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

The melatonin synthase-encoding gene ASMT mediates poplar resistance to drought stress and fungi Dothiorella gregaria

In recent years, the increase in extreme climates, such as persistent high temperatures and drought, has adversely affected the growth and development of fast-growing trees. Melatonin (MT) plays an important role in plant responses to biotic and abiotic stresses, yet there is a lack of research on the specific role of limiting enzyme genes for MT biosynthesis in fast-growing woody plants. In this study, we investigated the function of PtoASMT, a key rate-limiting enzyme encoding gene for MT biosynthesis, which can be induced by drought, salt, and the phytohormones ABA, SA and JA. Our results show that: (1) PtoASMT was widely expressed in all tissues of poplar, but was highly expressed in petioles, moderately expressed in roots, stems, shoots and young leaves, exhibiting a typical diurnal expression rhythm in leaves, with the encoded protein localized on chloroplasts; (2) the content of MT was significantly promoted in overexpressing PtoASMT transgenic poplar plants, but there were no obvious differences in their growth and development; (3) overexpressing PtoASMT plants exhibited stronger drought tolerance, accumulating less reactive oxygen species (ROS) under drought stress relative to wild-type plants, whereas knockout PtoASMT plants were more sensitive and accumulated more ROS; (4) overexpressing PtoASMT plants were more resistant to fungi Dothiorella gregaria than WT plants, while knockout plants showed higher sensitivity; meanwhile, the expression of disease resistance-related genes (PRs and JAZ10) was significantly altered. We conclude that PtoASMT enhances the resistance of poplar to drought and Dothiorella gregaria by mediating MT biosynthesis in poplar. These findings contribute to a better understanding the role of ASMT gene in MT accumulation and stress resistance in poplar.

Role of Neurotransmitters (Biomediators) in Plant Responses to Stress

Plants possess a complex signaling system that enables them to sense and adapt to various environmental stressors, including abiotic factors like extreme temperatures, drought, salinity, and toxic heavy metals. While the roles of hormones and signaling molecules in plant stress responses are well established, the involvement of neurotransmitters-traditionally linked to animal nervous systems-in plant stress physiology is a relatively underexplored area. Recent findings indicate that neurotransmitters such as gamma-aminobutyric acid, glutamate, serotonin, and dopamine play crucial roles in several physiological processes within plants. They regulate ion channels, adjust stomatal movements, modulate the production of reactive oxygen species, and influence gene expression. Evidence suggests that these neurotransmitters enhance antioxidant defense mechanisms and regulate stress-responsive pathways vital for plant stress tolerance. Additionally, under stressful conditions, neurotransmitters have been shown to impact plant growth, development, and reproductive activities. This review aims to illuminate the emerging understanding of neurotransmitters as key biomediators in plant responses to abiotic stress.

Metabolic regulation mechanism of melatonin for reducing cadmium accumulation and improving quality in rice

Melatonin acts as a potential regulator of cadmium (Cd) tolerance in rice. However, its practical value in rice production remains unclear. To validate the hypothesis that melatonin affects Cd accumulation and rice quality, a series of experiments were conducted. The results showed that exogenous melatonin application was associated with reduced Cd accumulation (23-43%) in brown rice. Fourier transform infrared spectroscopy (FTIR) analysis showed that exogenous melatonin affected the rice protein secondary structure and starch short-range structure. Metabolomics based on LC-MS/MS revealed that exogenous melatonin altered the brown rice metabolic profile, decreased fatty acid metabolite content, but increased amino acid metabolite, citric acid, melatonin biosynthetic metabolite, and plant hormone contents. These findings indicate that exogenous melatonin can effectively reduced Cd accumulation and improve rice quality through metabolic network regulation, serving as an effective treatment for rice cultivated in Cd-contaminated soil.

RNA splicing modulates the postharvest physiological deterioration of cassava storage root

Rapid postharvest physiological deterioration (PPD) of cassava (Manihot esculenta Crantz) storage roots is a major constraint that limits the potential of this plant as a food and industrial crop. Extensive studies have been performed to explore the regulatory mechanisms underlying the PPD processes in cassava to understand their molecular and physiological responses. However, the exceptional functional versatility of alternative splicing (AS) remains to be explored during the PPD process in cassava. Here, we identified several aberrantly spliced genes during the early PPD stage. An in-depth analysis of AS revealed that the abscisic acid (ABA) biosynthesis pathway might serve as an additional molecular layer in attenuating the onset of PPD. Exogenous ABA application alleviated PPD symptoms through maintaining ROS generation and scavenging. Interestingly, the intron retention transcript of MeABA1 (ABA DEFICIENT 1) was highly correlated with PPD symptoms in cassava storage roots. RNA yeast 3-hybrid and RNA immunoprecipitation (RIP) assays showed that the serine/arginine-rich protein MeSCL33 (SC35-like splicing factor 33) binds to the precursor mRNA of MeABA1. Importantly, overexpressing MeSCL33 in cassava conferred improved PPD resistance by manipulating the AS and expression levels of MeABA1 and then modulating the endogenous ABA levels in cassava storage roots. Our results uncovered the pivotal role of the ABA biosynthesis pathway and RNA splicing in regulating cassava PPD resistance and proposed the essential roles of MeSCL33 for conferring PPD resistance, broadening our understanding of SR proteins in cassava development and stress responses.

Melatonin differentially refines the metabolome to improve seed formation during grain developmental stages and enhances yield in two contrasting rice cultivars, grown in arsenic-contaminated soil

The manuscript revealed the ameliorative effects of exogenous melatonin in two distinct reproductive stages, i.e., developing grains (20 days after pollination) and matured grains (40 days after pollination) in two contrasting indica rice genotypes, viz., Khitish (arsenic-susceptible) and Muktashri (arsenic-tolerant), irrigated with arsenic-contaminated water throughout their life-cycle. Melatonin administration improved yield-related parameters like rachis length, primary and secondary branch length, number of grains per panicle, number of filled and empty grains per panicle, grain length and breadth and 1000-grain per weight. Expression of GW2, which negatively regulates grain development, was suppressed, along with concomitant induction of positive regulators like GIF1, DEP1 and SPL14 in both Khitish and Muktashri. Melatonin lowered arsenic bioaccumulation in grains and tissue biomass, more effectively in Khitish. Unregulated production of reactive oxygen species, leading to cellular necrosis caused by arsenic, was reversed in presence of melatonin. Endogenous melatonin level was stimulated due to up-regulation of the key biosynthetic genes, SNAT and ASMT. Melatonin enhanced the production of diverse antioxidants like anthocyanins, flavonoids, total phenolics and ascorbic acid and also heightened the production of thiol-metabolites (cysteine, reduced glutathione, non-protein thiols and phytochelatin), ensuring effective chelation and arsenic detoxification. Altogether, our observation, supported by principal component analysis, proved that melatonin re-programs the antioxidative metabolome to enhance plant resilience against arsenic stress to mitigate oxidative damages and reduce arsenic translocation from the soil to tissue biomass and edible grains.

