Melatonin levels, determined by LC-ESI-MS/MS, fluctuate during the day/night cycle in Vitis vinifera cv Malbec: evidence of its antioxidant role in fruits

Melatonin-Producing Endophytic Bacteria from Grapevine Roots Promote the Abiotic Stress-Induced Production of Endogenous Melatonin in Their Hosts

Endophytes form symbiotic relationships with plants and constitute an important source of phytohormones and bioactive secondary metabolites for their hosts. To date, most studies of endophytes have focused on the influence of these microorganisms on plant growth and physiology and their role in plant defenses against biotic and abiotic stressors; however, to the best of our knowledge, the ability of endophytes to produce melatonin has not been reported. In the present study, we isolated and identified root-dwelling bacteria from three grapevine varieties and found that, when cultured under laboratory conditions, some of the bacteria strains secreted melatonin and tryptophan-ethyl ester. The endophytic bacterium Bacillus amyloliquefaciens SB-9 exhibited the highest level of in vitro melatonin secretion and also produced three intermediates of the melatonin biosynthesis pathway: 5-hydroxytryptophan, serotonin, and N-acetylserotonin. After B. amyloliquefaciens SB-9 colonization, the plantlets exhibited increased plant growth. Additionally, we found that, in grapevine plantlets exposed to salt or drought stress, colonization by B. amyloliquefaciens SB-9 increased the upregulation of melatonin synthesis, as well as that of its intermediates, but reduced the upregulation of grapevine tryptophan decarboxylase genes (VvTDCs) and a serotonin N-acetyltransferase gene (VvSNAT) transcription, when compared to the un-inoculated control. Colonization by B. amyloliquefaciens SB-9 was also able to counteract the adverse effects of salt- and drought-induced stress by reducing the production of malondialdehyde and reactive oxygen species (H₂O₂ and O₂^-) in roots. Therefore, our findings demonstrate the occurrence of melatonin biosynthesis in endophytic bacteria and provide evidence for a novel form of communication between beneficial endophytes and host plants via melatonin.

Endophytic Bacterium Pseudomonas fluorescens RG11 May Transform Tryptophan to Melatonin and Promote Endogenous Melatonin Levels in the Roots of Four Grape Cultivars

Endophytes have been verified to synthesize melatonin in vitro and promote abiotic stress-induced production of endogenous melatonin in grape (Vitis vinifera L.) roots. This study aimed to further characterize the biotransformation of tryptophan to melatonin in the endophytic bacterium Pseudomonas fluorescens RG11 and to investigate its capacity for enhancing endogenous melatonin levels in the roots of different grape cultivars. Using ultra performance liquid chromatography-tandem mass spectrometry combined with 15N double-labeled L-tryptophan as the precursor for melatonin, we detected isotope-labeled 5-hydroxytryptophan, serotonin, N-acetylserotonin, and melatonin, but tryptamine was not detected during the in vitro incubation of P. fluorescens RG11. Furthermore, the production capacity of these four compounds peaked during the exponential growth phase. RG11 colonization increased the endogenous levels of 5-hydroxytryptophan, N-acetylserotonin, and melatonin, but reduced those of tryptamine and serotonin, in the roots of the Red Globe grape cultivar under salt stress conditions. Quantitative real-time PCR revealed that RG11 reduced the transcription of grapevine tryptophan decarboxylase and serotonin N-acetyltransferase genes when compared to the un-inoculated control. These results correlated with decreased reactive oxygen species bursts and cell damage, which were alleviated by RG11 colonization under salt stress conditions. Additionally, RG11 promoted plant growth and enhanced the levels of endogenous melatonin in different grape cultivars. Intraspecific variation in the levels of melatonin precursors was found among four grape cultivars, and the associated root crude extracts appeared to significantly induce RG11 melatonin biosynthesis in vitro. Overall, this study provides useful information that enhances the existing knowledge of a potential melatonin synthesis pathway in rhizobacteria, and it reveals plant-rhizobacterium interactions that affect melatonin biosynthesis in plants subjected to abiotic stress conditions.

Melatonin in Mediterranean diet, a new perspective

The health-promoting properties of the Mediterranean diet have been attributed, at least in part, to the chemical diversity of plant foods. Among phytochemicals, polyphenols represent the paradigm of the relationship between healthy foods and reduced risk of chronic-degenerative diseases, although, in the past few years, a new element has enriched this scenario. Melatonin, and possibly other indoleamines recently discovered in some relevant Mediterranean foods, may represent a new factor contributing to the elucidation of the protective effects of diets rich in plant products. Therefore, in synergy with polyphenols and other bioactive phytochemicals (e.g. carotenoids and glucosinolates), melatonin may contribute to maximizing the benefits of healthy dietary styles. This brief survey deals with the occurrence of melatonin in the Mediterranean diet, with an emphasis on grape products, and focuses on the biological significance of dietary melatonin, an emerging and exciting topic in the field of nutritional sciences.

