Melatonin in higher plants: occurrence and possible functions

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.

Melatonin production in the sea star Echinaster brasiliensis (Echinodermata)

The primary hormone of the vertebrate pineal gland, melatonin, has been identified broadly throughout the tree of life, in animals, plants, and fungi, supporting a deep evolutionary origin for this signaling molecule. However, some key groups have not been studied. Echinoderms, deuterostome animals, are one of these groups. Herein we study the presence of melatonin and enzymes of its pathway in the sea star Echinaster brasiliensis. We demonstrate that E. brasiliensis produces endogenous melatonin, in the gonads, under a circadian pattern with a nocturnal peak of production. We also show that the enzymes arylalkylamine N-acetyltransferase (AANAT) and tryptophan hydroxylase (TPH) are present and are probably regulating the melatonin production.

Changes in melatonin levels in transgenic 'Micro-Tom' tomato overexpressing ovine AANAT and ovine HIOMT genes

In animals, the melatonin biosynthesis pathway has been well defined after the isolation and identification of the four key genes that are involved in the conversion of tryptophan to melatonin. In plants, there are special alternative catalyzing steps, and plant genes share very low homology with the animal genes. It was of interest to examine the phenotype of transgenic Micro-Tom tomato plants overexpressing the homologous sheep oAANAT and oHIOMT genes responsible for the last two steps of melatonin synthesis. The oAANAT transgenic plants have higher melatonin levels and lower indoleacetic acid (IAA) contents than control due to the competition for tryptophan, the same precursor for both melatonin and IAA. Therefore, the oAANAT lines lose the 'apical dominance' inferring that melatonin likely lacks auxin activity. The significantly higher melatonin content in oHIOMT lines than oAANAT lines provides new proof for the important role of ASMT in plant melatonin synthesis. In addition, the enhanced drought tolerance of oHIOMT lines will also be an important contribution for plant engineering.

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.

Vibrational and electronic spectroscopic studies of melatonin

We report the infrared absorption and Raman spectra of melatonin recorded with 488 and 632.8 nm excitations in 3600-2700 and 1700-70 cm(-1) regions. Further, we optimized molecular structure of the three conformers of melatonin within density functional theory calculations. Vibrational frequencies of all three conformers have also been calculated. Observed vibrational bands have been assigned to different vibrational motions of the molecules on the basis of potential energy distribution calculations and calculated vibrational frequencies. Observed band positions match well with the calculated values after scaling except NH stretching mode frequencies. It is found that the observed and calculated frequencies mismatch of NH stretching is due to intermolecular interactions between melatonin molecules.

Growth conditions determine different melatonin levels in Lupinus albus L

Melatonin, an indoleamine, which has recently been assigned several roles in plant physiology as a growth promoter, as rooting agent, and as antioxidant in senescence delay and cytoprotection, seems to have a relevant function in plant stress situations. The presence of melatonin increases the resistance of lupin plant tissues (Lupinus albus L.) against natural or artificially induced adverse situations. In this work, we studied the response of lupin plants in controlled stress situations (drought-, anaerobic-, pH-, and cold stress and using ZnSO4 , NaCl, and H2 O2 as chemical stressors) and measured the changes in endogenous melatonin levels in lupin plants. Also, the effect of abscisic acid, ethylene, and natural environmental conditions were evaluated. In general, nearly all stressful factors caused an increase in melatonin in the investigated organs. The chemical stress provoked by ZnSO4 or NaCl caused the most pronounced changes in the endogenous level of melatonin, followed by cold and drought stressors. In some cases, the level of melatonin increased 12-fold with respect to the levels in control plants, indicating that melatonin biosynthesis is upregulated in common stress situations, in which it may serve as a signal molecule and/or as a direct antistress agent due to its well-known antioxidative properties.

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.

Peripheral reproductive organ health and melatonin: ready for prime time

Melatonin has a wide variety of beneficial actions at the level of the gonads and their adnexa. Some actions are mediated via its classic membrane melatonin receptors while others seem to be receptor-independent. This review summarizes many of the published reports which confirm that melatonin, which is produced in the ovary, aids in advancing follicular maturation and preserving the integrity of the ovum prior to and at the time of ovulation. Likewise, when ova are collected for in vitro fertilization-embryo transfer, treating them with melatonin improves implantation and pregnancy rates. Melatonin synthesis as well as its receptors have also been identified in the placenta. In this organ, melatonin seems to be of particular importance for the maintenance of the optimal turnover of cells in the villous trophoblast via its ability to regulate apoptosis. For male gametes, melatonin has also proven useful in protecting them from oxidative damage and preserving their viability. Incubation of ejaculated animal sperm improves their motility and prolongs their viability. For human sperm as well, melatonin is also a valuable agent for protecting them from free radical damage. In general, the direct actions of melatonin on the gonads and adnexa of mammals indicate it is an important agent for maintaining optimal reproductive physiology.

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.

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.