The role of phytomelatonin in plant homeostasis, signaling, and crosstalk in abiotic stress mitigation

In recent years, there has been an increase in the study of phytomelatonin. Having numerous functions in animals, melatonin produced by plants (phytomelatonin) is also a multi-regulatory molecule with great potential in plant physiology and in mitigating abiotic stresses, such as drought, salinity, chilling, heat, chemical contamination, and UV-radiation stress. This review highlights the primary functions of phytomelatonin as an anti-stress molecule against abiotic stress. We discuss the role of phytomelatonin as a master regulator, oxidative stress manager, reactive oxygen species and reactive nitrogen species regulator, and defense compounds inducer. Although there exist a handful of reviews on the crosstalk of phytomelatonin with other signaling molecules like auxin, cytokinin, gibberellin, abscisic acid, ethylene, nitric oxide, jasmonic acid, and salicylic acid, this review looks at studies that have reported a few aspects of phytomelatonin with newly discovered signaling molecules along with classical signaling molecules with relation to abiotic stress tolerance. The research and applications of phytomelatonin with hydrogen sulfide, strigolactones, brassinosteroids, and polyamines are still in their nascent stage but hold a promising scope for the future. Additionally, this review states the recent developments in the signaling of phytomelatonin with nitrogen metabolism and nitrosative stress in plants.

Comparative genomics of N-acetyl-5-methoxytryptamine members in four Prunus species with insights into bud dormancy and abiotic stress responses in Prunus avium

KEY MESSAGE

This study provides novel insights into the evolution, diversification, and functions of melatonin biosynthesis genes in Prunus species, highlighting their potential role in regulating bud dormancy and abiotic stresses. The biosynthesis of melatonin (MEL) in plants is primarily governed by enzymatic reactions involving key enzymes such as serotonin N-acetyltransferase (SNAT), tryptamine 5-hydroxylase (T5H), N-acetylserotonin methyltransferase (ASMT) and tryptophan decarboxylase (TDC). In this study, we analyzed Melatonin genes in four Prunus species such as Prunus avium (Pavi), Prunus pusilliflora (Ppus), Prunus serulata (Pser), and Prunus persica (Pper) based on comparative genomics approach. Among the four Prunus species, a total of 29 TDCs, 998 T5Hs, 16 SNATs, and 115 ASMTs within the genome of four Prunus genomes. A thorough investigation of melatonin-related genes was carried out using systematic biological methods and comparative genomics. Through phylogenetic analysis, orthologous clusters, Go enrichment, syntenic relationship, and gene duplication analysis, we discovered both similarities and variations in Melatonin genes among these Prunus species. Additionally, our study revealed the existence of unique subgroup members in the Melatonin genes of these species, which were distinct from those found in Arabidopsis genes. Furthermore, the transcriptomic expression analysis revealed the potential significance of melatonin genes in bud dormancy regulation and abiotic stresses. Our extensive results offer valuable perspectives on the evolutionary patterns, intricate expansion, and functions of PavMEL genes. Given their promising attributes, PavTDCs, PavT5H, PavNAT, and three PavASMT genes warrant in-depth exploration as prime candidates for manipulating dormancy in sweet cherry. This was done to lay the foundation for future explorations into the structural and functional aspects of these factors in Prunus species. This study offers significant insights into the functions of ASMT, SNAT, T5H, and TDC genes and sheds light on their roles in Prunus avium. Moreover, it established a robust foundation for further exploration functional characterization of melatonin genes in fruit species.

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.

Seed pretreatment with melatonin confers cadmium tolerance to chickpea seedlings through cellular redox homeostasis and antioxidant gene expression improvement

Several phytoremediation strategies have been undertaken to alleviate cadmium (Cd)-mediated injury to crop yield resulting from agricultural land pollution. In the present study, the potentially beneficial effect of melatonin (Me) was appraised. Therefore, chickpea (Cicer arietinum L.) seeds were imbibed for 12 H in distilled water or Me (10 µM) solution. Then, the seeds germinated in the presence or the absence of 200 µM CdCl₂ for 6 days. Seedlings obtained from Me-pretreated seeds exhibited enhanced growth traits, reflected by fresh biomass and length increase. This beneficial effect was associated with a decreased Cd accumulation in seedling tissues (by 46 and 89% in roots and shoots, respectively). Besides, Me efficiently protected the cell membrane integrity of Cd-subjected seedlings. This protective effect was manifested by the decreased lipoxygenase activity and the subsequently reduced accumulation of 4-hydroxy-2-nonenal. Melatonin counteracted the Cd-mediated stimulation of the pro-oxidant NADPH-oxidase (90 and 45% decrease compared to non-pretreated Cd-stressed roots and shoots, respectively) and NADH-oxidase activities (almost 40% decrease compared to non-pretreated roots and shoots), preventing, thus, hydrogen peroxide overaccumulation (50 and 35% lesser than non-pretreated roots and shoots, respectively). Furthermore, Me enhanced the cellular content of pyridine nicotinamide reduced forms [NAD(P)H] and their redox state. This effect was associated with the Me-mediated stimulation of the glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase activities, concomitantly with the inhibition of NAD(P)H-consuming activities. These effects were accompanied by the up-regulation of G6PDH gene expression (45% increase in roots) and the down-regulation of the respiratory burst oxidase homolog protein F (RBOHF) gene expression (53% decrease in roots and shoots). Likewise, Me induced an increased activity and gene transcription of the Asada-Halliwell cycle, namely ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, concomitantly with a reduction of the glutathione peroxidase activity. This modulating effect led to the restoration of the redox homeostasis of the ascorbate and the glutathione pools. Overall, current results attest that seed pretreatment with Me is effective in Cd stress relief and can be a beneficial crop-protective approach.

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.

