Melatonin is involved in skotomorphogenesis by regulating brassinosteroid biosynthesis in rice plants

Overexpression of cassava melatonin receptor PMTR1 plays dual roles in development under light and dark conditions in Arabidopsis

KEY MESSAGE

MePMTR1 is involved in plant development and production as well as photosynthesis in plant. Melatonin is widely involved in plant growth and development as well as stress responses. Compared with the extending studies of melatonin in stress responses, the direct link between melatonin and plant development in the whole stages remains unclear. With the identification of phytomelatonin receptor PMTR1 in plants, melatonin signalling is becoming much clearer. However, the function of MePMTR1 in tropical crop cassava remains elusive. In this study, we found that overexpression of MePMTR1 showed larger biomass than wild type (WT), including higher number and area of leaves, weight, and accompanying with higher photosynthetic efficiency. Consistently, exogenous melatonin accelerated photosynthetic rate in Arabidopsis. In addition, MePMTR1-overexpressed plants exhibited more resistance to dark-induced senescence compared with WT, demonstrated by higher chlorophyll, lower hydrogen peroxide and superoxide content. In summary, this study illustrated that melatonin and its receptor regulate growth, development and senescence in plants, highlighting the potential application of melatonin and its receptor in improving crop yield and photosynthesis.

Melatonin modulates the tolerance of plants to water stress: morphological response of the molecular mechanism

Water stress (drought and waterlogging) leads to an imbalance in plant water distribution, disrupts cell homeostasis, and severely inhibits plant growth. Melatonin is a growth hormone that plants synthesise and has been shown to resist adversity in many plants. This review discusses the biosynthesis and metabolism of melatonin, as well as the changes in plant morphology and physiological mechanisms caused by the molecular defence process. Melatonin induces the expression of related genes in the process of plant photosynthesis under stress and protects the structural integrity of chloroplasts. Exogenous melatonin can maintain the dynamic balance of root ion exchange under waterlogging stress. Melatonin can repair mitochondria and alleviate damage caused by reactive oxygen species and reactive nitrogen species; and has a wide range of uses in the regulation of stress-specific genes and the activation of antioxidant enzyme genes. Melatonin improves the stability of membrane lipids in plant cells and maintains osmotic balance by regulating water channels. There is crosstalk between melatonin and other hormones, which jointly improve the ability of the root system to absorb water and breathe and promote plant growth. Briefly, as a multifunctional molecule, melatonin improves the tolerance of plants under water stress and promotes plant growth and development.

Current research and future directions of melatonin's role in seed germination

Seed germination is a complex process regulated by internal and external factors. Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous signaling molecule, playing an important role in regulating seed germination under normal and stressful conditions. In this review, we aim to provide a comprehensive overview on melatonin's effects on seed germination on the basis of existing literature. Under normal conditions, exogenous high levels of melatonin can suppress or delay seed germination, suggesting that melatonin may play a role in maintaining seed dormancy and preventing premature germination. Conversely, under stressful conditions (e.g., high salinity, drought, and extreme temperatures), melatonin has been found to accelerate seed germination. Melatonin can modulate the expression of genes involved in ABA and GA metabolism, thereby influencing the balance of these hormones and affecting the ABA/GA ratio. Melatonin has been shown to modulate ROS accumulation and nutrient mobilization, which can impact the germination process. In conclusion, melatonin can inhibit germination under normal conditions while promoting germination under stressful conditions via regulating the ABA/GA ratios, ROS levels, and metabolic enzyme activity. Further research in this area will deepen our understanding of melatonin's intricate role in seed germination and may contribute to the development of improved seed treatments and agricultural practices.

Serotonin: A frontline player in plant growth and stress responses

Serotonin is a well-studied pineal hormone that functions as a neurotransmitter in mammals and is found in varying amounts in diverse plant species. By modulating gene and phytohormonal crosstalk, serotonin has a significant role in plant growth and stress response, including root, shoot, flowering, morphogenesis, and adaptability responses to numerous environmental signals. Despite its prevalence and importance in plant growth and development, its molecular action, regulation and signalling processes remain unknown. Here, we highlight the current knowledge of the role of serotonin-mediated regulation of plant growth and stress response. We focus on serotonin and its regulatory connections with phytohormonal crosstalk and address their possible functions in coordinating diverse phytohormonal responses during distinct developmental phases, correlating with melatonin. Additionally, we have also discussed the possible role of microRNAs (miRNAs) in the regulation of serotonin biosynthesis. In summary, serotonin may act as a node molecule to coordinate the balance between plant growth and stress response, which may shed light on finding its key regulatory pathways for uncovering its mysterious molecular network.

