Melatonin content of pepper and tomato fruits: effects of cultivar and solar radiation

Presence of melatonin in foods of daily consumption: The benefit of this hormone for health

Melatonin (MLT) is a hormone that exists in all living organisms, including bacteria, yeast, fungi, animals, and plants, many of which are ingested daily in the diet. However, the exact concentrations of melatonin in each of the foods and the effect on health of the intake of foods rich in MLT are not known. Therefore, the aim of this review was to gather the available information on the melatonin content of different foods and to evaluate the effect that this hormone has on different pathologies. The amount of MLT may vary depending on the variety, origin, heat treatment, processing, and analysis technique, among other factors. Dietary interventions with foods rich in MLT report health benefits, but there is no evidence that hormone is partially responsible for the clinical improvement. Therefore, it is necessary to evaluate the MLT content in more foods, as well as the effect that cooking/processing has on the amount of MLT, to estimate its total intake in a typical diet and better explore its potential impact on the health.

An ecotype-specific effect of osmopriming and melatonin during salt stress in Arabidopsis thaliana

BACKGROUND

Natural populations of Arabidopsis thaliana exhibit phenotypic variations in specific environments and growth conditions. However, this variation has not been explored after seed osmopriming treatments. The natural variation in biomass production and root system architecture (RSA) was investigated across the Arabidopsis thaliana core collection in response to the pre-sawing seed treatments by osmopriming, with and without melatonin (Mel). The goal was to identify and characterize physiologically contrasting ecotypes.

RESULTS

Variability in RSA parameters in response to PEG-6000 seed osmopriming with and without Mel was observed across Arabidopsis thaliana ecotypes with especially positive impact of Mel addition under both control and 100 mM NaCl stress conditions. Two ecotypes, Can-0 and Kn-0, exhibited contrasted root phenotypes: seed osmopriming with and without Mel reduced the root growth of Can-0 plants while enhancing it in Kn-0 ones under both control and salt stress conditions. To understand the stress responses in these two ecotypes, main stress markers as well as physiological analyses were assessed in shoots and roots. Although the effect of Mel addition was evident in both ecotypes, its protective effect was more pronounced in Kn-0. Antioxidant enzymes were induced by osmopriming with Mel in both ecotypes, but Kn-0 was characterized by a higher responsiveness, especially in the activities of peroxidases in roots. Kn-0 plants experienced lower oxidative stress, and salt-induced ROS accumulation was reduced by osmopriming with Mel. In contrast, Can-0 exhibited lower enzyme activities but the accumulation of proline in its organs was particularly high. In both ecotypes, a greater response of antioxidant enzymes and proline accumulation was observed compared to mechanisms involving the reduction of Na+ content and prevention of K+ efflux.

CONCLUSIONS

In contrast to Can-0, Kn-0 plants grown from seeds osmoprimed with and without Mel displayed a lower root sensitivity to NaCl-induced oxidative stress. The opposite root growth patterns, enhanced by osmopriming treatments might result from different protective mechanisms employed by these two ecotypes which in turn result from adaptive strategies proper to specific habitats from which Can-0 and Kn-0 originate. The isolation of contrasting phenotypes paves the way for the identification of genetic factors affecting osmopriming efficiency.

Melatonin Combined with Wax Treatment Enhances Tolerance to Chilling Injury in Red Bell Pepper

Bell peppers (Capsicum annuum L.) are prone to chilling injury (CI) when stored at temperatures below 7 °C. Melatonin, a natural plant regulator, plays a critical role in defending against different pre- and post-harvest abiotic stresses, including those associated with cold storage. This study aimed to assess the effects of applying exogenous melatonin alone and in combination with a commercial wax on the CI tolerance, postharvest life, and potential biomarker search of red bell peppers. In the initial experiment, the effective melatonin concentration to reduce CI effects was determined. Peppers were sprayed with either distilled water (control) or a melatonin aqueous solution (M100 = 100 μM or M500 = 500 μM) and then stored for 33 d at 4 °C, followed by 2 d at 20 °C. The M500 treatment proved to be more effective in reducing fruit CI incidence (superficial scalds) and metabolic rate, while weight loss, softening, and color were comparable to the control. A second experiment assessed the potential synergistic effects of a combined melatonin and commercial wax treatment on pepper CI and quality. Fruits were sprayed with distilled water (control), melatonin (M500), commercial wax (Wax), or the combined treatment (Wax + M500) and stored for 28 d at 4 °C, followed by 2 d at 20 °C. The Wax + M500 was the most effective in significantly reducing the incidence of fruit CI symptoms and calyx fungal infection. Furthermore, this combined treatment enhanced fruit weight loss prevention compared with individual melatonin or wax treatment. Also, Wax + M500-treated peppers exhibited notable proline accumulation, indicative of a metabolic response counteracting the cold effects, resulting in better fruit stress acclimation. This treatment also preserved the peppers' color and antioxidant capacity. In summary, these findings highlight the suitability of applying a combined Wax + M500 treatment as a highly effective strategy to enhance the CI tolerance of peppers and extend their postharvest life.

Ascorbate peroxidase in fruits and modulation of its activity by reactive species

Ascorbate peroxidase (APX) is one of the enzymes of the ascorbate-glutathione cycle and is the key enzyme that breaks down H2O2 with the aid of ascorbate as an electron source. APX is present in all photosynthetic eukaryotes from algae to higher plants and, at the cellular level, it is localized in all subcellular compartments where H2O2 is generated, including the apoplast, cytosol, plastids, mitochondria, and peroxisomes, either in soluble form or attached to the organelle membranes. APX activity can be modulated by various post-translational modifications including tyrosine nitration, S-nitrosation, persulfidation, and S-sulfenylation. This allows the connection of H2O2 metabolism with other relevant signaling molecules such as NO and H2S, thus building a complex coordination system. In both climacteric and non-climacteric fruits, APX plays a key role during the ripening process and during post-harvest, since it participates in the regulation of both H2O2 and ascorbate levels affecting fruit quality. Currently, the exogenous application of molecules such as NO, H2S, H2O2, and, more recently, melatonin is seen as a new alternative to maintain and extend the shelf life and quality of fruits because they can modulate APX activity as well as other antioxidant systems. Therefore, these molecules are being considered as new biotechnological tools to improve crop quality in the horticultural industry.

