Melatonin in Plants - Diversity of Levels and Multiplicity of Functions

The case of tryptamine and serotonin in plants: a mysterious precursor for an illustrious metabolite

Indolamines are tryptophan-derived specialized metabolites belonging to the huge and ubiquitous indole alkaloids group. Serotonin and melatonin are the best-characterized members of this family, given their many hormonal and physiological roles in animals. Following their discovery in plants, the study of plant indolamines has flourished and their involvement in important processes, including stress responses, growth and development, and reproduction, has been proposed, leading to their classification as a new category of phytohormones. However, the complex indolamine puzzle is far from resolved, particularly the biological roles of tryptamine, the early serotonin precursor representing the central hub of many downstream indole alkaloids. Tryptophan decarboxylase, which catalyzes the synthesis of tryptamine, strictly regulates the flux of carbon and nitrogen from the tryptophan pool into the indolamine pathway. Furthermore, tryptamine accumulates to high levels in the reproductive organs of many plant species and therefore cannot be classed as a mere intermediate but rather as an end product with potentially important functions in fruits and seeds. This review summarizes current knowledge on the role of tryptamine and its close relative serotonin, emphasizing the need for a clear understanding of the functions of, and mutual relations between, these indolamines and their biosynthesis pathways in plants.

Exogenous melatonin improves the salt tolerance of cotton by removing active oxygen and protecting photosynthetic organs

BACKGROUND

As damage to the ecological environment continues to increase amid unreasonable amounts of irrigation, soil salinization has become a major challenge to agricultural development. Melatonin (MT) is a pleiotropic signal molecule and indole hormone, which alleviates the damage of abiotic stress to plants. MT has been confirmed to eliminate reactive oxygen species (ROS) by improving the antioxidant system and reducing oxidative damage under adversity. However, the mechanism by which exogenous MT mediates salt tolerance by regulating the photosynthetic capacity and ion balance of cotton seedlings still remains unknown. In this study, the regulatory effects of MT on the photosynthetic system, osmotic modulators, chloroplast, and anatomical structure of cotton seedlings were determined under 0-500 μM MT treatments with salt stress induced by treatment with 150 mM NaCl.

RESULTS

Salt stress reduces the chlorophyll content, net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate, PSII photochemical efficiency, PSII actual photochemical quantum yield, the apparent electron transfer efficiency, stomata opening, and biomass. In addition, it increases non-photochemical quenching. All of these responses were effectively alleviated by exogenous treatment with MT. Exogenous MT reduces oxidative damage and lipid peroxidation by reducing salt-induced ROS and protects the plasma membrane from oxidative toxicity. MT also reduces the osmotic pressure by reducing the salt-induced accumulation of Na⁺ and increasing the contents of K⁺ and proline. Exogenous MT can facilitate stomatal opening and protect the integrity of cotton chloroplast grana lamella structure and mitochondria under salt stress, protect the photosynthetic system of plants, and improve their biomass. An anatomical analysis of leaves and stems showed that MT can improve xylem and phloem and other properties and aides in the transportation of water, inorganic salts, and organic substances. Therefore, the application of MT attenuates salt-induced stress damage to plants. Treatment with exogenous MT positively increased the salt tolerance of cotton seedlings by improving their photosynthetic capacity, stomatal characteristics, ion balance, osmotic substance biosynthetic pathways, and chloroplast and anatomical structures (xylem vessels and phloem vessels).

CONCLUSIONS

Our study attributes help to protect the structural stability of photosynthetic organs and increase the amount of material accumulation, thereby reducing salt-induced secondary stress. The mechanisms of MT-induced plant tolerance to salt stress provide a theoretical basis for the use of MT to alleviate salt stress caused by unreasonable irrigation, fertilization, and climate change.

Melatonin delays ovarian aging in mice by slowing down the exhaustion of ovarian reserve

Studies have shown that melatonin (MLT) can delay ovarian aging, but the mechanism has not been fully elucidated. Here we show that granulosa cells isolated from mice follicles can synthesize MLT; the addition of MLT in ovary culture system inhibited follicle activation and growth; In vivo experiments indicated that injections of MLT to mice during the follicle activation phase can reduce the number of activated follicles by inhibiting the PI3K-AKT-FOXO3 pathway; during the early follicle growth phase, MLT administration suppressed follicle growth and atresia, and multiple pathways involved in folliculogenesis, including PI3K-AKT, were suppressed; MLT deficiency in mice increased follicle activation and atresia, and eventually accelerated age-related fertility decline; finally, we demonstrated that prolonged high-dose MLT intake had no obvious adverse effect. This study presents more insight into the roles of MLT in reproductive regulation that endogenous MLT delays ovarian aging by inhibiting follicle activation, growth and atresia.

Melatonin promotes seed germination under salt stress by regulating ABA and GA3 in cotton (Gossypium hirsutum L.)

Although previous studies have found that melatonin can promote seed germination, the phytohormone regulation mechanism by which exogenous melatonin mediates salt tolerance during cotton seed germination is still largely unknown. The effects of melatonin on germination traits and physiological parameters of GXM9 cotton seeds (Gossypium hirsutum L.) under three salt stress treatments (CK, germination of seeds pretreated with water alone; S, germination of seeds pretreated in 150 mM NaCl under salt stress; SM, germination of seeds pretreated in 20 μM melatonin under 150 mM NaCl solution) in the laboratory was investigated. The results showed that salt stress (150 mM) inhibited cotton seed germination and endogenous melatonin accumulation, and pretreatment with 20 μM exogenous melatonin enhanced the cotton germination rate and hypocotyl length as well as the content of endogenous melatonin during seed germination. This suggests that exogenous melatonin promotes seed germination from a morphological perspective. The contents of starch, α-amylase (EC3.3.1.1), β-galactosidase (EC3.2.1.23), abscisic acid (ABA), and gibberellin (GA) were determined simultaneously. The results showed that the α-amylase and β-galactosidase contents in the cotton seeds decreased by 56.97% and 20.18%, respectively, under salt stress compared with the control, while the starch content increased by 11.53% compared with the control at day 7. The ABA content increased by 25.18% and GA content decreased by 27.99% under salt stress compared with the control at 24 h. When exogenous melatonin was applied to the cotton seeds, the content of α-amylase and β-galactosidase increased by 121.77% and 32.76%, respectively, whereas the starch contents decreased by 13.55% compared with the S treatment at day 7. Similarly, the ABA content increased by 12.20% and the GA content increased by 4.77% at 24 h. To elucidate the molecular mechanism by which melatonin promotes seed germination under salt stress, the effects of ABA- and GA-related genes on plant hormone signal transduction were analyzed by quantitative real-time PCR and RNA sequencing. The results indicated that melatonin regulated the expression of ABA and GA genes in the plant signal transduction pathway, induced embryo root development and seed germination, and alleviated dormancy. The expression of the ABA signaling gene GhABF2 was up-regulated and GhDPBF2 was down-regulated, and the expression of GA signaling genes (e.g., GhGID1C and GhGID1B) was up-regulated by melatonin. In conclusion, melatonin enhances salt tolerance in cotton seeds by regulating ABA and GA and by mediating the expression of hormone-related genes in plant hormone signal transduction. This should help us to explore the regulatory mechanisms of cotton resistance and provide a foundation for the cultivation of new varieties.

Exogenous melatonin positively regulates lignin biosynthesis in Camellia sinensis

Melatonin (MT) is a bioactive molecule that can regulate various developmental processes. Changes in lignin content play important roles in plant growth and development. Herein, quantitative analysis and histochemical staining showed that lignin content significantly increased over time, and melatonin treatment triggered the lignification at 8 and 16 d in tea leaves. The POD activity participated in lignin formation had also been significantly improved. The effect of melatonin on the increase of lignin content was attenuation over time. Sequencing results based on transcriptome at 8 and 16 d showed that 5273 and 3019 differentially expressed genes (DEGs) were identified in CK1 vs. MT1 and CK2 vs. MT2, respectively. A total of 67 DEGs were annotated to lignin biosynthesis, and 38 and 9 genes were significantly up-regulated under melatonin treatment, respectively. Some transcription factor genes such as MYB were also identified among the two pairwise comparisons, which might relate to lignin metabolism. Melatonin increased the degree of lignification in tea leaves by modifying the enzyme genes expression involved in lignin synthesis pathway. These results provide a reference for further study on the molecular mechanism of the dynamic changes of lignin content induced by melatonin treatment in tea plants.