Serotonin receptor 2C and insulin secretion

Type 2 diabetes mellitus (T2DM) describes a group of metabolic disorders characterized by defects in insulin secretion and insulin sensitivity. Insulin secretion from pancreatic β-cells is an important factor in the etiology of T2DM, though the complex regulation and mechanisms of insulin secretion from β-cells remains to be fully elucidated. High plasma levels of serotonin (5-hydroxytryptamine; 5-HT) have been reported in T2DM patients, though the potential effect on insulin secretion is unclear. However, it is known that the 5-HT receptor 2C (5-HT(2C)R) agonist, mCPP, decreases plasma insulin concentration in mice. As such, we aimed to investigate the expression of the 5-HT(2C)R in pancreatic islets of diabetic mice and the role of 5-HT(2C)R signaling in insulin secretion from pancreatic β-cells. We found that 5-HT(2C)R expression was significantly increased in pancreatic islets of db/db mice. Furthermore, treatment with a 5-HT(2C)R antagonist (SB242084) increased insulin secretion from pancreatic islets isolated from db/db mice in a dose-dependent manner, but had no effect in islets from control mice. The effect of a 5-HT(2C)R agonist (mCPP) and antagonist (SB242084) were further studied in isolated pancreatic islets from mice and Min-6 cells. We found that mCPP significantly inhibited insulin secretion in Min-6 cells and isolated islets in a dose-dependent manner, which could be reversed by SB242084 or RNA interference against 5-HT(2C)R. We also treated Min-6 cells with palmitic acid for 24 h, and found that the expression of 5-HT(2C)R increased in a dose-dependent manner; furthermore, the inhibition of insulin secretion in Min-6 cells induced by palmitic acid could be reversed by SB242084 or RNA interference against 5-HT(2C)R. Taken together, our data suggests that increased expression of 5-HT(2C)R in pancreatic β-cells might inhibit insulin secretion. This unique observation increases our understanding of T2DM and suggests new avenues for potential treatment.

Melatonin promotes water-stress tolerance, lateral root formation, and seed germination in cucumber (Cucumis sativus L.)

A comprehensive investigation was carried out to determine the changes that occurred in water-stressed cucumber (Cucumis sativus L.) in response to melatonin treatment. We examined the potential roles of melatonin during seed germination and root generation and measured its effect on reactive oxygen species (ROS) levels, antioxidant enzyme activities, and photosynthesis. Melatonin alleviated polyethylene glycol induced inhibition of seed germination, with 100 μm melatonin-treated seeds showing the greatest germination rate. Melatonin stimulated root generation and vitality and increased the root:shoot ratio; therefore, melatonin may have an effect on strengthening cucumber roots. Melatonin treatment significantly reduced chlorophyll degradation. Seedlings treated with 100 μm melatonin clearly showed a higher photosynthetic rate, thus reversing the effect of water stress. Furthermore, the ultrastructure of chloroplasts in water-stressed cucumber leaves was maintained after melatonin treatment. The antioxidant levels and activities of the ROS scavenging enzymes, i.e., superoxide dismutase, peroxidase, and catalase, were also increased by melatonin. These results suggest that the adverse effects of water stress can be minimized by the application of melatonin.

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.

Melatonin in antinociception: its therapeutic applications

The intensity of pain sensation exhibits marked day and night variations. Since the intensity of pain perception is low during dark hours of the night when melatonin levels are high, this hormone has been implicated as one of the prime antinociceptive substances. A number of studies have examined the antinociceptive role of melatonin in acute, inflammatory and neuropathic pain animal models. It has been demonstrated that melatonin exerts antinociceptive actions by acting at both spinal cord and supraspinal levels. The mechanism of antinociceptive actions of melatonin involves opioid, benzodiazepine, α(1)- and α(2)-adrenergic, serotonergic and cholinergic receptors. Most importantly however, the involvement of MT(1)/MT(2) melatonergic receptors in the spinal cord has been well documented as an antinociceptive mechanism in a number of animal models of pain perception. Exogenous melatonin has been used effectively in the management of pain in medical conditions such as fibromyalgia, irritable bowel syndrome and migraine and cluster headache. Melatonin has been tried during surgical operating conditions and has been shown to enhance both preoperative and post-operative analgesia. The present review discusses the available evidence indicating that melatonin, acting through MT(1)/MT(2) melatonin receptors, plays an important role in the pathophysiological mechanism of pain.

Melatonin promotes adventitious root regeneration in in vitro shoot tip explants of the commercial sweet cherry rootstocks CAB-6P (Prunus cerasus L.), Gisela 6 (P. cerasus × P. canescens), and MxM 60 (P. avium × P. mahaleb)

The objectives of this study were to test the effects of melatonin (N-acetyl-5-methoxytryptamine), a natural compound of edible plants on the rooting of certain commercial sweet cherry rootstocks. Shoot tip explants from previous in vitro cultures of the cherry rootstocks CAB-6P (Prunus cerasus L.), Gisela 6 (P. cerasus × P. canescens), and M × M 60 (P. avium × P. mahaleb) were included in the experiment. The effect of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) alone or in combination with melatonin was tested concerning their rooting potential. Seven concentrations of melatonin (0, 0.05, 0.1, 0.5, 1, 5, and 10 μM) alone or in combination with 5.71 μM of IAA or 4.92 μM of IBA were tested. For each rootstock, 21 treatments were included. The explants were grown in glass tubes containing 10 mL of substrate. The parameters measured include rooting percentage, number of roots per rooted explant, root length, and callus formation. The data presented in this study show that melatonin has a rooting promoting effect at a low concentration but a growth inhibitory effect at high concentrations. In the absence of auxin, 1 μM melatonin had auxinic response concerning the number and length of roots, but 10 μM melatonin was inhibitory to rooting in all the tested rootstocks. The final conclusion of this experiment is that exogenously applied melatonin acted as a rooting promoter and its action was similar to that of IAA.