Functions of melatonin in plants: a review

The number of studies on melatonin in plants has increased significantly in recent years. This molecule, with a large set of functions in animals, has also shown great potential in plant physiology. This review outlines the main functions of melatonin in the physiology of higher plants. Its role as antistress agent against abiotic stressors, such as drought, salinity, low and high ambient temperatures, UV radiation and toxic chemicals, is analyzed. The latest data on their role in plant-pathogen interactions are also discussed. Both abiotic and biotic stresses produce a significant increase in endogenous melatonin levels, indicating its possible role as effector in these situations. The existence of endogenous circadian rhythms in melatonin levels has been demonstrated in some species, and the data, although limited, suggest a central role of this molecule in the day/night cycles in plants. Finally, another aspect that has led to a large volume of research is the involvement of melatonin in aspects of plant development regulation. Although its role as a plant hormone is still far of from being fully established, its involvement in processes such as growth, rhizogenesis, and photosynthesis seems evident. The multiple changes in gene expression caused by melatonin point to its role as a multiregulatory molecule capable of coordinating many aspects of plant development. This last aspect, together with its role as an alleviating-stressor agent, suggests that melatonin is an excellent prospect for crop improvement.

Melatonin in Arabidopsis thaliana acts as plant growth regulator at low concentrations and preserves seed viability at high concentrations

Since the discovery of melatonin in plants, several roles have been described for different species, organs, and developmental stages. Arabidopsis thaliana, being a model plant species, is adequate to contribute to the elucidation of the role of melatonin in plants. In this work, melatonin was monitored daily by UHPLC-MS/MS in leaves, in order to study its diurnal accumulation as well as the effects of natural and artificial light treatments on its concentration. Furthermore, the effects of exogenous application of melatonin to assess its role in seed viability after heat stress and as a regulator of growth and development of vegetative tissues were evaluated. Our results indicate that melatonin contents in Arabidopsis were higher in plants growing under natural radiation when compared to those growing under artificial conditions, and its levels were not diurnally-regulated. Exogenous melatonin applications prolonged seed viability after heat stress conditions. In addition, melatonin applications retarded leaf senescence. Its effects as growth promoter were dose and tissue-dependent; stimulating root growth at low concentrations and decreasing leaf area at high doses.

Melatonin regulates carbohydrate metabolism and defenses against Pseudomonas syringae pv. tomato DC3000 infection in Arabidopsis thaliana

Melatonin has been reported to promote plant growth and development. Our experiments with Arabidopsis thaliana showed that exogenous applications of this molecule mediated invertase inhibitor (C/VIF)-regulated invertase activity and enhanced sucrose metabolism. Hexoses were accumulated in response to elevated activities by cell wall invertase (CWI) and vacuolar invertase (VI). Analyses of sugar metabolism-related genes revealed differential expression during plant development that was modulated by melatonin. In particular, C/VIF1 and C/VIF2 were strongly down-regulated by exogenous feeding. We also found the elevated CWI activity in melatonin-treated Arabidopsis improved the factors (cellulose, xylose, and galactose) for cell wall reinforcement and callose deposition during Pseudomonas syringae pv. tomato DC3000 infection, therefore, partially induced the pathogen resistance. However, CWI did not involve in salicylic acid (SA)-regulated defense pathway. Taken together, this study reveals that melatonin plays an important role in invertase-related carbohydrate metabolism, plant growth, and pathogen defense.

Yeast contribution to melatonin, melatonin isomers and tryptophan ethyl ester during alcoholic fermentation of grape musts

Melatonin (MEL) has been found in some medicinal and food plants, including grapevine, a commodity of particular interest for the production of wine, a beverage of economic relevance. It has also been suggested that MEL in wine may, at least in part, contribute to the health-promoting properties attributed to this beverage and, possibly, to other traditional Mediterranean foodstuffs. After a preliminary screening of 9 yeast strains in laboratory medium, three selected strains (Saccharomyces cerevisiae EC1118, Torulaspora delbrueckii CBS1146(T) and Zygosaccharomyces bailii ATCC36947(T) ) were inoculated in experimental musts obtained from 2 white (Moscato and Chardonnay) and 2 red (Croatina and Merlot) grape varieties. The production of MEL, melatonin isomers (MIs) and tryptophan ethyl ester (TEE) was monitored during the alcoholic fermentation. The screening showed that the three investigated strains produced the highest concentrations of MEL and two MIs in optimal growth conditions. However, MEL and MIs were not produced in oenological conditions, but the three strains synthesized high concentrations of a new MI and TEE in musts.

Tryptophan-ethylester, the false (unveiled) melatonin isomer in red wine

Among the food plants, the presence of melatonin in grapes (Vitis vinifera L.) deserves particular attention because of the production of wine, an alcoholic beverage of economic relevance and with putative healthy effects. Furthermore, melatonin isomers have been detected in wine too. Recently, one of these isomers has been identified as tryptophan-ethylester, a compound with the same molecular weight of melatonin. In this Commentary, we briefly comment the source(s) of tryptophan-ethylester in wine and the putative nutritional role(s).