Mediterranean Diet and Melatonin: A Systematic Review

The Mediterranean diet (MD) has beneficial effects on human health, which is evidenced by the observation of lower incidence rates of chronic diseases in Mediterranean countries. The MD dietary pattern is rich in antioxidants, such as melatonin, which is a hormone produced mainly by the pineal gland and controls several circadian rhythms. Additionally, melatonin is found in foods, such as fruit and vegetables. The purpose of this systematic review was to assess the melatonin content in Mediterranean foods and to evaluate the influence of the MD on melatonin levels in both humans and model organisms. A comprehensive search was conducted in four databases (PubMed, Scopus, Cochrane Library and Web of Science) and data were extracted. A total of 31 records were chosen. MD-related foods, such as tomatoes, olive oil, red wine, beer, nuts, and vegetables, showed high melatonin contents. The consumption of specific MD foods increases melatonin levels and improves the antioxidant status in plasma.

Melatonin in different food samples: Recent update on distribution, bioactivities, pretreatment and analysis techniques

Melatonin (MLT) plays a significant role on maintaining the basic physiological functions and regulating various metabolic processes in plentiful organisms. Recent years have witnessed an increase in MLT's share in global market with its affluent functions. However, the worrisome quality issues and inappropriate or excessive application of MLT take place inevitably. In addition, its photosensitive properties, oxidation, complex substrate concentration and trace levels leave exact detection of MLT doubly difficult. Therefore, it is essential to exploit precise, sensitive and stable extraction and detection methods to resolve above questions. In this study, we reviewed the distribution and bioactivities of MLT and conducted a comprehensive overview of the developments of pretreatment and analysis methods for MLT in food samples since 2010. Commonly used pretreatment methods for MLT include not only traditional techniques, but also novel ones, such as solid-phase extraction, QuEChERS, microextraction by packed sorbent, solid phase microextraction, liquid phase microextraction, and so on. Analysis methods include liquid chromatography coupled with different detectors, GC methods, capillary electrophoresis, sensors, and so on. The advantages and disadvantages of different techniques have been compared and the development tendency was prospected.

Interactions of melatonin, reactive oxygen species, and nitric oxide during fruit ripening: an update and prospective view

Fruit ripening is a physiological process that involves a complex network of signaling molecules that act as switches to activate or deactivate certain metabolic pathways at different levels, not only by regulating gene and protein expression but also through post-translational modifications of the involved proteins. Ethylene is the distinctive molecule that regulates the ripening of fruits, which can be classified as climacteric or non-climacteric according to whether or not, respectively, they are dependent on this phytohormone. However, in recent years it has been found that other molecules with signaling potential also exert regulatory roles, not only individually but also as a result of interactions among them. These observations imply the existence of mutual and hierarchical regulations that sometimes make it difficult to identify the initial triggering event. Among these 'new' molecules, hydrogen peroxide, nitric oxide, and melatonin have been highlighted as prominent. This review provides a comprehensive outline of the relevance of these molecules in the fruit ripening process and the complex network of the known interactions among them.

Is Melatonin the "Next Vitamin D"?: A Review of Emerging Science, Clinical Uses, Safety, and Dietary Supplements

Melatonin has become a popular dietary supplement, most known as a chronobiotic, and for establishing healthy sleep. Research over the last decade into cancer, Alzheimer's disease, multiple sclerosis, fertility, PCOS, and many other conditions, combined with the COVID-19 pandemic, has led to greater awareness of melatonin because of its ability to act as a potent antioxidant, immune-active agent, and mitochondrial regulator. There are distinct similarities between melatonin and vitamin D in the depth and breadth of their impact on health. Both act as hormones, affect multiple systems through their immune-modulating, anti-inflammatory functions, are found in the skin, and are responsive to sunlight and darkness. In fact, there may be similarities between the widespread concern about vitamin D deficiency as a "sunlight deficiency" and reduced melatonin secretion as a result of "darkness deficiency" from overexposure to artificial blue light. The trend toward greater use of melatonin supplements has resulted in concern about its safety, especially higher doses, long-term use, and application in certain populations (e.g., children). This review aims to evaluate the recent data on melatonin's mechanisms, its clinical uses beyond sleep, safety concerns, and a thorough summary of therapeutic considerations concerning dietary supplementation, including the different formats available (animal, synthetic, and phytomelatonin), dosing, timing, contraindications, and nutrient combinations.

Melatonin mediated activation of MAP kinase pathway may reduce DNA damage stress in plants: A review

Melatonin is an important biomolecule found in diverse groups of organisms. Under different abiotic stresses, the synthesis of melatonin is markedly increased suggesting pivotal roles of melatonin in plants enduring stresses. Being an endogenous signaling molecule with antioxidant activity, melatonin alters many physiological responses and is found to be involved in regulating DNA damage responses. However, the molecular mechanisms of melatonin in response to DNA damage have not yet been studied. The present review aims to provide insights into the molecular mechanisms of melatonin in response to DNA damage in plants. We propose that the MAP kinase pathway is involved in regulating melatonin dependent response of plants under DNA damage stress. Where melatonin might activate MAPK via H₂ O₂ or Ca²⁺ dependent pathways. The activated MAPK in turn might phosphorylate and activate SOG1 and repressor type MYBs to mitigate DNA damage under abiotic stress.

Melatonin: First-line soldier in tomato under abiotic stress current and future perspective

Melatonin is a natural, multifunctional, nontoxic, regulatory, and ubiquitous biomolecule, having low molecular weight and pleiotropic effects in the plant kingdom. It is a recently discovered plant master regulator which has a crucial role under abiotic stress conditions (salinity, drought, heat, cold, alkalinity, acid rain, ozone, and metals stress). In the solanaceous family, the tomato is highly sensitive to abiotic stresses that affect its growth and development, ultimately hampering production and productivity. Melatonin acts as a strong antioxidant, bio-stimulator, and growth regulator, facilitating photosynthesis, delaying leaf senescence, and increasing the antioxidant enzymes system through direct scavenging of reactive oxygen species (ROS) under abiotic stresses. In addition, melatonin also boosts morphological traits such as vegetative growth, leaf photosynthesis, root architecture system, mineral nutrient elements, and antioxidant activities in tomato plants, confirming their tolerances against salinity, drought, heat, cold, alkalinity, acid rain, chemical, pathogen, and metals stress. In this review, an attempt has been made to summarize the potential role of melatonin for tomato plant endurance towards abiotic stresses, along with the known relationship between the two.