Escherichia coli RimI Encodes Serotonin N-Acetyltransferase Activity and Its Overexpression Leads to Enhanced Growth and Melatonin Biosynthesis

Serotonin N-acetyltransferase (SNAT) functions as the penultimate or final enzyme in melatonin biosynthesis, depending on the substrate. The Escherichia coli orthologue of archaeal SNAT from Thermoplasma volcanium was identified as RimI (EcRimI), with 42% amino acid similarity to archaeal SNAT. EcRimI has been reported to be an N-acetyltransferase enzyme. Here, we investigated whether EcRimI also exhibits SNAT enzyme activity. To achieve this goal, we purified recombinant EcRimI and examined its SNAT enzyme kinetics. As expected, EcRimI showed SNAT activity toward various amine substrates including serotonin and 5-methoxytryptamine, with K_m and V_max values of 531 μM and 528 pmol/min/mg protein toward serotonin and 201 μM and 587 pmol/min/mg protein toward 5-methoxytryptamine, respectively. In contrast to the rimI mutant E. coli strain that showed no growth defect, the EcRimI overexpression strain exhibited a 2-fold higher growth rate than the control strain after 24 h incubation in nutrient-rich medium. The EcRimI overexpression strain produced more melatonin than the control strain in the presence of 5-methoxytryptamine. The enhanced growth effect of EcRimI overexpression was also observed under cadmium stress. The higher growth rate associated with EcRimI expression was attributed to increased protein N-acetyltransferase activity, increased synthesis of melatonin, or the combined effects of both.

Exogenous Melatonin Enhances the Yield and Secondary Metabolite Contents of Prunella vulgaris by Modulating Antioxidant System, Root Architecture and Photosynthetic Capacity

Melatonin (MT) plays a number of key roles in regulating plant growth and secondary metabolite accumulation. Prunella vulgaris is an important traditional Chinese herbal medicinal plant which is used for the treatment of lymph, goiter, and mastitis. However, the effect of MT on the yield and medicinal component content of P. vulgaris remains still unclear. In this research, we have examined the influence of different concentrations of MT (0, 50, 100, 200, 400 μM) on the physiological characteristics, secondary metabolite contents, and yield of P. vulgaris biomass. The results showed that 50-200 μM MT treatment had a positive effect on P. vulgaris. MT treatment at 100 μM greatly increased the activities of superoxide dismutase and peroxidase, the contents of soluble sugar and proline, and obviously decreased the relative electrical conductivity, the contents of malondialdehyde and hydrogen peroxide of leaves. Furthermore, it markedly promoted the growth and development of the root system, increased the content of photosynthetic pigments, improved the performance of photosystems I and II and the coordination of both photosystems, and enhanced the photosynthetic capacity of P. vulgaris. In addition, it significantly increased the dry mass of whole plant and spica and promoted the accumulation of total flavonoids, total phenolics, caffeic acid, ferulic acid, rosmarinic acid, and hyperoside in the spica of P. vulgaris. These findings demonstrated that the application of MT could effectively activate the antioxidant defense system of P. vulgaris, protect the photosynthetic apparatus from photooxidation damage, and improve the photosynthetic capacity and the root absorption capacity, thereby promoting the yield and accumulation of secondary metabolites in P. vulgaris.

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.