Phytomelatonin in stress management in agriculture

Melatonin was discovered as a pineal gland hormone in animals and is now more significantly known as a signaling molecule in plants' biotic and abiotic stressors. Melatonin has been traced back to prokaryotic organisms during evolution and its primary function of antioxidant scavenging free radicals in photosynthetic prokaryotic bacteria is a lesser explored and exciting area for further research globally. The authors at IIT Delhi are trying to establish its potential role in stress management in agriculture. The present manuscript addresses the biosynthetic pathways hitherto suggested by scientists. In this manuscript, the potential scope of melatonin in agriculture as a growth promoter, post-harvest loss inhibitor, and signaling and quality improvement molecule is envisaged.

Melatonin confers heavy metal-induced tolerance by alleviating oxidative stress and reducing the heavy metal accumulation in Exophiala pisciphila, a dark septate endophyte (DSE)

Background

Melatonin (MT), ubiquitous in almost all organisms, functions as a free radical scavenger. Despite several reports on its role as an antioxidant in animals, plants, and some microorganisms, extensive studies in filamentous fungi are limited. Based upon the role of melatonin as an antioxidant, we investigated its role in heavy metal-induced stress tolerance in Exophiala pisciphila, a dark septate endophyte (DSE), by studying the underlying mechanisms in alleviating oxidative stress and reducing heavy metal accumulation.

Results

A significant decrease in malondialdehyde (MDA) and oxygen free radical (OFR) in E. pisciphila was recorded under Cd, Zn, and Pb stresses as compared to the control. Pretreatment of E. pisciphila with 200.0 μM exogenous melatonin significantly increased the activity of superoxide dismutase (SOD) under Zn and Pb stresses. Pretreatment with 200.0 μM melatonin also lowered Cd, Zn, and Pb concentrations significantly. Melatonin production was enhanced by Cd, Cu, and Zn after 2 d, and melatonin biosynthetic enzyme genes, E. pisciphila tryptophan decarboxylase (EpTDC1) and serotonin N-acetyltransferase (EpSNAT1), were transcriptionally upregulated. The overexpression of EpTDC1 and N-acetylserotonin O-methyltransferase (EpASMT1) in Escherichia coli and Arabidopsis thaliana enhanced its heavy metal-induced stress tolerance. The overexpression of EpTDC1 and EpASMT1 reduced the Cd accumulation in the whole A. thaliana plants, especially in the roots.

Conclusions

Melatonin conferred heavy metal-induced stress tolerance by alleviating oxidative stress, activating antioxidant enzyme SOD, and reducing heavy metal accumulation in E. pisciphila. Melatonin biosynthetic enzyme genes of E. pisciphila also played key roles in limiting excessive heavy metal accumulation in A. thaliana. These findings can be extended to understand the role of melatonin in other DSEs associated with economically important plants and help develop new strategies in sustainable agriculture practice where plants can grow in soils contaminated with heavy metals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02098-1.

Bioregulators: unlocking their potential role in regulation of the plant oxidative defense system

Plant bioregulators play an important role in managing oxidative stress tolerance in plants. Utilizing their ability in stress sensitive crops through genetic engineering will be a meaningful approach to manage food production under the threat of climate change. Exploitation of the plant defense system against oxidative stress to engineer tolerant plants in the climate change scenario is a sustainable and meaningful strategy. Plant bioregulators (PBRs), which are important biotic factors, are known to play a vital role not only in the development of plants, but also in inducing tolerance in plants against various environmental extremes. These bioregulators include auxins, gibberellins, cytokinins, abscisic acid, brassinosteroids, polyamines, strigolactones, and ascorbic acid and provide protection against the oxidative stress-associated reactive oxygen species through modulation or activation of a plant's antioxidant system. Therefore, exploitation of their functioning and accumulation is of considerable significance for the development of plants more tolerant of harsh environmental conditions in order to tackle the issue of food security under the threat of climate change. Therefore, this review summarizes a new line of evidence that how PBRs act as inducers of oxidative stress resistance in plants and how they could be modulated in transgenic crops via introgression of genes. Reactive oxygen species production during oxidative stress events and their neutralization through an efficient antioxidants system is comprehensively detailed. Further, the use of exogenously applied PBRs in the induction of oxidative stress resistance is discussed. Recent advances in engineering transgenic plants with modified PBR gene expression to exploit the plant defense system against oxidative stress are discussed from an agricultural perspective.

Melatonin: A master regulator of plant development and stress responses

Melatonin is a pleiotropic molecule with multiple functions in plants. Since the discovery of melatonin in plants, numerous studies have provided insight into the biosynthesis, catabolism, and physiological and biochemical functions of this important molecule. Here, we describe the biosynthesis of melatonin from tryptophan, as well as its various degradation pathways in plants. The identification of a putative melatonin receptor in plants has led to the hypothesis that melatonin is a hormone involved in regulating plant growth, aerial organ development, root morphology, and the floral transition. The universal antioxidant activity of melatonin and its role in preserving chlorophyll might explain its anti-senescence capacity in aging leaves. An impressive amount of research has focused on the role of melatonin in modulating postharvest fruit ripening by regulating the expression of ethylene-related genes. Recent evidence also indicated that melatonin functions in the plant's response to biotic stress, cooperating with other phytohormones and well-known molecules such as reactive oxygen species and nitric oxide. Finally, great progress has been made towards understanding how melatonin alleviates the effects of various abiotic stresses, including salt, drought, extreme temperature, and heavy metal stress. Given its diverse roles, we propose that melatonin is a master regulator in plants.

Metabolite and Transcriptome Profiling Analysis Revealed That Melatonin Positively Regulates Floral Scent Production in Hedychium coronarium

Melatonin is a pleiotropic molecule that regulates a variety of developmental processes. Floral volatiles are important features of flowers that facilitate flower-visitor interactions by attracting pollinators, structure flower-visitor communities, and play defensive roles against plant and flower antagonists. Aside from their role in plants, floral volatiles are an essential ingredient in cosmetics, perfumes, pharmaceuticals, and flavorings. Herein, integrated metabolomic and transcriptomic approaches were carried out to analyze the changes triggered by melatonin exposure during the Hedychium coronarium flower development stages. Quantitative analysis of the volatiles of H. coronarium flowers revealed that volatile organic compound emission was significantly enhanced after melatonin exposure during the half bloom (HS), full bloom (FB) and fade stage (FS). Under the melatonin treatment, the emission of volatile contents was highest during the full bloom stage of the flower. Variable importance in projection (VIP) analysis and partial least-squares discriminant analysis (PLS-DA) identified 15 volatile compounds with VIP > 1 that were prominently altered by the melatonin treatments. According to the transcriptome sequencing data of the HS, FB, and FS of the flowers, 1,372, 1,510, and 1,488 differentially expressed genes were identified between CK-HS and 100MT-HS, CK-FB and 100MT-FB, and CK-FS and 100MT-FS, respectively. Among the significant differentially expressed genes (DEGs), 76 were significantly upregulated and directly involved in the floral scent biosynthesis process. In addition, certain volatile organic compounds were substantially linked with various DEGs after combining the metabolome and transcriptome datasets. Moreover, some transcription factors, such as MYB and bHLH, were also significantly upregulated in the comparison, which might be related to the floral aroma mechanism. Our results suggested that melatonin increased floral aroma production in H. coronarium flowers by modifying the expression level of genes involved in the floral scent biosynthesis pathway. These findings serve as a foundation for future research into the molecular mechanisms underlying the dynamic changes in volatile contents induced by melatonin treatment in H. coronarium.