Feasibility of using melatonin content in pepper (Capsicum annuum) seeds as a physiological marker of chilling stress tolerance

The presence of melatonin, a known animal hormone, has been confirmed in many evolutionary distant organisms, including higher plants. It is known that melatonin increases tolerance to stress factors as a wide spectrum antioxidant. Tolerant genotypes have generally higher melatonin content than sensitive ones, and exposure to stressful conditions is known to increase endogenous melatonin levels. However, endogenous melatonin levels in seeds have never been used to select genotypes tolerant to abiotic stresses. Thus, in this study, the existence of possible relationship between seed melatonin levels of 28 pepper (Capsicum annuum L.) genotypes and their germination and emergence performance under chilling conditions (15°C) was investigated. The results indicated that these parameters were much better for pepper genotypes with higher seed melatonin contents while those having less than 2ngg-1 additionally exhibited elevated levels of MDA and H2 O2 but lower antioxidant enzyme activities. Thus, a positive relationship between seed melatonin content and chilling stress tolerance has been shown, suggesting a possible use of endogenous melatonin levels as a criterion in selecting chilling stress tolerant varieties. To save considerable time, money and labour, it is recommended that genotypes with lower melatonin contents are excluded from breeding programmes that aim to develop new stress tolerant genotypes.

The role of melatonin in tomato stress response, growth and development

Melatonin has attracted widespread attention after its discovery in higher plants. Tomato is a key model economic crop for studying fleshy fruits. Many studies have shown that melatonin plays important role in plant stress resistance, growth, and development. However, the research progress on the role of melatonin and related mechanisms in tomatoes have not been systematically summarized. This paper summarizes the detection methods and anabolism of melatonin in tomatoes, including (1) the role of melatonin in combating abiotic stresses, e.g., drought, heavy metals, pH, temperature, salt, salt and heat, cold and drought, peroxidation hydrogen and carbendazim, etc., (2) the role of melatonin in combating biotic stresses, such as tobacco mosaic virus and foodborne bacillus, and (3) the role of melatonin in tomato growth and development, such as fruit ripening, postharvest shelf life, leaf senescence and root development. In addition, the future research directions of melatonin in tomatoes are explored in combination with the role of melatonin in other plants. This review can provide a theoretical basis for enhancing the scientific understanding of the role of melatonin in tomatoes and the improved breeding of fruit crops.

Melatonin triggers tissue-specific changes in anthocyanin and hormonal contents during postharvest decay of Angeleno plums

Plum is a stone fruit that stands out for having a short shelf-life because of its high susceptibility to rapid deterioration. Part of this deterioration is explained by fruit overripening. Recently, the role of melatonin in delaying postharvest decay has been investigated but its regulatory function during overripening is still under extensive debate. In this study, to understand physiological events taking place in plums overripening and elucidate the role of melatonin on the postharvest quality of these fruits and its relationship to other plant hormones, Angeleno plums were sprayed with 10^-4 M of melatonin solution immediately after harvest. We carried out tissue-specific (mesocarp and exocarp) analysis of total phenols and anthocyanin quantification, as well as the evaluation of different phytohormones by LC-MS/MS and fruit quality parameters. Results showed that during postharvest, endogenous melatonin contents decreased both in the mesocarp and the exocarp of Angeleno plums. Likewise, plum firmness also decreased and a strong correlation was found for this parameter with jasmonic acid (JA) and cytokinins. Conversely, after exogenous melatonin application, endogenous melatonin content increased both in mesocarp and exocarp but it had a differential effect depending on the plum tissue. Indeed, total phenol and anthocyanin contents arose by 21% and 58%, respectively, in the mesocarp after melatonin treatment but no variations were found in the exocarp of Angeleno plums. Hormonal analysis of Angeleno mesocarp also revealed an increase in the JA and its precursor, 12-oxo-phytodienoic acid (OPDA), on the fourth day after melatonin application as well as a positive correlation between melatonin and gibberellin 1 (GA₁). These results suggest that melatonin may be acting as a signal molecule increasing phenolic compounds contents through direct regulation and by signaling with other phytohormones. Therefore, this research provides valuable information for understanding the regulatory role of melatonin and its relationship with plant hormones during overripening to contribute to improve the postharvest quality of plums.

Extraction and quantification of melatonin in cornelian cherry (Cornus mas L.) by ultra-fast liquid chromatography coupled to fluorescence detector (UFLC-FD)

Wild edible plants (WEPs) can be widely found in the world and defined as native species that grow naturally in their natural habitat. They have become part of the traditional food as human diet and used in folk medicine to treat diseases. They are very rich in terms of nutraceuticals. Melatonin is a natural hormone providing several benefits for human health. It has functions such as regulating growth and development and increasing tolerance to environmental stress factors in plants. It is stated that the serum melatonin level in humans increases after intake of foods containing melatonin. This study examined the presence of melatonin in wild grown cornelian cherry fruits by UFLC-FD and determined suitable extraction and chromatographic conditions. The optimum mobile phase, excitation/emission wavelength, and extraction solvent were determined as methanol: water: acetic acid, 275/345 nm, and methanol: water: HCl, respectively. Melatonin content in fruits ranged from 130.82 to 201.84 ng g−1 in fresh fruit.

Foliar melatonin stimulates cotton boll distribution characteristics by modifying leaf sugar metabolism and antioxidant activities during drought conditions

Drought is a severe abiotic stress affecting the plant's antioxidant system and interrupting compatible solute translocation processes, which leads to low productivity. Melatonin acts as a common growth regulator enhancing the plants defense system and regulates sugar metabolism in challenging environments. Melatonin treatments enable plants to be tolerant to abiotic stresses via enhancing their recovery potential, but its impact using various concentrations has not yet been studied in leaf physiological aspects when applied to cotton foliage during their peak flowering and boll loading stage. The overall objective of this research was to facilitate cotton boll distribution characteristics by modifying cotton leaf sugar metabolism and antioxidant activity by applying foliar melatonin (0, 25, 50, and 100 μmol l^-1 ) under drought levels with a relative soil water content of 75%, 60%, and 45% ± 5 (FC1, FC2, and FC3, respectively). Higher rates of melatonin application (100 μmol l^-1 ) enhanced boll distribution characteristics and controlled the boll shedding rate during drought conditions. An increase in melatonin rates proved to be more helpful in stimulating cotton sympodial leaf physiological attributes, including leaf gas exchange parameters, sugar metabolism, proline content, and antioxidants defense system as compared with less or no melatonin application during all FC conditions and showed the most significant effect at a higher melatonin concentration (M100) at 7-21 DAF. The total proline content and antioxidant activity were enhanced in the M100 treatment during all FC levels, which caused a reduction in the total malondialdehyde (MDA) contents and hydrogen peroxide (H₂ O₂ ) concentrations in cotton leaves. Moreover, sugar metabolism responsible genes GhSusA and SPS2 showed an upsurge in expression levels and enhanced sucrose degradation in M100 treatments during all FC levels. Furthermore, cotton boll attributes showed also a positive relation with leaf physiological and gas exchange attributes. The results suggested that foliar melatonin application during the flowering initiation stage improved the overall performance and is helpful for cotton crops productivity against drought stress.