Melatonin in plants and other phototrophs: advances and gaps concerning the diversity of functions

Melatonin is synthesized in Alphaproteobacteria, Cyanobacteria, Dinoflagellata, Euglenoidea, Rhodophyta, Phae ophyta, and Viridiplantae. The biosynthetic pathways have been identified in dinoflagellates and plants. Other than in dinoflagellates and animals, tryptophan is not 5-hydroxylated in plants but is first decarboxylated. Serotonin is formed by 5-hydroxylation of tryptamine. Serotonin N-acetyltransferase is localized in plastids and lacks homology to the vertebrate aralkylamine N-acetyltransferase. Melatonin content varies considerably among species, from a few picograms to several micrograms per gram, a strong hint for different actions of this indoleamine. At elevated levels, the common and presumably ancient property as an antioxidant may prevail. Although melatonin exhibits nocturnal maxima in some phototrophs, it is not generally a mediator of the signal 'darkness'. In various plants, its formation is upregulated by visible and/or UV light. Increases are often induced by high or low temperature and several other stressors including drought, salinity, and chemical toxins. In Arabidopsis, melatonin induces cold- and stress-responsive genes. It has been shown to support cold resistance and to delay experimental leaf senescence. Transcriptome data from Arabidopsis indicate upregulation of genes related to ethylene, abscisic acid, jasmonic acid, and salicylic acid. Auxin-like actions have been reported concerning root growth and inhibition, and hypocotyl or coleoptile lengthening, but effects caused by melatonin and auxins can be dissected. Assumptions on roles in flower morphogenesis and fruit ripening are based mainly on concentration changes. Whether or not melatonin will find a place in the phytohormone network depends especially on the identification of molecular signals regulating its synthesis, high-affinity binding sites, and signal transduction pathways.

Circadian changes in endogenous concentrations of indole-3-acetic acid, melatonin, serotonin, abscisic acid and jasmonic acid in Characeae (Chara australis Brown)

Giant-celled Characeae (Chara australis Brown), grown for 4 months on 12/12 hr day/night cycle and summer/autumn temperatures, exhibited distinct concentration maxima in auxin (indole-3-acetic acid; IAA), melatonin and serotonin about 4 hr after subjective daybreak. These concentration peaks persisted after 3 day pretreatment in continuous darkness: confirming a circadian rhythm, rather than a response to "light on." The plants pretreated for 3 d in continuous light exhibited several large IAA concentration maxima throughout the 24 hr. The melatonin and serotonin concentrations decreased and were less synchronized with IAA. Chara plants grown on 9/15 hr day/night cycle for 4 months and winter/spring temperatures contained much smaller concentrations of IAA, melatonin and serotonin. The IAA concentration maxima were observed in subjective dark phase. Serotonin concentration peaks were weakly correlated with those of IAA. Melatonin concentration was low and mostly independent of circadian cycle. The "dark" IAA concentration peaks persisted in plants treated for 3 d in the dark. The plants pretreated for 3 d in the light again developed more IAA concentration peaks. In this case the concentration maxima in melatonin and serotonin became more synchronous with those in IAA. The abscisic acid (ABA) and jasmonic acid (JA) concentrations were also measured in plants on winter regime. The ABA concentration did not exhibit circadian pattern, while JA concentration peaks were out of phase with those of IAA. The data are discussed in terms of crosstalk between metabolic pathways.

A new balancing act: The many roles of melatonin and serotonin in plant growth and development

Melatonin and serotonin are indoleamines first identified as neurotransmitters in vertebrates; they have now been found to be ubiquitously present across all forms of life. Both melatonin and serotonin were discovered in plants several years after their discovery in mammals, but their presence has now been confirmed in almost all plant families. The mechanisms of action of melatonin and serotonin are still poorly defined. Melatonin and serotonin possess important roles in plant growth and development, including functions in chronoregulation and modulation of reproductive development, control of root and shoot organogenesis, maintenance of plant tissues, delay of senescence, and responses to biotic and abiotic stresses. This review focuses on the roles of melatonin and serotonin as a novel class of plant growth regulators. Their roles in reproductive and vegetative plant growth will be examined including an overview of current hypotheses and knowledge regarding their mechanisms of action in specific responses.

Melatonin: plant growth regulator and/or biostimulator during stress?

Melatonin regulates the growth of roots, shoots, and explants, to activate seed germination and rhizogenesis and to delay induced leaf senescence. The antioxidant properties of melatonin would seem to explain, at least partially, its ability to fortify plants subjected to abiotic stress. In this Review we examine recent data on the gene-regulation capacity of melatonin that point to many interesting features, such as the upregulation of anti-stress genes and recent aspects of the auxin-independent effects of melatonin as a plant growth regulator. This, together with the recent data on endogenous melatonin biosynthesis induction by environmental factors, makes melatonin an interesting candidate for use as a natural biostimulating treatment for field crops.

Extrapineal melatonin: sources, regulation, and potential functions

Endogenous melatonin is synthesized from tryptophan via 5-hydroxytryptamine. It is considered an indoleamine from a biochemical point of view because the melatonin molecule contains a substituted indolic ring with an amino group. The circadian production of melatonin by the pineal gland explains its chronobiotic influence on organismal activity, including the endocrine and non-endocrine rhythms. Other functions of melatonin, including its antioxidant and anti-inflammatory properties, its genomic effects, and its capacity to modulate mitochondrial homeostasis, are linked to the redox status of cells and tissues. With the aid of specific melatonin antibodies, the presence of melatonin has been detected in multiple extrapineal tissues including the brain, retina, lens, cochlea, Harderian gland, airway epithelium, skin, gastrointestinal tract, liver, kidney, thyroid, pancreas, thymus, spleen, immune system cells, carotid body, reproductive tract, and endothelial cells. In most of these tissues, the melatonin-synthesizing enzymes have been identified. Melatonin is present in essentially all biological fluids including cerebrospinal fluid, saliva, bile, synovial fluid, amniotic fluid, and breast milk. In several of these fluids, melatonin concentrations exceed those in the blood. The importance of the continual availability of melatonin at the cellular level is important for its physiological regulation of cell homeostasis, and may be relevant to its therapeutic applications. Because of this, it is essential to compile information related to its peripheral production and regulation of this ubiquitously acting indoleamine. Thus, this review emphasizes the presence of melatonin in extrapineal organs, tissues, and fluids of mammals including humans.

