Arabidopsis serotonin N-acetyltransferase knockout mutant plants exhibit decreased melatonin and salicylic acid levels resulting in susceptibility to an avirulent pathogen

Melatonin-Nitric Oxide Crosstalk in Plants and the Prospects of NOMela as a Nitric Oxide Donor

Melatonin regulates vital physiological processes in animals, such as the circadian cycle, sleep, locomotion, body temperature, food intake, and sexual and immune responses. In plants, melatonin modulates seed germination, longevity, circadian cycle, photoperiodicity, flowering, leaf senescence, postharvest fruit storage, and resistance against biotic and abiotic stresses. In plants, the effect of melatonin is mediated by various regulatory elements of the redox network, including RNS and ROS. Similarly, the radical gas NO mediates various physiological processes, like seed germination, flowering, leaf senescence, and stress responses. The biosynthesis of both melatonin and NO takes place in mitochondria and chloroplasts. Hence, both melatonin and nitric oxide are key signaling molecules governing their biological pathways independently. However, there are instances when these pathways cross each other and the two molecules interact with each other, resulting in the formation of N-nitrosomelatonin or NOMela, which is a nitrosated form of melatonin, discovered recently and with promising roles in plant development. The interaction between NO and melatonin is highly complex, and, although a handful of studies reporting these interactions have been published, the exact molecular mechanisms governing them and the prospects of NOMela as a NO donor have just started to be unraveled. Here, we review NO and melatonin production as well as RNS-melatonin interaction under normal and stressful conditions. Furthermore, for the first time, we provide highly sensitive, ozone-chemiluminescence-based comparative measurements of the nitric oxide content, as well as NO-release kinetics between NOMela and the commonly used NO donors CySNO and GSNO.

The Physiological and Molecular Mechanisms of Exogenous Melatonin Promote the Seed Germination of Maize (Zea mays L.) under Salt Stress

Salt stress caused by high concentrations of Na+ and Cl- in soil is one of the most important abiotic stresses in agricultural production, which seriously affects grain yield. The alleviation of salt stress through the application of exogenous substances is important for grain production. Melatonin (MT, N-acetyl-5-methoxytryptamine) is an indole-like small molecule that can effectively alleviate the damage caused by adversity stress on crops. Current studies have mainly focused on the effects of MT on the physiology and biochemistry of crops at the seedling stage, with fewer studies on the gene regulatory mechanisms of crops at the germination stage. The aim of this study was to explain the mechanism of MT-induced salt tolerance at physiological, biochemical, and molecular levels and to provide a theoretical basis for the resolution of MT-mediated regulatory mechanisms of plant adaptation to salt stress. In this study, we investigated the germination, physiology, and transcript levels of maize seeds, analyzed the relevant differentially expressed genes (DEGs), and examined salt tolerance-related pathways. The results showed that MT could increase the seed germination rate by 14.28-19.04%, improve seed antioxidant enzyme activities (average increase of 11.61%), and reduce reactive oxygen species accumulation and membrane oxidative damage. In addition, MT was involved in regulating the changes of endogenous hormones during the germination of maize seeds under salt stress. Transcriptome results showed that MT affected the activity of antioxidant enzymes, response to stress, and seed germination-related genes in maize seeds under salt stress and regulated the expression of genes related to starch and sucrose metabolism and phytohormone signal transduction pathways. Taken together, the results indicate that exogenous MT can affect the expression of stress response-related genes in salt-stressed maize seeds, enhance the antioxidant capacity of the seeds, reduce the damage induced by salt stress, and thus promote the germination of maize seeds under salt stress. The results provide a theoretical basis for the MT-mediated regulatory mechanism of plant adaptation to salt stress and screen potential candidate genes for molecular breeding of salt-tolerant maize.

Melatonin: The Multifaceted Molecule in Plant Growth and Defense

Melatonin (MEL), a hormone primarily known for its role in regulating sleep and circadian rhythms in animals, has emerged as a multifaceted molecule in plants. Recent research has shed light on its diverse functions in plant growth and defense mechanisms. This review explores the intricate roles of MEL in plant growth and defense responses. MEL is involved in plant growth owing to its influence on hormone regulation. MEL promotes root elongation and lateral root formation and enhances photosynthesis, thereby promoting overall plant growth and productivity. Additionally, MEL is implicated in regulating the circadian rhythm of plants, affecting key physiological processes that influence plant growth patterns. MEL also exhibits antioxidant properties and scavenges reactive oxygen species, thereby mitigating oxidative stress. Furthermore, it activates defense pathways against various biotic stressors. MEL also enhances the production of secondary metabolites that contribute to plant resistance against environmental changes. MEL's ability to modulate plant response to abiotic stresses has also been extensively studied. It regulates stomatal closure, conserves water, and enhances stress tolerance by activating stress-responsive genes and modulating signaling pathways. Moreover, MEL and nitric oxide cooperate in stress responses, antioxidant defense, and plant growth. Understanding the mechanisms underlying MEL's actions in plants will provide new insights into the development of innovative strategies for enhancing crop productivity, improving stress tolerance, and combating plant diseases. Further research in this area will deepen our knowledge of MEL's intricate functions and its potential applications in sustainable agriculture.

The OsICS1 is directly regulated by OsWRKY6 and increases resistance against Xanthomonas oryzae pv. oryzae

KEY MESSAGE

OsICS1 but not OsICS1-L mediates the rice response to Xoo inoculation, with its overexpression increasing resistance against this pathogen. OsICS1 but not OsICS-L is directly upregulated by OsWRKY6. Rice (Oryza sativa) is a staple crop for about half of the global population and is particularly important in the diets of people living in Asia, Latin America, and Africa. This crop is continually threatened by bacterial leaf blight disease caused by Xanthomonas oryzae pv. oryzae (Xoo), which drastically reduces yields; therefore, it is needed to elucidate the plant's resistance mechanisms against Xoo. Isochorismate synthase (ICS1) generates salicylic acid (SA) and increases resistance against bacterial disease. The OsICS1 is differently annotated in rice genome databases and has not yet been functionally characterized in the context of Xoo infection. Here, we report that the expression of the OsICS1 is directly regulated by OsWRKY6 and increases plant resistance against Xoo. Inoculation with Xoo increased the expression of OsICS1 but not that of the long variant of OsICS1 (OsICS1-L). OsWRKY6 directly activated the OsICS1 promoter but not the OsICS1-L promoter. OsICS1 overexpression in rice increased resistance against Xoo through the induction of SA-dependent bacterial defense genes. These data show that OsICS1 promotes resistance against Xoo infection.

Drought Stress Alleviator Melatonin Reconfigures Water-Stressed Barley (Hordeum vulgare L.) Plants' Photosynthetic Efficiency, Antioxidant Capacity, and Endogenous Phytohormone Profile

The production of crops is severely limited by water scarcity. We still do not fully understand the underlying mechanism of exogenous melatonin (MT)-mediated water stress tolerance in barley. This study is the first of its kind to show how MT can potentially mitigate changes in barley's physio-biochemical parameters caused by water deficiency. Barley was grown under three irrigation levels (100%, 70%, and 30% of field capacity) and was foliar sprayed with 70 μM MT. The results showed that exogenously applied MT protected the photosynthetic apparatus by improving photosynthetic pigment content, photochemical reactions of photosynthesis, Calvin cycle enzyme activity, gas exchange capacity, chlorophyll fluorescence system, and membrane stability index. Furthermore, the increased levels of salicylic acid, gibberellins, cytokinins, melatonin, and indole-3-acetic acid, as well as a decrease in abscisic acid, indicated that foliar-applied MT greatly improved barley water stress tolerance. Additionally, by increasing the activity of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase and decreasing hydrogen peroxide content, lipid peroxidation, and electrolyte leakage, MT application lessened water stress-induced oxidative stress. According to the newly discovered data, MT application improves barley water stress tolerance by reprogramming endogenous plant hormone production and antioxidant activity, which enhances membrane stability and photosynthesis. This study unraveled MT's crucial role in water deficiency mitigation, which can thus be applied to water stress management.