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

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.

Melatonin in plants: what we know and what we don’t

Melatonin is an endogenous micromolecular compound of indoleamine with multiple physiological functions in various organisms. In plants, melatonin is involved in growth and development, as well as in responses to biotic and abiotic stresses. Furthermore, melatonin functions in phytohormone-mediated signal transduction pathways. There are multiple melatonin biosynthesis pathways, and the melatonin content in plants is greatly affected by intrinsic genetic characteristics and external environmental factors. Although melatonin biosynthesis has been extensively studied in model plants, it remains uncharacterized in most plants. This article focuses on current knowledge on the biosynthesis, regulation and application of melatonin, particularly for fruit quality and preservation. In addition, it highlights the links between melatonin and other hormones, as well as future research directions.

Mitigation of salt stress response in upland cotton (Gossypium hirsutum) by exogenous melatonin

As a pleiotropic signal molecule, melatonin is ubiquitous throughout the animal and plant kingdoms and plays important roles in the regulation of plant growth, development, and responses to environmental stresses. In this study, we quantified the endogenous melatonin levels in upland cotton (Gossypium hirsutum L.), using high-performance liquid chromatography-tandem mass spectrometry. The melatonin concentrations in root, stem, and leaf were 150.60, 37.92, and 40.58 ng g fresh weight^- 1, respectively. The effects of exogenous melatonin (1 µM) on plant growth, photosynthesis, antioxidant enzyme activity, and ion homeostasis in upland cotton seedlings exposed to 100 mM NaCl treatment were determined. Pretreatment (prior to exposure to salt stress) of seedlings with exogenous melatonin significantly alleviated plant growth inhibition by salt stress and maintained an improved photosynthetic capacity. The application of melatonin also significantly reduced the salt-induced oxidative damage, possibly through the accumulation of osmotic regulatory substances and the activation of antioxidant enzymes. We also showed that exogenous melatonin regulated the expression of stress-responsive and ion-channel genes under salinity, which could contribute to improved salt tolerance in cotton. Taken together, our study provides evidence that cotton contains endogenous melatonin, and it may have unraveled crucial evidence of the role of melatonin in cotton against salt stress.

Melatonin Improves Cotton Salt Tolerance by Regulating ROS Scavenging System and Ca2 + Signal Transduction

As one of the cash crops, cotton is facing the threat of abiotic stress during its growth and development. It has been reported that melatonin is involved in plant defense against salt stress, but whether melatonin can improve cotton salt tolerance and its molecular mechanism remain unclear. We investigated the role of melatonin in cotton salt tolerance by silencing melatonin synthesis gene and exogenous melatonin application in upland cotton. In this study, applicating of melatonin can improve salt tolerance of cotton seedlings. The content of endogenous melatonin was different in cotton varieties with different salt tolerance. The inhibition of melatonin biosynthesis related genes and endogenous melatonin content in cotton resulted in the decrease of antioxidant enzyme activity, Ca²⁺ content and salt tolerance of cotton. To explore the protective mechanism of exogenous melatonin against salt stress by RNA-seq analysis. Melatonin played an important role in the resistance of cotton to salt stress, improved the salt tolerance of cotton by regulating antioxidant enzymes, transcription factors, plant hormones, signal molecules and Ca²⁺ signal transduction. This study proposed a regulatory network for melatonin to regulate cotton's response to salt stress, which provided a theoretical basis for improving cotton's salt tolerance.

Phytomelatonin: An overview of the importance and mediating functions of melatonin against environmental stresses

Recently, melatonin has gained significant importance in plant research. The presence of melatonin in the plant kingdom has been known since 1995. It is a molecule that is conserved in a wide array of evolutionary distant organisms. Its functions and characteristics have been found to be similar in both plants and animals. The review focuses on the role of melatonin pertaining to physiological functions in higher plants. Melatonin regulates physiological functions regarding auxin activity, root, shoot, and explant growth, activates germination of seeds, promotes rhizogenesis (growth of adventitious and lateral roots), and holds up impelled leaf senescence. Melatonin is a natural bio-stimulant that creates resistance in field crops against various abiotic stress, including heat, chemical pollutants, cold, drought, salinity, and harmful ultra-violet radiation. The full potential of melatonin in regulating physiological functions in higher plants still needs to be explored by further research.

Production system influences tomato phenolics and indoleamines in a cultivar-specific manner

Tomato (Solanum lycopersicum) fruit is a rich source of health-promoting compounds, and epidemiological studies show that tomato consumption may reduce the risk of chronic diseases. This study compared the effect of genotype, production system, and their interaction on eight tomato varieties grown in the open-field (OF) or net-house (NH), a structure completely covered with a 50-mesh screen to reduce pest and wind damage, in South Texas. The NH structure reduced solar radiation up to ~30% and decreased wind speed by 6.44 km/h compared with conditions measured in the OF. We simultaneously analyzed 16 phenolics and indoleamines using ultra-high-performance liquid chromatography coupled with quadrupole time-of-flight high-resolution mass spectrometry (UHPLC/ESI-HR-QTOFMS). The chemometric analysis showed a distinct difference between NH- and OF-grown tomatoes irrespective of the variety. The melatonin and serotonin contents showed a cultivar-specific effect of the production system. Likewise, the effect of cultivation systems on levels of phenolic acids and flavonoids varied based on tomato cultivar. Among the studied phenolic acids, significantly enhanced levels of sinapic acid were observed in OF-grown tomatoes. Similarly, we detected a considerable genotypic effect on gallic acid, p-coumaric acid, ferulic acid, sinapic acid, and naringin. The interaction of cultivar and production system substantially affected gallic acid, protocatechuic acid, sinapic acid, and apigenin. However, further studies need to be performed to explore the environment-specific effects on the total composition. In summary, our results indicate that the production system plays an important role in tomato composition beyond the natural genetic variation among cultivars.