Exogenous melatonin improves salt stress adaptation of cotton seedlings by regulating active oxygen metabolism

Melatonin is a small-molecule indole hormone that plays an important role in participating in biotic and abiotic stress resistance. Melatonin has been confirmed to promote the normal development of plants under adversity stress by mediating physiological regulation mechanisms. However, the mechanisms by which exogenous melatonin mediates salt tolerance via regulation of antioxidant activity and osmosis in cotton seedlings remain largely unknown. In this study, the regulatory effects of melatonin on reactive oxygen species (ROS), the antioxidant system, and osmotic modulators of cotton seedlings were determined under 0-500 µM melatonin treatments with salt stress induced by 150 mM NaCl treatment. Cotton seedlings under salt stress exhibited an inhibition of growth, excessive hydrogen peroxide (H₂O₂), superoxide anion (O₂ ^-), and malondialdehyde (MDA) accumulations in leaves, increased activity levels of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), and elevated ascorbic acid (AsA) and glutathione (GSH) content in leaves. However, the content of osmotic regulators (i.e., soluble sugars and proteins) in leaves was reduced under salt stress. This indicates high levels of ROS were produced, and the cell membrane was damaged. Additionally, osmotic regulatory substance content was reduced, resulting in osmotic stress, which seriously affected cotton seedling growth under salt stress. However, exogenous melatonin at different concentrations reduced the contents of H₂O₂, O₂ ^-, and MDA in cotton leaves, increased the activity of antioxidant enzymes and the content of reductive substances (i.e., AsA and GSH), and promoted the accumulation of osmotic regulatory substances in leaves under salt stress. These results suggest that melatonin can inhibit ROS production in cotton seedlings, improve the activity of the antioxidant enzyme system, raise the content of osmotic regulation substances, reduce the level of membrane lipid peroxidation, and protect the integrity of the lipid membrane under salt stress, which reduces damage caused by salt stress to seedlings and effectively enhances inhibition of salt stress on cotton seedling growth. These results indicate that 200 µM melatonin treatment has the best effect on the growth and salt tolerance of cotton seedlings.

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

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

Role of Melatonin in Plant Tolerance to Soil Stressors: Salinity, pH and Heavy Metals

Melatonin (MT) is a pleiotropic molecule with diverse and numerous actions both in plants and animals. In plants, MT acts as an excellent promotor of tolerance against abiotic stress situations such as drought, cold, heat, salinity, and chemical pollutants. In all these situations, MT has a stimulating effect on plants, fomenting many changes in biochemical processes and stress-related gene expression. Melatonin plays vital roles as an antioxidant and can work as a free radical scavenger to protect plants from oxidative stress by stabilization cell redox status; however, MT can alleviate the toxic oxygen and nitrogen species. Beyond this, MT stimulates the antioxidant enzymes and augments antioxidants, as well as activates the ascorbate–glutathione (AsA–GSH) cycle to scavenge excess reactive oxygen species (ROS). In this review, we examine the recent data on the capacity of MT to alleviate the effects of common abiotic soil stressors, such as salinity, alkalinity, acidity, and the presence of heavy metals, reinforcing the general metabolism of plants and counteracting harmful agents. An exhaustive analysis of the latest advances in this regard is presented, and possible future applications of MT are discussed.

Genome-Wide Identification and Expression Profile of the SNAT Gene Family in Tobacco (Nicotiana tabacum)

Melatonin plays key roles in development and confers stress tolerance to plants. Serotonin N-acetyltransferase (SNAT) is either the enzyme involved in the last step or the penultimate enzyme of phytomelatonin biosynthesis. To date, SNAT genes have not been characterized in tobacco (Nicotiana tabacum), an economically important plant species. The sequence of the Acetyltransf_7 conserved domain was used as a query sequence, and 12 NtSNAT candidate genes were in turn identified in the genome of tobacco. These NtSNATs could be divided into two groups based on the phylogenetic tree. NtSNAT1 and NtSNAT2 clustered together with the other typical SNATs, but the other 10 NtSNATs separately clustered outside of the typical SNATs. These 10 NtSNATs have only motif 1, whereas representative SNATs, such as NtSNAT1 and NtSNAT2 or a SNAT from cyanobacteria, have five motifs. In addition, NtSNAT1 and NtSNAT2 are highly homologous to the characterized OsSNAT1, 62.95 and 71.36%, respectively; however, the homology between the other 10 NtSNAT genes and OsSNAT1 is low. Concomitantly, it is hypothesized that NtSNAT1 and NtSNAT2 are the homolog of SNATs, whereas the other 10 candidates could be considered NtSNAT-like genes. Furthermore, both Nicotiana tomentosiformis and Nicotiana sylvestris, two diploid ancestor species of N. tabacum, have two SNAT candidates; therefore, it is speculated that gene rearrangement or deletion during the process of genomic stabilization after whole-genome duplication or polyploidization led to the preservation of NtSNAT1 and NtSNAT2 during the evolution of tobacco from the ancestral diploid to the allotetraploid. NtSNAT and NtSNAT-like genes were differentially expressed in all organs under different stress conditions, indicating that these genes potentially associated with plant growth and development and stress resistance. Under different stress conditions, the expression of NtSNAT1 was significantly upregulated upon high-temperature and cadmium stresses, while the expression of NtSNAT2 did not significantly increase under any of the tested stress treatments. These results provide valuable information for elucidating the evolutionary relationship of SNAT genes in tobacco and genetic resources for improving tobacco production in the future.

Exogenous supplementation of melatonin alters representative organic acids and enzymes of respiratory cycle as well as sugar metabolism during arsenic stress in two contrasting indica rice cultivars

The objective of the present investigation was to understand the impact of exogenously applied melatonin on mitochondrial respiration and sugar metabolism in two contrasting rice cultivars, viz., Khitish (arsenic-susceptible) and Muktashri (arsenic-tolerant) under arsenic-stress. Melatonin effectively restored the level of organic acids like pyruvic acid, malic acid and more particularly citric acid by 33 % in Khitish which were lowered during arsenic-stress, whereas their levels were further elevated in Muktashri to provide energy for defence against arsenic-induced injury. Arsenic-exposure led to a significant inhibition in enzyme activities as well as corresponding transcript level of key respiratory enzymes, viz., pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, intriguingly more prominently in case of Khitish. Conversely, melatonin supplementation, irrespective of cultivars, considerably improved the activity of the above enzymes and corresponding gene expressions during stress, indicating acceleration in the rate of Krebs cycle. Melatonin supplementation also stimulated the accumulation of total soluble sugars by 62 % and 25 %, reducing sugars by 50 % and 44 % and non-reducing sugars by 75 % and 14 % in Khitish and Muktashri respectively, concomitant with higher activities of acid invertase, sucrose synthase and sucrose phosphate synthase enzymes, along with the expression of corresponding genes. Enhanced starch accumulation via regulation of alpha amylase and starch phosphorylase activities and gene expression, by melatonin also contributed towards better stress tolerance. Overall, this work illustrated the efficacy of melatonin in the regulation of representative organic acids and enzymes of respiratory cycle along with starch and sugar metabolism in rice cultivars under arsenic toxicity.

Melatonin in business with abiotic stresses in plants

Melatonin (MEL) is the potential biostimulator molecule, governing multiple range of growth and developmental processes in plants, particularly under different environmental constrains. Mainly, its role is considered as an antioxidant molecule that copes with oxidative stress through scavenging of reactive oxygen species and modulation of stress related genes. It also enhances the antioxidant enzyme activities and thus helps in regulating the redox hemostasis in plants. Apart from its broad range of antioxidant functions, it is involved in the regulation of various physiological processes such as germination, lateral root growth and senescence in plants. Moreover this multifunctional molecule takes much interest due to its recent identification and characterization of receptorCandidate G-protein-Coupled Receptor 2/Phytomelatonin receptor(CAND2/PMTR1) in Arabidopsis thaliana. In this compiled work, different aspects of melatonin in plants such as melatonin biosynthesis and detection in plants, signaling pathway, modulation of stress related genes and physiological role of melatonin under different environmental stresses have been dissected in detail.

Melatonin overcomes MCR-mediated colistin resistance in Gram-negative pathogens

Background: Emergence, prevalence and widely spread of plasmid-mediated colistin resistance in Enterobacteriaceae strongly impairs the clinical efficacy of colistin against life-threatening bacterial infections. Combinations of antibiotics and FDA-approved non-antibiotic agents represent a promising means to address the widespread emergence of antibiotic-resistant pathogens. Methods: Herein, we investigated the synergistic activity between melatonin and antibiotics against MCR (mobilized colistin resistance)-positive Gram-negative pathogens through checkerboard assay and time-killing curve. Molecular mechanisms underlying its mode of action were elucidated. Finally, we assessed the in vivo efficacy of melatonin in combination with colistin against drug-resistant Gram-negative bacteria. Results: Melatonin, which has been approved for treating sleep disturbances and circadian disorders, substantially potentiates the activity of three antibiotics, particularly colistin, against MCR-expressing pathogens without enhancing its toxicity. This is evidence that the combination of colistin with melatonin enhances bacterial outer membrane permeability, promotes oxidative damage and inhibits the effect of efflux pumps. In three animal models infected by mcr-1-carrying E. coli, melatonin dramatically rescues colistin efficacy. Conclusion: Our findings revealed that melatonin serves as a promising colistin adjuvant against MCR-positive Gram-negative pathogens.