Unraveling the regulatory role of MYC2 on ASMT gene expression in wheat: Implications for melatonin biosynthesis and drought tolerance

Recognized for its multifaceted functions, melatonin is a hormone found in both animals and plants. In the plant kingdom, it plays diverse roles, regulating growth, development, and stress responses. Notably, melatonin demonstrates its significance by mitigating the effects of abiotic stresses like drought. However, understanding the precise regulatory mechanisms controlling melatonin biosynthesis genes, especially during monocots' response to stresses, requires further exploration. Seeking to understand the molecular basis of drought stress tolerance in wheat, we analyzed RNA-Seq libraries of wheat exposed to drought stress using bioinformatics methods. In light of our findings, we identified that the Myelocytomatosis oncogenes 2 (MYC2) transcription factor is a hub gene upstream of a main melatonin biosynthesis gene, N-acetylserotonin methyltransferase (ASMT), in the wheat drought response-gene network. Promoter analysis of the ASMT gene suggested that it might be a target gene of MYC2. We conducted a set of molecular and physiochemical assays along with robust machine learning approaches to elevate those findings further. MYC2 and ASMT were co-regulated under Jasmonate, drought, and a combination of them in the leaf tissues of wheat was detected. A meaningful correlation was observed among gene expression profiles, melatonin contents, photosynthetic activities, antioxidant enzyme activities, H2 O2 levels, and plasma membrane damage. The results indicated an evident relationship between jasmonic acid and the melatonin biosynthesis pathway. Moreover, it seems that the MYC2-ASMT module might contribute to wheat drought tolerance by regulating melatonin contents.

Experimental approaches in studying active biomolecules modulating fruit ripening: Melatonin as a case study

Phytohormones are naturally occurring small organic molecules found at low concentrations in plants. They perform essential functions in growth and developmental processes, from organ initiation to senescence, including fruit ripening. These regulatory molecules are studied using different experimental approaches, such as performing exogenous applications, evaluating endogenous levels, and/or obtaining genetically modified lines. Here, we discuss the advantages and limitations of current experimental approaches used to study active biomolecules modulating fruit ripening, focusing on melatonin. Although melatonin has been implicated in fruit ripening in several model fruit crops, current knowledge is affected by the different experimental approaches used, which have given different and sometimes even contradictory results. The methods of application and the doses used have produced different results in studies based on exogenous applications, while different measurement methods and ways of expressing results explain most of the variability in studies using correlative analyses. Furthermore, studies on genetically modified crops have focused on tomato (Solanum lycopersicum L.) plants only. However, TILLING and CRISPR methodologies are becoming essential tools to complement the results from the experimental approaches described above. This will not only help the scientific community better understand the role of melatonin in modulating fruit ripening, but it will also help develop technological advances to improve fruit yield and quality in major crops. The combination of various experimental approaches will undoubtedly lead to a complete understanding of the function of melatonin in fruit ripening in the near future, so that this knowledge can be effectively transferred to the field.

Phytomelatonin: A key regulator of redox and phytohormones signaling against biotic/abiotic stresses

Plants being sessile in nature, are exposed to unwarranted threats as a result of constantly changing environmental conditions. These adverse factors can have negative impacts on their growth, development, and yield. Hormones are key signaling molecules enabling cells to respond rapidly to different external and internal stimuli. In plants, melatonin (MT) plays a critical role in the integration of various environmental signals and activation of stress-response networks to develop defense mechanisms and plant resilience. Additionally, melatonin can tackle the stress-induced alteration of cellular redox equilibrium by regulating the expression of redox hemostasis-related genes and proteins. The purpose of this article is to compile and summarize the scientific research pertaining to MT's effects on plants' resilience to biotic and abiotic stresses. Here, we have summarized that MT exerts a synergistic effect with other phytohormones, for instance, ethylene, jasmonic acid, and salicylic acid, and activates plant defense-related genes against phytopathogens. Furthermore, MT interacts with secondary messengers like Ca²⁺, nitric oxide, and reactive oxygen species to regulate the redox network. This interaction triggers different transcription factors to alleviate stress-related responses in plants. Hence, the critical synergic role of MT with diverse plant hormones and secondary messengers demonstrates phytomelatonin's importance in influencing multiple mechanisms to contribute to plant resilience against harsh environmental factors.

Integrated transcriptome and metabolome analysis unveil the response mechanism in wild rice (Zizania latifolia griseb.) against sheath rot infection

Sheath rot disease (SRD) is one of the most devastating diseases of Manchurian wild rice (MWR) (Zizania latifolia Griseb). Pilot experiments in our laboratory have shown that an MWR cultivar "Zhejiao NO.7"exhibits signs of SRD tolerance. To explore the responses of Zhejiao No. 7 to SRD infection, we used a combined transcriptome and metabolome analysis approach. A total of 136 differentially accumulated metabolites (DAMs, 114 up- and 22 down-accumulated in FA compared to CK) were detected. These up-accumulated metabolites were enriched in tryptophan metabolism, amino acid biosynthesis, flavonoids, and phytohormone signaling. Transcriptome sequencing results showed the differential expression of 11,280 genes (DEGs, 5,933 up-, and 5,347 downregulated in FA compared to CK). The genes expressed in tryptophan metabolism, amino acid biosynthesis, phytohormone biosynthesis and signaling, and reactive oxygen species homeostasis confirmed the metabolite results. In addition, genes related to the cell wall, carbohydrate metabolism, and plant-pathogen interaction (especially hypersensitive response) showed changes in expression in response to SRD infection. These results provide a basis for understanding the response mechanisms in MWR to FA attack that can be used for breeding SRD-tolerant MWR.

Melatonin treatment delays postharvest senescence of broccoli with regulation of carotenoid metabolism

The effect of melatonin treatment on the carotenoid metabolism in broccoli florets during storage was explored. The results indicated that 100 µmol/L of melatonin maintained the sensory quality of broccoli florets, which retarded the increase of the L* value and the decrease of the H value. Melatonin treatment increased the activities of tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT), leading to the enrichment of endogenous melatonin content in broccoli florets. Meanwhile, the treatment inhibited the concentrations of β-carotene, β-cryptoxanthin, zeaxanthin and lutein, which was beneficial in delaying the yellowing of broccoli. In addition, a series of carotenoid biosynthetic genes such as BoPSY, BoPDS, BoZDS, BoLCYβ and BoZEP was also suppressed by melatonin. Further analysis revealed that the lower carotenoid content and the down-regulated BoNCED expression in treated broccoli resulted in less accumulation of abscisic acid precursors, inhibiting abscisic acid production during the yellowing process.

Melatonin: A promising candidate for maintaining food security under the threat of phytopathogens

Plant immune response is tightly controlled by an interplay of various phytohormones and plant growth regulators. Among them, the role of salicylic acid, jasmonic acid, and ethylene is well established while some others such as nitric oxide, polyamines, and hydrogen sulfide have appeared to be key regulators of plant immunity. In addition, some other chemicals, such as melatonin (N-acetyl-5-methoxytryptamine), are apparently turning out to be the novel regulators of plant defense responses. Melatonin has shown promising results in enhancing resistance of plants to a variety of pathogens including fungi, bacteria, and viruses, however, the molecular mechanism of melatonin-mediated plant immune regulation is currently elusive. Evidence gathered so far indicates that melatonin regulates plant immunity by (1) facilitating the maintenance of ROS homeostasis, (2) interacting with other phytohormones and growth regulators, and (3) inducing the accumulation of defense molecules. Therefore, engineering crops with improved melatonin production could enhance crop productivity under stress conditions. This review extends our understanding of the multifaceted role of melatonin in the regulation of plant defense response and presents a putative pathway of melatonin functioning and its interaction with phytohormones during biotic stress.

Phytomelatonin interferes with flavonols biosynthesis to regulate ROS production and stomatal closure in tobacco

Flavonols are well-known antioxidants that prevent stomatal closure via interfering with ROS signaling. Phytomelatonin regulates stomatal closure, but the signaling pathways are still largely unknown. Here, we investigated the role of flavonols in phytomelatonin-mediated stomatal closure in tobacco plants. The application of melatonin induced stomatal closure through NADPH oxidase-mediated ROS production. Transgenic tobacco plants overexpressing soybean GmSNAT1 (coding for serotonin N-acetyltransferase that catalyzes the penultimate step in phytomelatonin biosynthesis) had higher phytomelatonin concentration, accumulated more ROS in guard cells and were more sensitive to melatonin-induced stomatal closure than the wild-type plants, which was associated with the higher expression of PMTR1-homologous genes. Exogenous melatonin decreased flavonol concentrations in guard cells and the expression of flavonoid-related genes in wild-type and transgenic tobacco plants, and these inhibitory effects were more obvious in GmSNAT1-overexpressing plants than the wild type. However, the melatonin-mediated stomatal closure and ROS production were diminished by the application of kaempferol (a type of flavonol). Additionally, transgenic tobacco plants with increased expression of NtFLS (encoding flavonol synthase) were less sensitive to melatonin-induced stomatal closure. In conclusion, phytomelatonin hampers the biosynthesis of flavonols in guard cells, which results in high concentration of ROS and induces stomatal closure in tobacco plants.