Culinary–medicinal mushrooms: a review of organic compounds and bioelements with antioxidant activity

There are about 3000 species of mushrooms, which have a high amount of substances that are beneficial to human health, such as antioxidants. It is well known that oxidative stress plays an important role in the etiopathogenesis of many diseases, including cancer, cardiovascular disorders, and diseases of the central nervous system. One way to prevent homeostasis disorders that occur as a result of excessive production of pro-oxidative substances is to include the ingredients having antioxidant properties in the diet. Several compounds, such as those with phenolic and indole derivatives as well as carotenoids and some vitamins, exhibit antioxidant activity. These substances are present in many foods, including mushrooms. In addition, they have certain unique compounds that are not found in other sources (e.g., norbadione A). The present work discusses selected ingredients exhibiting antioxidant activity, which are found in various species of mushrooms as wells as describes the content of these compounds in the extracts obtained from mushrooms using artificial digestive juice.

Melatonin rich foods in our diet: food for thought or wishful thinking?

Melatonin continues to generate interest in the scientific community and the general public. In recent years, there has been growing interest in the possibility that melatonin present in human foods may have physiological effects. This has led to the promotion of "melatonin-rich" foods and "phyto-melatonin". The night time secretion of endogenous melatonin from the pineal gland provides a daily circadian signal which is detected by receptors in various tissues. In animals the changing circadian pattern of melatonin secretion across seasons is important to them to program their reproductive behaviours to ensure optimal reproductive success, while in humans it probably plays a prominent role in anchoring sleep to the night period. When melatonin is administered in non-physiological, milligram amounts to humans, the onset of sleep can be manipulated and in larger doses anti-oxidant properties may emerge. Melatonin-rich foods are considered in this context too, but the question remains whether the amounts of melatonin in the food can be expected to be high enough to realistically change sleep or have antioxidant properties. In this review, papers reporting the effects of ingestion of melatonin-rich food on plasma or saliva melatonin or its urinary metabolite are critically evaluated. Unfortunately many of the papers are compromised by poor experimental design and assay methodologies and uncritical evaluation of results. The conclusion drawn from this review is that it is wishful thinking to expect that the amount of melatonin in "melatonin-rich" foods will impact on sleep or have any other physiological impact.

The growth and development of cress (Lepidium sativum) affected by blue and red light

Today, the use of light emitting diodes (LEDs) are rapidly increasing in horticulture industry as a result of technological advancements. Lighting systems play an important role in the commercial greenhouse productions. As an artificial source of light, LED lamps can contribute to the better and faster growth of horticulture products such as vegetables grown in greenhouses. In this study, the effects of red and blue light spectrums were implemented and performed as a pot experiment under the cultivation-without-soil condition in greenhouse based on a completely random plan with 3 lighting treatments including natural light (control), 60% red light +40% blue light (60R:40B), and 90% red light +10% blue light (90R:10B), repeated 3 times. The results showed that the application of blue and red lights affected the fresh and dry weights of cress as well as its biomass, demonstrating a considerable increase compared to the plants grown under natural sunlight condition. In this regard, the fresh weight of the plant under the 60R:40B treatment had 57.11% increase compared to the natural light treatment. Compared to the control sample, the dry weight had 26.06% increase under 90R:10B treatment. The highest extent of biomass was observed under the 60R:40B lighting treatment with a value of 1.51 (g per kg dry weight of the plant), which was a 68.87% increase compared to the natural light treatment. Under the 60R:40B treatment, cress had its highest length at 19.76 cm. Under the similar treatment, the cress leaf had a total area of 56.78 cm² which was the largest. The stem diameter and the number of leaves under the 60R:40B treatment had their highest values at 3.28 mm and 8.16, respectively. Accordingly, a growing trend was observed with 56.7 and 61.27% increase compared to the control treatment. Furthermore, the biochemical traits of cress, the amount of a, b and total chlorophyll, the amount of anthocyanin and phenolic contents under the application of red and blue light were at their highest values compared to the control treatment. The highest amount of chlorophyll was observed under 60R:40B treatment as 15.09 mg g⁻¹ FW leaf. Moreover, the phenolic contents and the amount of anthocyanin were of significant difference at 1% level of likelihood compared to the control treatment. Therefore, the vegetative growth of cress was substantially affected by red and blue lights, resulting in the enhancement of the plant's biochemical features compared to control condition via adjusting the lighting quality and impacts of each red and blue light spectrum on their specific receptors. As a result, the presence of both lighting spectrums is essential for expanding and increasing the quality of the plant. At the large scale, this technology is capable of improving the commercial greenhouse production performance while helping farmers achieve maximum products. This particular combination of lights is one of the beneficial features of LED lighting systems intended for different types of commercial greenhouse productions, especially the valuable greenhouse products.

Melatonin Mediates Enhancement of Stress Tolerance in Plants

Melatonin is a multifunctional signaling molecule, ubiquitously distributed in different parts of plants and responsible for stimulating several physiological responses to adverse environmental conditions. In the current review, we showed that the biosynthesis of melatonin occurred in plants by themselves, and accumulation of melatonin fluctuated sharply by modulating its biosynthesis and metabolic pathways under stress conditions. Melatonin, with its precursors and derivatives, acted as a powerful growth regulator, bio-stimulator, and antioxidant, which delayed leaf senescence, lessened photosynthesis inhibition, and improved redox homeostasis and the antioxidant system through a direct scavenging of reactive oxygen species (ROS) and reactive nitrogen species (RNS) under abiotic and biotic stress conditions. In addition, exogenous melatonin boosted the growth, photosynthetic, and antioxidant activities in plants, confirming their tolerances against drought, unfavorable temperatures, salinity, heavy metals, acid rain, and pathogens. However, future research, together with recent advancements, would support emerging new approaches to adopt strategies in overcoming the effect of hazardous environments on crops and may have potential implications in expanding crop cultivation against harsh conditions. Thus, farming communities and consumers will benefit from elucidating food safety concerns.