The role of melatonin in plant growth and metabolism, and its interplay with nitric oxide and auxin in plants under different types of abiotic stress

Melatonin is a pleiotropic signaling molecule that reduces the adverse effects of abiotic stresses, and enhances the growth and physiological function of many plant species. Several recent studies have demonstrated the pivotal role of melatonin in plant functions, specifically its regulation of crop growth and yield. However, a comprehensive understanding of melatonin, which regulates crop growth and yield under abiotic stress conditions, is not yet available. This review focuses on the progress of research on the biosynthesis, distribution, and metabolism of melatonin, and its multiple complex functions in plants and its role in the mechanisms of metabolism regulation in plants grown under abiotic stresses. In this review, we focused on the pivotal role of melatonin in the enhancement of plant growth and regulation of crop yield, and elucidated its interactions with nitric oxide (NO) and auxin (IAA, indole-3-acetic acid) when plants are grown under various abiotic stresses. The present review revealed that the endogenousapplication of melatonin to plants, and its interactions with NO and IAA, enhanced plant growth and yield under various abiotic stresses. The interaction of melatonin with NO regulated plant morphophysiological and biochemical activities, mediated by the G protein-coupled receptor and synthesis genes. The interaction of melatonin with IAA enhanced plant growth and physiological function by increasing the levels of IAA, synthesis, and polar transport. Our aim was to provide a comprehensive review of the performance of melatonin under various abiotic stresses, and, therefore, further explicate the mechanisms that plant hormones use to regulate plant growth and yield under abiotic stresses.

Ectopic expression of HIOMT improves tolerance and nitrogen utilization efficiency in transgenic apple under drought stress

Melatonin enhances plant tolerance to various environmental stressors. Although exogenous application of melatonin has been investigated, the role of endogenous melatonin metabolism in the response of apples to drought stress and nutrient utilization remains unclear. Here, we investigated the effects of ectopically expressing the human melatonin synthase gene HIOMT on transgenic apple plants under drought stress conditions. The tolerance of transgenic apple lines that ectopically expressed HIOMT improved significantly under drought conditions. After 10 days of natural drought stress treatment, the transgenic apple plants showed higher relative water content, chlorophyll levels and Fv/Fm, and lower relative electrolyte leakage and hydrogen peroxide accumulation, than wild-type plants. The activities of peroxidase, superoxide dismutase and catalase, as well as genes in the ascorbate-glutathione cycle, increased more in transgenic apple plants than in the wild-type. The ectopic expression of HIOMT also markedly alleviated the inhibitory effects of long-term drought stress on plant growth, photosynthetic rate and chlorophyll concentrations in apple plants. The uptake and utilization of 15N increased markedly in the transgenic lines under long-term moderate drought stress. Drought stress sharply reduced the activity of enzymes involved in nitrogen metabolism, but ectopic expression of HIOMT largely reversed that response. The expression levels of genes of nitrogen metabolism and uptake were more upregulated in transgenic apple plants than the wild-type. Overall, our study demonstrates that ectopic expression of HIOMT enhanced the tolerance of apple plants to drought stress, and transgenic apple plants showed improved growth due to higher nutrient utilization efficiency under drought conditions.

Human Naa50 Shows Serotonin N-Acetyltransferase Activity, and Its Overexpression Enhances Melatonin Biosynthesis, Resulting in Osmotic Stress Tolerance in Rice

A new clade of serotonin N-acetyltransferase (SNAT), the penultimate enzyme in the melatonin biosynthetic pathway, has been reported in the archaeon Thermoplasma volcanium. The closest homolog of archaea SNAT in human was an N-alpha-acetyltransferase50 (Naa50). To determine whether human Naa50 (hNaa50) shows SNAT enzyme activity, we chemically synthesized and expressed the hNaa50 gene in Escherichia coli, followed by Ni²⁺ affinity purification. Purified recombinant hNaa50 showed SNAT activity (K_m and V_max values of 986 μM and 1800 pmol/min/mg protein, respectively). To assess its in vivo function, hNaa50 was overexpressed in rice (hNaa50-OE). The transgenic rice plants produced more melatonin than nontransgenic wild-type rice, indicating that hNaa50 is functionally coupled with melatonin biosynthesis. Due to its overproduction of melatonin, hNaa50-OE had a higher tolerance against osmotic stress than the wild type. Enhanced expression of the chaperone genes BIP1 and CNX in hNaa50-OE plants was responsible for the increased tolerance. It is concluded that hNaa50 harbors serotonin N-acetyltransferase enzyme activity in addition to its initial N-alpha-acetyltransferase, suggesting the bifunctionality of the hNaa50 enzyme toward serotonin and protein substrates. Consequently, ectopic overexpression of hNaa50 in rice enhanced melatonin synthesis, indicating that hNaa50 is in fact involved in melatonin biosynthesis.

Genome-wide identification of the B3 gene family in soybean and the response to melatonin under cold stress

INTRODUCTION

Melatonin is a multipotent molecule that exists widely in animals and plants and plays an active regulatory role in abiotic stresses. The B3 superfamily is a ubiquitous transcription factor with a B3 functional domain in plants, which can respond temporally to abiotic stresses by activating defense compounds and plant hormones. Despite the fact that the B3 genes have been studied in a variety of plants, their role in soybean is still unknown.

METHODS

The regulation of melatonin on cold resistance of soybean and the response of B3 genes to cold stress were investigated by measuring biochemical indexes of soybean. Meanwhile, the genome-wide identification of B3 gene family was conducted in soybean, and B3 genes were analyzed based on phylogeny, motifs, gene structure, collinearity, and cis-regulatory elements analysis.

RESULTS

We found that cold stress-induced oxidative stress in soybean by producing excessive reactive oxygen species. However, exogenous melatonin treatment could increase the content of endogenous melatonin and other hormones, including IAA and ABA, and enhance the antioxidative system, such as POD activity, CAT activity, and GSH/GSSG, to scavenge ROS. Furthermore, the present study first revealed that melatonin could alleviate the response of soybean to cold stress by inducing the expression of B3 genes. In addition, we first identified 145 B3 genes in soybean that were unevenly distributed on 20 chromosomes. The B3 gene family was divided into 4 subgroups based on the phylogeny tree constructed with protein sequence and a variety of plant hormones and stress response cis-elements were discovered in the promoter region of the B3 genes, indicating that the B3 genes were involved in several aspects of the soybean stress response. Transcriptome analysis and results of qRT-PCR revealed that most GmB3 genes could be induced by cold, the expression of which was also regulated by melatonin. We also found that B3 genes responded to cold stress in plants by interacting with other transcription factors.