Dark secrets of phytomelatonin

Phytomelatonin is a newly identified plant hormone, and its primary functions in plant growth and development remain relatively poorly appraised. Phytomelatonin is a master regulator of reactive oxygen species (ROS) signaling and acts as a darkness signal in circadian stomatal closure. Plants exhibit at least three interrelated patterns of interaction between phytomelatonin and ROS production. Exogenous melatonin can induce flavonoid biosynthesis, which might be required for maintenance of antioxidant capacity under stress, after harvest, and in leaf senescence conditions. However, several genetic studies have provided direct evidence that phytomelatonin plays a negative role in the biosynthesis of flavonoids under non-stress conditions. Phytomelatonin delays flowering time in both dicot and monocot plants, probably via its receptor PMTR1 and interactions with the gibberellin, strigolactone, and ROS signaling pathways. Furthermore, phytomelatonin signaling also functions in hypocotyl and shoot growth in skotomorphogenesis and ultraviolet B (UV-B) exposure; the G protein α-subunit (Arabidopsis GPA1 and rice RGA1) and constitutive photomorphogenic1 (COP1) are important signal components during this process. Taken together, these findings indicate that phytomelatonin acts as a darkness signal with important regulatory roles in circadian stomatal closure, flavonoid biosynthesis, flowering, and hypocotyl and shoot growth.

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 signaling molecule for protein quality control via chaperone, autophagy, and ubiquitin-proteasome systems in plants

Physiological effects mediated by melatonin are attributable to its potent antioxidant activity as well as its role as a signaling molecule in inducing a vast array of melatonin-mediated genes. Here, we propose melatonin as a signaling molecule essential for protein quality control (PQC) in plants. PQC occurs by the coordinated activities of three systems: the chaperone network, autophagy, and the ubiquitin-proteasome system. With regard to the melatonin-mediated chaperone pathway, melatonin increases thermotolerance by induction of heat shock proteins and confers endoplasmic reticulum stress tolerance by increasing endoplasmic reticulum chaperone proteins. In chloroplasts, melatonin-induced chaperones, including Clps and CpHSP70s, play key roles in the PQC of chloroplast-localized proteins, such as Lhcb1, Lhcb4, and RBCL, during growth. Melatonin regulates PQC by autophagy processes, in which melatonin induces many autophagy (ATG) genes and autophagosome formation under stress conditions. Finally, melatonin-mediated plant stress tolerance is associated with up-regulation of stress-induced transcription factors, which are regulated by the ubiquitin-proteasome system. In this review, we propose that melatonin plays a pivotal role in PQC and consequently functions as a pleiotropic molecule under non-stress and adverse conditions in plants.

Molecular mechanisms and evolutionary history of phytomelatonin in flowering

Flowering is a critical stage in plant life history, which is coordinated by environmental signals and endogenous cues. Phytomelatonin is a widely distributed indoleamine present in all living organisms and plays pleiotropic roles in plant growth and development. Recent evidence has established that phytomelatonin could modulate flowering in many species, probably in a concentration-dependent manner. Phytomelatonin seems to associate with floral meristem identification and floral organ formation, and the fluctuation of phytomelatonin might be important for flowering. Regarding the underlying mechanisms, phytomelatonin interacts with the central components of floral gene regulatory networks directly or indirectly, including the MADS-box gene family, phytohormones, and reactive oxygen species (ROS). From an evolutionary point of view, the actions of phytomelatonin in flowering probably evolved during the period of the diversification of flowering plants and could be regarded as a functional extension of its primary activities. The presumed evolutionary history of phytomelatonin-modulated flowering is proposed, presented in the chronological order of the appearance of phytomelatonin and core flowering regulators, namely DELLA proteins, ROS, and phytohormones. Further efforts are needed to address some intriguing aspects, such as the exploration of the association between phytomelatonin and photoperiodic flowering, phytomelatonin-related floral MADS-box genes, the crosstalk between phytomelatonin and phytohormones, as well as its potential applications in agriculture.

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.

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.

Comparative Physiological and Transcriptome Analysis Reveal the Molecular Mechanism of Melatonin in Regulating Salt Tolerance in Alfalfa (Medicago sativa L.)