Functional roles of melatonin in plants, and perspectives in nutritional and agricultural science

The presence of melatonin in plants is universal. Evidence has confirmed that a major portion of the melatonin is synthesized by plants themselves even though a homologue of the classic arylalkylamine N-acetyltransferase (AANAT) has not been identified as yet in plants. Thus, the serotonin N-acetylating enzyme in plants may differ greatly from the animal AANAT with regard to sequence and structure. This would imply multiple evolutionary origins of enzymes with these catalytic properties. A primary function of melatonin in plants is to serve as the first line of defence against internal and environmental oxidative stressors. The much higher melatonin levels in plants compared with those found in animals are thought to be a compensatory response by plants which lack means of mobility, unlike animals, as a means of coping with harsh environments. Importantly, remarkably high melatonin concentrations have been measured in popular beverages (coffee, tea, wine, and beer) and crops (corn, rice, wheat, barley, and oats). Billions of people worldwide consume these products daily. The beneficial effects of melatonin on human health derived from the consumption of these products must be considered. Evidence also indicates that melatonin has an ability to increase the production of crops. The mechanisms may involve the roles of melatonin in preservation of chlorophyll, promotion of photosynthesis, and stimulation of root development. Transgenic plants with enhanced melatonin content could probably lead to breakthroughs to increase crop production in agriculture and to improve the general health of humans.

Chronopharmacognosy

This study aims to review the concept of biological rhythms in medicinal plants. Dictionariesgenerally define pharmacognosy as the subject of the study of crude drugs of plant and animal origin. The name is derived from the Greek words pharmakon (drug) and gnosis (knowledge). Today pharmacognosy is also defined as the study of physical, chemical, biochemical and biological properties of drugs, drug substances, or potential drugs or drug substances of natural origin, as well as the search for new drugs from natural sources. Also, another important phenomenon to be taken care of in the production of therapeutic compounds in medicinal plants is the use of circardian clock. The circardian clock is studied by chronobiology, which can be defined as a field of science that examines periodic (cyclic) phenomena in living organisms and their adaptation to solar and lunar related rhythms. Thus, it is the scientific study of the effect of time on living systems and of biological rhythms. Also rhythmic oscillations in plants lead to the enormous production of particular compounds in plants at particular time, which may or may not produce any therapeutic effect in humans. Thus, the study of chronobiology and pharmacognosy can be put together as chronopharmacognosy.

Application of LC and LC-MS to the analysis of melatonin and serotonin in edible plants

Melatonin is a neurohormone produced by the pineal gland of animals. Serotonin is a monoamine neurotransmitter and one of the precursors of melatonin biosynthesis. These two indoleamines have recently been reported to have widespread occurrence in many edible plants. Consuming foodstuffs containing melatonin and serotonin could raise their physiologic concentrations in blood and enhance human health. Literature concerning analytical methods suitable for determination of melatonin and serotonin in edible plants is limited, although several liquid chromatographic (LC) techniques have been used for their quantification. Liquid chromatography-mass spectrometry (LC-MS) methods combine selectivity, sensitivity, and high precision, and enable the simultaneous determination of melatonin and serotonin. This work reviews LC and LC-MS techniques used to determine melatonin and serotonin, and the available data on melatonin and serotonin levels in edible plants.

Melatonin combats molecular terrorism at the mitochondrial level

The intracellular environmental is a hostile one. Free radicals and related oxygen and nitrogen-based oxidizing agents persistently pulverize and damage molecules in the vicinity of where they are formed. The mitochondria especially are subjected to frequent and abundant oxidative abuse. The carnage that is left in the wake of these oxygen and nitrogen-related reactants is referred to as oxidative damage or oxidative stress. When mitochondrial electron transport complex inhibitors are used, e.g., rotenone, 1-methyl-1-phenyl-1,2,3,6-tetrahydropyridine, 3-nitropropionic acid or cyanide, pandemonium breaks loose within mitochondria as electron leakage leads to the generation of massive amounts of free radicals and related toxicants. The resulting oxidative stress initiates a series of events that leads to cellular apoptosis. To alleviate mitochondrial destruction and the associated cellular implosion, the cell has at its disposal a variety of free radical scavengers and antioxidants. Among these are melatonin and its metabolites. While melatonin stimulates several antioxidative enzymes it, as well as its metabolites (cyclic 3-hydroxymelatonin, N¹-acetyl-N²-formyl-5-methoxykynuramine and N¹-acetyl-5-methoxykynuramine), likewise effectively neutralize free radicals. The resulting cascade of reactions greatly magnifies melatonin's efficacy in reducing oxidative stress and apoptosis even in the presence of mitochondrial electron transport inhibitors. The actions of melatonin at the mitochondrial level are a consequence of melatonin and/or any of its metabolites. Thus, the molecular terrorism meted out by reactive oxygen and nitrogen species is held in check by melatonin and its derivatives.