Diffuse light affects the contents of vitamin C, phenolic compounds and free amino acids in lettuce plants

Enhancement of the diffuse solar radiation to which lettuce plants were exposed clearly affected the vitamin C content and the quantitative and qualitative patterns of phenolic compounds and free amino acids (AA) in the leaves. Although the enhanced level of diffuse light was detrimental to the contents of vitamin C and total phenolic compounds, lowering them by 10-46% and 8-11%, respectively, the content of di-caffeoyltartaric acid increased from 0.26 ± 0.19 to 0.52 ± 0.10 μmol 100 g-1 f.w. for plants harvested in summer. The effect of diffuse light on AA depended on the total amount of global radiation incident on the plants. Considering the lowest amount of global radiation, the enhanced diffuse light increased the AA content from 766 ± 89 to 849 ± 90 μmol 100 g-1 f.w. By contrast, under the highest level of global radiation, diffuse light decreased the amount of AA from 990 ± 16 to 830 ± 76 μmol 100 g-1 f.w.

Melatonin Is a Potential Target for Improving Post-Harvest Preservation of Fruits and Vegetables

Melatonin is a ubiquitous molecule distributed in nature and not only plays an important role in animals and humans but also has extensive functions in plants, such as delaying senescence, exerting antioxidant effects, regulating growth and development, and facilitating plant adaption to stress conditions. Endogenous melatonin is widespread in fruits and vegetables and plays prominent roles in the ripening and post-harvest process of fruits and vegetables. Exogenous application of melatonin removes excess reactive oxygen species from post-harvest fruits and vegetables by increasing antioxidant enzymes, non-enzymatic antioxidants, and enzymes related to oxidized protein repair. Moreover, exogenous application of melatonin can increase endogenous melatonin to augment its effects on various physiological processes. Many previous reports have demonstrated that application of exogenous melatonin improves the post-harvest preservation of fruits and vegetables. Although overproduction of melatonin in plants via transgenic approaches could be a potential means for improving the post-harvest preservation of fruits and vegetables, efforts to increase endogenous melatonin in plants are limited. In this review, we summarize the recent progress revealing the role and action mechanisms of melatonin in post-harvest fruits and vegetables and provide future directions for the utilization of melatonin to improve the post-harvest preservation of fruits and vegetables.

Impact of Salicylic Acid Content and Growing Environment on Phytoprostane and Phytofuran (Stress Biomarkers) in Oryza sativa L

Phytoprostanes (PhytoPs) and phytofurans (PhytoFs) are oxylipins synthesized by nonenzymatic peroxidation of α-linolenic acid. These compounds are biomarkers of oxidative degradation in plant foods. In this research, the effect of environment and supplementation with salicylic acid (SA) on PhytoPs and PhytoFs was monitored by ultra-high-performance liquid chromatography coupled to electrospray ionization and triple quadrupole mass spectrometry (UHPLC-ESI-QqQ-MS/MS) on seven rice genotypes from Oryza sativa L. subsp. japonica. The plastic cover environment and spray application with 1 and 15 mM SA produced a reduction in the concentration of most of these newly established stress biomarkers [9-F_1t-PhytoP, ent-16-F_1t-PhytoP, ent-16- epi-16-F_1t-PhytoP, 9-D_1t-PhytoP, 9- epi-9-D_1t-PhytoP, 16-B₁-PhytoP, 9-L₁-PhytoP, ent-16( RS)-9- epi-ST-Δ¹⁴-10-PhytoF, ent-9( RS)-12- epi-ST-Δ¹⁰-13-PhytoF, and ent-16( RS)-13- epi-ST-Δ¹⁴-9-PhytoF] by 60.7% on average. The modification observed in the level of PhytoPs and PhytoFs differed according to the specific oxylipins and genotype, demonstrating a close linkage between genetic features and resistance to abiotic stress, to some extent mediated by the sensitivity of plants to the plant hormone SA that participates in the physiological response of higher plants to stress. Thus, in plants exposed to stressing factors, SA contribute to modulating the redox balance, minimizing the oxidation of fatty acids and thus the syntheis of oxylipins. These results indicated that SA could be a promising tool for managing the thermotolerance of rice crop. However, it remains necessary to study the mechanism of action of PhytoPs and PhytoFs in biochemical processes related to the defense of plants and define their role as stress biomarkers through a nonenzymatic pathway.

Melatonin and Its Effects on Plant Systems

Melatonin (N-acetyl-5-methoxytryptamine) is a nontoxic biological molecule produced in a pineal gland of animals and different tissues of plants. It is an important secondary messenger molecule, playing a vital role in coping with various abiotic and biotic stresses. Melatonin serves as an antioxidant in postharvest technology and enhances the postharvest life of fruits and vegetables. The application of exogenous melatonin alleviated reactive oxygen species and cell damage induced by abiotic and biotic stresses by means of repairing mitochondria. Additionally, the regulation of stress-specific genes and the activation of pathogenesis-related protein and antioxidant enzymes genes under biotic and abiotic stress makes it a more versatile molecule. Besides that, the crosstalk with other phytohormones makes inroads to utilize melatonin against non-testified stress conditions, such as viruses and nematodes. Furthermore, different strategies have been discussed to induce endogenous melatonin activity in order to sustain a plant system. Our review highlighted the diverse roles of melatonin in a plant system, which could be useful in enhancing the environmental friendly crop production and ensure food safety.

Dietary Melatonin Supplementation Could Be a Promising Preventing/Therapeutic Approach for a Variety of Liver Diseases

In the therapeutic strategies, the role of diet is a well-established factor that can also have an important role in liver diseases. Melatonin, identified in animals, has many antioxidant properties and it was after discovered also in plants, named phytomelatonin. These substances have a positive effect during aging and in pathological conditions too. In particular, it is important to underline that the amount of melatonin produced by pineal gland in human decreases during lifetime and its reduction in blood could be related to pathological conditions in which mitochondria and oxidative stress play a pivotal role. Moreover, it has been indicated that melatonin/phytomelatonin containing foods may provide dietary melatonin, so their ingestion through balanced diets could be sufficient to confer health benefits. In this review, the classification of liver diseases and an overview of the most important aspects of melatonin/phytomelatonin, concerning the differences among their synthesis, their presence in foods and their role in health and diseases, are summarized. The findings suggest that melatonin/phytomelatonin supplementation with diet should be considered important in preventing different disease settings, in particular in liver. Currently, more studies are needed to strengthen the potential beneficial effects of melatonin/phytomelatonin in liver diseases and to better clarify the molecular mechanisms of action.