Bioactive compounds derived from the yeast metabolism of aromatic amino acids during alcoholic fermentation

Metabolites resulting from nitrogen metabolism in yeast are currently found in some fermented beverages such as wine and beer. Their study has recently attracted the attention of researchers. Some metabolites derived from aromatic amino acids are bioactive compounds that can behave as hormones or even mimic their role in humans and may also act as regulators in yeast. Although the metabolic pathways for their formation are well known, the physiological significance is still far from being understood. The understanding of this relevance will be a key element in managing the production of these compounds under controlled conditions, to offer fermented food with specific enrichment in these compounds or even to use the yeast as nutritional complements.

Phytomelatonin: Discovery, Content, and Role in Plants

Melatonin (N-acetyl-5-methoxytryptamine) is an indolic compound derived from tryptophan. Usually identified as a neurotransmitter or animal hormone, this compound was detected in plants in 1995. Interest in knowing the melatonin content of plants and its possible role therein is growing, as indicated by the increasing number of related publications. Melatonin is present in all plant species studied, with large variations in its level depending on the plant organ or tissue. It seems to be more abundant in aromatic plants and in leaves than in seeds. Regarding its physiological function in plants, melatonin shows auxin activity and is an excellent antioxidant, regulating the growth of roots, shoots, and explants, activating seed germination and rhizogenesis (lateral- and adventitious-roots), and delaying induced leaf senescence. Its ability to strengthen plants subjected to abiotic stress such as drought, cold, heat, salinity, chemical pollutants, herbicides, and UV radiation makes melatonin an interesting candidate for use as a natural biostimulating substance for treating field crops.

Arabidopsis transcriptome analysis reveals key roles of melatonin in plant defense systems

Melatonin is a ubiquitous molecule and exists across kingdoms including plant species. Studies on melatonin in plants have mainly focused on its physiological influence on growth and development, and on its biosynthesis. Much less attention has been drawn to its affect on genome-wide gene expression. To comprehensively investigate the role(s) of melatonin at the genomics level, we utilized mRNA-seq technology to analyze Arabidopsis plants subjected to a 16-hour 100 pM (low) and 1 mM (high) melatonin treatment. The expression profiles were analyzed to identify differentially expressed genes. 100 pM melatonin treatment significantly affected the expression of only 81 genes with 51 down-regulated and 30 up-regulated. However, 1 mM melatonin significantly altered 1308 genes with 566 up-regulated and 742 down-regulated. Not all genes altered by low melatonin were affected by high melatonin, indicating different roles of melatonin in regulation of plant growth and development under low and high concentrations. Furthermore, a large number of genes altered by melatonin were involved in plant stress defense. Transcript levels for many stress receptors, kinases, and stress-associated calcium signals were up-regulated. The majority of transcription factors identified were also involved in plant stress defense. Additionally, most identified genes in ABA, ET, SA and JA pathways were up-regulated, while genes pertaining to auxin responses and signaling, peroxidases, and those associated with cell wall synthesis and modifications were mostly down-regulated. Our results indicate critical roles of melatonin in plant defense against various environmental stresses, and provide a framework for functional analysis of genes in melatonin-mediated signaling pathways.

Melatonin synthesis in rice seedlings in vivo is enhanced at high temperatures and under dark conditions due to increased serotonin N-acetyltransferase and N-acetylserotonin methyltransferase activities

Temperature and light are important environmental factors for plant growth and development. The final two enzymes in the melatonin synthesis pathway in plants are serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT), which have thermophilic characteristics. Thus, the effects of temperature and light on melatonin synthesis in rice seedlings were investigated. Here, we demonstrated that melatonin levels increased as temperature increased when rice seedlings were exposed to various temperatures for 1 hr. Moreover, the relative melatonin levels were higher in the dark. For example, exposure of rice seedlings to 1-hr darkness at 55°C resulted in a melatonin yield of 4.9 ng/g fresh weight (fw), compared with 2.95 ng/g fw under light conditions. Temperature-dependent melatonin synthesis was closely associated with an increase in both SNAT and ASMT activities, but not with transcript levels of melatonin biosynthetic genes. The daily melatonin levels in field-grown rice plants were unaffected as the positive effect of the relatively high temperature during the day was counteracted by the negative effect of the high light. The opposite effect occurred during the night, in which the positive effect of darkness on melatonin synthesis was counteracted by the negative effect of a low temperature.