DISCUSSION

We found that melatonin regulates the response of soybean to cold stress by regulating the expression of the transcription factor B3 gene, and we identified 145 B3 genes in soybean. These findings further elucidate the potential role of the B3 gene family in soybean to resist low-temperature stress and provide valuable information for soybean functional genomics study.

Genome-wide analysis for the melatonin trait associated genes and SNPs in dairy goat (Capra hircus) as the molecular breeding markers

Previous studies have reported that the endogenous melatonin level is positively associated with the quality and yield of milk of cows. In the current study, a total of 34,921 SNPs involving 1,177 genes were identified in dairy goats by using the whole genome resequencing bulked segregant analysis (BSA) analysis. These SNPs have been used to match the melatonin levels of the dairy goats. Among them, 3 SNPs has been identified to significantly correlate with melatonin levels. These 3 SNPs include CC genotype 147316, GG genotype 147379 and CC genotype 1389193 which all locate in the exon regions of ASMT and MT2 genes. Dairy goats with these SNPs have approximately 5-fold-higher melatonin levels in milk and serum than the average melatonin level detected in the current goat population. If the melatonin level impacts the milk production in goats as in cows, the results strongly suggest that these 3 SNPs can serve as the molecular markers to select the goats having the improved milk quality and yield. This is a goal of our future study.

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.

Melatonin regulates gene expressions through activating auxin synthesis and signaling pathways

BACKGROUND

Both melatonin and indole-3-acetic acid (IAA) are derived from tryptophan. And the most interesting and unsolved puzzle in melatonin research is that what is the relationship between melatonin and auxin?

METHODS

In this study, we performed transcriptome analysis with a time series method to disclose the connection of the two metabolites in soybean.

RESULTS

Our results reveal that melatonin and IAA treatments cause substantial overlaps in gene expression changes. Common genes of melatonin and IAA treatments could be sorted into clusters with very similar expression tendency. A KEGG assay showed that exogenous applied melatonin enriched differentially expressed genes in auxin biosynthesis and signaling pathways. For details, melatonin up-regulates several YUCCA genes which participate in auxin biosynthesis; melatonin also enhances expression levels of auxin receptor coding genes, such as TIR1, AFB3 and AFB5; dozens of genes involved in auxin transport, such as AUXI and PIN, are regulated by melatonin similarly as by auxin; auxin-responsive genes, such as IAA, ARF, GH3 and SAUR-like genes, intensively respond to melatonin as well as to auxin. A DR5 promoter mediated GUS staining assay showed that low concentration of melatonin could induce auxin biosynthesis in a dosage manner, whereas high concentration of melatonin would eliminate such effect. At last, gene ontology (GO) analysis suggests that melatonin treatment has similar characteristics as auxin treatment in many processes. However, the two molecules still keep their own features respectively. For example, melatonin takes part in stress responses, while IAA treatment enriches the GO terms that related to cell growth.

CONCLUSION

Taken together, exogenous applied melatonin, if not exceeds the appropriate concentration, could promote auxin responses range from biosynthesis to signaling transduction. Thus, our research is a key part to explain the auxin-like roles of melatonin in regulating plant growth.

Recent advances and mechanistic insights on Melatonin-mediated salt stress signaling in plants

Salinity stress is one of the major abiotic constraints that limit plant growth and yield, which thereby is a serious concern to world food security. It adversely affects crop production by inducing hyperosmotic stress and ionic toxicity as well as secondary stresses such as oxidative stress, all of which disturb optimum physiology and metabolism. Nonetheless, various strategies have been employed to improve salt tolerance in crop plants, among which the application of Melatonin (Mel) could also be used as it has demonstrated promising results. The ongoing experimental evidence revealed that Mel is a pleiotropic signaling molecule, which besides being involved in various growth and developmental processes also mediates environmental stress responses. The current review systematically discusses and summarizes how Mel mediates the response of plants under salt stress and could optimize the balance between plant growth performances and stress responses. Specifically, it covers the latest advances of Mel in fine-tuning the signaling in plants. Furthermore, it highlights plant-built tolerance of salt stress by manifesting the biosynthesis of Mel, its cross talks with nitric oxide (NO), and Mel as a multifaceted antioxidant molecule.

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.

Functions and prospects of melatonin in plant growth, yield, and quality

Melatonin (N-acetyl-5-methoxytryptamine) is an indole molecule widely found in animals and plants. It is well known that melatonin improves plant resistance to various biotic and abiotic stresses due to its potent free radical scavenging ability while being able to modulate plant signaling and response pathways through mostly unknown mechanisms. In recent years, an increasing number of studies have shown that melatonin plays a crucial role in improving crop quality and yield by participating in the regulation of various aspects of plant growth and development. Here, we review the effects of melatonin on plant vegetative growth and reproductive development, and systematically summarize its molecular regulatory network. Moreover, the effective concentrations of exogenously applied melatonin in different crops or at different growth stages of the same crop are analysed. In addition, we compare endogenous phytomelatonin concentrations in various crops and different organs, and evaluate a potential function of phytomelatonin in plant circadian rhythms. The prospects of different approaches in regulating crop yield and quality through exogenous application of appropriate concentrations of melatonin, endogenous modification of phytomelatonin metabolism-related genes, and the use of nanomaterials and other technologies to improve melatonin utilization efficiency are also discussed.

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

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

Functional Characterization of Arylalkylamine N-Acetyltransferase, a Pivotal Gene in Antioxidant Melatonin Biosynthesis from Chlamydomonas reinhardtii

Arylalkylamine N-acetyltransferase (AANAT) is a pivotal enzyme in melatonin biosynthesis that catalyzes the conversion of serotonin to N-acetylserotonin. Homologs of animal AANAT genes are present in animals, but not in plants. An AANAT homolog was found in Chlamydomonas reinhardtii, but not other green algae. The characteristics of C. reinhardtii AANAT (CrAANAT) are unclear. Here, full-length CrAANAT was chemically synthesized and expressed in Escherichia coli. Recombinant CrAANAT exhibited AANAT activity with a Km of 247 μM and Vmax of 325 pmol/min/mg protein with serotonin as the substrate. CrAANAT was localized to the cytoplasm in tobacco leaf cells. Transgenic rice plants overexpressing CrAANAT (CrAANAT-OE) exhibited increased melatonin production. CrAANAT-OE plants showed a longer seed length and larger second leaf angle than wild-type plants, indicative of the involvement of brassinosteroids (BRs). As expected, BR biosynthesis- and signaling-related genes such as D2, DWARF4, DWARF11, and BZR1 were upregulated in CrAANAT-OE plants. Therefore, an increased endogenous melatonin level by ectopic overexpression of CrAANAT seems to be closely associated with BR biosynthesis, thereby influencing seed size.