Melatonin improves the survival of cryopreserved callus of Rhodiola crenulata

An important aspect of the function of melatonin seems to be the mediation of stress caused by environmental and chemical factors. In the cryopreservation process, environmental changes including osmotic injury, desiccation, and low temperature can impose a series of stresses on plants. In this study, we evaluated the role of melatonin in stress protection during the process of cryopreservation using callus of an endangered plant species Rhodiola crenulata. The survival rate of the cryopreserved callus significantly increased when the callus was pretreated for 5 days with 0.1 μm melatonin prior to freezing in liquid nitrogen. Analysis of antioxidative activity following the pretreatment of callus with 0.1 μm melatonin showed a significant reduction in malondialdehyde production during various steps of cryopreservation. Enhanced peroxidase and catalase activity was observed in the callus after pretreatment with 0.1 μm melatonin compared to the control. These observations provide new evidence of the antioxidant/anti-stress function of melatonin, and it is the first report of its potential application in the preservation of elite endangered germplasm through the process of cryopreservation.

Brain serotonin system in the coordination of food intake and body weight

An inverse relationship between brain serotonin and food intake and body weight has been known for more than 30 years. Specifically, augmentation of brain serotonin inhibits food intake, while depletion of brain serotonin promotes hyperphagia and weight gain. Through the decades, serotonin receptors have been identified and their function in the serotonergic regulation of food intake clarified. Recent refined genetic studies now indicate that a primary mechanism through which serotonin influences appetite and body weight is via serotonin 2C receptor (5-HT(2C)R) and serotonin 1B receptor (5-HT(1B)R) influencing the activity of endogenous melanocortin receptor agonists and antagonists at the melanocortin 4 receptor (MC4R). However, other mechanisms are also possible and the challenge of future research is to delineate them in the complete elucidation of the complex neurocircuitry underlying the serotonergic control of appetite and body weight.

Melatonin decreases cell proliferation and induces melanogenesis in human melanoma SK-MEL-1 cells

Melatonin is an indoleamine synthesized in the pineal gland, and after its release into the blood, it has an extensive repertoire of biological activities, including antitumoral properties. In this study, we found that melatonin reduced the growth of the human melanoma cells SK-MEL-1. The antiproliferative effect was associated with an alteration in the progression of the phases of the cell cycle and also with an increase in tyrosinase activity, the key regulatory enzyme of melanogenesis. Antagonists for melatonin membrane receptors (luzindole and 4-P-PDOT) and the general G-coupled receptor inhibitor, pertussis toxin, did not prevent the melatonin-induced cell growth arrest; this suggests a mechanism independent of G-coupled membrane receptors. In contrast, p38 mitogen-activated protein kinase (p38 MAPK) signaling pathway seems to play a significant role in cell growth inhibition by melatonin. The indoleamine-induced phosphorylation of p38 MAPK and the effect on cell proliferation were abrogated by the specific inhibitor SB203580. Furthermore, comparative studies with known antioxidants such as N-acetyl-l-cysteine and trolox indicate that the growth of SK-MEL-1 cells is highly sensitive to antioxidants.

Changes in the levels of indoleamine phytochemicals during véraison and ripening of wine grapes

Melatonin and serotonin have previously been described in mature wine grapes and finished wines, but the metabolism of these signalling molecules in the development of wine grapes has not previously been investigated. We harvested wine grapes at different stages of development from lag phase through véraison from eight different commercial vineyards representing a diversity of growing conditions, management practices, merlot varietals and localized ecosystems to determine whether different patterns in melatonin and serotonin can be found in wine grapes during seed development and berry maturation. Melatonin was detected in 45% of the fully developed purple, postvéraison grapes but only found in 23% of prelag phase samples. However, the actual concentration of melatonin was highest in wine grapes harvested at the early stage of véraison when the seed is developing. Serotonin was not detected in any of the prelag phase grapes but was consistently detected in 30-35% of grapes harvested during the véraison transition at consistent levels of about 8-10 mug/g. Interestingly, the nitrogen storage compound gamma-aminobutyric acid was also found at about 115 mug/g in 77% of early stage green grapes and declined in both prevalence and concentration with ripening. Together, these data are indicative of a potential role for these molecules in the development and maturation of wine grapes.