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

We evaluated the effect of cultivar and solar radiation on the melatonin content of Capsicum annuum (pepper) and Solanum lycopersicum (tomato) fruits. The melatonin content of red pepper fruits ranged from 31 to 93ngg(-1) (dry weight). The melatonin content of tomato ranged from 7.5 to 250ngg(-1) (dry weight). We also studied the effect of ripeness on melatonin content and identified one group of pepper cultivars in which the melatonin content increased as the fruit ripened and another in which it decreased as the fruit ripened. Under shade conditions, the melatonin content in most of tomato cultivars tended to increase (up to 135%), whereas that of most pepper cultivars decreased (to 64%). Overall, the results also demonstrated that the melatonin content of the fruits was not related to carbon fluxes from leaves.

Cellular localization and kinetics of the rice melatonin biosynthetic enzymes SNAT and ASMT

Serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT) are the final two enzymes in the melatonin synthesis pathway in plants. Although their corresponding genes have been cloned, their cellular localization and enzymatic characteristics are unknown. Using confocal microscopy, we showed that SNAT protein is localized in chloroplasts, whereas ASMT is expressed in the cytoplasm. In vitro measurement of ASMT enzyme activity revealed a peak of activity in roots, but SNAT enzyme activity was not detected in any plant tissues. This may be attributed in part to an effect of chlorophyll because SNAT enzyme activity was greatly inhibited by chlorophyll in a dose-dependent manner. Because the SNAT protein of cyanobacteria is thermophilic, we examined the effect of temperature on the activity of the rice SNAT and ASMT enzymes. Purified recombinant rice SNAT and ASMT enzymes had an optimum temperature for activity of 55°C. The Km and Vmax values for SNAT at 55°C were 270 μm and 3.3 nmol/min/mg protein, whereas the Km and Vmax for ASMT were 222 μm and 9 nmol/min/mg protein, respectively. The catalytic efficiency (Vmax /Km ) values of SNAT and ASMT were 16-fold and 4054-fold higher at 55°C than at 30°C suggestive of increased melatonin production at high temperature in plants.

Occurrence, diel patterns, and the influence of melatonin on the photosynthetic performance of cultured Symbiodinium

Dinoflagellata is the earliest phylum in which true circadian regulation of melatonin rhythms has been convincingly demonstrated. Here, diel profiling of melatonin in a cultured member of this phylum belonging to the genus Symbiodinium indicated that melatonin levels oscillate with significant nocturnal peaks. However, unlike in other previously studied dinoflagellate species, the diel rhythmicity of melatonin in Symbiodinium did not persist under constant dark conditions. Thus, the oscillating pattern of melatonin in Symbiodinium is presumed not to be driven by endogenous circadian control of melatonin production, but rather by changes in the daily photocycle, most likely through a mechanism involving the enhanced photo-consumption of melatonin by free radicals. Although direct interactions of melatonin with detrimental radicals have been previously studied in several basal species, including dinoflagellates, none of these investigations addressed the effects that this molecule may have on photosynthesis, a major source of radical species in unicellular algae. In the present work, real-time monitoring of oxygen evolution in Symbiodinium cultures indicated a significant decrease in photosynthesis rates upon treatment with various doses of melatonin. Analyses of chlorophyll a fluorescence and xanthophyll cycle activity confirmed this effect and further revealed that this slowdown may occur through an enhanced engagement of photoprotective mechanisms in melatonin-treated cells. These findings are of great importance as they demonstrate that in certain photoautotroph species, the interactions of elevated melatonin levels with photosynthesis may extend beyond the general purpose of antioxidant protection.

Melatonin and its potential biological functions in the fruits of sweet cherry

Melatonin is a well-known molecule which possesses many beneficial effects on human health. Many agriculture products provide natural melatonin in the diet. Cherry is one such fruit as they are rich in melatonin. In order to understand the biological roles of melatonin in cherry fruit, melatonin synthesis and its changes over 24 hr period were systematically monitored both during their development and in the ripe cherries in two cultivars, 'Hongdeng' (Prunus avium L. cv. Hongdeng) and 'Rainier' (Prunus avium L. cv. Rainier). It was found that both darkness and oxidative stress induced melatonin synthesis, which led to dual melatonin synthetic peaks during a 24 hr period. The high levels of malondialdehyde induced by high temperature and high intensity light exposure were directly related to up-regulated melatonin production. A primary function of melatonin in cherry fruits is speculated to be as an antioxidant to protect the cherry from the oxidative stress. Importantly, plant tryptophan decaboxylase gene (PaTDC) was identified in cherry fruits. Our data shows that PaTDC expression is positively related to the melatonin production in the cherry. This provides additional information to suggest that tryptophan decaboxylase is a rate-limiting enzyme of melatonin synthesis in plants.