An evolutionary view of melatonin synthesis and metabolism related to its biological functions in plants

Plant melatonin research is a rapidly developing field. A variety of isoforms of melatonin's biosynthetic enzymes are present in different plants. Due to the different origins, they exhibit independent responses to the variable environmental stimuli. The locations for melatonin biosynthesis in plants are chloroplasts and mitochondria. These organelles have inherited their melatonin biosynthetic capacities from their bacterial ancestors. Under ideal conditions, chloroplasts are the main sites of melatonin biosynthesis. If the chloroplast pathway is blocked for any reason, the mitochondrial pathway will be activated for melatonin biosynthesis to maintain its production. Melatonin metabolism in plants is a less studied field; its metabolism is quite different from that of animals even though they share similar metabolites. Several new enzymes for melatonin metabolism in plants have been cloned and these enzymes are absent in animals. It seems that the 2-hydroxymelatonin is a major metabolite of melatonin in plants and its level is ~400-fold higher than that of melatonin. In the current article, from an evolutionary point of view, we update the information on plant melatonin biosynthesis and metabolism. This review will help the reader to understand the complexity of these processes and promote research enthusiasm in these fields.

Melatonin and Mesenchymal Stem Cells as a Key for Functional Integrity for Liver Cancer Treatment

Hepatocellular carcinoma (HCC) is the most common hepatobiliary malignancy with limited therapeutic options. On the other hand, melatonin is an indoleamine that modulates a variety of potential therapeutic effects. In addition to its important role in the regulation of sleep–wake rhythms, several previous studies linked the biologic effects of melatonin to various substantial endocrine, neural, immune and antioxidant functions, among others. Furthermore, the effects of melatonin could be influenced through receptor dependent and receptor independent manner. Among the other numerous physiological and therapeutic effects of melatonin, controlling the survival and differentiation of mesenchymal stem cells (MSCs) has been recently discussed. Given its controversial interaction, several previous reports revealed the therapeutic potential of MSCs in controlling the hepatocellular carcinoma (HCC). Taken together, the intention of the present review is to highlight the effects of melatonin and mesenchymal stem cells as a key for functional integrity for liver cancer treatment. We hope to provide solid piece of information that may be helpful in designing novel drug targets to control HCC.

Exogenous melatonin improves glutathione content, redox state and increases essential oil production in two Salvia species under drought stress

This research was conducted to understand the influence of foliar applied melatonin (0, 50, 100, 150 and 200 μM) on two Salvia species (Salvia nemorosa L., and Salvia reuterana Boiss) under conditions of water stress. Water stress was applied using a reduced irrigation strategy based on re-watering at 80%, 60% and 40% of the field capacity (FC). Increasing water stress, while significantly enhancing malondialdehyde (MDA), H2O2, electrolyte leakage, oxidized glutathione (GSSG), and total glutathione (GT), reduced glutathione (GSH), catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) and glutathione reductase (GR) activities, which led to a marked reduction in fluorescence (Fv/Fm). Foliar application of melatonin alleviated the oxidative stress by increasing GT, CAT, POD, SOD and GR activities and reducing GSSG. In particular, melatonin heightened GSH content as well as the ratio of GSH/GSSG when compared to non-sprayed water stressed plants. Melatonin-treated plants had significantly lower SOD and POD activities than control plants under drought stress, while the CAT activity was enhanced with the foliar treatment. Essential oil yield of both Salvia species increased with the decrease in irrigation from 80% to 60% FC but diminished with the more severe water deficit (40% FC). Essential oil components of Salvia nemorosa were β- caryophyllene, germacrene- B, spathulenol, and cis- β- farnesene, while (E) - β- ocimene, α- gurjnnene, germacrene-D, hexyl acetate and aromadendrene was the major constituents of Salvia reuterana. When plants were subjected to water deficit, melatonin treatment increased the concentration and composition of the essential oil. In particular, melatonin treatments improved the primary oil components in both species when compared to non-melatonin treated plants. In conclusion, reduced irrigation regimes as well as melatonin treatments resulted in a significant improvement of essential oil production and composition in both Salvia species.

Melatonin foliar sprays elicit salinity stress tolerance and enhance fruit yield and quality in strawberry (Fragaria × ananassa Duch.)

The increasing salinity in soils and irrigation water is a major concern for growers of strawberry, a salt-sensitive horticultural crop. The hormone melatonin (N-acetyl-5-methoxytryptamine) is involved in many biological processes and affects plant responses to environmental stresses. The effects of weekly 100 and 200 μM melatonin sprays on leaf composition parameters (photosynthetic pigment and macronutrient concentrations, oxidative stress markers, antioxidant defense systems and abscisic acid concentrations), fruit yield and quality parameters (soluble solids, total acidity, ascorbic acid, total antioxidants and phenolics and sugars), and leaf and fruit melatonin have been studied in strawberry grown under non-saline, moderate and intense salinity conditions (0, 40 and 80 mM NaCl, respectively). Salinity led to decreases in yield, fruit quality parameters and leaf photosynthetic pigments and macronutrient concentrations, as well as to increases in oxidative stress, with melatonin foliar application alleviating all these changes. On the other hand, salinity led to increases in the leaf levels of antioxidant enzymes, abscisic acid and melatonin, with foliar applications of melatonin boosting those increases. In the absence of salinity stress, melatonin led to smaller changes in all parameters in the same direction to that observed in the presence of salinity. Furthermore, melatonin resulted in increases in strawberry fruit yield and quality, especially in plants grown under salinity. Results indicate that the effects of melatonin application are associated with a boost in leaf antioxidant enzymes and abscisic acid, and support that the application of melatonin is a promising tool for mitigating salt stress in strawberry.

Melatonin Priming Alleviates Aging-Induced Germination Inhibition by Regulating β-oxidation, Protein Translation, and Antioxidant Metabolism in Oat (Avena sativa L.) Seeds

Although melatonin has been reported to play an important role in regulating metabolic events under adverse stresses, its underlying mechanisms on germination in aged seeds remain unclear. This study was conducted to investigate the effect of melatonin priming (MP) on embryos of aged oat seeds in relation to germination, ultrastructural changes, antioxidant responses, and protein profiles. Proteomic analysis revealed, in total, 402 differentially expressed proteins (DEPs) in normal, aged, and aged + MP embryos. The downregulated DEPs in aged embryos were enriched in sucrose metabolism, glycolysis, β-oxidation of lipid, and protein synthesis. MP (200 μM) turned four downregulated DEPs into upregulated DEPs, among which, especially 3-ketoacyl-CoA thiolase-like protein (KATLP) involved in the β-oxidation pathway played a key role in maintaining TCA cycle stability and providing more energy for protein translation. Furthermore, it was found that MP enhanced antioxidant capacity in the ascorbate-glutathione (AsA-GSH) system, declined reactive oxygen species (ROS), and improved cell ultrastructure. These results indicated that the impaired germination and seedling growth of aged seeds could be rescued to a certain level by melatonin, predominantly depending on β-oxidation, protein translation, and antioxidant protection of AsA-GSH. This work reveals new insights into melatonin-mediated mechanisms from protein profiles that occur in embryos of oat seeds processed by both aging and priming.

Towards a Sustainable Agriculture: Strategies Involving Phytoprotectants against Salt Stress

Salinity is one of the main constraints for agriculture productivity worldwide. This important abiotic stress has worsened in the last 20 years due to the increase in water demands in arid and semi-arid areas. In this context, increasing tolerance of crop plants to salt stress is needed to guarantee future food supply to a growing population. This review compiles knowledge on the use of phytoprotectants of microbial origin (arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria), osmoprotectants, melatonin, phytohormones and antioxidant metabolism-related compounds as alleviators of salt stress in numerous plant species. Phytoprotectants are discussed in detail, including their nature, applicability, and role in the plant in terms of physiological and phenotype effects. As a result, increased crop yield and crop quality can be achieved, which in turn positively impact food security. Herein, efforts from academic and industrial sectors should focus on defining the treatment conditions and plant-phytoprotectant associations providing higher benefits.