A cysteine-rich secretory protein involves in phytohormone melatonin mediated plant resistance to CGMMV

BACKGROUND

Melatonin is considered to be a polyfunctional master regulator in animals and higher plants. Exogenous melatonin inhibits plant infection by multiple diseases; however, the role of melatonin in Cucumber green mottle mosaic virus (CGMMV) infection remains unknown.

RESULTS

In this study, we demonstrated that exogenous melatonin treatment can effectively control CGMMV infection. The greatest control effect was achieved by 3 days of root irrigation at a melatonin concentration of 50 μM. Exogenous melatonin showed preventive and therapeutic effects against CGMMV infection at early stage in tobacco and cucumber. We utilized RNA sequencing technology to compare the expression profiles of mock-inoculated, CGMMV-infected, and melatonin+CGMMV-infected tobacco leaves. Defense-related gene CRISP1 was specifically upregulated in response to melatonin, but not to salicylic acid (SA). Silencing CRISP1 enhanced the preventive effects of melatonin on CGMMV infection, but had no effect on CGMMV infection. We also found exogenous melatonin has preventive effects against another Tobamovirus, Pepper mild mottle virus (PMMoV) infection.

CONCLUSIONS

Together, these results indicate that exogenous melatonin controls two Tobamovirus infections and inhibition of CRISP1 enhanced melatonin control effects against CGMMV infection, which may lead to the development of a novel melatonin treatment for Tobamovirus control.

Melatonin Treatment of Strawberry Fruit during Storage Extends Its Post-Harvest Quality and Reduces Infection Caused by Botrytis cinerea

Gray mold is a main disease of strawberry fruit (Fragaria × xananassa cv. Camino Real) caused by Botrytis cinerea, which leads to marketable value losses in the supply chain. The purpose of this study was to investigate the effects of exogenous melatonin (MT) on the physicochemical quality, antioxidant defense system, and disease resistance of strawberry fruit to B. cinerea infection. The results revealed that strawberry fruit immersed in 100 µM MT for 15 min effectively maintained its brightness and delayed the change in fruit color. MT also maintained the level of titratable acidity and slowed down the increase of total soluble solids in strawberry fruit. Moreover, strawberries immersed in MT maintained a fresh weight and fruit firmness, as well as reduced B. cinerea infection when compared to the untreated control fruit and fruit treated with 5% NaOCl. In addition, MT increased the accumulation of DPPH scavenging capacity and the activity of antioxidant enzymes (SOD, POD, and APX) with the exception of CAT. The same effect was also observed in strawberry fruit after immersion in MT and followed by B. cinerea inoculation. These findings demonstrated that exogenous MT could effectively maintain the postharvest quality of strawberries, even when the fruit was inoculated with B. cinerea.

The role of phytomelatonin receptor 1-mediated signaling in plant growth and stress response

Phytomelatonin is a pleiotropic signaling molecule that regulates plant growth, development, and stress response. In plant cells, phytomelatonin is synthesized from tryptophan via several consecutive steps that are catalyzed by tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acyltransferase (SNAT), and N-acetylserotonin methyltransferase (ASMT) and/or caffeic acid-3-O-methyltransferase (COMT). Recently, the identification of the phytomelatonin receptor PMTR1 in Arabidopsis has been considered a turning point in plant research, with the function and signal of phytomelatonin emerging as a receptor-based regulatory strategy. In addition, PMTR1 homologs have been identified in several plant species and have been found to regulate seed germination and seedling growth, stomatal closure, leaf senescence, and several stress responses. In this article, we review the recent evidence in our understanding of the PMTR1-mediated regulatory pathways in phytomelatonin signaling under environmental stimuli. Based on structural comparison of the melatonin receptor 1 (MT1) in human and PMTR1 homologs, we propose that the similarity in the three-dimensional structure of the melatonin receptors probably represents a convergent evolution of melatonin recognition in different species.

Genetic and evolutionary dissection of melatonin response signaling facilitates the regulation of plant growth and stress responses

The expansion of gene families during evolution could generate functional diversity among their members to regulate plant growth and development. Melatonin, a phylogenetically ancient molecule, is vital for many aspects of a plant's life. Understanding the functional diversity of the molecular players involved in melatonin biosynthesis, signaling, and metabolism will facilitate the regulation of plant phenotypes. However, the molecular mechanism of melatonin response signaling elements in regulating this network still has many challenges. Here, we provide an in-depth analysis of the functional diversity and evolution of molecular components in melatonin signaling pathway. Genetic analysis of multiple mutants in plant species will shed light on the role of gene families in melatonin regulatory pathways. Phylogenetic analysis of these genes was performed, which will facilitate the identification of melatonin-related genes for future study. Based on the abovementioned signal networks, the mechanism of these genes was summarized to provide reference for studying the regulatory mechanism of melatonin in plant phenotypes. We hope that this work will facilitate melatonin research in higher plants and finely tuned spatio-temporal regulation of melatonin signaling.

Melatonin as a Possible Natural Anti-Viral Compound in Plant Biocontrol

Melatonin is a multifunctional and ubiquitous molecule. In animals, melatonin is a hormone that is involved in a wide range of physiological activities and is also an excellent antioxidant. In plants, it has been considered a master regulator of multiple physiological processes as well as of hormonal homeostasis. Likewise, it is known for its role as a protective biomolecule and activator of tolerance and resistance against biotic and abiotic stress in plants. Since infections by pathogens such as bacteria, fungi and viruses in crops result in large economic losses, interest has been aroused in determining whether melatonin plays a relevant role in plant defense systems against pathogens in general, and against viruses in particular. Currently, several strategies have been applied to combat infection by pathogens, one of them is the use of eco-friendly chemical compounds that induce systemic resistance. Few studies have addressed the use of melatonin as a biocontrol agent for plant diseases caused by viruses. Exogenous melatonin treatments have been used to reduce the incidence of several virus diseases, reducing symptoms, virus titer, and even eradicating the proliferation of viruses such as Tobacco Mosaic Virus, Apple Stem Grooving Virus, Rice Stripe Virus and Alfalfa Mosaic Virus in tomato, apple, rice and eggplant, respectively. The possibilities of using melatonin as a possible natural virus biocontrol agent are discussed.

Creation of a Plant Metabolite Spectral Library for Untargeted and Targeted Metabolomics

Large-scale high throughput metabolomic technologies are indispensable components of systems biology in terms of discovering and defining the metabolite parts of the system. However, the lack of a plant metabolite spectral library limits the metabolite identification of plant metabolomic studies. Here, we have created a plant metabolite spectral library using 544 authentic standards, which increased the efficiency of identification for untargeted metabolomic studies. The process of creating the spectral library was described, and the mzVault library was deposited in the public repository for free download. Furthermore, based on the spectral library, we describe a process of creating a pseudo-targeted method, which was applied to a proof-of-concept study of Arabidopsis leaf extracts. As authentic standards become available, more metabolite spectra can be easily incorporated into the spectral library to improve the mzVault package.

Melatonin-mediated temperature stress tolerance in plants

Global climate changes cause extreme temperatures and a significant reduction in crop production, leading to food insecurity worldwide. Temperature extremes (including both heat and cold stresses) is one of the most limiting factors in plant growth and development and severely affect plant physiology, biochemical, and molecular processes. Biostimulants like melatonin (MET) have a multifunctional role that acts as a "defense molecule" to safeguard plants against the noxious effects of temperature stress. MET treatment improves plant growth and temperature tolerance by improving several defense mechanisms. Current research also suggests that MET interacts with other molecules, like phytohormones and gaseous molecules, which greatly supports plant adaptation to temperature stress. Genetic engineering via overexpression or CRISPR/Cas system of MET biosynthetic genes uplifts the MET levels in transgenic plants and enhances temperature stress tolerance. This review highlights the critical role of MET in plant production and tolerance against temperature stress. We have documented how MET interacts with other molecules to alleviate temperature stress. MET-mediated molecular breeding would be great potential in helping the adverse effects of temperature stress by creating transgenic plants.