Analytical tools for elucidating the biological role of melatonin in plants by LC-MS/MS

Melatonin (MT) presence in higher plants was recently discovered and the knowledge of its function in vivo is limited. Several studies have recently shown the occurrence of MT and related compounds in grapes and wines. The analysis of MT in plants and foods represents a highly challenging task due to its wide concentration range, the difficulty in the selection of the extraction solvents because of its amphipathic nature, and the fact that it reacts quickly with other matrix components. Thus, sample processing factors; preparation/cleanup procedures; and chromatographic/detection parameters, such as HILIC and reverse phase (C(8) and C(18)) chromatographic modes, ESI, and atmospheric pressure chemical ionization (APCI) in both negative and positive modes were evaluated. Taken together, we have demonstrated that optimal conditions were quite different for each of the matrices under study. A sonication-mediated extraction step was necessary for grape skin (100% v/v methanol) and plant tissues (50% v/v methanol), while wine and must required a SPE preconcentration step. HILIC-(+) APCI ionization was better for MT standards, while C(8) -(+) APCI was the best choice for grape skin and C(18) -(+ESI) was suitable for wine. On the other hand, C(8) -(+)ESI was the most appropriate for vegetal tissues of Arabidopsis thaliana. Proposed methods were validated and the LODs were in the low picogram levels range. The optimized approaches were applied to the determination of MT and its isomer in different vegetal/food samples; levels found within the range: 4.9-440 ng/g.

Melatonin-rich transgenic rice plants exhibit resistance to herbicide-induced oxidative stress

To examine whether melatonin-rich plants can defend against oxidative stress, we subjected melatonin-rich transgenic (MRT) rice plants to the singlet-oxygen-generating herbicide butafenacil. Both MRT and transgenic control (TC; expressing the vector only) rice seeds germinated and grew equally well in continuous dark on half-strength Murashige and Skoog (MS) medium containing 0.1 μm butafenacil. However, after transferring the seedlings to light, the TCs rapidly necrotized, whereas the MRT seedlings showed resistant phenotypes. Seven-day-old MRT seedlings treated with 0.1 μm butafenacil were resistant to the herbicide and contained high chlorophyll levels and low malondialdehyde and hydrogen peroxide contents compared with the TCs. As they did before the herbicide treatment, the MRT plants also produced much more melatonin after the herbicide treatment than the TCs. In addition, the MRT plants exhibited higher superoxide dismutase and catalase activities before and after the herbicide treatment compared with the TCs. This is the first report showing that MRT plants exhibit resistance against a peroxidizing herbicide that acts by generating reactive oxygen species (ROS) that kill plants. This result indicates that melatonin scavenges ROS efficiently in vivo in the transgenic plants, leading to oxidative stress resistance.

Melatonin, melatonin isomers and stilbenes in Italian traditional grape products and their antiradical capacity

Although polyphenols represent the paradigm of the health-promoting effects ascribed to grape products, recently, attention has been paid to dietary melatonin, significantly present in Mediterranean foods. In this work, we measured melatonin, its isomers, stilbenes (trans- and cis-resveratrol and their glucosides, piceids) and total polyphenols in some different grape products (red, white and dessert wines, grape juices and Modena balsamic vinegars) of distinct Italian areas. We also evaluated their antiradical activity by DPPH(·) and ABTS(·+) assays. For indoleamine analysis, the separation was carried out on a 1.7-μm C18 BEH column and the detection performed by means of mass spectrometry with electrospray ionization in positive ion mode with multiple reaction monitoring. The confirmation of the peak identity was accomplished by injection into the high-resolution system (Orbitrap) using accurate mass measurements (error below 1.0 ppm). Mass spectrometry analyses revealed, for the first time, the presence of melatonin in dessert wines and balsamic vinegars, as well as the occurrence of three different melatonin isomers in grape products.

Kinetic analysis of purified recombinant rice N-acetylserotonin methyltransferase and peak melatonin production in etiolated rice shoots

Rice (Oryza sativa) N-acetylserotonin methyltransferase (osASMT), the last enzyme in the synthesis of melatonin, was expressed in Escherichia coli and purified. We then characterized its enzyme kinetics, which is the first time this has been performed in plants. Purified glutathione S-transferase (GST)-fused recombinant osASMT (GST-osASMT) and GST-free osASMT showed specific enzyme activities of 6.6 and 12.6 pmol/min per mg protein, respectively. When evaluated by the Lineweaver-Burk equation, GST-free osASMT exhibited a K(m) of 864 μm. An in vitro enzyme assay of purified osASMT showed melatonin formation to be proportional to the enzyme and substrate concentrations, as well as time. Unlike animal ASMT, high substrate concentrations did not inhibit the activity of osASMT. Finally, melatonin biosynthesis in rice seedlings was affected by light intensity, with etiolated shoots grown in continuous darkness producing more melatonin than shoots grown in continuous light. The level of melatonin in relation to the light intensity closely paralleled the mRNA level of osASMT in the shoots, suggesting that endogenous melatonin is upregulated in darkness, as is the case in animals.

Exogenous melatonin affects photosynthesis in characeae Chara australis

Melatonin was found in the fresh water characeae Chara australis. The concentrations (~4 μg/g of tissue) were similar in photosynthesizing cells, independent of their position on the plant and rhizoids (roots) without chloroplasts. Exogenous melatonin, added at 10 μM to the artificial pond water, increased quantum yield of photochemistry of photosystem II by 34%. The increased efficiency appears to be due to the amount of open reaction centers of photosystem II, rather than increased efficiency of each reaction center. More open reaction centers reflect better functionality of all photosynthetic transport chain constituents. We suggest that melatonin protection against reactive oxygen species covers not only chlorophyll, but also photosynthetic proteins in general.