Jerte Valley cherry-enriched diets improve nocturnal rest and increase 6-sulfatoxymelatonin and total antioxidant capacity in the urine of middle-aged and elderly humans

Tryptophan, serotonin, and melatonin, present in Jerte Valley cherries, participate in sleep regulation and exhibit antioxidant properties. The effect of the intake of seven different Jerte Valley cherry cultivars on the sleep-wake cycle, 6-sulfatoxymelatonin levels, and urinary total antioxidant capacity in middle-aged and elderly participants was evaluated. Volunteers were subjected to actigraphic monitoring to record and display the temporal patterns of their nocturnal activity and rest. 6-sulfatoxymelatonin and total antioxidant capacity were quantified by enzyme-linked immunosorbent assay and colorimetric assay kits, respectively. The intake of each of the cherry cultivars produced beneficial effects on actual sleep time, total nocturnal activity, assumed sleep, and immobility. Also, there were significant increases in 6-sulfatoxymelatonin levels and total antioxidant capacity in urine after the intake of each cultivar. These findings suggested that the intake of Jerte Valley cherries exerted positive effect on sleep and may be seen as a potential nutraceutical tool to counteract oxidation.

Melatonin and serotonin in flowers and fruits of Datura metel L

Datura metel is a plant that contains several different neurologically active phytochemicals which affect human health. On-going research has examined the potential role of the human neuroindoles, melatonin and serotonin, in medicinal plants with neurological efficacy. In this report, we describe the quantification of melatonin and serotonin in flowers and developing fruits of Datura metel and the effects of cold stress on the levels of these neuroindoles in the reproductive tissues of this plant. Melatonin and serotonin were found at the highest levels in the least developed flower buds with decreasing concentrations as the flower buds matured. Cold stress significantly increased the concentration of melatonin in young flower buds. In the developing fruit, melatonin was present at relatively stable, high concentrations for the first 10 days after anthesis. After 10-15 days, the ovule had grown to a sufficient size for excision and analysis and melatonin was found to be at the highest concentrations in the developing ovule with minimal concentrations of the neuroindoles in the fleshy fruit. Together, these data indicate that melatonin may play a role in protecting the reproductive tissues during flower and seed formation in a Datura species.

Circadian rhythm of Z- and E-2-beta-D: -glucopyranosyloxy-4-methoxy cinnamic acids and herniarin in leaves of Matricaria chamomilla

Chamomile (Matricaria chamomilla) in the above-ground organs synthesizes and accumulates (Z)- and (E)-2-beta-D: -glucopyranosyloxy-4-methoxy cinnamic acids (GMCA), the precursors of phytoanticipin herniarin (7-methoxycoumarin). The diurnal rhythmicity of the sum of GMCA (maximum before daybreak) and herniarin (acrophase at 10 h 21 min of circadian time) was observed under artificial lighting conditions LD 12:12. The acrophase is the time point of the maximum of the sinusoidal curve fitted to the experimental data. In continuous light, the circadian rhythms of both compounds were first described with similar acrophases of endogenous rhythms; a significantly different result from that in synchronized conditions. The rhythms' mesor (the mean value of the sinusoidal curve fitted to the experimental data) under free-running conditions was not influenced. Abiotic stress under synchronized conditions decreased the average content of GMCA to half of the original level and eliminated the rhythmicity. In contrast, the rhythm of herniarin continued, though its content significantly increased. Nitrogen deficiency resulted in a significant increase in GMCA content, which did not manifest any rhythmicity while the rhythm of herniarin continued. Circadian control of herniarin could be considered as a component of the plant's specialized defence mechanisms.

Cloning and characterization of a Chlamydomonas reinhardtii cDNA arylalkylamine N-acetyltransferase and its use in the genetic engineering of melatonin content in the Micro-Tom tomato

Melatonin is found in a wide variety of plant species. Several investigators have studied the physiological roles of melatonin in plants. However, its role is not well understood because of the limited information on its biosynthetic pathway. To clarify melatonin biosynthesis in plants, we isolated a cDNA-coded arylalkylamine N-acetyltransferase (AANAT), a possible limiting enzyme for melatonin biosynthesis, from Chlamydomonas reinhardtii (designated as CrAANAT). The predicted amino acid sequence of CrAANAT shares 39.0% homology to AANAT from Ostreococcus tauri and lacks cAMP-dependent protein kinase phosphorylation sites in the N- and C-terminal regions that are conserved in vertebrates. The enzyme activity of CrAANAT was confirmed by in vitro assay using Escherichia coli. Transgenic plants constitutively expressing the CrAANAT were produced using Micro-Tom, a model cultivar of tomato (Solanum lycopersicum L.). The transgenic Micro-Tom exhibited higher melatonin content compared with wild type, suggesting that melatonin was synthesized from serotonin via N-acetylserotonin in plants. Moreover, the melatonin-rich transgenic Micro-Tom can be used to elucidate the role of melatonin in plant development.

Bioactivity of Grape Chemicals for Human Health

Grapevine ( Vitis vinifera) products, grape and grape juice, represent a valuable source of bioactive phytochemicals, synthesized by three secondary metabolic pathways (phenylpropanoid, isoprenoid and alkaloid biosynthetic routes) and stored in different plant tissues. In the last decades, compelling evidence suggested that regular consumption of these products may contribute to reducing the incidence of chronic illnesses, such as cancer, cardiovascular diseases, ischemic stroke, neurodegenerative disorders and aging, in a context of the Mediterranean dietary tradition. The health benefits arising from grape product intake can be ascribed to the potpourri of biologically active chemicals occurring in grapes. Among them, the recently discovered presence of melatonin adds a new element to the already complex grape chemistry. Melatonin, and its possible synergistic action with the great variety of polyphenols, contributes to further explaining the observed health benefits associated with regular grape product consumption.