Protocorm-like-body extract of Phalaenopsis aphrodite combats watermelon fruit blotch disease

Bacterial fruit blotch, caused by the seedborne gram-negative bacterium Acidovorax citrulli, is one of the most destructive bacterial diseases of cucurbits (gourds) worldwide. Despite its prevalence, effective and reliable means to control bacterial fruit blotch remain limited. Transcriptomic analyses of tissue culture-based regeneration processes have revealed that organogenesis-associated cellular reprogramming is often associated with upregulation of stress- and defense-responsive genes. Yet, there is limited evidence supporting the notion that the reprogrammed cellular metabolism of the regenerated tissued confers bona fide antimicrobial activity. Here, we explored the anti-bacterial activity of protocorm-like-bodies (PLBs) of Phalaenopsis aphrodite. Encouragingly, we found that the PLB extract was potent in slowing growth of A. citrulli, reducing the number of bacteria attached to watermelon seeds, and alleviating disease symptoms of watermelon seedlings caused by A. citrulli. Because the anti-bacterial activity can be fractionated chemically, we predict that reprogrammed cellular activity during the PLB regeneration process produces metabolites with antibacterial activity. In conclusion, our data demonstrated the antibacterial activity in developing PLBs and revealed the potential of using orchid PLBs to discover chemicals to control bacterial fruit blotch disease.

Transcriptome and hormone Analyses reveal that melatonin promotes adventitious rooting in shaded cucumber hypocotyls

Melatonin, a multi-regulatory molecule, stimulates root generation and regulates many aspects of plant growth and developmental processes. To gain insight into the effects of melatonin on adventitious root (AR) formation, we use cucumber seedings subjected to one of three treatments: EW (hypocotyl exposed and irrigated with water), SW (hypocotyl shaded and irrigated with water) and SM (hypocotyl shaded and irrigated with 100 µM melatonin). Under shaded conditions, melatonin induced significant AR formation in the hypocotyl. To explore the mechanism of this melatonin-induced AR formation, we used transcriptome analysis to identify 1296 significant differentially expressed genes (DEGs). Comparing SM with SW, a total of 774 genes were upregulated and 522 genes were downregulated. The DEGs were classified among different metabolic pathways, especially those connected with the synthesis of secondary metabolites, with hormone signal transduction and with plant-pathogen interactions. Analyses indicate exogenous melatonin increased contents of endogenous auxin, jasmonic acid, salicylic acid, cytokinin and abscisic acid levels during AR formation. This study indicates melatonin promotes AR formation in cucumber seedings by regulating the expressions of genes related to hormone synthesis, signaling and cell wall formation, as well as by increasing the contents of auxin, cytokinin, jasmonic acid, salicylic acid and abscisic acid. This research elucidates the molecular mechanisms of melatonin's role in promoting AR formation in the hypocotyl of cucumber seedings under shaded conditions.

Melatonin biosynthesis and signal transduction in plants in response to environmental conditions

Melatonin, the most widely distributed hormone in nature, plays important roles in plants. Many physiological processes in plants are linked to melatonin, including seed germination, anisotropic cell growth, and senescence. Compared with animals, different plants possess diverse melatonin biosynthetic pathways and regulatory networks. Whereas melatonin biosynthesis in animals is known to be regulated by ambient signals, little is known about how melatonin biosynthesis in plants responds to environmental signals. Plants are affected by numerous environmental factors, such as light, temperature, moisture, carbon dioxide, soil conditions, and nutrient availability at all stages of development and in different tissues. Melatonin content exhibits dynamic changes that affect plant growth and development. Melatonin plays various species-specific roles in plant responses to different environmental conditions. However, much remains to be learned, as not all environmental factors have been studied, and little is known about the mechanisms by which these factors influence melatonin biosynthesis. In this review, we provide a detailed, systematic description of melatonin biosynthesis and signaling and of the roles of melatonin in plant responses to different environmental factors, providing a reference for in-depth research on this important issue.

Melatonin as a regulator of plant ionic homeostasis: implications for abiotic stress tolerance

Melatonin is a highly conserved and ubiquitous molecule that operates upstream of a broad array of receptors in animal systems. Since melatonin was discovered in plants in 1995, hundreds of papers have been published revealing its role in plant growth, development, and adaptive responses to the environment. This paper summarizes the current state of knowledge of melatonin's involvement in regulating plant ion homeostasis and abiotic stress tolerance. The major topics covered here are: (i) melatonin's control of H+-ATPase activity and its implication for plant adaptive responses to various abiotic stresses; (ii) regulation of the reactive oxygen species (ROS)-Ca2+ hub by melatonin and its role in stress signaling; and (iii) melatonin's regulation of ionic homeostasis via hormonal cross-talk. We also show that the properties of the melatonin molecule allow its direct scavenging of ROS, thus preventing negative effects of ROS-induced activation of ion channels. The above 'desensitization' may play a critical role in preventing stress-induced K+ loss from the cytosol as well as maintaining basic levels of cytosolic Ca2+ required for optimal cell operation. Future studies should focus on revealing the molecular identity of transporters that could be directly regulated by melatonin and providing a bioinformatic analysis of evolutionary aspects of melatonin sensing and signaling.

Anabolism and signaling pathways of phytomelatonin

Phytomelatonin is a small multifunctional molecule found ubiquitously in plants, which plays an important role in plant growth, development, and biotic and abiotic stress responses. The classical biosynthetic and metabolic pathways of phytomelatonin have been elucidated, and uncovering alternative pathways has deepened our understanding of phytomelatonin synthesis. Phytomelatonin functions mainly via two pathways. In the direct pathway, phytomelatonin mediates the stress-induced reactive oxygen species burst through its strong antioxidant capacity. In the indirect pathway, phytomelatonin acts as a signal to activate signaling cascades and crosstalk with other plant hormones. The phytomelatonin receptor PMTR1/CAND2 was discovered in 2018, which enhanced our understanding of phytomelatonin function. This review summarizes the classical and potential pathways involved in phytomelatonin synthesis and metabolism. To elucidate the functions of phytomelatonin, we focus on the crosstalk between phytomelatonin and other phytohormones. We propose two models to explain how PMTR1 transmits the phytomelatonin signal through the G protein and MAPK cascade. This review will facilitate the identification of additional signaling molecules that function downstream of the phytomelatonin signaling pathway, thus improving our understanding of phytomelatonin signal transmission.

Phytomelatonin as a central molecule in plant disease resistance

Melatonin is an essential phytohormone in the regulation of many plant processes, including during plant development and in response to stress. Pathogen infections cause serious damage to plants and reduce agricultural production. Recent studies indicate that melatonin plays important roles in alleviating bacterial, fungal, and viral diseases in plants and post-harvest fruits. Herein, we summarize information related to the effects of melatonin on plant disease resistance. Melatonin, reactive oxygen species, and reactive nitrogen species form a complex loop in plant-pathogen interaction to regulate plant disease resistance. Moreover, crosstalk of melatonin with other phytohormones including salicylic acid, jasmonic acid, auxin, and abscisic acid further activates plant defense genes. Melatonin plays an important role not only in plant immunity but also in alleviating pathogenicity. We also summarize the known processes by which melatonin mediates pathogenicity via negatively regulating the expression levels of genes related to cell viability as well as virulence-related genes. The multiple mechanisms underlying melatonin influences on both plant immunity and pathogenicity support the recognition of the essential nature of melatonin in plant-pathogen interactions, highlighting phytomelatonin as a critical molecule in plant immune responses.