Melatonin promotes seminal root elongation and root growth in transgenic rice after germination

The effect of melatonin on root growth after germination was examined in transgenic rice seedlings expressing sheep serotonin N-acetyltransferase (NAT). Enhanced melatonin levels were found in T(3) homozygous seedlings because of the ectopic overexpression of sheep NAT, which is believed to be the rate-limiting enzyme in melatonin biosynthesis in animals. Compared with wild-type rice seeds, the transgenic rice seeds showed enhanced seminal root growth and an analogous number of adventitious roots 4 and 10 days after seeding on half-strength Murashige and Skoog medium. The enhanced initial seminal root growth in the transgenic seedlings matched their increased root biomass well. We also found that treatment with 0.5 and 1 μM melatonin promoted seminal root growth of the wild type under continuous light. These results indicate that melatonin plays an important role in regulating both seminal root length and root growth after germination in monocotyledonous rice plants. This is the first report on the effects of melatonin on root growth in gain-of-function mutant plants that produce high levels of melatonin.

Melatonin suppresses proinflammatory mediators in lipopolysaccharide-stimulated CRL1999 cells via targeting MAPK, NF-κB, c/EBPβ, and p300 signaling

Melatonin is an indoleamine secreted by the pineal gland as well as a plant-derived product that exerts potential anti-inflammatory properties, but the mechanisms of action remain unclear. Here, we investigated the roles of melatonin in regulation of proinflammatory mediators and identified the underlying mechanisms in human vascular smooth muscle (VSM) cell line CRL1999 stimulated by lipopolysaccharide (LPS). We found that treatment with melatonin significantly inhibited the production and expression of TNF-α and interleukin (IL)-1β, cyclooxygenase-2 (COX-2), inducible nitric oxide synthase, prostaglandin E(2) (PGE2), and nitric oxide (NO) in a dose-dependent manner. Moreover, we also found that the suppression of proinflammatory mediators by melatonin was mediated through inhibition of MAPK, NF-κB, c/EBPβ, and p300 signaling in LPS-stimulated CRL1999 cells. Treatment with melatonin markedly inhibited phosphorylation of ERK1/2, JNK, p38 MAPK, IκB-α, and c/EBPβ, blocked binding of NF-κB and c/EBPβ to promoters, and suppressed p300 histone acetyltransferase (HAT) activity and p300 HAT-mediated NF-κB acetylation. Transfection with an ERK-, IκB-, or c/EBPβ-specific siRNA or pretreatment with an ERK-, p38 MAPK-, or p300-selective inhibitor considerably abrogated the melatonin-mediated inhibition of proinflammatory mediators. Conversely, exogenous overexpression of a constitutively active p300, but not its HAT mutant, effectively reversed the melatonin-mediated inhibitions. Collectively, these results indicate that melatonin suppresses proinflammatory mediators by simultaneously targeting the multiple signaling such as ERK/p38 MAPK, c/EBPβ, NF-κB, and p300, in LPS-stimulated VSM cell line CRL1999, and suggest that melatonin is a potential candidate compound for the treatment of proinflammatory disorders.

Emergence of naturally occurring melatonin isomers and their proposed nomenclature

Melatonin was considered to be the sole member of this natural family. The emergence of naturally occurring melatonin isomers (MIs) has opened an exciting new research area. Currently, several MIs have been identified in wine, and these molecules are believed to be synthesized by either yeasts or bacteria. A tentative nomenclature for the MIs is proposed in this article. It will be important to explore whether all organisms have the capacity to synthesize MIs, especially under the conditions of environmental stress. These isomers probably share many of the biological functions of melatonin, but their activities seem to exceed those of melatonin. On basis of the limited available information, it seems that MIs differ in their biosynthetic pathways from melatonin. Especially in those compounds in which the aliphatic side chain is not attached to ring atom 3, the starting material may not be tryptophan. Also, the metabolic pathways of MIs remain unknown. This, therefore, is another promising area of research to explore. It is our hypothesis that MIs would increase the performance of yeasts and probiotic bacteria during the processes of fermentation. Therefore, yeasts producing elevated levels of these isomers might have a superior alcohol tolerance and be able to produce higher levels of alcohol. This can be tested by comparing existing yeast strains differing in alcohol tolerance. Selection for MIs may become a strategy for isolating more resistant yeast and Lactobacillus strains, which can be of interest for industrial alcohol production and quality improvements in bacterially fermented foods such as kimchi.

Content of melatonin and other antioxidants in grape-related foodstuffs: measurement using a MEPS-HPLC-F method

The strong antioxidant activity of melatonin is well known and it is important to investigate its presence and levels in different foodstuffs, for the purpose of evaluating their nutraceutical properties. As a contribution towards this goal, an original analytical method has been developed for the simultaneous determination of melatonin and other indolic and phenolic antioxidants (including trans- and cis-resveratrol, ferulic acid, tryptophan, serotonin and 5-hydroxyindoleacetic acid) in grape-related foodstuffs and beverages: namely grape, grape juice, must, wine and grappa (Italian pomace brandy). These foodstuffs represent an important part of the diet, both traditionally and in recent times, especially in Mediterranean countries and could be (at least in part) responsible for the beneficial effects involved in the 'French paradox'. The analytical method is based on high-performance liquid chromatography coupled to fluorescence detection, exploiting the native fluorescence of the analytes. A C8 column was used as the stationary phase, while the mobile phase was composed of acidic phosphate buffer and acetonitrile; fluorescence intensity was monitored at λ=386nm while exciting at λ=298nm. The sample pretreatment was carried out by a fast and reliable microextraction by packed sorbent (MEPS) procedure. After validation, the method was applied to the analysis of melatonin and other antioxidants in food and beverages derived from grape, with very good results being obtained. Thus, this methodology may represent a promising tool for the evaluation of the antioxidant properties of nutraceuticals and functional foods.