Profiling of melatonin in the model tomato (Solanum lycopersicum L.) cultivar Micro-Tom

Melatonin exists in a considerable variety of plant species. However, the physiological roles of melatonin in plants are not well understood. In this study, the distribution and accumulation of melatonin during leaf and fruit development were analyzed in Micro-Tom, a model cultivar of tomato (Solanum lycopersicum L.). Melatonin was extracted using an acetone-methanol method and measured by enzyme-linked immunosorbent assay. Melatonin was detected in leaves, stems, roots, flowers, fruits, seedlings and seeds in the range of 1.5-66.6 ng/g fresh weight, with seeds containing the highest concentration of melatonin. In fruits and leaves, melatonin concentrations varied depending on the developmental stage, suggesting that melatonin controls some of the processes involved in plant maturation.

Exogenously applied melatonin stimulates root growth and raises endogenous indoleacetic acid in roots of etiolated seedlings of Brassica juncea

Exogenous melatonin was applied to etiolated seedlings of wild leaf mustard (Brassica juncea) and the effect on root growth and endogenous indole-3-acetic acid (IAA) levels determined. The results show that 0.1microM melatonin has a stimulatory effect on root growth, while 100microM is inhibitory. Furthermore, the stimulatory effect was only detectable in young seedlings (2-d old). Older seedlings (4-d old) appear to be less susceptible to both the stimulatory and the inhibitory effect of melatonin. Exogenous application of 0.1microM melatonin also raised the endogenous levels of free IAA in roots, while higher concentrations had no significant effect. The specific mechanism that causes exogenous melatonin to increase the amount of free IAA in roots, paired with a stimulation of root growth, remains to be uncovered.

Phytomelatonin: a review

Melatonin (N-acetyl-5-methoxytryptamine) has been detected in a number of plant species. Indeed, there exists evidence that this classically-considered animal indole is actually both synthesized in and taken up by plants. Among the actions that melatonin may carry out in plant tissues, its role as an antioxidant or growth promoter is most strongly supported by the experimental evidence. Other suggested functional implications include the co-ordination of photoperiodic responses and regulation of plant reproductive physiology, defence of plant cells against apoptosis induced by harsh environmental conditions, its participation as a free radical scavenging agent and/or up-regulator of certain protective enzymes in the senescent process. This review presents a detailed summary of the investigations that have been performed to date in the plant melatonin (phytomelatonin) field. The purpose of this summary is to bring the reader up to date on what is known about melatonin in plants and to encourage plant scientists to investigate this novel research topic; this would certainly assist in solving the numerous questions that still remain regarding the role of melatonin in plants.

Assessment of different sample processing procedures applied to the determination of melatonin in plants

INTRODUCTION

Melatonin, an indoleamine well known in vertebrates and structurally related to other important substances such as tryptophan or indole-3-acetic acid, is also present in the plant kingdom although its specific function(s) remain to be established. The emerging field of melatonin studies in plants has progressed very slowly, mainly due to the problems associated with melatonin quantification in plants.

OBJECTIVE

Two commonly used procedures for plant samples are compared. The analytical characteristics of both procedures are quantitatively presented using different solvents and small amounts of fresh biological material, and the respective recovery rates and quantitative limits are presented. Some improvements are suggested.

METHODOLOGY

Two different sample extraction procedures were compared: a direct-sample extraction (DSE) and a homogenised- sample extraction (HSE). Melatonin was then determined in the respective plant samples by HPLC with fluorescence detection.

RESULTS

Using the DSE procedure, more than 94% melatonin was recovered from standard solutions, whereas levels higher than 93% were recovered from the spiked plant samples, with little difference between ethyl acetate and chloroform extractions. In the case of HSE, the recoveries of melatonin were approximately half and never higher than 55%. The ultrasonic treatment proposed in the DSE procedure showed different levels of efficiency (2-20%), depending on the sample.

CONCLUSION

This study has established that, with the direct sample extraction procedure, higher recovery rates are obtained both in standard solutions and in plant samples. The straightforwardness and reproducibility of the extraction procedure is accompanied by the high sensitivity obtained with fluorescence detection.

Protective effect of melatonin against chlorophyll degradation during the senescence of barley leaves

Melatonin (N-acetyl-5-methoxytryptamine) is a highly conserved molecule whose presence is not exclusive to the animal kingdom. Indeed, numerous studies have demonstrated its presence in plants, where the possible role(s) of this indoleamine is (are) under active investigation. The present work aims to further our knowledge in this respect and presents the results of a study of the effect that melatonin has on foliar senescence. Barley leaves treated with melatonin solutions clearly slowed down the senescence process, as estimated from the chlorophyll lost in leaves. This effect of melatonin was concentration dependent, with an optimal response being obtained at 1 mm melatonin, after 48 hr of incubation in darkness. The already known effects of the phytohormones, kinetin, and abscisic acid, were also assayed. Of the phytohormone and melatonin combinations assayed, 1 mm melatonin presented the best protection against senescence. The levels of endogenous melatonin in control leaves were measured by liquid chromatography with fluorescence detection and in leaves treated with different exogenous melatonin concentrations (to demonstrate the absorption capacity of leaves). The possible physiological implications of this newly revealed action of melatonin in foliar senescence are discussed.