Melatonin Function and Crosstalk with Other Phytohormones under Normal and Stressful Conditions

Melatonin was discovered in plants in the late nineties, but its role, signaling, and crosstalk with other phytohormones remain unknown. Research on melatonin in plants has risen dramatically in recent years and the role of this putative plant hormone under biotic and abiotic stress conditions has been reported. In the present review, we discuss the main functions of melatonin in the growth and development of plants, its role under abiotic stresses, such as water stress (waterlogging and drought), extreme temperature (low and high), salinity, heavy metal, and light-induced stress. Similarly, we also discuss the role of melatonin under biotic stresses (antiviral, antibacterial, and antifungal effects). Moreover, the present review meticulously discusses the crosstalk of melatonin with other phytohormones such as auxins, gibberellic acids, cytokinins, ethylene, and salicylic acid under normal and stressful conditions and reports melatonin receptors and signaling in plants. All these aspects of melatonin suggest that phytomelatonin is a key player in crop improvement and biotic and abiotic stress regulation.

Genome-wide identification, characterization of Serotonin N-acetyltransferase and deciphering its importance under development, biotic and abiotic stress in soybean

Serotonin N-acetyltransferase (SNAT) is the penultimate enzyme involved in plant melatonin biosynthesis. Identifying its expression under development and stress will reveal the regulatory role in the soybean. To identify and characterize SNAT, we employed genome-wide analysis, gene structure, cis-acting elements, expression, and enzyme activity. We identified seven putative genes by genome-wide analysis and found chloroplast signal peptides in three GmSNATs. To elucidate GmSNATs role, expression datasets of more than a hundred samples related to circadian rhythm, developmental stages, and stress conditions were analysed. Notably, the expression of GmSNAT1 did not show significant expression during biotic and abiotic stress. The GmSNAT1 sequence showed 67.8 and 72.2 % similarities with OsSNAT and AtSNAT, respectively. The Km and Vmax of the purified recombinant GmSNAT1 were 657 μM and 3780 pmol/min/mg, respectively. To further understand the GmSNAT1 role, we supplemented different concentrations of serotonin and melatonin to in-vitro cultures and seed priming. These studies revealed that the GmSNAT1 expression was significantly up-regulated at higher concentrations of serotonin and down-regulated at higher melatonin concentrations. We speculate that a high concentration of melatonin during abiotic, biotic stress, and in-vitro cultures are responsible for regulating GmSNAT1 expression, which may regulate them at the enzyme level during stress in soybean.

ELONGATED HYPOCOTYL 5-mediated suppression of melatonin biosynthesis is alleviated by darkness and promotes cotyledon opening

Melatonin (N-acetyl-5-methoxytryptamine) biosynthesis in plants is induced by darkness and high-intensity light; however, the underlying transcriptional mechanisms and upstream signalling pathways are unknown. We identified a dark-induced and highly expressed melatonin synthetase in Arabidopsis thaliana, AtSNAT6, encoding serotonin N-acetyltransferase. We assessed melatonin content and AtSNAT6 expression in mutants lacking key regulators of light/dark signalling. AtCOP1 (CONSTITUTIVE PHOTOMORPHOGENIC 1) and AtHY5 (ELONGATED HYPOCOTYL 5), which control light/dark transition and photomorphogenesis, promoted and suppressed melatonin biosynthesis, respectively. Using EMSA and ChIP-qPCR analysis, we showed that AtHY5 inhibits AtSNAT6 expression directly. An analysis of melatonin content in snat6 hy5 double mutant and AtHY5+AtSNAT6-overexpressing plants confirmed the regulatory function of AtHY5 and AtSNAT6 in melatonin biosynthesis. Exogenous melatonin further inhibited cotyledon opening in hy5 mutant and AtSNAT6-overexpressing seedlings, but partially reversed the promotion of cotyledon opening in AtHY5-overexpressing seedlings and snat6. Additionally, CRISPR/Cas9-mediated mutation of AtSNAT6 increased cotyledon opening in hy5 mutant, and overexpression of AtSNAT6 decreased cotyledon opening in AtHY5-overexpressing seedlings via changing melatonin biosynthesis, confirming that AtHY5 decreased melatonin-mediated inhibition of cotyledon opening. Our data provide new insights into the regulation of melatonin biosynthesis and its function in cotyledon opening.

Insight into melatonin-mediated response and signaling in the regulation of plant defense under biotic stress

Melatonin plays a crucial role in the mitigation of plant biotic stress through induced defense responses and pathogen attenuation. Utilizing the current knowledge of signaling and associated mechanism of this phytoprotectant will be invaluable in sustainable plant disease management. Biotic stress in plants involves complex regulatory networks of various sensory and signaling molecules. In this context, the polyfunctional, ubiquitous-signaling molecule melatonin has shown a regulatory role in biotic stress mitigation in plants. The present review conceptualized the current knowledge concerning the melatonin-mediated activation of the defense signaling network that leads to the resistant or tolerant phenotype of the infected plants. Fundamentals of signaling networks involved in melatonin-induced reactive oxygen species (ROS) or reactive nitrogen species (RNS) scavenging through enzymatic and non-enzymatic antioxidants have also been discussed. Increasing evidence has suggested that melatonin acts upstream of mitogen-activated proteinase kinases in activation of defense-related genes and heat shock proteins that provide immunity against pathogen attack. Besides, the direct application of melatonin on virulent fungi and bacteria showed disrupted spore morphology, destabilization of cell ultrastructure, reduced biofilm formation, and enhanced mortality that led to attenuate disease symptoms on melatonin-treated plants. The transcriptome analysis has revealed the down-regulation of pathogenicity genes, metabolism-related genes, and up-regulation of fungicide susceptibility genes in melatonin-treated pathogens. The activation of melatonin-mediated systemic acquired resistance (SAR) through cross-talk with salicylic acid (SA), jasmonic acid (JA) has been essential for viral disease management. The high endogenous melatonin concentration has also been correlated with the up-regulation of genes involved in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). The present review highlights the versatile functions of melatonin towards direct inhibition of pathogen propagule along with active participation in mediating oxidative burst and simulating PTI, ETI and SAR responses. The hormonal cross-talk involving melatonin mediated biotic stress tolerance through defense signaling network suggests its suitability in a sustainable plant protection system.

The Antioxidant Cyclic 3-Hydroxymelatonin Promotes the Growth and Flowering of Arabidopsis thaliana

In plants, melatonin is metabolized into several compounds, including the potent antioxidant cyclic 3-hydroxymelatonin (3-OHM). Melatonin 3-hydroxylase (M3H), a member of the 2-oxo-glutarate-dependent enzyme family, is responsible for 3-OHM biosynthesis. Although rice M3H has been cloned, its roles are unclear, and no homologs in other plant species have been characterized. Here, we cloned and characterized Arabidopsis thaliana M3H (AtM3H). The purified recombinant AtM3H exhibited K_m and V_max values of 100 μM and 20.7 nmol/min/mg protein, respectively. M3H was localized to the cytoplasm, and its expression peaked at night. Based on a 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, 3-OHM exhibited 15-fold higher antioxidant activity than melatonin. An Arabidopsis M3H knockout mutant (m3h) produced less 3-OHM than the wildtype (WT), thus reducing antioxidant activity and biomass and delaying flowering. These defects were caused by reduced expression of FLOWERING LOCUS T (FT) and gibberellin-related genes, which are responsible for flowering and growth. Exogenous 3-OHM, but not exogenous melatonin, induced FT expression. The peak of M3H expression at night matched the FT expression pattern. The WT and m3h exhibited similar responses to salt stress and pathogens. Collectively, our findings indicate that 3-OHM promotes growth and flowering in Arabidopsis.

OsCOMT, encoding a caffeic acid O-methyltransferase in melatonin biosynthesis, increases rice grain yield through dual regulation of leaf senescence and vascular development

Melatonin, a natural phytohormone in plants, plays multiple critical roles in plant growth and stress responses. Although melatonin biosynthesis-related genes have been suggested to possess diverse biological functions, their roles and functional mechanisms in regulating rice grain yield remain largely unexplored. Here, we uncovered the roles of a caffeic acid O-methyltransferase (OsCOMT) gene in mediating rice grain yield through dual regulation of leaf senescence and vascular development. In vitro and in vivo evidence revealed that OsCOMT is involved in melatonin biosynthesis. Transgenic assays suggested that OsCOMT significantly delays leaf senescence at the grain filling stage by inhibiting degradation of chlorophyll and chloroplast, which, in turn, improves photosynthesis efficiency. In addition, the number and size of vascular bundles in the culms and leaves were significantly increased in the OsCOMT-overexpressing plants, while decreased in the knockout plants, suggesting that OsCOMT plays a positive role in vascular development of rice. Further evidence indicated that OsCOMT-mediated vascular development might owe to the crosstalk between melatonin and cytokinin. More importantly, we found that OsCOMT is a positive regulator of grain yield, and overexpression of OsCOMT increase grain yield per plant even in a high-yield variety background, suggesting that OsCOMT can be used as an important target for enhancing rice yield. Our findings shed novel insights into melatonin-mediated leaf senescence and vascular development and provide a possible strategy for genetic improvement of rice grain yield.