Regulation of physiological aspects in plants by hydrogen sulfide and nitric oxide under challenging environment

Plants are exposed to a plethora of abiotic stresses such as drought, salinity, heavy metal and temperature stresses at different stages of their life cycle, from germination to seedling till the reproductive phase. As protective mechanisms, plants release signaling molecules that initiate a cascade of stress-signaling events, leading either to programmed cell death or plant acclimation. Hydrogen sulfide (H₂ S) and nitric oxide (NO) are considered as new 'gasotransmitter' molecules that play key roles in regulating gene expression, posttranslational modification (PTM), as well as cross-talk with other hormones. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulation and environmental stress in plants. These molecules are also involved in a large array of stress responses and act synergistically or antagonistically as signaling components, depending on their respective concentration. This study provides a comprehensive update on the signaling interplay between H₂ S and NO in the regulation of various physiological processes under multiple abiotic stresses, modes of action and effects of exogenous application of these two molecules under drought, salt, heat and heavy metal stresses. However, the complete picture of the signaling cascades mediated by H₂ S and NO is still elusive. Recent researches indicate that during certain plant processes, such as stomatal closure, H₂ S could act upstream of NO signaling or downstream of NO in response to abiotic stresses by improving antioxidant activity in most plant species. In addition, PTMs of antioxidative pathways by these two molecules are also discussed.

Direct comparison of Arabidopsis gene expression reveals different responses to melatonin versus auxin

BACKGROUND

Melatonin (N-acetyl-5-methoxytryptamine) in plants, regulates shoot and root growth and alleviates environmental stresses. Melatonin and the phyto-hormone auxin are tryptophan-derived compounds. However, it largely remains controversial as to whether melatonin and auxin act through similar or overlapping signalling and regulatory pathways.

RESULTS

Here, we have used a promoter-activation study to demonstrate that, unlike auxin (1-naphthalene acetic acid, NAA), melatonin neither induces Direct repeat 5 DR5 expression in Arabidopsis thaliana roots under normal growth conditions nor suppresses the induction of Alternative oxidase 1a AOX1a in leaves upon Antimycin A treatment, both of which are the hallmarks of auxin action. Additionally, comparative global transcriptome analysis conducted on Arabidopsis treated with melatonin or NAA revealed differences in the number and types of differentially expressed genes. Auxin (4.5 μM) altered the expression of a diverse and large number of genes whereas melatonin at 5 μM had no significant effect but melatonin at 100 μM had a modest effect on transcriptome compared to solvent-treated control. Interestingly, the prominent category of genes differentially expressed upon exposure to melatonin trended towards biotic stress defence pathways while downregulation of key genes related to photosynthesis was observed.

CONCLUSION

Together these findings indicate that though they are both indolic compounds, melatonin and auxin act through different pathways to alter gene expression in Arabidopsis thaliana. Furthermore, it appears that effects of melatonin enable Arabidopsis thaliana to prioritize biotic stress defence signalling rather than growth. These findings clear the current confusion in the literature regarding the relationship of melatonin and auxin and also have greater implications of utilizing melatonin for improved plant protection.

Melatonin-Nitric Oxide Crosstalk and Their Roles in the Redox Network in Plants

Melatonin, an amine hormone highly conserved during evolution, has a wide range of physiological functions in animals and plants. It is involved in plant growth, development, maturation, and aging, and also helps ameliorate various types of abiotic and biotic stresses, including salt, drought, heavy metals, and pathogens. Melatonin-related growth and defense responses of plants are complex, and involve many signaling molecules. Among these, the most important one is nitric oxide (NO), a freely diffusing amphiphilic biomolecule that can easily cross the cell membrane, produce rapid signal responses, and participate in a wide variety of physiological reactions. NO-induced S-nitrosylation is also involved in plant defense responses. NO interacts with melatonin as a long-range signaling molecule, and helps regulate plant growth and maintain oxidative homeostasis. Exposure of plants to abiotic stresses causes the increase of endogenous melatonin levels, with the consequent up-regulation of melatonin synthesis genes, and further increase of melatonin content. The application of exogenous melatonin causes an increase in endogenous NO and up-regulation of defense-related transcription factors, resulting in enhanced stress resistance. When plants are infected by pathogenic bacteria, NO acts as a downstream signal to lead to increased melatonin levels, which in turn induces the mitogen-activated protein kinase (MAPK) cascade and associated defense responses. The application of exogenous melatonin can also promote sugar and glycerol production, leading to increased levels of salicylic acid and NO. Melatonin and NO in plants can function cooperatively to promote lateral root growth, delay aging, and ameliorate iron deficiency. Further studies are needed to clarify certain aspects of the melatonin/NO relationship in plant physiology.

Bacterial and Fungal Communities Are Differentially Modified by Melatonin in Agricultural Soils Under Abiotic Stress

An extensive body of evidence from the last decade has indicated that melatonin enhances plant resistance to a range of biotic and abiotic stressors. This has led to an interest in the application of melatonin in agriculture to reduce negative physiological effects from environmental stresses that affect yield and crop quality. However, there are no reports regarding the effects of melatonin on soil microbial communities under abiotic stress, despite the importance of microbes for plant root health and function. Three agricultural soils associated with different land usage histories (pasture, canola or wheat) were placed under abiotic stress by cadmium (100 or 280 mg kg-1 soil) or salt (4 or 7 g kg-1 soil) and treated with melatonin (0.2 and 4 mg kg-1 soil). Automated Ribosomal Intergenic Spacer Analysis (ARISA) was used to generate Operational Taxonomic Units (OTU) for microbial community analysis in each soil. Significant differences in richness (α diversity) and community structures (β diversity) were observed between bacterial and fungal assemblages across all three soils, demonstrating the effect of melatonin on soil microbial communities under abiotic stress. The analysis also indicated that the microbial response to melatonin is governed by the type of soil and history. The effects of melatonin on soil microbes need to be regarded in potential future agricultural applications.

cis-Cinnamic acid is a natural plant growth-promoting compound

Agrochemicals provide vast potential to improve plant productivity, because they are easy to implement at low cost while not being restricted by species barriers as compared with breeding strategies. Despite the general interest, only a few compounds with growth-promoting activity have been described so far. Here, we add cis-cinnamic acid (c-CA) to the small portfolio of existing plant growth stimulators. When applied at low micromolar concentrations to Arabidopsis roots, c-CA stimulates both cell division and cell expansion in leaves. Our data support a model explaining the increase in shoot biomass as the consequence of a larger root system, which allows the plant to explore larger areas for resources. The requirement of the cis-configuration for the growth-promoting activity of CA was validated by implementing stable structural analogs of both cis- and trans-CA in this study. In a complementary approach, we used specific light conditions to prevent cis/trans-isomerization of CA during the experiment. In both cases, the cis-form stimulated plant growth, whereas the trans-form was inactive. Based on these data, we conclude that c-CA is an appealing lead compound representing a novel class of growth-promoting agrochemicals. Unraveling the underlying molecular mechanism could lead to the development of innovative strategies for boosting plant biomass.

Melatonin facilitates lateral root development by coordinating PAO-derived hydrogen peroxide and Rboh-derived superoxide radical

Melatonin, a phytochemical, can regulate lateral root (LR) formation, but the downstream signaling of melatonin remains elusive. Here we investigated the roles of hydrogen peroxide (H2O2) and superoxide radical (O2•‾) in melatonin-promoted LR formation in tomato (Solanum lycopersicum) roots by using physiological, histochemical, bioinformatic, and biochemical approaches. The increase in endogenous melatonin level stimulated reactive oxygen species (ROS)-dependent development of lateral root primordia (LRP) and LR. Melatonin promoted LRP/LR formation and modulated the expression of cell cycle genes (SlCDKA1, SlCYCD3;1, and SlKRP2) by stimulating polyamine oxidase (PAO)-dependent H2O2 production and respiratory burst oxidase homologue (Rboh)-dependent O2•‾ production, respectively. Screening of SlPAOs and SlRbohs gene family combined with gene expression analysis suggested that melatonin-promoted LR formation was correlated to the upregulation of SlPAO1, SlRboh3, and SlRboh4 in LR-emerging zone. Transient expression analysis confirmed that SlPAO1 was able to produce H2O2 while SlRboh3 and SlRboh4 were capable of producing O2•‾. Melatonin-ROS signaling cassette was also found in the regulation of LR formation in rice root and lateral hyphal branching in fungi. These results suggested that SlPAO1-H2O2 and SlRboh3/4-O2•‾ acted as downstream of melatonin to regulate the expression of cell cycle genes, resulting in LRP initiation and LR development. Such findings uncover one of the regulatory pathways for melatonin-regulated LR formation, which extends our knowledge for melatonin-regulated plant intrinsic physiology.