Distribution of melatonin in different zones of lupin and barley plants at different ages in the presence and absence of light

In animals, melatonin (N-acetyl-5-methoxytryptamine) has several physiological roles, mostly related with circadian and seasonal rhythms. In 1995, it was detected in a variety of edible plants, and it is known that melatonin from plant foods is absorbed from the gastrointestinal tract and incorporated in the blood stream. This indoleamine also crosses the blood-brain barrier and the placenta, being incorporated at the subcellular level. The possibility of modulating blood melatonin levels in mammals and avians through the ingestion of plant foodstuffs seems to be an interesting prospect. However, data concerning the melatonin content of edible plants are scarce and have not been contrasted. Obtained with very different analytical techniques, in some cases inappropriate, the quantitative data show a high degree of variation. Possibly for the first time in plants, we have used liquid chromatography with time-of-flight/mass spectrometry to identify melatonin. This sophisticated technique, combined with the more commonly used liquid chromatography with fluorescence detection for melatonin quantification, has permitted us to describe the distribution of this compound in different organs and zones in plants. Also, changes in melatonin levels with age and the possible influence of a light/dark photoperiod or constant darkness on its levels are studied. The proposal, applied here to lupin (Lupinus albus L.) and barley (Hordeum vulgare L.), may also serve as a model for application to other plant foodstuffs.

Plant neurobiology: from sensory biology, via plant communication, to social plant behavior

In plants, numerous parameters of both biotic and abiotic environments are continuously monitored. Specialized cells are evolutionary-optimized for effective translation of sensory input into developmental and motoric output. Importantly, diverse physical forces, influences, and insults induce immediate electric responses in plants. Recent advances in plant cell biology, molecular biology, and sensory ecology will be discussed in the framework of recently initiated new discipline of plant sciences, namely plant neurobiology.

Presowing seed treatment with melatonin protects red cabbage seedlings against toxic copper ion concentrations

One of the targets of modern plant physiology is to identify tools for improving seed germination and plant growth under unfavorable environmental conditions. Seeds of Brassica oleracea rubrum were pretreated with melatonin at concentrations: 1, 10, and 100 microM using a hydropriming method. Air-dried seeds of each experimental variants that were nonpretreated (control), hydroprimed (H) or hydroprimed with melatonin (HM1, HM10, and HM100) were germinated in darkness for 3 days at 25 degrees C. Young seedlings were then transferred to the light and grown for an additional 5 days. Both germination and growth tests were performed in water and in CuSO(4) water solutions in concentrations of 0.5 and 1 mM. H, HM1 and HM10 improved seed germination both in water and in the presence of Cu(2+). One or 10 microM melatonin eliminated the inhibitory effect of the 0.5 mM metal concentration on the fresh weight of seedlings. HM100 had a negative effect; thus seed germination was lower and seedlings had poor establishment. The toxic effect of Cu(2+) manifested by membrane peroxidation and DNA endoreplication blocking in the seedlings grown from nontreated (control) and H seeds was not observed in the seedlings grown from HM1 and HM10 seeds; in contrast, HM100 enhanced the toxic effect of Cu(2+).

Photo-degradation of melatonin: influence of argon, hydrogenperoxide, and ethanol

When organic compounds are irradiated with UV light at 254 nm, part of their covalent bonds can dissociate if the compound absorbs light at that wavelength. Therefore, photo-degradation depends strongly on the wavelength used. The energy of a light quanta at 254 nm amounts to approximately 110 kcal/mol quanta, which is in many cases higher than the binding energy of a variety of covalent bonds. As a consequence, the absorbing molecule is degraded. As melatonin absorbs light at 254 nm, this compound is vulnerable to UV light. In order to minimize undesired effects of other absorbing substances, we used as solvent mostly pure water and analyzed the influence of lambda irr = 254 nm on the disappearance of the educt (melatonin) as well as on the appearance of products derived from melatonin in the presence of oxygen, argon, hydrogen peroxide, and ethanol by UV-VIS spectroscopy and high-performance liquid chromatography (HPLC) technique. N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) appears to be the main, but obviously not the only product of photo-degradation of melatonin, independently of whether the system contains oxygen or not. If the system contains additionally hydrogen peroxide, a very strong oxidant, the hydroxyl radical (*OH), is formed. Under such conditions, melatonin is not solely photo-degraded but also attacked by the formed *OH which interact similarly with the formed main photo-product AFMK. Ethanol, as a potent scavenger of *OH, efficiently blocks the effect of this aggressive radical even at low concentrations of that scavenger (0.1% v/v) but is less effective in preventing photo-degradation of melatonin.