Transcriptome and physiological analysis of increase in drought stress tolerance by melatonin in tomato

Drought stress seriously affects tomato growth, yield and quality. Previous reports have pointed out that melatonin (MT) can alleviate drought stress damage to tomato. To better understand the possible physiological and molecular mechanisms, chlorophyll fluorescence parameters and leaf transcriptome profiles were analyzed in the "Micro Tom" tomato cultivar with or without melatonin irrigation under normal and drought conditions. Polyethylene glycol 6000 (PEG6000) simulated continuous drought treatment reduced plant height, but melatonin treatment improved plant growth rate. Physiological parameter measurements revealed that the drought-induced decreases in maximum efficiency of photosystem II (PSII) photochemistry, the effective quantum yield of PSII, electron transfer rate, and photochemical quenching value caused by PEG6000 treatment were alleviated by melatonin treatment, which suggests a protective effect of melatonin on PSII. Comparative transcriptome analysis identified 447, 3982, 4526 and 3258 differentially expressed genes (DEGs) in the comparative groups plus-melatonin vs. minus-melatonin (no drought), drought vs. no drought (minus-melatonin), drought vs. no drought (melatonin) and plus-melatonin vs. minus-melatonin (drought), respectively. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analysis revealed that DEGs in the four comparative groups were involved in multiple metabolic processes and closely related to hormone signal transduction and transcription factors. Transcriptome data revealed that melatonin changed the expression pattern of most hormone signal transduction related DEGs induced by drought, and improved plant drought resistance by down-regulating the expression of linoleic acid catabolic enzyme genes. These results provide new insights into a probable mechanism of the melatonin-induced protection of photosynthesis and enhancement of drought tolerance in tomato plants.

The phytomelatonin receptor PMTR1 regulates seed development and germination by modulating abscisic acid homeostasis in Arabidopsis thaliana

Melatonin is known to involve multiple physiological actions in plants. Herein, we found that exogenous melatonin inhibited the Arabidopsis seedling growth through the elevated abscisic acid (ABA) levels, and the elevated ABA was ascribed to the upregulation of 9-cis-epoxycarotenoid dioxygenase genes (NCEDs) in the ABA biosynthesis pathway. We also found that the overexpression lines of the melatonin receptor gene PMTR1 (also known as Cand2) yielded smaller seeds and germinated slower than the wild type, whereas PMTR1-knockout mutants produced larger seeds and germinated faster than the wild type. During the seed development, the accumulation peak of ABA was higher in the PMTR1-knockout mutant, while it was lower in the PMTR1-overexpression line than that in the wild type. In the dry seeds and imbibed seeds, the PMTR1-overexpression line accumulated higher ABA levels, while the PMTR1-knockout contained less ABA than the wild type. In summary, our findings suggest that PMTR1 is involved in ABA-mediated seed development and germination in Arabidopsis.

Functional Characterization of Serotonin N-Acetyltransferase in Archaeon Thermoplasma volcanium

Serotonin N-acetyltransferase is the penultimate enzyme in the melatonin biosynthetic pathway that catalyzes serotonin into N-acetylserotonin. Many SNAT genes have been cloned and characterized from organisms ranging from bacteria to plants and mammals. However, to date, no SNAT gene has been identified from Archaea. In this study, three archaeal SNAT candidate genes were synthesized and expressed in Escherichia coli, and SNAT enzyme activity was measured using their purified recombinant proteins. Two SNAT candidate genes, from Methanoregulaceae (Archaea) and Pyrococcus furiosus, showed no SNAT enzyme activity, whereas a SNAT candidate gene from Thermoplasma volcanium previously named TvArd1 exhibited SNAT enzyme activity. The substrate affinity and the maximum reaction rate of TvSNAT toward serotonin were 621 μM and 416 pmol/min/mg protein, respectively. The highest amine substrate was tyramine, followed by tryptamine, serotonin, and 5-methoxytryptamine, which were similar to those of plant SNAT enzymes. Homologs of TvSNAT were found in many Archaea families. Ectopic overexpression of TvSNAT in rice resulted in increased melatonin content, antioxidant activity, and seed size in conjunction with the enhanced expression of seed size-related gene. This study is the first to report the discovery of SNAT gene in Archaea. Future research avenues include the cloning of TvSNAT orthologs in different phyla, and identification of their regulation and functions related to melatonin biosynthesis in living organisms.

How do plants defend themselves against pathogens-Biochemical mechanisms and genetic interventions

In agro-ecosystem, plant pathogens hamper food quality, crop yield, and global food security. Manipulation of naturally occurring defense mechanisms in host plants is an effective and sustainable approach for plant disease management. Various natural compounds, ranging from cell wall components to metabolic enzymes have been reported to protect plants from infection by pathogens and hence provide specific resistance to hosts against pathogens, termed as induced resistance. It involves various biochemical components, that play an important role in molecular and cellular signaling events occurring either before (elicitation) or after pathogen infection. The induction of reactive oxygen species, activation of defensive machinery of plants comprising of enzymatic and non-enzymatic antioxidative components, secondary metabolites, pathogenesis-related protein expression (e.g. chitinases and glucanases), phytoalexin production, modification in cell wall composition, melatonin production, carotenoids accumulation, and altered activity of polyamines are major induced changes in host plants during pathogen infection. Hence, the altered concentration of biochemical components in host plants restricts disease development. Such biochemical or metabolic markers can be harnessed for the development of "pathogen-proof" plants. Effective utilization of the key metabolites-based metabolic markers can pave the path for candidate gene identification. This present review discusses the valuable information for understanding the biochemical response mechanism of plants to cope with pathogens and genomics-metabolomics-based sustainable development of pathogen proof cultivars along with knowledge gaps and future perspectives to enhance sustainable agricultural production.

Melatonin-Mediated Abiotic Stress Tolerance in Plants

Melatonin is a multi-functional molecule that is ubiquitous in all living organisms. Melatonin performs essential roles in plant stress tolerance; its application can reduce the harmful effects of abiotic stresses. Plant melatonin biosynthesis, which usually occurs within chloroplasts, and its related metabolic pathways have been extensively characterized. Melatonin regulates plant stress responses by directly inhibiting the accumulation of reactive oxygen and nitrogen species, and by indirectly affecting stress response pathways. In this review, we summarize recent research concerning melatonin biosynthesis, metabolism, and antioxidation; we focus on melatonin-mediated tolerance to abiotic stresses including drought, waterlogging, salt, heat, cold, heavy metal toxicity, light and others. We also examine exogenous melatonin treatment in plants under abiotic stress. Finally, we discuss future perspectives in melatonin research and its applications in plants.

Functional Characterization of Serotonin N-Acetyltransferase Genes (SNAT1/2) in Melatonin Biosynthesis of Hypericum perforatum

Hypericum perforatum is a traditional medicinal plant that contains various secondary metabolites. As an active component in H. perforatum, melatonin plays important role in plant antioxidation, growth, and photoperiod regulation. Serotonin N-acetyltransferase (SNAT) is the key enzyme involved in the last or penultimate step of phytomelatonin biosynthesis. A total of 48 members of SNAT family were screened and analyzed based on the whole genome data of H. perforatum, and two SNAT genes (HpSNAT1 and HpSNAT2) were functionally verified to be involved in the biosynthesis of melatonin. It was found that HpSNAT1 and HpSNAT2 were highly expressed in the leaves and showed obvious responses to high salt and drought treatment. Subcellular localization analysis indicated that these two proteins were both localized in the chloroplasts by the Arabidopsis protoplasts transient transfection. Overexpression of HpSNAT1 and HpSNAT2 in Arabidopsis (SNAT) and H. perforatum (wild-type) resulted in melatonin content 1.9-2.2-fold and 2.5-4.2-fold higher than that in control groups, respectively. Meanwhile, SNAT-overexpressing Arabidopsis plants showed a stronger ability of root growth and scavenging endogenous reactive oxygen species. In this study, the complete transgenic plants of H. perforatum were obtained through Agrobacterium-mediated genetic transformation for the first time, which laid a significant foundation for further research on the function of key genes in H. perforatum.