Melatonin and Expression of Tryptophan Decarboxylase Gene (TDC) in Herbaceous Peony (Paeonia lactiflora Pall.) Flowers

Melatonin is a bioactive, edible ingredient that promotes human health and exists widely in plants, but little is known about its biosynthetic routes and underlying molecular mechanisms in the herbaceous peony. In this contribution, we found that herbaceous peony flowers are rich in melatonin that is found in the greatest quantities in the white series, followed by the ink series, the red series and then the pink series. On this basis, the melatonin content fluctuates during flower development and peaks during the bloom stage. Moreover, it is apparent that sun exposure and blue light induce melatonin production whereas green light restrains it during a 24-h light/dark cycle of melatonin content, as there were ‘dual peaks’ at 2 p.m. and 2 a.m. Additionally, the corresponding expression pattern of the herbaceous peony tryptophan decarboxylase gene (TDC) was positively related with melatonin production. These results suggest that color series, development stage and light play an important role in melatonin accumulation, and that TDC is a rate-limiting gene in melatonin biosynthesis.

Tolerance to Stress Combination in Tomato Plants: New Insights in the Protective Role of Melatonin

Abiotic stresses such as drought, heat or salinity are major causes of yield loss worldwide. Recent studies have revealed that the acclimation of plants to a combination of different environmental stresses is unique and therefore cannot be directly deduced from studying the response of plants to each of the different stresses applied individually. The efficient detoxification of reactive oxygen species (ROS) is thought to play a key role in enhancing the tolerance of plants to abiotic stresses. Here, we report on the role of melatonin in the protection of the photosynthetic apparatus through the increase in ROS detoxification in tomato plants grown under the combination of salinity and heat, two of the most common abiotic stresses known to act jointly. Plants treated with exogenous melatonin showed a different modulation in the expression on some antioxidant-related genes and their related enzymes. More specifically, ascorbate peroxidase, glutathione reductase, glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase (APX, GR, GPX and Ph-GPX, resepctively) showed an antagonistic regulation as compared to plants that did not receive melatonin. This translated into a better antioxidant capacity and to a lesser ROS accumulation under stress combination. The performance of the photosynthesis parameters and the photosystems was also increased in plants treated with exogenous melatonin under the combination of salinity and heat. In accordance with these findings, tomato plants treated with melatonin were found to grow better under stress combination that the non-treated ones. Our study highlights the important role that exogenous melatonin plays in the acclimation of plants to a combination of two different abiotic stresses, and how this compound can specifically regulate oxidative stress-related genes and enzymes to increase plant tolerance.

Cadmium-induced melatonin synthesis in rice requires light, hydrogen peroxide, and nitric oxide: Key regulatory roles for tryptophan decarboxylase and caffeic acid O-methyltransferase

In plants, melatonin production is induced by stimuli such as cold and drought, and cadmium (Cd) is the best elicitor of melatonin production in rice. However, the mechanism by which Cd induces melatonin synthesis in plants remains unknown. We challenged rice seedlings with Cd under different light conditions and found that continuous light produced the highest levels of melatonin, while continuous dark failed to induce melatonin production. Transcriptional and translational induction of tryptophan decarboxylase contributed to the light induction of melatonin during Cd treatment, whereas the protein level of light-induced caffeic acid O-methyltransferase (COMT) was decreased by Cd treatment. In analogy, COMT enzyme activity was inhibited in vitro by Cd in a dose-dependent manner. Notably, the Cd-induced melatonin synthesis was significantly impaired by treatment with either an H₂ O₂ production inhibitor (DPI) or an NO scavenger (cPTIO). The combination of both inhibitors almost completely abolished Cd-induced melatonin synthesis, suggesting an absolute requirement for H₂ O₂ and NO. However, neither serotonin nor N-acetylserotonin (NAS) was induced by H₂ O₂ alone. In contrast, NO significantly induced serotonin production but not NAS or melatonin production. This indicated that serotonin did not enter chloroplasts, where serotonin N-acetyltransferase (SNAT) is constitutively expressed. This suggests that chloroplastidic SNAT expression prevents increased melatonin production after exposure to stress, ultimately leading to the maintenance of a steady-state melatonin level inside cells.

Dietary Sources and Bioactivities of Melatonin

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.

Melatonin and resveratrol reverse the toxic effect of high boron (B) and modulate biochemical parameters in pepper plants (Capsicum annuum L.)

The objectives of this research were to test a possible involvement of melatonin (MEL) and resveratrol (RES) in restoring growth and to control boron (B) toxicity in peppers. The plants were subjected to four different nutrient solution treatments as following: 1) half-strength Hoagland's nutrient solution (Control), 2) half-strength Hoagland's nutrient solution+100 μM B (100 μMB), 3) half-strength Hoagland's nutrient solution+100 μM boron+100 μMresveratrol (100 μMRES), and 4) half-strength Hoagland's nutrient solution+100 μM B+1 μMmelatonin (1 μM MEL). Pepper plants subjected to B excess (100 μM) for 68 days (d) exhibited visible B toxicity symptoms, reduced rate of photosynthesis (Pn) and reduced dry weight (DW), while their leaf and fruit had the greatest increase of B concentration. The reduction of photosynthesis was restored, the reduction of DW was prevented, while the B leaf and fruit accumulation was moderated with the application of both 100 μMresveratrol (RES) and 1 μMmelatonin (MEL). Moreover, plants exposed to MEL and/or RES displayed no visible B toxicity symptoms. The present study revealed a novel role of MEL and/or RES in the adaptation of pepper plants to B excess based on plant growth, physiological and biochemical criteria.

Nonlinear stabilization for a class of time delay systems via inverse optimality approach

This paper is devoted to obtain a stabilizing optimal nonlinear controller based on the well known Control Lyapunov-Krasovskii Functional (CLKF) approach, aimed to solve the inverse optimality problem for a class of nonlinear time delay systems. To determine sufficient conditions for the Bellman's equation solution of the system under consideration, the CLKF and the inverse optimality approach are considered in this paper. In comparison with previous results, this scheme allows us to obtain less conservative controllers, implying energy saving (in terms of average power consumption for a specific thermo-electrical process). Sufficient delay-independent criteria in terms of CLKF is obtained such that the closed-loop nonlinear time-delay system is guaranteed to be local Asymptotically Stable. To illustrate the effectiveness of the theoretical results, a comparative study with an industrial PID controller tuned by the Ziegler-Nichols methodology (Z-N) and a Robust-PID tuned by using the D-partition method is presented by online experimental tests for an atmospheric drying process with time delay in its dynamics.