Melatonin as an antibiotic: new insights into the actions of this ubiquitous molecule

The incidence of serious infections caused by multidrug-resistant gram-positive and gram-negative bacteria has been increasing rapidly worldwide despite advances in antibacterial therapy in the last two decades. Among multidrug-resistant gram-positive and gram-negative bacteria, methicillin-resistant Staphylococcus aureus, carbapenem-resistant Pseudomonas aeruginosa and Acinetobacter baumannii are of great importance, because they have emerged as primary nosocomial pathogens in hospital outbreaks. In this study, we investigated whether melatonin has antibacterial effects against these microorganisms in vitro. The minimum inhibitory concentration of melatonin was determined using a standard microdilution method at 24 and 48 hr. Melatonin inhibited microbial growth at both 24 and 48 hr; but results showed that melatonin had antibacterial effects against these microorganisms after 48 hr of incubation in lower doses [concentrations between 31.25 to 125 microg/mL (0.13-0.53 mM)]. Also, it was determined that melatonin has a more potent antimicrobial effect on gram-negative microorganism. Among possible mechanisms, it is concluded that melatonin showed antibacterial effects by reducing intracellular substrates.

Melatonin in Plants: More Studies are Necessary

Melatonin (N-acetyl-5-methoxytryptamine) is a biogenic indoleamine structurally related with other important substances such as tryptophan, serotonin, indole-3-acetic acid (IAA). In mammals, birds, reptiles and fish melatonin is a biological modulator of several timing (circadian) processes such as mood, sleep, sexual behavior, immunological status, etc. Since its discovery in plants in 1995 several physiological roles, including a possible role in flowering, circadian rhythms and photoperiodicity and as growth-regulator have been postulated. Recently, a possible role in rhizogenesis in lupin has also been proposed. Here, these actions of melatonin in plant development are commented on and some other interesting recent data concerning melatonin in plants are also discussed. The need for more investigation into melatonin and plants is presented as an obvious conclusion.

The mode of action of thidiazuron: auxins, indoleamines, and ion channels in the regeneration of Echinacea purpurea L

The biochemical mechanisms underlying thidiazuron (TDZ)-induced regeneration in plant cells have not been clearly elucidated. Exposure of leaf explants of Echinacea purpurea to a medium containing TDZ results in undifferentiated cell proliferation and differentiated growth as mixed shoot organogenesis and somatic embryogenesis. The current studies were undertaken to determine the potential roles of auxin, indoleamines, and ion signaling in the dedifferentiation and redifferentiation of plant cells. E. purpurea leaf explants were found to contain auxin and the related indoleamine neurotransmitters, melatonin, and serotonin. The levels of these endogenous indoleamines were increased by exposure to TDZ associated with the induction of regeneration. The auxin-transport inhibitor 2,3,5-triiodobenzoic acid and auxin action inhibitor, p-chlorophenoxyisobutyric acid decreased the TDZ-induced regeneration but increased concentrations of endogenous serotonin and melatonin. As well, inhibitors of calcium and sodium transport significantly reduced TDZ-induced morphogenesis while increasing endogenous indoleamine content. These data indicate that TDZ-induced regeneration is the manifestation of a metabolic cascade that includes an initial signaling event, accumulation, and transport of endogenous plant signals such as auxin and melatonin, a system of secondary messengers, and a concurrent stress response.

Melatonin promotes adventitious- and lateral root regeneration in etiolated hypocotyls of Lupinus albus L

Melatonin is a well-known animal substance, which has recently been detected in plant tissues. However, there are only a few studies concerning its possible physiological role in plants. In this paper, we investigate the possible effect of melatonin on the regeneration of lateral and adventious roots in etiolated hypocotyls of Lupinus albus L. compared with the effect of indole-3-acetic acid. We performed this study by measuring both molecules in roots. Six-day-old derooted lupin hypocotyls immersed in several melatonin or indole-3-acetic acid concentrations were used to induce roots. A macro- and microscopic study of the histological origin of the adventitious and lateral roots was made, while melatonin and indole-3-acetic acid in the roots were quantified using liquid chromatography with fluorescence detection. The data show that both melatonin and indole-3-acetic acid induced the appearance of root primordia from pericicle cells, modifying the pattern of distribution of adventitious or lateral roots, the time-course, the number and length of adventitious roots, and the number of lateral roots. Melatonin and indole-3-acetic acid were detected and quantified in lupin primary roots, where both molecules were present in similar concentrations. The physiological effect of exogenous melatonin as root promoter was demonstrated, its action being similar to that of indole-3-acetic acid.

Inhibition of proliferation and induction of apoptosis by melatonin in human myeloid HL-60 cells

Melatonin is an indoleamine that is synthesized in the pineal gland and has an extensive repertoire of biological activities. In the present study, we found that melatonin reduced the growth of the human myeloid leukemia cells HL-60, inhibiting progression from G(1) to S phase of the cell cycle and increasing apoptotic cell death. Furthermore, melatonin treatment elevated cytochrome c release from mitochondria and augmented caspase-3 and caspase-9 activities. Upregulation of Bax and downregulation of Bcl-2 was also observed upon melatonin treatment. The effects of melatonin were found not to be mediated by membrane receptors for the indoleamine. Together, our results suggest that melatonin reduces the viability of HL-60 cells via induction of apoptosis primarily through regulation of Bax/Bcl-2 expression.