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

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

Melatonin-Induced Protection Against Plant Abiotic Stress: Mechanisms and Prospects

Global warming in this century increases incidences of various abiotic stresses restricting plant growth and productivity and posing a severe threat to global food production and security. The plant produces different osmolytes and hormones to combat the harmful effects of these abiotic stresses. Melatonin (MT) is a plant hormone that possesses excellent properties to improve plant performance under different abiotic stresses. It is associated with improved physiological and molecular processes linked with seed germination, growth and development, photosynthesis, carbon fixation, and plant defence against other abiotic stresses. In parallel, MT also increased the accumulation of multiple osmolytes, sugars and endogenous hormones (auxin, gibberellic acid, and cytokinins) to mediate resistance to stress. Stress condition in plants often produces reactive oxygen species. MT has excellent antioxidant properties and substantially scavenges reactive oxygen species by increasing the activity of enzymatic and non-enzymatic antioxidants under stress conditions. Moreover, the upregulation of stress-responsive and antioxidant enzyme genes makes it an excellent stress-inducing molecule. However, MT produced in plants is not sufficient to induce stress tolerance. Therefore, the development of transgenic plants with improved MT biosynthesis could be a promising approach to enhancing stress tolerance. This review, therefore, focuses on the possible role of MT in the induction of various abiotic stresses in plants. We further discussed MT biosynthesis and the critical role of MT as a potential antioxidant for improving abiotic stress tolerance. In addition, we also addressed MT biosynthesis and shed light on future research directions. Therefore, this review would help readers learn more about MT in a changing environment and provide new suggestions on how this knowledge could be used to develop stress tolerance.

Crosstalk between Melatonin and Reactive Oxygen Species in Plant Abiotic Stress Responses: An Update

Melatonin acts as a multifunctional molecule that takes part in various physiological processes, especially in the protection against abiotic stresses, such as salinity, drought, heat, cold, heavy metals, etc. These stresses typically elicit reactive oxygen species (ROS) accumulation. Excessive ROS induce oxidative stress and decrease crop growth and productivity. Significant advances in melatonin initiate a complex antioxidant system that modulates ROS homeostasis in plants. Numerous evidences further reveal that melatonin often cooperates with other signaling molecules, such as ROS, nitric oxide (NO), and hydrogen sulfide (H₂S). The interaction among melatonin, NO, H₂S, and ROS orchestrates the responses to abiotic stresses via signaling networks, thus conferring the plant tolerance. In this review, we summarize the roles of melatonin in establishing redox homeostasis through the antioxidant system and the current progress of complex interactions among melatonin, NO, H₂S, and ROS in higher plant responses to abiotic stresses. We further highlight the vital role of respiratory burst oxidase homologs (RBOHs) during these processes. The complicated integration that occurs between ROS and melatonin in plants is also discussed.

Overexpression of MdASMT9, an N-acetylserotonin methyltransferase gene, increases melatonin biosynthesis and improves water-use efficiency in transgenic apple

Improving apple water-use efficiency (WUE) is increasingly desirable in the face of global climate change. Melatonin is a pleiotropic molecule that functions in plant development and stress tolerance. In apple, exogenous application of melatonin has been largely investigated, but melatonin biosynthesis and its physiological roles remain elusive. In the plant biosynthetic pathway of melatonin, the last and key step is that N-acetylserotonin methyltransferase (ASMT) converts N-acetylserotonin into melatonin. Here, we identified an apple ASMT gene, MdASMT9, using homology-based cloning and in vitro enzyme assays. Overexpression of MdASMT9 significantly increased melatonin accumulation in transgenic apple lines. Moreover, an enhanced WUE was observed in the MdASMT9-overexpressing apple lines. Under well-watered conditions, this increase in WUE was attributed to an enhancement of photosynthetic rate and stomatal aperture via a reduction in abscisic acid biosynthesis. By contrast, under long-term moderate water deficit conditions, regulations in photoprotective mechanisms, stomatal behavior, osmotic adjustment and antioxidant activity enhanced the WUE in transgenic apple lines. Taken together, our findings shed light on the positive effect of MdASMT9 on improving WUE of apple by modulating melatonin biosynthesis.

Introducing melatonin to the horticultural industry: physiological roles, potential applications, and challenges

Melatonin (N-acetyl-5-methoxytryptamine) is an emerging biomolecule that influences horticultural crop growth, flowering, fruit ripening, postharvest preservation, and stress protection. It functions as a plant growth regulator, preservative and antimicrobial agent to promote seed germination, regulate root system architecture, influence flowering and pollen germination, promote fruit production, ensure postharvest preservation, and increase resistance to abiotic and biotic stresses. Here, we highlight the potential applications of melatonin in multiple aspects of horticulture, including molecular breeding, vegetative reproduction, production of virus-free plants, food safety, and horticultural crop processing. We also discuss its effects on parthenocarpy, autophagy, and arbuscular mycorrhizal symbiosis. Together, these many features contribute to the promise of melatonin for improving horticultural crop production and food safety. Effective translation of melatonin to the horticultural industry requires an understanding of the challenges associated with its uses, including the development of economically viable sources.

Potential roles of melatonin and ABA on apple dwarfing in semi-arid area of Xinjiang China

Dwarfing is a typic breeding trait for mechanical strengthening and relatively high yield in modern apple orchards. Clarification of the mechanisms associated with dwarfing is important for use of molecular technology to breed apple. Herein, we identified four dwarfing apple germplasms in semi-arid area of Xinjiang, China. The internodal distance of these four germplasms were significantly shorter than non-dwarfing control. Their high melatonin (MT) contents are negatively associated with their malondialdehyde (MDA) levels and oxidative damage. In addition, among the detected hormones including auxin (IAA), gibberellin (GA), brassinolide (BR), zeatin-riboside (ZR), and abscisic acid (ABA), only ABA and ZR levels were in good correlation with the dwarfing phenotype. The qPCR results showed that the expression of melatonin synthetic enzyme genes MdASMT1 and MdSNAT5, ABA synthetic enzyme gene MdAAO3 and degradative gene MdCYP707A, ZR synthetic enzyme gene MdIPT5 all correlated well with the enhanced levels of MT, ABA and the reduced level of of ZR in the dwarfing germplasms. Furthermore, the significantly higher expression of ABA marker genes (MdRD22 and MdRD29) and the lower expression of ZR marker genes (MdRR1 and MdRR2) in all the four dwarf germplasms were consistent with the ABA and ZR levels. Considering the yearly long-term drought occurring in Xinjiang, China, it seems that dwarfing with high contents of MT and ABA may be a good strategy for these germplasms to survive against drought stress. This trait of dwarfing may also benefit apple production and breeding in this semi-arid area.