5-Methoxyindole, a Chemical Homolog of Melatonin, Adversely Affects the Phytopathogenic Fungus Fusarium graminearum

Fusarium graminearum is a destructive fungal pathogen that threatens the production and quality of wheat, and controlling this pathogen is a significant challenge. As the cost-effective homolog of melatonin, 5-methoxyindole showed strong activity against F. graminearum. In the present study, our results showed the strong adverse activity of 5-methoxyindole against F. graminearum by inhibiting its growth, formation, and conidia germination. In addition, 5-methoxyindole could induce malformation, reactive oxygen species (ROS) accumulation, and cell death in F. graminearum hyphae and conidia. In response to 5-methoxyindole, F. graminearum genes involved in scavenging reactive oxygen species were significantly downregulated. Overall, these findings reveal the mechanism of antifungal action of melatonin-homolog 5-methoxyindole. To the best of our knowledge, this is the first report that a novel melatonin homolog confers strong antifungal activity against F. graminearum, and 5-methoxyindole is a potential compound for protecting wheat plants from F. graminearum infection.

Exogenous application of melatonin to plants, algae, and harvested products to sustain agricultural productivity and enhance nutritional and nutraceutical value: A meta-analysis

Melatonin is produced by plants, algae, and animals. Worldwide studies show diverse positive effects of exogenous melatonin on plants, edible plant products, and algae, but the potential of melatonin to enhance food and feed systems through these positive effects remains largely unexplored. Through a meta-analysis of about 25,000 observations, we show for the first time that exogenous application of melatonin significantly increases crop productivity and yields, and enhances the nutritional and nutraceutical value of edible plant products and algae by regulating diverse biological functions. We demonstrate that melatonin can improve plants, edible plant products, and algae under various current climate change scenarios, environmental pollution factors, and other stresses by about 7% to nearly 30%, on average, depending on the stressor. We also analyze various technical/methodological factors influencing the desired outcomes and identify conditions that offer optimal enhancement. We show that the positive effect of melatonin on plants and edible plant products varies among species, genera, and families, and strongly depends on the concentration of melatonin and treatment duration. The effect of melatonin is slightly lower on the monocot clade Commelinids than on the eudicot clades Asterids and Rosids. We also show that its stimulatory effect on plants depends on cultivation system, with a larger effect obtained in hydroponic systems. However, it does not depend on application stage (seed or vegetative), application route (foliage, roots, or seed), and whether the cultivation system is ex vivo or in vivo. This is the first meta-analysis examining the effects of melatonin on plants, edible plant products, and algae, and offers a scientific and technical roadmap facilitating sustainable food and feed production through the application of exogenous melatonin.

Melatonin Ameliorates Thermotolerance in Soybean Seedling through Balancing Redox Homeostasis and Modulating Antioxidant Defense, Phytohormones and Polyamines Biosynthesis

Global warming is impacting the growth and development of economically important but sensitive crops, such as soybean (Glycine max L.). Using pleiotropic signaling molecules, melatonin can relieve the negative effects of high temperature by enhancing plant growth and development as well as modulating the defense system against abiotic stresses. However, less is known about how melatonin regulates the phytohormones and polyamines during heat stress. Our results showed that high temperature significantly increased ROS and decreased photosynthesis efficiency in soybean plants. Conversely, pretreatment with melatonin increased plant growth and photosynthetic pigments (chl a and chl b) and reduced oxidative stress via scavenging hydrogen peroxide and superoxide and reducing the MDA and electrolyte leakage contents. The inherent stress defense responses were further strengthened by the enhanced activities of antioxidants and upregulation of the expression of ascorbate-glutathione cycle genes. Melatonin mitigates heat stress by increasing several biochemicals (phenolics, flavonoids, and proline), as well as the endogenous melatonin and polyamines (spermine, spermidine, and putrescine). Furthermore, the positive effects of melatonin treatment also correlated with a reduced abscisic acid content, down-regulation of the gmNCED3, and up-regulation of catabolic genes (CYP707A1 and CYP707A2) during heat stress. Contrarily, an increase in salicylic acid and up-regulated expression of the defense-related gene PAL2 were revealed. In addition, melatonin induced the expression of heat shock protein 90 (gmHsp90) and heat shock transcription factor (gmHsfA2), suggesting promotion of ROS detoxification via the hydrogen peroxide-mediated signaling pathway. In conclusion, exogenous melatonin improves the thermotolerance of soybean plants and enhances plant growth and development by activating antioxidant defense mechanisms, interacting with plant hormones, and reprogramming the biochemical metabolism.

Melatonin Application Modifies Antioxidant Defense and Induces Endoreplication in Maize Seeds Exposed to Chilling Stress

The aim of the study was to demonstrate the biostimulating effect of exogenous melatonin (MEL) applied to seeds via hydroconditioning. It was indicated that only well-chosen application technique and MEL dose guarantees success concerning seed germination and young seedlings growth under stress conditions. For maize seed, 50 μM of MEL appeared to be the optimal dose. It improved seed germination and embryonic axes growth especially during chilling stress (5 °C/14 days) and during regeneration after its subsided. Unfortunately, MEL overdosing lowered IAA level in dry seeds and could disrupt the ROS-dependent signal transduction pathways. Very effective antioxidant MEL action was confirmed by low level of protein oxidative damage and smaller quantity of lipid oxidation products in embryonic axes isolated from seeds pre-treated with MEL and then exposed to cold. The stimulatory effects of MEL on antioxidant enzymes: SOD, APX and GSH-PX and on GST-a detoxifying enzyme, was also demonstrated. It was indicated for the first time, that MEL induced defence strategies against stress at the cytological level, as appearing endoreplication in embryonic axes cells even in the seeds germinating under optimal conditions (preventive action), but very intensively in those germinating under chilling stress conditions (intervention action), and after stress removal, to improve regeneration.

Melatonin increases leaf disease resistance and saponin biosynthesis in Panax notogiseng

Panax notoginseng (Bruk.) FH Chen is a valuable traditional herb in China, with saponins being the main medicinal components in its roots. However, leaf diseases are a major factor limiting growth and production of P. notoginseng. Melatonin is a ubiquitous signaling molecule associated with abiotic stress resistance. In this study, we investigated the role of melatonin in leaf disease resistance of P. notoginseng in field conditions. Additionally, saponin concentrations were analyzed to evaluate the suitability of melatonin use in agricultural practice. Our results showed that exogenous application of melatonin promoted the endogenous phytomelatonin accumulation via upregulation of genes involved in its biosynthesis. The application of 10 μM melatonin decreased the incidence of leaf diseases (gray mold, round spot, and black spot) by about 40% compared with the solvent control, which might have been due to the increased expression of genes associated with immunity and disease resistance. Furthermore, concentrations of saponins and expression of their biosynthesis-related genes were significantly increased by melatonin. Taken together, the data presented here suggested that melatonin could be used in agricultural management of P. notoginseng because it increased leaf disease resistance and biosynthesis of saponins.

PAL-mediated SA biosynthesis pathway contributes to nematode resistance in wheat

The pathogen cereal cyst nematode (CCN) is deleterious to Triticeae crops and is a threat to the global crop yield. Accession no. 1 of Aegilops variabilis, a relative of Triticum aestivum (bread wheat), is highly resistant to CCN. Our previous study demonstrated that the expression of the phenylalanine ammonia lyase (PAL) gene AevPAL1 in Ae. variabilis is strongly induced by CCN. PAL, the first enzyme of phenylpropanoid metabolism, is involved in abiotic and biotic stress responses. However, its role in plant-CCN interaction remains unknown. In the present study, we proved that AevPAL1 helps to confer CCN resistance through affecting the synthesis of salicylic acid (SA) and downstream secondary metabolites. The silencing of AevPAL1 increased the incidence of CCN infection in roots and decreased the accumulation of SA and phenylalanine (Phe)-derived specialized metabolites. The exogenous pre-application of SA also improved CCN resistance. Additionally, the functions of PAL in phenylpropanoid metabolism correlated with tryptophan decarboxylase (TDC) functioning in tryptophan metabolism pathways. The silencing of either AevPAL1 or AevTDC1 exhibited a concomitant reduction in the expression of both genes and the contents of metabolites downstream of PAL and TDC. These results suggested that AevPAL1, possibly in coordination with AevTDC1, positively contributes to CCN resistance by altering the downstream secondary metabolites and SA content in Ae. variabilis. Moreover, AevPAL1 overexpression significantly enhanced CCN resistance in bread wheat and did not exhibit significant negative effects on yield-related traits, suggesting that AevPAL1 is valuable for the genetic improvement of CCN resistance in bread wheat.