Development and Validation of New Ultra-High-Performance Liquid Chromatography-Hybrid Quadrupole-Orbitrap Mass Spectrometry Method for Determination of Melatonin in Fruits

A new reliable analytical method based on ultra-high-performance liquid chromatography coupled with hybrid quadrupole-Orbitrap mass spectrometry was developed for the analysis of melatonin (MEL) in food products. In-house validation was performed using deuterated melatonin (MEL-d4) as an internal standard to ensure method selectivity and accuracy and to evaluate the efficiency of a robust ethyl acetate extraction technique used for sample preparation. The analysis of 18 tart cherry varieties and 28 tomato varieties was performed at optimized conditions. The method was linear (R(2)> 0.99) over the concentration range of 5-200 pg/g. A very low limit of quantification (10 pg/g) was provided for both analyzed matrices. The determined average recoveries (102 and 110%) and the values of intraday repeatability (6.30 and 10.9%) for cherry and tomato matrices, respectively, indicated a good accuracy and precision. The elaborated procedure proved the absence of MEL in any of tart cherries (<10 pg/g), whereas the concentration levels in tomatoes were found to be in the range of <10-149 pg/g, where the highest concentrations were determined in "Cherry," "Cherry Red" and "Rome" tomatoes grown in the Netherlands.

Effect of Cultivar, Temperature, and Environmental Conditions on the Dynamic Change of Melatonin in Mulberry Fruit Development and Wine Fermentation

High levels of melatonin have been reported in various foods but not in mulberry or its wine. This study investigated the dynamic changes of melatonin levels during mulberry fruit development and ethanol fermentation of 2 different colored mulberry cultivars ("Hongguo2ˮ Morus nigra, black and "Baiyuwangˮ Morus alba, white) at 2 fermentation temperatures (16 and 25 °C). Our results showed that the melatonin level increased in the beginning of mulberry development but decreased in the end. The MnTDC gene expression level correlated with melatonin production, which implied that TDC may be the rate-limiting enzyme of the melatonin biosynthetic process in mulberries. During mulberry fermentation, the melatonin concentration increased rapidly in the beginning and then decreased gradually. Low temperature delayed the melatonin production during fermentation. A relatively high level of melatonin was found in "Hongguo2ˮ compared with "Baiyuwangˮ during fruit development and fermentation. The variation of melatonin correlated with the ethanol production rate, suggesting that melatonin may participate in physiological regulation of Saccharomyces cerevisiae during the fermentation stage.

Melatonin: Current Status and Future Perspectives in Plant Science

Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous molecule with pleiotropic actions in different organisms. It performs many important functions in human, animals, and plants; these range from regulating circadian rhythms in animals to controlling senescence in plants. In this review, we summarize the available information regarding the presence of melatonin in different plant species, along with highlighting its biosynthesis and mechanisms of action. We also collected the available information on the effects of melatonin application on commercially important crops to improve their growth and development. Additionally, we have identified many new aspects where melatonin may have possible roles in plants, for example, its function in improving the storage life and quality of fruits and vegetables, its role in vascular reconnection during the grafting process and nutrient uptake from roots by modifying root architecture. Another potentially important aspect is the production of melatonin-rich food crops (cereals, fruits, and vegetables) through combination of conventional and modern breeding approaches, to increase plant resistance against biotic and abiotic stress, leading to improved crop yields, and the nutraceutical value of produce to solve food security issues.

Melatonin biosynthesis requires N-acetylserotonin methyltransferase activity of caffeic acid O-methyltransferase in rice

Caffeic acid O-methyltransferase (COMT) methylates N-acetylserotonin into melatonin; that is, it has N-acetylserotonin O-methyltransferase (ASMT) activity. The ASMT activity of COMT was first detected in Arabidopsis thaliana COMT (AtCOMT). To confirm the involvement of COMT on melatonin synthesis in other plant species, the ASMT activity of a COMT from rice (Oryza sativa) (OsCOMT) was evaluated. Purified recombinant OsCOMT protein from Escherichia coli was used to validate the high ASMT activity of OsCOMT, similar to that of AtCOMT. The K m and V max values for the ASMT activity of OsCOMT were 243 µM and 2400 pmol min(-1) mg protein(-1), which were similar to those of AtCOMT. Similar to AtCOMT, OsCOMT was localized in the cytoplasm. In vitro ASMT activity was significantly inhibited by either caffeic acid or quercetin in a dose-dependent manner. Analogously, in vivo production of melatonin was significantly inhibited by quercetin in 4-week-old detached rice leaves. Lastly, the transgenic rice plants overexpressing rice COMT showed an increase in melatonin levels whereas transgenic rice plants suppressing the rice COMT had a significant decrease on melatonin levels, suggestive of the direct role of COMT in melatonin biosynthesis in plants.

Chloroplastic and cytoplasmic overexpression of sheep serotonin N-acetyltransferase in transgenic rice plants is associated with low melatonin production despite high enzyme activity

Serotonin N-acetyltransferase (SNAT), the penultimate enzyme in melatonin biosynthesis, catalyzes the conversion of serotonin into N-acetylserotonin. Plant SNAT is localized in chloroplasts. To test SNAT localization effects on melatonin synthesis, we generated transgenic rice plants overexpressing a sheep (Ovis aries) SNAT (OaSNAT) in their chloroplasts and compared melatonin biosynthesis with that of transgenic rice plants overexpressing OaSNAT in their cytoplasm. To localize the OaSNAT in chloroplasts, we used a chloroplast targeting sequence (CTS) from tobacco protoporphyrinogen IX oxidase (PPO), which expresses in chloroplasts. The purified recombinant CTS:OaSNAT fusion protein was enzymatically functional and localized in chloroplasts as confirmed by confocal microscopic analysis. The chloroplast-targeted CTS:OaSNAT lines and cytoplasm-expressed OaSNAT lines had similarly high SNAT enzyme activities. However, after cadmium and butafenacil treatments, melatonin production in rice leaves was severalfold lower in the CTS:OaSNAT lines than in the OaSNAT lines. Notably, enhanced SNAT enzyme activity was not directly proportional to the production of N-acetylserotonin, melatonin, or 2-hydroxymelatonin, suggesting that plant SNAT has a role in the homeostatic regulation of melatonin rather than in accelerating melatonin synthesis.