Melatonin Induced Cold Tolerance in Plants: Physiological and Molecular Responses

Cold stress is one of the most limiting factors for plant growth and development. Cold stress adversely affects plant physiology, molecular and biochemical processes by determining oxidative stress, poor nutrient and water uptake, disorganization of cellular membranes and reduced photosynthetic efficiency. Therefore, to recover impaired plant functions under cold stress, the application of bio-stimulants can be considered a suitable approach. Melatonin (MT) is a critical bio-stimulant that has often shown to enhance plant performance under cold stress. Melatonin application improved plant growth and tolerance to cold stress by maintaining membrane integrity, plant water content, stomatal opening, photosynthetic efficiency, nutrient and water uptake, redox homeostasis, accumulation of osmolytes, hormones and secondary metabolites, and the scavenging of reactive oxygen species (ROS) through improved antioxidant activities and increase in expression of stress-responsive genes. Thus, it is essential to understand the mechanisms of MT induced cold tolerance and identify the diverse research gaps necessitating to be addressed in future research programs. This review discusses MT involvement in the control of various physiological and molecular responses for inducing cold tolerance. We also shed light on engineering MT biosynthesis for improving the cold tolerance in plants. Moreover, we highlighted areas where future research is needed to make MT a vital antioxidant conferring cold tolerance to plants.

Grape Small Auxin Upregulated RNA (SAUR) 041 Is a Candidate Regulator of Berry Size in Grape

Grape (Vitis vinifera) is an important horticultural crop that can be used to make juice and wine. However, the small size of the berry limits its yield. Cultivating larger berry varieties can be an effective way to solve this problem. As the largest family of auxin early response genes, SAUR (small auxin upregulated RNA) plays an important role in the growth and development of plants. Berry size is one of the important factors that determine grape quality. However, the SAUR gene family’s function in berry size of grape has not been studied systematically. We identified 60 SAUR members in the grape genome and divided them into 12 subfamilies based on phylogenetic analysis. Subsequently, we conducted a comprehensive and systematic analysis on the SAUR gene family by analyzing distribution of key amino acid residues in the domain, structural features, conserved motifs, and protein interaction network, and combined with the heterologous expression in Arabidopsis and tomato. Finally, the member related to grape berry size in SAUR gene family were screened. This genome-wide study provides a systematic analysis of grape SAUR gene family, further understanding the potential functions of candidate genes, and provides a new idea for grape breeding.

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

Melatonin is a ubiquitous indolamine, largely investigated for its key role in the regulation of several physiological processes in both animals and plants. In the last century, it was reported that this molecule may be produced in high concentrations by several species belonging to the plant kingdom and stored in specialized tissues. In this review, the main information related to the chemistry of melatonin and its metabolism has been summarized. Furthermore, the biosynthetic pathway characteristics of animal and plant cells have been compared, and the main differences between the two systems highlighted. Additionally, in order to investigate the distribution of this indolamine in the plant kingdom, distribution cluster analysis was performed using a database composed by 47 previously published articles reporting the content of melatonin in different plant families, species and tissues. Finally, the potential pharmacological and biostimulant benefits derived from the administration of exogenous melatonin on animals or plants via the intake of dietary supplements or the application of biostimulant formulation have been largely discussed.

Melatonin promotes Arabidopsis primary root growth in an IAA-dependent manner

Melatonin has been characterized as a growth regulator in plants. Melatonin shares tryptophan as the precursor with the auxin indole-3-acetic acid (IAA), but the interplay between melatonin and IAA remains controversial. In this study, we aimed to dissect the relationship between melatonin and IAA in regulating Arabidopsis primary root growth. We observed that melatonin concentrations ranging from 10-9 to 10-6 M functioned as IAA mimics to promote primary root growth in Arabidopsis wild type, as well as in pin-formed (pin) single and double mutants. Transcriptome analysis showed that changes in gene expression after melatonin and IAA treatment were moderately correlated. Most of the IAA-regulated genes were co-regulated by melatonin, indicating that melatonin and IAA regulated a similar subset of genes. Melatonin partially rescued primary root growth defects in pin single and double mutant plants. However, melatonin treatment had little effect on primary root growth in the presence of high concentrations of auxin biosynthesis inhibitors, or polar transport inhibitor, and could not rescue the root length defect of the IAA biosynthesis quintuple mutant yucQ. Therefore, we propose that melatonin promotes primary root growth in an IAA-dependent manner.

Enhancement of Nicotiana tabacum Resistance Against Dehydration-Induced Leaf Senescence via Metabolite/Phytohormone-Gene Regulatory Networks Modulated by Melatonin

Melatonin (MEL) is a pleiotropic agent with crucial functions reported in a variety of stress responses and developmental processes. Although MEL involvement in plant defense against natural leaf senescence has been widely reported, the precise regulatory mechanisms by which it delays stress-induced senescence remain unclear. In this study, we found that foliar spraying of melatonin markedly ameliorated dehydration-induced leaf senescence in Nicotiana tabacum, accompanied by attenuated oxidative damage, expression of senescence-related genes, and reduced endogenous ABA production. Metabolite profiling indicated that melatonin-treated plants accumulated higher concentrations of sugars, sugar alcohol, and organic acids, but fewer concentrations of amino acids in the leaves, than untreated plants after exposure to dehydration. Gene expression analysis revealed that the delayed senescence of stressed plants achieved by melatonin treatment might be partially ascribed to the upregulated expression of genes involved in ROS scavenging, chlorophyll biosynthesis, photosynthesis, and carbon/nitrogen balances, and downregulated expression of senescence-associated genes. Furthermore, hormone responses showed an extensively modulated expression, complemented by carotenoid biosynthesis regulation to achieve growth acceleration in melatonin-treated plants upon exposure to dehydration stress. These findings may provide more comprehensive insights into the role of melatonin in alleviating leaf senescence and enhancing dehydration resistance.

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

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

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.