Melatonin-Induced Transcriptome Variation of Rapeseed Seedlings under Salt Stress

Salt stress inhibits the production of all crop species, including rapeseed (Brassica napus L.), the second most widely planted oil crop species. Although melatonin was confirmed to alleviate salt stress in rapeseed seedlings recently, the mechanism governing the expression levels remains unknown. Therefore, the melatonin-induced transcriptome variation of salt-stressed seedlings was explored. In this study, the transcriptomes of leaves and roots under control (CK), salt (125 mM NaCl, ST) and melatonin (125 mM NaCl plus 50 µM melatonin, MS) treatments were evaluated by using next-generation sequencing techniques. After conducting comparisons of gene expression in the roots and leaves between MS and ST, the differentially expressed gene (DEG) pools were screened. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses highlighted the significant pathways, which were mainly related to plant hormone synthesis and signal transduction, lignin and fatty acid metabolism. The functional genes in the objective KEGG pathways were identified. Furthermore, members of several transcription factor (TF) families participated in the response process. Combined with the hormone (campesterol (CS), jasmonic acid (JA), and gibberellic acid 3 (GA3)) contents measured in the seedlings, it could be concluded that melatonin induced changes in the intrinsic hormone metabolic network, which promoted seedling growth. Thus, this study identified new candidate genes and pathways active during the interactions between melatonin and salt stress, which provide clues for disclosing melatonin’s function in resistance to salt injury. Our results contribute to developing a practical method for sustainable agriculture on saline lands.

Suppression of Melatonin 2-Hydroxylase Increases Melatonin Production Leading to the Enhanced Abiotic Stress Tolerance against Cadmium, Senescence, Salt, and Tunicamycin in Rice Plants

Melatonin 2-hydroxylase (M2H) catalyzes the conversion of melatonin into 2hydroxymelatonin (2OHM), which is present in plants at a higher concentration than melatonin. Although M2H has been cloned, the in vivo function of its product is unknown. Here, we generated stable T₂ homozygous transgenic rice plants in which expression of endogenous M2H was suppressed (RNAi lines). However, we failed to generate M2H overexpression transgenic rice due to failure of somatic embryogenesis. The M2H transcript level showed a diurnal rhythm with a peak at night concomitantly with the peak concentration of 2OHM. RNAi rice showed a reduced M2H mRNA level and 2OHM and melatonin concentrations. The unexpected decrease in the melatonin concentration was caused by redirection of melatonin into cyclic 3hydroxymelatonin via a detour catabolic pathway. Thus, the decrease in the melatonin concentration in M2H RNAi rice led to slowed seedling growth and delayed germination. By contrast, the transient increase in the melatonin concentration was of greater magnitude in the M2H RNAi than the wild-type rice upon cadmium treatment due to possible suppression of melatonin degradation. Due to its higher concentration of melatonin, the M2H RNAi rice displayed tolerance to senescence, salt, and tunicamycin stresses. Therefore, the increase in the melatonin concentration caused by suppression of melatonin degradation or by overexpression of melatonin biosynthetic genes enhances stress tolerance in rice.

Melatonin as a Chemical Substance or as Phytomelatonin Rich-Extracts for Use as Plant Protector and/or Biostimulant in Accordance with EC Legislation

Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous molecule present in animals and plants, and also in bacteria and fungi. In plants, it has an important regulatory and protective role in the face of different stress situations in which it can be involved, mainly due to its immobility. Both in the presence of biotic and abiotic stressors, melatonin exerts protective action in which, through significant changes in gene expression, it activates a stress tolerance response. Its anti-stress role, along with other outstanding functions, suggests its possible use in active agricultural management. This review establishes considerations that are necessary for its possible authorization. The particular characteristics of this substance and its categorization as plant biostimulant are discussed, and also the different legal aspects within the framework of the European Community. The advantages and disadvantages are also described of two of its possible applications, as a plant protector or biostimulant, in accordance with legal provisions.

Nitric oxide and hydrogen sulfide in plants: which comes first?

Nitric oxide (NO) is a signal molecule regarded as being involved in myriad functions in plants under physiological, pathogenic, and adverse environmental conditions. Hydrogen sulfide (H2S) has also recently been recognized as a new gasotransmitter with a diverse range of functions similar to those of NO. Depending on their respective concentrations, both these molecules act synergistically or antagonistically as signals or damage promoters in plants. Nevertheless, available evidence shows that the complex biological connections between NO and H2S involve multiple pathways and depend on the plant organ and species, as well as on experimental conditions. Cysteine-based redox switches are prone to reversible modification; proteomic and biochemical analyses have demonstrated that certain target proteins undergo post-translational modifications such as S-nitrosation, caused by NO, and persulfidation, caused by H2S, both of which affect functionality. This review provides a comprehensive update on NO and H2S in physiological processes (seed germination, root development, stomatal movement, leaf senescence, and fruit ripening) and under adverse environmental conditions. Existing data suggest that H2S acts upstream or downstream of the NO signaling cascade, depending on processes such as stomatal closure or in response to abiotic stress, respectively.

Melatonin in Medicinal and Food Plants: Occurrence, Bioavailability, and Health Potential for Humans

Melatonin is a widespread molecule among living organisms involved in multiple biological, hormonal, and physiological processes at cellular, tissue, and organic levels. It is well-known for its ability to cross the blood–brain barrier, and renowned antioxidant effects, acting as a free radical scavenger, up-regulating antioxidant enzymes, reducing mitochondrial electron leakage, and interfering with proinflammatory signaling pathways. Detected in various medicinal and food plants, its concentration is widely variable. Plant generative organs (e.g., flowers, fruits), and especially seeds, have been proposed as having the highest melatonin concentrations, markedly higher than those found in vertebrate tissues. In addition, seeds are also rich in other substances (lipids, sugars, and proteins), constituting the energetic reserve for a potentially growing seedling and beneficial for the human diet. Thus, given that dietary melatonin is absorbed in the gastrointestinal tract and transported into the bloodstream, the ingestion of medicinal and plant foods by mammals as a source of melatonin may be conceived as a key step in serum melatonin modulation and, consequently, health promotion.

Melatonin Synthesis and Function: Evolutionary History in Animals and Plants

Melatonin is an ancient molecule that can be traced back to the origin of life. Melatonin's initial function was likely that as a free radical scavenger. Melatonin presumably evolved in bacteria; it has been measured in both α-proteobacteria and in photosynthetic cyanobacteria. In early evolution, bacteria were phagocytosed by primitive eukaryotes for their nutrient value. According to the endosymbiotic theory, the ingested bacteria eventually developed a symbiotic association with their host eukaryotes. The ingested α-proteobacteria evolved into mitochondria while cyanobacteria became chloroplasts and both organelles retained their ability to produce melatonin. Since these organelles have persisted to the present day, all species that ever existed or currently exist may have or may continue to synthesize melatonin in their mitochondria (animals and plants) and chloroplasts (plants) where it functions as an antioxidant. Melatonin's other functions, including its multiple receptors, developed later in evolution. In present day animals, via receptor-mediated means, melatonin functions in the regulation of sleep, modulation of circadian rhythms, enhancement of immunity, as a multifunctional oncostatic agent, etc., while retaining its ability to reduce oxidative stress by processes that are, in part, receptor-independent. In plants, melatonin continues to function in reducing oxidative stress as well as in promoting seed germination and growth, improving stress resistance, stimulating the immune system and modulating circadian rhythms; a single melatonin receptor has been identified in land plants where it controls stomatal closure on leaves. The melatonin synthetic pathway varies somewhat between plants and animals. The amino acid, tryptophan, is the necessary precursor of melatonin in all taxa. In animals, tryptophan is initially hydroxylated to 5-hydroxytryptophan which is then decarboxylated with the formation of serotonin. Serotonin is either acetylated to N-acetylserotonin or it is methylated to form 5-methoxytryptamine; these products are either methylated or acetylated, respectively, to produce melatonin. In plants, tryptophan is first decarboxylated to tryptamine which is then hydroxylated to form serotonin.