Light-regulated melatonin biosynthesis in rice during the senescence process in detached leaves

The effect of light on melatonin biosynthesis was examined in detached rice (Oryza sativa cv. Asahi) leaves during the senescence process. The detached leaves were exposed to senescence treatment either in constant darkness or in constant light, and subjected to HPLC analysis for melatonin and its precursors. Higher melatonin levels were detected in rice leaves under constant light while very low levels were observed in constant darkness. Levels of the melatonin intermediates, tryptamine, serotonin, and N-acetylserotonin significantly decreased in the dark compared to those in the light. Furthermore, relative mRNA levels of melatonin biosynthetic genes and their corresponding proteins decreased accordingly in constant darkness. The most striking difference between constant light and dark was observed in levels of the protein tryptamine 5-hydroxylase. These results suggest that melatonin biosynthesis during senescence is dependent on light signals in rice leaves, contrary to the response found in animals.

Daily variation in melatonin synthesis and arylalkylamine N-acetyltransferase activity in the nematode Caenorhabditis elegans

Melatonin influences circadian rhythms and seasonal behavioral changes in vertebrates; it is synthesized from serotonin by N-acetylation by arylalkylamine N-acetyltransferase (AA-NAT) and O-methylation by N-acetylserotonin methyltransferase. However, its physiology and function in invertebrate models are less understood. In this work, we studied daily variations in melatonin synthesis and AA-NAT activity in the nematode Caenorhabditis elegans. Under light-dark conditions (LD), a rhythmic pattern of melatonin levels was observed, with higher levels toward the middle of the night, peaking at zeitgeber time (ZT) 18, and with a minimum value around ZT0-6. AA-NAT activity showed a diurnal and circadian fluctuation with higher levels of activity during the early night, both under LD and constant darkness conditions. A peak was found around ZT12 and circadian time (CT) 12. In addition, we investigated whether this nocturnal AA-NAT activity is inhibited by light. Our results show that both white and blue light pulses significantly inhibited AA-NAT activity at ZT18. This work demonstrates the daily fluctuation of melatonin synthesis and AA-NAT activity in the adult nematode C. elegans. In summary, this study takes additional advantage of an extremely useful invertebrate model system, which has only recently been exploited for circadian studies.

Monitoring melatonin and its isomer in Vitis vinifera cv. Malbec by UHPLC-MS/MS from grape to bottle

Several studies have shown the presence of melatonin and related compounds in grapes and wines. The latter provides evidence of the possibility to enhance the nutraceutical properties of premium wines. However, there are many external factors that can influence the levels of this indolamine in grape and wines. In this study, the monitoring of melatonin and its tentatively identified isomer was carried out during the entire winemaking process in Vitis vinifera cv. Malbec by ultra high-performance liquid chromatography-tandem mass spectrometry. Laboratory and pilot studies were carried out to elucidate the role of grape, yeasts, and tryptophan in the evolution of the indolamines during the fermentation process. Melatonin was detected in grape extract within the range 120-160 ng/g while its isomer was found in musts and finished wines. Our results demonstrate that Saccaromyces cervisiae plays a decisive role in contributing to the content of melatonin and its isomer in wine.

Tryptamine 5-hydroxylase-deficient Sekiguchi rice induces synthesis of 5-hydroxytryptophan and N-acetyltryptamine but decreases melatonin biosynthesis during senescence process of detached leaves

Melatonin biosynthesis was examined in Sekiguchi mutant rice lacking functional tryptamine 5-hydroxylase (T5H) activity, which is the terminal enzyme for serotonin biosynthesis in rice. During senescence process, the leaves of Sekiguchi mutant rice produced more tryptamine and N-acetyltryptamine compared with the wild-type Asahi leaves. Even though T5H activity is absent, Sekiguchi leaves produce low levels of serotonin derived from 5-hydroxytryptophan, which was found to be synthesized during senescence process. Accordingly, both rice cultivars exhibited similar levels of N-acetylserotonin until 6 days of senescence induction; however, only Asahi leaves continued to accumulate N-acetylserotonin after 6 days. In contrast, a large amount of N-acetyltryptamine was accumulated in Sekiguchi leaves, indicating that tryptamine was efficiently utilized as substrate by the rice arylalkylamine N-acetyltransferase enzyme. An increase in N-acetyltryptamine in Sekiguchi had an inhibitory effect on synthesis of melatonin because little melatonin was produced in Sekiguchi leaves at 6 days of senescence induction, even in the presence of equivalent levels of N-acetylserotonin in both cultivars. The exogenous treatment of 0.1 mmN-acetyltryptamine during senescence process completely blocked melatonin synthesis.