Pre treatment with Bacillus subtilis mitigates drought induced photo-oxidative damages in okra by modulating antioxidant system and photochemical activity

Growth promoting potential of Bacillus subtilis (BS) in drought stressed Abelmoschus esculentus (L.) Moench (okra) was assessed by measuring the chlorophyll stability index (CSI), chlorophyll a (Chl-a) fluorescence, leaf osmotic potential and lipid peroxidation by malondialdehyde content, emission of reactive oxygen species (ROS), osmolyte content and the activity of non-enzyme and enzyme antioxidants. BS treatment significantly increased the leaf osmotic potential, osmolyte production and the activity of non-enzyme and enzyme antioxidants under drought stress. BS treatment mitigated the drought-induced reduction in Chl a fluorescence and CSI. Concomitant increase in total sugar, proline, non-enzyme antioxidants [glutathione and ascorbate] and enzyme antioxidants like superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase and dehydroascorbate reductase modulate the intracellular ROS concentration in okra to resist the stress induced oxidative damage in BS treated plants led to fast recovery and less photodamage.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-00982-8.

Plant-derived melatonin from food: a gift of nature

In recent years, people have become increasingly interested in bioactive ingredients from plants, especially antioxidant molecules such as melatonin, which are beneficial to human health. The purpose of this article is to provide new information on plant-derived foods with a high content of melatonin. We comprehensively summarize the content of melatonin in plant-derived foods and discuss the factors that influence melatonin levels to provide new ideas on enhancement. Additionally, we describe the biosynthetic pathway of melatonin and identify its major functions. Medicinal herbs are often rich in melatonin while many vegetables and fruits exhibit somewhat lower levels with wide variations among species. The genetic traits of plants, the phenological stage of the cultivar, the photoperiod, the level of stress to which the plants are exposed at the time of harvest, exposure to agrochemicals and determination methods are the main factors affecting the melatonin content. To date, standardization of uniform sampling times and the use of suitable pretreatments as well as determination methods have not been achieved. The results of the studies reviewed highlight the potentially important role of plant melatonin in influencing the progression of human diseases. Based on the health promotional aspects of melatonin, consuming foods containing higher concentrations of tryptophan and melatonin is suggested.

A Beginner's Guide to Osmoprotection by Biostimulants

Water is indispensable for the life of any organism on Earth. Consequently, osmotic stress due to salinity and drought is the greatest threat to crop productivity. Ongoing climate change includes rising temperatures and less precipitation over large areas of the planet. This is leading to increased vulnerability to the drought conditions that habitually threaten food security in many countries. Such a scenario poses a daunting challenge for scientists: the search for innovative solutions to save water and cultivate under water deficit. A search for formulations including biostimulants capable of improving tolerance to this stress is a promising specific approach. This review updates the most recent state of the art in the field.

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.

Melatonin metabolism, signaling and possible roles in plants

Melatonin is a multifunctional biomolecule found in both animals and plants. In this review, the biosynthesis, levels, signaling, and possible roles of melatonin and its metabolites in plants is summarized. Tryptamine 5-hydroxylase (T5H), which catalyzes the conversion of tryptamine into serotonin, has been proposed as a target to create a melatonin knockout mutant presenting a lesion-mimic phenotype in rice. With a reduced anabolic capacity for melatonin biosynthesis and an increased catabolic capacity for melatonin metabolism, all plants generally maintain low melatonin levels. Some plants, including Arabidopsis and Nicotiana tabacum (tobacco), do not possess tryptophan decarboxylase (TDC), the first committed step enzyme required for melatonin biosynthesis. Major melatonin metabolites include cyclic 3-hydroxymelatonin (3-OHM) and 2-hydroxymelatonin (2-OHM). Other melatonin metabolites such as N¹ -acetyl-N² -formyl-5-methoxykynuramine (AFMK), N-acetyl-5-methoxykynuramine (AMK) and 5-methoxytryptamine (5-MT) are also produced when melatonin is applied to Oryza sativa (rice). The signaling pathways of melatonin and its metabolites act via the mitogen-activated protein kinase (MAPK) cascade, possibly with Cand2 acting as a melatonin receptor, although the integrity of Cand2 remains controversial. Melatonin mediates many important functions in growth stimulation and stress tolerance through its potent antioxidant activity and function in activating the MAPK cascade. The concentration distribution of melatonin metabolites appears to be species specific because corresponding enzymes such as M2H, M3H, catalases, indoleamine 2,3-dioxygenase (IDO) and N-acetylserotonin deacetylase (ASDAC) are differentially expressed among plant species and even among different tissues within species. Differential levels of melatonin and its metabolites can lead to differential physiological effects among plants when melatonin is either applied exogenously or overproduced through ectopic overexpression.

Melatonin as Master Regulator in Plant Growth, Development and Stress Alleviator for Sustainable Agricultural Production: Current Status and Future Perspectives

Melatonin, a multifunctional signaling molecule, is ubiquitously distributed in different parts of a plant and responsible for stimulating several physiochemical responses against adverse environmental conditions in various plant systems. Melatonin acts as an indoleamine neurotransmitter and is primarily considered as an antioxidant agent that can control reactive oxygen and nitrogen species in plants. Melatonin, being a signaling agent, induces several specific physiological responses in plants that might serve to enhance photosynthesis, growth, carbon fixation, rooting, seed germination and defense against several biotic and abiotic stressors. It also works as an important modulator of gene expression related to plant hormones such as in the metabolism of indole-3-acetic acid, cytokinin, ethylene, gibberellin and auxin carrier proteins. Additionally, the regulation of stress-specific genes and the activation of pathogenesis-related protein and antioxidant enzyme genes under stress conditions make it a more versatile molecule. Because of the diversity of action of melatonin, its role in plant growth, development, behavior and regulation of gene expression it is a plant’s master regulator. This review outlines the main functions of melatonin in the physiology, growth, development and regulation of higher plants. Its role as anti-stressor agent against various abiotic stressors, such as drought, salinity, temperatures, UV radiation and toxic chemicals, is also analyzed critically. Additionally, we have also identified many new aspects where melatonin may have possible roles in plants, for example, its function in improving the storage life and quality of fruits and vegetables, which can be useful in enhancing the environmentally friendly crop production and ensuring food safety.