The Role of Melatonin in Salt Stress Responses

Melatonin, an indoleamine widely found in animals and plants, is considered as a candidate phytohormone that affects responses to a variety of biotic and abiotic stresses. In plants, melatonin has a similar action to that of the auxin indole-3-acetic acid (IAA), and IAA and melatonin have the same biosynthetic precursor, tryptophan. Salt stress results in the rapid accumulation of melatonin in plants. Melatonin enhances plant resistance to salt stress in two ways: one is via direct pathways, such as the direct clearance of reactive oxygen species; the other is via an indirect pathway by enhancing antioxidant enzyme activity, photosynthetic efficiency, and metabolite content, and by regulating transcription factors associated with stress. In addition, melatonin can affect the performance of plants by affecting the expression of genes. Interestingly, other precursors and metabolite molecules associated with melatonin can also increase the tolerance of plants to salt stress. This paper explores the mechanisms by which melatonin alleviates salt stress by its actions on antioxidants, photosynthesis, ion regulation, and stress signaling.

Melatonin-deficient rice plants show a common semidwarf phenotype either dependent or independent of brassinosteroid biosynthesis

Melatonin-deficient rice with a semidwarf erect-leaf phenotype was created by suppressing serotonin N-acetyltransferase 2 (SNAT2). We generated an RNAi transgenic rice that suppressed tryptophan decarboxylase (TDC), which encodes the first TDC enzyme committed step for melatonin biosynthesis in plants catalyzing the conversion of tryptophan into tryptamine, to determine whether other transgenic rice with downregulated melatonin biosynthetic genes exhibited the same erect-leaf phenotype as the snat2 RNAi rice. The TDC RNAi rice produced significantly less melatonin than the wild type and exhibited a semidwarf phenotype, but no erect-leaf phenotype was observed. In contrast, tryptamine 5-hydroxylase (T5H) knockout Sekiguchi rice and caffeic acid O-methyltransferase (COMT) RNAi rice seedlings were semidwarf phenotypes with erect leaves, as was the snat2 RNAi rice due to a melatonin deficiency. All RNAi rice plants showing erect-leaf phenotypes had lower expression levels of the DWARF4 gene, which is a key enzyme for brassinosteroid (BR) biosynthesis, leading to lower BR levels than their respective wild types. Suppressing melatonin synthesis did not alter the contents of indole 3-acetic acid (IAA), suggesting the irrelevance of melatonin deficiency to IAA biosynthesis. These data indicate that a semidwarf seedling is a common rice phenotype by the lack of melatonin synthesis with or without BR suppression in a melatonin biosynthetic gene-specific manner.

Cancer Chemotherapy by Novel Bio-active Natural Products: Looking Towards the Future

Background:

Cancer is the second leading cause of death globally and accounted for 8.8 million deaths annually in humans. Lung, prostate, colorectal, stomach and liver cancer are the most common types of cancer in men, while breast, colorectal, lung, cervix and stomach cancer are the most common among women. Numerous drugs that the US Food and Drug Administration (FDA) have approved for use in cancer therapy are derived from plants, including taxanes such as paclitaxel and vinca alkaloids such as vincristine and vinblastine. Still, there is an intense need for a search for numerous bioactive sources to develop a novel anti-cancer drug to overcome this chronic disorder. About more than thirty plants derived natural products have been isolated till date and are currently under clinical trials. As per literature survey from various journals and texts has been found to be novel medicinal agents from bioactive sources are clinically active against various types of cancer cells.

Conclusion:

Current review has been highlighted on the novel medicinal agents from plant sources have potential effects against many types of cancer, which have been supported by clinical trials. The main findings of these active novel medicinal agents were also summarized and discussed here.

Direct visualization of location and uptake of applied melatonin and serotonin in living tissues and their redistribution in plants in response to thermal stress

Melatonin and serotonin are important phytochemicals enabling plants to redirect growth in response to environmental stresses. Despite much research on their biosynthetic routes, localization of their biosynthetic enzymes and recent identification of a phytomelatonin receptor, localization of the molecules themselves has to date not been possible. Elucidation of their locations in living tissues can provide an effective tool to facilitate indolamine research across systems including both plants and animals. In this study, we employed a novel technique, quantum dot nanoparticles, to directly visualize melatonin and serotonin in axenic roots. Melatonin was absorbed through epidermal cells, travelled laterally, and accumulated in endodermal and rapidly dividing pericycle cells. Serotonin was absorbed by cells proximal to the crown with rapid polar movement toward the root tip. Thermal stress disrupted localization and dispersed melatonin and serotonin across cells. These data demonstrate the natural movement of melatonin and serotonin in roots directing cell growth and suggest that plants have a mechanism to disperse the indolamines throughout tissues as antioxidants in response to environmental stresses.

Comparative physiological responses and transcriptome analysis reveal the roles of melatonin and serotonin in regulating growth and metabolism in Arabidopsis

BACKGROUND

Melatonin and serotonin are well-known signaling molecules that mediate multiple physiological activities in plants, including stress defense, growth, development, and morphogenesis, but their underlying mechanisms have not yet been thoroughly elucidated. In this study, we investigated the roles of melatonin and serotonin in modulating plant growth and defense by integrating physiological and transcriptome analyses in Arabidopsis.

RESULTS

Moderate concentrations of melatonin and serotonin did not affect primary root (PR) growth but markedly induced lateral root (LR) formation. Both melatonin and serotonin locally induced the expression of the cell-wall-remodeling-related genes LBD16 and XTR6, thereby inducing LR development. Our data support the idea that melatonin and serotonin lack any auxin-like activity. Treatment with 50 μM serotonin significantly improved PSII activity, and the transcriptome data supported this result. Melatonin and serotonin slightly affected glycolysis and the TCA cycle; however, they markedly regulated the catabolism of several key amino acids, thereby affecting carbon metabolism and energy metabolism. Melatonin and serotonin improved iron (Fe) deficiency tolerance by inducing Fe-responsive gene expression.

CONCLUSIONS

Overall, our results from the physiological and transcriptome analyses reveal the roles of melatonin and serotonin in modulating plant growth and stress responses and provide insight into novel crop production strategies using these two phytoneurotransmitters.

Promising Antineoplastic Actions of Melatonin

Melatonin is an endogenous indoleamine with an incredible variety of properties and activities. In recent years, an increasing number of studies have investigated this indoleamine’s interaction with cancerous cells. In particular, it seems that melatonin not only has the ability to improve the efficacy of many drugs used in chemotherapy but also has a direct inhibitory action on neoplastic cells. Many publications underlined the ability of melatonin to suppress the proliferation of various cancer cells or to modulate the expression of membrane receptors on these cells, thereby reducing tumor aggressiveness to metastasize. In addition, while melatonin has antiapoptotic actions in normal cells, in many cancer cells it has proapoptotic effects; these dichotomous actions have gained the interest of researchers. The increasing focus on melatonin in the field of oncology and the growing number of studies on this topic require a deep understanding of what we already know about the antineoplastic actions of melatonin. This information would be of value for potential use of melatonin against neoplastic diseases.

Melatonin induction and its role in high light stress tolerance in Arabidopsis thaliana

In plants, melatonin is a potent bioactive molecule involved in the response against various biotic and abiotic stresses. However, little is known of its defensive role against high light (HL) stress. In this study, we found that melatonin was transiently induced in response to HL stress in Arabidopsis thaliana with a simultaneous increase in the expression of melatonin biosynthetic genes, including serotonin N-acetyltransferase1 (SNAT1). Transient induction of melatonin was also observed in the flu mutant, a singlet oxygen (¹ O₂ )-producing mutant, upon light exposure, suggestive of melatonin induction by chloroplastidic ¹ O₂ against HL stress. An Arabidopsis snat1 mutant was devoid of melatonin induction upon HL stress, resulting in high susceptibility to HL stress. Exogenous melatonin treatment mitigated damage caused by HL stress in the snat1 mutant by reducing O₂^- production and increasing the expression of various ROS-responsive genes. In analogy, an Arabidopsis SNAT1-overexpressing line showed increased tolerance of HL stress concomitant with a reduction in malondialdehyde and ion leakage. A complementation line expressing an Arabidopsis SNAT1 genomic fragment in the snat1 mutant completely restored HL stress susceptibility in the snat1 mutant to levels comparable to that of wild-type Col-0 plants. The results of the analysis of several Arabidopsis genetic lines reveal for the first time at the genetic level that melatonin is involved in conferring HL stress tolerance in plants.