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

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

Bacteriostatic Potential of Melatonin: Therapeutic Standing and Mechanistic Insights

Diseases caused by pathogenic bacteria in animals (e.g., bacterial pneumonia, meningitis and sepsis) and plants (e.g., bacterial wilt, angular spot and canker) lead to high prevalence and mortality, and decomposition of plant leaves, respectively. Melatonin, an endogenous molecule, is highly pleiotropic, and accumulating evidence supports the notion that melatonin's actions in bacterial infection deserve particular attention. Here, we summarize the antibacterial effects of melatonin in vitro, in animals as well as plants, and discuss the potential mechanisms. Melatonin exerts antibacterial activities not only on classic gram-negative and -positive bacteria, but also on members of other bacterial groups, such as Mycobacterium tuberculosis. Protective actions against bacterial infections can occur at different levels. Direct actions of melatonin may occur only at very high concentrations, which is at the borderline of practical applicability. However, various indirect functions comprise activation of hosts' defense mechanisms or, in sepsis, attenuation of bacterially induced inflammation. In plants, its antibacterial functions involve the mitogen-activated protein kinase (MAPK) pathway; in animals, protection by melatonin against bacterially induced damage is associated with inhibition or activation of various signaling pathways, including key regulators such as NF-κB, STAT-1, Nrf2, NLRP3 inflammasome, MAPK and TLR-2/4. Moreover, melatonin can reduce formation of reactive oxygen and nitrogen species (ROS, RNS), promote detoxification and protect mitochondrial damage. Altogether, we propose that melatonin could be an effective approach against various pathogenic bacterial infections.

Melatonin elevated Sclerotinia sclerotiorum resistance via modulation of ATP and glucosinolate biosynthesis in Brassica rapa ssp. pekinensis

Sclerotinia stem rot is a common disease found in Brassica rapa that is caused by the necrotic plant pathogen Sclerotinia sclerotiorum. Melatonin (MT) has known biological activity and effectively relieved this type of Sclerotinia stem rot in B. rapa. To better understand the mechanisms behind MT-induced S. sclerotiorum resistance in B. rapa, we performed both proteomic and metabolomic analysis. Our results showed that during S. sclerotiorum infection, thiamine synthesis was activated and defended against it. In infected leaves, ribosomal synthesis-related proteins responded positively to MT treatment. Integrated proteomic and metabolomic analysis showed that amino acid metabolism was activated by MT treatment. After MT treatment, adenosine-triphosphate (ATP) content and the activity of antioxidant enzymes were both increased in B. rapa infected leaves. Cysteine synthase, sulfur transfer-related proteins, and glucosinolate (GS) were all increased after MT treatment in infected B. rapa leaves. Taken together, these results indicated that B. rapa leaves promoted thiamine formation to defend against S. sclerotiorum infection. Moreover, MT helped further induce antioxidant activation in B. rapa in an ATP-dependent manner and stimulating GS biosynthesis to well inhibit the S. sclerotiorum infection. SIGNIFICANCE: Melatonin (MT) has biological activity and effectively relieved the Sclerotinia stem rot of Brassica rapa caused by the necrotic plant pathogen Sclerotinia sclerotiorum. In order to reveal the molecular mechanisms of MT-induced S. sclerotiorum resistance in B. rapa, comprehensive proteomic and metabolomic analyses were conducted. The integration analysis of omic-data illustrated that the modulation of ATP and glucosinolate biosynthesis induced by MT administration helped to defend the infection of S. sclerotiorum in B. rapa. Our results will provide insights into MT-induced anti-fungal mechanism and therapeutic strategies to mitigate Sclerotinia stem rot of B. rapa, thereby increasing plant yield and decreasing economic losses.

Melatonin as a regulatory hub of plant hormone levels and action in stress situations

Melatonin, a molecule first discovered in animal tissues, plays an important role in multiple physiological responses as a possible plant master regulator. It mediates responses to different types of stress, both biotic and abiotic. Melatonin reduces the negative effects associated with stressors, improving the plant response by increasing plant stress tolerance. When plants respond to stress situations, they use up a large amount of plant resources through a set of perfectly synchronized actions. Responses mediated by melatonin use the plant's hormones to, after adequate modulation, counteract and overcome the negative action of the stressor. In this paper, we review melatonin-plant hormone relationships. Factors that trigger the stress response and the central role of melatonin are analysed. An extensive analysis of current studies shows that melatonin modulates the metabolism of plant hormones (biosynthesis and catabolism), the rise or fall in their endogenous levels, the regulation of signalling elements and how melatonin affects the final response of auxin, gibberellins, cytokinins, abscisic acid, ethylene, salicylic acid, jasmonates, brassinosteroids, polyamines and strigolactones. Lastly, a general overview of melatonin's actions and its regulatory role at a global level is provided and proposals for future research are made.

Melatonin Enhances the Tolerance and Recovery Mechanisms in Brassica juncea (L.) Czern. Under Saline Conditions

Melatonin has been recently known to stimulate plant growth and induce protective responses against different abiotic stresses. However, the mechanisms behind exogenous melatonin pretreatment and restoration of plant vigor from salinity stress remain poorly understood. The present study aimed to understand the effects of exogenous melatonin pretreatment on salinity-damaged green mustard (Brassica juncea L. Czern.) seedlings in terms of oxidative stress regulation and endogenous phytohormone production. Screening of several melatonin concentrations (0, 0.1, 1, 5, and 10 μM) on mustard growth showed that the 1 μM concentration revealed an ameliorative increase of plant height, leaf length, and leaf width. The second study aimed at determining how melatonin application can recover salinity-damaged plants and studying its effects on physiological and biochemical parameters. Under controlled environmental conditions, mustard seedlings were irrigated with distilled water or 150 mM of NaCl for 7 days. This was followed by 1 μM of melatonin application to determine its recovery impact on the damaged plants. Furthermore, several physiological and biochemical parameters were examined in stressed and unstressed seedlings with or without melatonin application. Our results showed that plant height, leaf length/width, and stem diameter were enhanced in 38-day-old salinity-stressed plants under melatonin treatment. Melatonin application obviously attenuated salinity-induced reduction in gas exchange parameters, relative water content, and amino acid and protein levels, as well as antioxidant enzymes, such as superoxide dismutase and catalase. H₂O₂ accumulation in salinity-damaged plants was reduced by melatonin treatment. A decline in abscisic acid content and an increase in salicylic acid content were observed in salinity-damaged seedlings supplemented with melatonin. Additionally, chlorophyll content decreased during the recovery period in salinity-damaged plants by melatonin treatment. This study highlighted, for the first time, the recovery impact of melatonin on salinity-damaged green mustard seedlings. It demonstrated that exogenous melatonin supplementation significantly improved the physiologic and biochemical parameters in salinity-damaged green mustard seedlings.

Evolution-Driven Versatility of N Terminal Acetylation in Photoautotrophs

N terminal protein α-acetylation (NTA) is a pervasive protein modification that has recently attracted renewed interest. Early studies on NTA were mostly conducted in yeast and metazoans, providing a detailed portrait of the modification, which was indirectly applied to all eukaryotes. However, new findings originating from photosynthetic organisms have expanded our knowledge of this modification, revealing strong similarities as well as idiosyncratic features. Here, we review the most recent advances on NTA and its dedicated machinery in photosynthetic organisms. We discuss the cytosolic and unique plastid NTA machineries and their critical biological roles in development, stress responses, protein translocation, and stability. These new findings suggest that the multitasking plastid and cytosolic machineries evolved to support the specific needs of photoautotrophs.