to animals and plants. Eggs and fish are higher melatonin-containing food groups in animal foods, whereas in plant foods, nuts are with the highest content of melatonin. Some kinds of mushrooms, cereals and germinated legumes or seeds are also good dietary sources of melatonin. It has been proved that the melatonin concentration in human serum could significantly increase after the consumption of melatonin containing food. Furthermore, studies show that melatonin exhibits many bioactivities, such as antioxidant activity, anti-inflammatory characteristics, boosting immunity, anticancer activity, cardiovascular protection, anti-diabetic, anti-obese, neuroprotective and anti-aging activity. This review summaries the dietary sources and bioactivities of melatonin, with special attention paid to the mechanisms of action.

https://www.mdpi.com/2072-6643/9/4/367/pdf

Dietary Sources and Bioactivities of Melatonin.

Melatonin is a phylogenetically ancient molecule. It is ubiquitously present in almost all organisms from primitive photosynthetic bacteria to humans. Its original primary function is presumable to be that of an antioxidant with other functions of this molecule having been acquired during evolution. The synthetic pathway of melatonin in vertebrates has been extensively studied. It is common knowledge that serotonin is acetylated to form N-acetylserotonin by arylalkylamine N-acetyltransferase (AANAT) or arylamine N-acetyltransferase (SNAT or NAT) and N-acetylserotonin is, subsequently, methylated to melatonin by N-acetylserotonin O-methyltransferase (ASMT; also known as hydroxyindole-O-methyltransferase, HIOMT). This is referred to as a classic melatonin synthetic pathway. Based on new evidence, we feel that this classic melatonin pathway is not generally the prevailing route of melatonin production. An alternate pathway is known to exist, in which serotonin is first O-methylated to 5-methoxytryptamine (5-MT) and, thereafter, 5-MT is N-acetylated to melatonin. Here, we hypothesize that the alternate melatonin synthetic pathway may be more important in certain organisms and under certain conditions. Evidence strongly supports that this alternate pathway prevails in some plants, bacteria, and, perhaps, yeast and may also occur in animals.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/jpi.12336

On the significance of an alternate pathway of melatonin synthesis via 5-methoxytryptamine: comparisons across species

Melatonin, a derivative of tryptophan, was first detected in plant species in 1995 and it has been shown to be a diverse regulator during plant growth and development, and in stress responses. Recently, great progress has been made towards determining the detailed functions of melatonin in plant responses to abiotic stress. Melatonin priming improves plant tolerance to cold, heat, salt, and drought stresses through regulation of genes involved in the DREB/CBF, HSF, SOS, and ABA pathways, respectively. As a scavenger of free radicals, melatonin also directly detoxifies reactive oxygen species, thus alleviating membrane oxidation. Abiotic stress-inhibited photosynthesis is partially recovered and metabolites accumulate in the presence of melatonin, leading to improved plant growth, delayed leaf senescence, and increased stress tolerance. In this review, we summarize the interactions of melatonin with phytohormones to regulate downstream gene expression, protein stabilization, and epigenetic modification in plants. Finally, we consider the need for, and approaches to, the identification of melatonin receptors and components during signaling transduction pathways.

/pdf/phytomelatonin-a-universal-abiotic-stress-regulator-1jxx8ivem0.pdf

Phytomelatonin: A universal abiotic stress regulator

In this study, the effect of baking conditions and dough formulations on phenolic compounds, antioxidant capacity and color of cookies made from anthocyanin-rich corn flour were investigated. The control cookies were prepared from dark-red popping corn, blue popping corn and blue-standard corn flour according AACC method 10–54 and baked at 200 °C for 7 and 10 min. In control corn cookies, the content of total flavonoids and anthocyanins, as free water-soluble phenolic compounds, was reduced by applied baking conditions. Our results show that citric acid significantly increased the total flavonoids and anthocyanins content in the cookies prepared from blue popping corn and blue-standard corn. Compared with the control cookies baked at 200 °C for 7 min, the cookies prepared with 0.5 g/100 g added citric acid and baked at 150 °C for 12 min had higher contents of total flavonoids and anthocyanins by 60 and 70%, respectively. However, as a result of Maillard reaction inhibition at low pH, antioxidant capacity of anthocyanins-rich blue popping corn and blue standard corn cookies with 0.5 and 1 g/100 g citric acid was lower by 56 and 39%, respectively, compared to antioxidant capacity of their control cookies baked at 200 °C for 10 min.

Effects of baking conditions and dough formulations on phenolic compound stability, antioxidant capacity and color of cookies made from anthocyanin-rich corn flour

Determination of melatonin and its isomer in foods by liquid chromatography tandem mass spectrometry.

Model studies on the role of 5-hydroxymethyl-2-furfural in acrylamide formation from asparagine

Coffee, as a source of acrylamide, needs to be investigated in depth to understand the contribution of different precursors. This study aimed to investigate the contributions of sucrose decomposition and lipid oxidation on acrylamide formation in coffee during roasting. Coffee beans and model systems were used to monitor the accumulation of neo-formed carbonyls during heating through sucrose decomposition and lipid oxidation. High resolution mass spectrometry analyses confirmed the formation of 5-hydroxymethylfurfural (HMF) and 3,4-dideoxyosone, which were identified as the major sugar decomposition products in both roasted coffee and model systems. Among others, 2-octenal, 2,4-decadienal, 2,4-heptadienal, 4-hydroxynonenal, and 4,5-epoxy-2-decenal were identified in relatively high quantities in roasted coffee. Formation and elimination of HMF in coffee during roasting had a kinetic pattern similar to those of acrylamide. Its concentration rapidly increased within 10 min followed by an exponential decrease afterward. The amount of lipid oxidation products tended to increase linearly during roasting. It was concluded from the results that roasting formed a pool of neo-formed carbonyls from sucrose decomposition and lipid oxidation, and they play certain role on acrylamide formation in coffee.

Tolgahan Kocadağlı

Papers

Effects of baking conditions and dough formulations on phenolic compound stability, antioxidant capacity and color of cookies made from anthocyanin-rich corn flour

Determination of melatonin and its isomer in foods by liquid chromatography tandem mass spectrometry.

Model studies on the role of 5-hydroxymethyl-2-furfural in acrylamide formation from asparagine

In depth study of acrylamide formation in coffee during roasting: role of sucrose decomposition and lipid oxidation

Multiresponse kinetic modelling of Maillard reaction and caramelisation in a heated glucose/wheat flour system.