Flavonoids inhibit both rice and sheep serotonin N-acetyltransferases and reduce melatonin levels in plants

The plant melatonin biosynthetic pathway has been well characterized, but inhibitors of melatonin synthesis have not been well studied. Here, we found that flavonoids potently inhibited plant melatonin synthesis. For example, flavonoids including morin and myricetin significantly inhibited purified, recombinant sheep serotonin N-acetyltransferase (SNAT). Flavonoids also dose-dependently and potently inhibited purified rice SNAT1 and SNAT2. Thus, myricetin (100 μmol/L) reduced rice SNAT1 and SNAT2 activity 7- and 10-fold, respectively, and also strongly inhibited the N-acetylserotonin methyltransferase activity of purified, recombinant rice caffeic acid O-methyltransferase. To explore the in vivo effects, rice leaves were treated with flavonoids and then cadmium. Flavonoid-treated leaves had lower melatonin levels than the untreated control. To explore the direct roles of flavonoids in melatonin biosynthesis, we first functionally characterized a putative rice flavonol synthase (FLS) in vitro and generated flavonoid-rich transgenic rice plants that overexpressed FLS. Such plants produced more flavonoids but less melatonin than the wild-type, which suggests that flavonoids indeed inhibit plant melatonin biosynthesis.

Zinc finger of Arabidopsis thaliana 6 is involved in melatonin-mediated auxin signaling through interacting INDETERMINATE DOMAIN15 and INDOLE-3-ACETIC ACID 17

Although accumulating evidence demonstrates the cross talk between melatonin and auxin as derivatives of tryptophan, the underlying signaling events remain unclear. In this study, we found that melatonin and auxin mediated the transcriptional levels of zinc finger of Arabidopsis thaliana (ZAT6) in a mutually antagonistic manner. ZAT6 negatively modulated the endogenous auxin level, and ZAT6 knockdown plants were less sensitive to melatonin-regulated auxin biosynthesis, indicating its involvement in melatonin-mediated auxin accumulation. Additionally, the identification of INDETERMINATE DOMAIN15 (IDD15) and INDOLE-3-ACETIC ACID 17 (IAA17) in Arabidopsis that interacted with ZAT6 in vivo provided new insight of ZAT6-mediated auxin signaling. Further investigation showed that ZAT6 repressed the transcription activation of IDD15 on the YUC2 promoter, while ZAT6 inhibited the interaction of TRANSPORT INHIBITOR RESPONSE 1 (TIR1) and IAA17 through competitively binding to IAA17. Thus, both auxin synthesis and the auxin response were negatively modulated by ZAT6. Taken together, ZAT6 is involved in melatonin-mediated auxin signaling through forming an interacting complex of auxin signaling pathway in Arabidopsis.

The Role of Phyto-Melatonin and Related Metabolites in Response to Stress

Plant hormone candidate melatonin has been widely studied in plants under various stress conditions, such as heat, cold, salt, drought, heavy metal, and pathogen attack. Under stress, melatonin usually accumulates sharply by modulating its biosynthesis and metabolic pathways. Beginning from the precursor tryptophan, four consecutive enzymes mediate the biosynthesis of tryptamine or 5-hydroxytryptophan, serotonin, N-acetylserotonin or 5-methoxytryptamine, and melatonin. Then, the compound is catabolized into 2-hydroxymelatonin, cyclic-3-hydroxymelatonin, and N¹-acetyl-N²-formyl-5-methoxyknuramine through 2-oxoglutarate-dependent dioxygenase catalysis or reaction with reactive oxygen species. As an ancient and powerful antioxidant, melatonin directly scavenges ROS induced by various stress conditions. Furthermore, it confreres stress tolerance by activating the plant's antioxidant system, alleviating photosynthesis inhibition, modulating transcription factors that are involved with stress resisting, and chelating and promoting the transport of heavy metals. Melatonin is even proven to defense against pathogen attacks for the plant by activating other stress-relevant hormones, like salicylic acid, ethylene, and jasmonic acid. Intriguingly, other precursors and metabolite molecules involved with melatonin also can increase stress tolerance for plant except for unconfirmed 5-methoxytryptamine, cyclic-3-hydroxymelatonin, and N¹-acetyl-N²-formyl-5-methoxyknuramine. Therefore, the precursors and metabolites locating at the whole biosynthesis and catabolism pathway of melatonin could contribute to plant stress resistance, thus providing a new perspective for promoting plant stress tolerance.

Global insights of protein responses to cold stress in plants: Signaling, defence, and degradation

Cold stress (CS) as one of the unfavorable abiotic tensions proceeds different aspects of plant responses. These responses are generated through CS effects on crucial processes such as photosynthesis, energy metabolism, ROS homeostasis, membrane fluidity and cell wall architecture. As a tolerance response, plants apply proteins in various strategies such as transferring the message of cold entrance named as signaling, producing defensive and protective molecules against the stress and degrading some unfavorable or unnecessary proteins to produce other required ones. A change in one part of these networks can irritate alternations in others. These strategies as acclimation mechanisms are conducted through gene expression reprogramming to provide a new adjusted metabolic homeostasis dependent on the stress severity and duration and plant species. Investigating protein alterations in metabolic pathways and their role in adjusting cellular components from upstream to downstream levels can provide a profound knowledge of plants tolerance mechanism against the damaging effects of CS. In this review, we summarized the activity of some cold-responsive proteins from the perception phase to tolerance response against CS.

Premature leaf senescence 3, encoding a methyltransferase, is required for melatonin biosynthesis in rice

Premature leaf senescence in rice is one of the most common factors affecting the plant's development and yield. Although methyltransferases are involved in diverse biological functions, their roles in rice leaf senescence have not been previously reported. In this study, we identified the premature leaf senescence 3 (pls3) mutant in rice, which led to early leaf senescence and early heading date. Further investigations revealed that premature leaf senescence was triggered by the accumulation of reactive oxygen species. Using physiological analysis, we found that chlorophyll content was reduced in the pls3 mutant leaves, while hydrogen peroxide (H2 O2 ) and malondialdehyde levels were elevated. Consistent with these findings, the pls3 mutant exhibited hypersensitivity to exogenous hydrogen peroxide. The expression of other senescence-associated genes such as Osh36 and RCCR1 was increased in the pls3 mutant. Positional cloning indicated the pls3 phenotype was the result of a mutation in OsMTS1, which encodes an O-methyltransferase in the melatonin biosynthetic pathway. Functional complementation of OsMTS1 in pls3 completely restored the wild-type phenotype. We found leaf melatonin content to be dramatically reduced in pls3, and that exogenous application of melatonin recovered the pls3 mutant's leaf senescence phenotype to levels comparable to that of wild-type rice. Moreover, overexpression of OsMTS1 in the wild-type plant increased the grain yield by 15.9%. Our results demonstrate that disruption of OsMTS1, which codes for a methyltransferase, can trigger leaf senescence as a result of decreased melatonin production.

Natural Antioxidants: Multiple Mechanisms to Protect Skin From Solar Radiation

Human skin exposed to solar ultraviolet radiation (UVR) results in a dramatic increase in the production of reactive oxygen species (ROS). The sudden increase in ROS shifts the natural balance toward a pro-oxidative state, resulting in oxidative stress. The detrimental effects of oxidative stress occur through multiple mechanisms that involve alterations to proteins and lipids, induction of inflammation, immunosuppression, DNA damage, and activation of signaling pathways that affect gene transcription, cell cycle, proliferation, and apoptosis. All of these alterations promote carcinogenesis and therefore, regulation of ROS levels is critical to the maintenance of normal skin homeostasis. Several botanical products have been found to exhibit potent antioxidant capacity and the ability to counteract UV-induced insults to the skin. These natural products exert their beneficial effects through multiple pathways, including some known to be negatively affected by solar UVR. Aging of the skin is also accelerated by UVR exposure, in particular UVA rays that penetrate deep into the epidermis and the dermis where it causes the degradation of collagen and elastin fibers via oxidative stress and activation of matrix metalloproteinases (MMPs). Because natural compounds are capable of attenuating some of the UV-induced aging effects in the skin, increased attention has been generated in the area of cosmetic sciences. The focus of this review is to cover the most prominent phytoproducts with potential to mitigate the deleterious effects of solar UVR and suitability for use in topical application.