Overexpression of MzASMT 1, a Gene From Malus zumi Mats, Enhances Salt Tolerance in Transgenic Tobacco

Melatonin, widely found in various plants as a new antioxidant, could protect plants from various biotic and/or abiotic stresses, including salt stress. MzASMT 1 (KJ123721), a gene from Malus zumi Mats, is a key enzyme required for melatonin synthesis. However, whether the overexpression of MzASMT 1 could regulate the synthesis of melatonin and improve the salt tolerance in tobacco remains unknown. In this study, the overexpression of MzASMT 1 in tobacco increased the melatonin content, and the transgenic lines owned higher salt tolerance capacity. The transgenic lines overexpressing MzASMT 1 exhibited lower degree of leaf wilting; much more fresh weight; higher plant height; longer root; higher relative water content (RWC) of leaves, stem, and root; and higher chlorophyll content and Fv/Fm, which makes transgenic lines better adapt to salt stress. The transgenic lines also had higher accumulation of proline, lower accumulation of malondialdehyde (MDA), and improved antioxidant systems, which protected plants from cell damage and oxidative stress due to excess reactive oxygen species (ROS) accumulation under salt treatment. The transcription of salt response genes was much more highly activated in transgenic lines than in wild type under salt stress. The above results contributed to the understanding of functions for MzASMT 1 in tobacco under salt stress and provided a new choice for the application of MzASMT 1 in improving plant salt tolerance.

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-Induced Water Stress Tolerance in Plants: Recent Advances

Water stress (drought and waterlogging) is severe abiotic stress to plant growth and development. Melatonin, a bioactive plant hormone, has been widely tested in drought situations in diverse plant species, while few studies on the role of melatonin in waterlogging stress conditions have been published. In the current review, we analyze the biostimulatory functions of melatonin on plants under both drought and waterlogging stresses. Melatonin controls the levels of reactive oxygen and nitrogen species and positively changes the molecular defense to improve plant tolerance against water stress. Moreover, the crosstalk of melatonin and other phytohormones is a key element of plant survival under drought stress, while this relationship needs further investigation under waterlogging stress. In this review, we draw the complete story of water stress on both sides-drought and waterlogging-through discussing the previous critical studies under both conditions. Moreover, we suggest several research directions, especially for waterlogging, which remains a big and vague piece of the melatonin and water stress puzzle.

Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

Water stress (drought and waterlogging) is severe abiotic stress to plant growth and development. Melatonin, a bioactive plant hormone, has been widely tested in drought situations in diverse plant species, while few studies on the role of melatonin in waterlogging stress conditions have been published. In the current review, we analyze the biostimulatory functions of melatonin on plants under both drought and waterlogging stresses. Melatonin controls the levels of reactive oxygen and nitrogen species and positively changes the molecular defense to improve plant tolerance against water stress. Moreover, the crosstalk of melatonin and other phytohormones is a key element of plant survival under drought stress, while this relationship needs further investigation under waterlogging stress. In this review, we draw the complete story of water stress on both sides-drought and waterlogging-through discussing the previous critical studies under both conditions. Moreover, we suggest several research directions, especially for waterlogging, which remains a big and vague piece of the melatonin and water stress puzzle.

Global transcriptomic network of melatonin regulated root growth in Arabidopsis

Melatonin functions as a plant growth regulator in a concentration-dependent manner. In this study, we investigated the effects of melatonin on root growth and dissected underlined mechanisms. The results showed that melatonin up to 1000 μM inhibited primary root growth, but promoted lateral root development. Through RNA sequencing analysis, functions of differentially expressed genes were mainly involved in stress response, signaling transduction, transport, hormone metabolism and amino acid metabolism. Genes involving in jasmonate (JA), brassinosteroid (BR) and cytokinin (CK) biosynthesis were inhibited, but these in ethylene (ET), strigolactone (SL) and gibberellins (GA) biosynthetic pathways were activated after melatonin treatment. The majority of zinc finger proteins (ZFPs), Calmodulin-like (CMLs), NAM, ATAF1/2, and CUC2 (NACs) and ubiquitination related genes (RING/U-box and F-box) were upregulated, which possibly acted downstream of integrated hormone signals to mediate root growth. This study characterized melatonin modulated networks in regulating root growth.

Crystal structure of Oryza sativa TDC reveals the substrate specificity for TDC-mediated melatonin biosynthesis

Plant tryptophan decarboxylase (TDC) is a type II Pyridoxal-5'-phosphate-dependent decarboxylase (PLP_DC) that could be used as a target to genetically improve crops. However, lack of accurate structural information on plant TDC hampers the understanding of its decarboxylation mechanisms. In the present study, the crystal structures of Oryza sativa TDC (OsTDC) in its complexes with pyridoxal-5'-phosphate, tryptamine and serotonin were determined. The structures provide detailed interaction information between TDC and its substrates. The Y359 residue from the loop gate is a proton donor and forms a Lewis acid-base pair with serotonin/tryptamine, which is associated with product release. The H214 residue is responsible for PLP binding and proton transfer, and its proper interaction with Y359 is essential for OsTDC enzyme activity. The extra hydrogen bonds formed between the 5-hydroxyl group of serotonin and the backbone carboxyl groups of F104 and P105 explain the discrepancy between the catalytic activity of TDC in tryptophan and in 5-hydroxytryptophan. In addition, an evolutionary analysis revealed that type II PLP_DC originated from glutamic acid decarboxylase, potentially as an adaptive evolution of mechanism in organisms in extreme environments. This study is, to our knowledge, the first to present a detailed analysis of the crystal structure of OsTDC in these complexes. The information regarding the catalytic mechanism described here could facilitate the development of protocols to regulate melatonin levels and thereby contribute to crop improvement efforts to improve food security worldwide.

Nano-ZnO-Induced Drought Tolerance Is Associated with Melatonin Synthesis and Metabolism in Maize

The applications of ZnO nanoparticles in agriculture have largely contributed to crop growth regulation, quality enhancement, and induction of stress tolerance, while the underlying mechanisms remain elusive. Herein, the involvement of melatonin synthesis and metabolism in the process of nano-ZnO induced drought tolerance was investigated in maize. Drought stress resulted in the changes of subcellular ultrastructure, the accumulation of malondialdehyde and osmolytes in leaf. The nano-ZnO (100 mg L^-1) application promoted the melatonin synthesis and activated the antioxidant enzyme system, which alleviated drought-induced damage to mitochondria and chloroplast. These changes were associated with upregulation of the relative transcript abundance of Fe/Mn SOD, Cu/Zn SOD, APX, CAT, TDC, SNAT, COMT, and ASMT induced by nano-ZnO application. It was suggested that modifications in endogenous melatonin synthesis were involved in the nano-ZnO induced drought tolerance in maize.

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.