As a high-quality legume forage, alfalfa is restricted by various abiotic stresses during its growth and development. Melatonin is a multifunctional signaling molecule that involves in plant defense against multiple stresses. However, little is known about its downstream signaling pathway and regulatory mechanisms in salt stress of alfalfa. In this study, we investigated the protective effects and key regulatory pathways of melatonin on alfalfa under salt tolerance. The results showed that melatonin promoted the growth of alfalfa seedlings under salt stress, as demonstrated by higher plant height, leaf area, and fresh weight. Melatonin treatment resulted in an increase in the photosynthetic capacity and starch content of alfalfa. Moreover, melatonin decreased cell membrane damage and reactive oxygen species (ROS) accumulation by enhancing antioxidant defense activity under salt stress conditions. Transcriptome sequencing (RNA-seq) analysis revealed that melatonin mainly induced the transcription of genes involved in Ca²⁺ signaling (cyclic nucleotide gated channel, CNGCs; cam modulin/calmodulin-like protein, CAM/CMLs and calcium-dependent protein kinase, CDPKs), starch and sucrose metabolism (α-amylase, AMYs; β-amylase, BAMs; starch synthase, SSs and sucrose synthase, SUSs), plant hormone signal transduction (auxin/indole acetic acid protein, AUX/IAAs; ABA receptor, PYL4; protein phosphatase 2C, PP2Cs; scarecrow-like protein, SCLs and ethylene-responsive transcription factor 1B, ERF1B), and key transcription factors (C3Hs, MYBs, ERFs, and WRKYs). Specifically, we focused on starch and sucrose metabolism and plant hormone signal transduction pathways. The interactions between melatonin and other phytohormones occurred via regulation of the expression of genes involved in hormone signaling pathways. In addition, melatonin increased the contents of endogenous melatonin, auxin, gibberellic acid (GA₃), salicylic acid, brassinosteroids, and ethylene, while decreasing the abscisic acid content under salt stress. In summary, this study established a regulatory network for melatonin-induced key signaling pathways and functional genes under salt stress and provided a theoretical basis for salt tolerance breeding in alfalfa.

Molecular Regulation of Antioxidant Melatonin Biosynthesis by Brassinosteroid Acting as an Endogenous Elicitor of Melatonin Induction in Rice Seedlings

Gibberellic acid (GA) was recently shown to induce melatonin synthesis in rice. Here, we examined whether brassinosteroids (BRs) also induce melatonin synthesis because BRs and GA show redundancy in many functions. Among several plant hormones, exogenous BR treatment induced melatonin synthesis by twofold compared to control treatment, whereas ethylene, 6-benzylaminopurine (BA), and indole-3-acetic acid (IAA) showed negligible effects on melatonin synthesis. Correspondingly, BR treatment also induced a number of melatonin biosynthetic genes in conjunction with the suppression of melatonin catabolic gene expression. Several transgenic rice plants with downregulated BR biosynthesis-related genes, such as DWARF4, DWARF11, and RAV-Like1 (RAVL1), were generated and exhibited decreased melatonin synthesis, indicating that BRs act as endogenous elicitors of melatonin synthesis. Notably, treatment with either GA or BR fully restored melatonin synthesis in the presence of paclobutrazol, a GA biosynthesis inhibitor. Moreover, exogenous BR treatment partially restored melatonin synthesis in both RAVL1 and Gα RNAi transgenic rice plants, whereas GA treatment fully restored melatonin synthesis comparable to wild type in RAVL1 RNAi plants. Taken together, our results highlight a role of BR as an endogenous elicitor of melatonin synthesis in a GA-independent manner in rice plants.

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.

Function, Mechanism, and Application of Plant Melatonin: An Update with a Focus on the Cereal Crop, Barley (Hordeum vulgare L.)

Melatonin is a multiple-function molecule that was first identified in animals and later in plants. Plant melatonin regulates versatile processes involved in plant growth and development, including seed germination, root architecture, flowering time, leaf senescence, fruit ripening, and biomass production. Published reviews on plant melatonin have been focused on two model plants: (1) Arabidopsis and (2) rice, in which the natural melatonin contents are quite low. Efforts to integrate the function and the mechanism of plant melatonin and to determine how plant melatonin benefits human health are also lacking. Barley is a unique cereal crop used for food, feed, and malt. In this study, a bioinformatics analysis to identify the genes required for barley melatonin biosynthesis was first performed, after which the effects of exogenous melatonin on barley growth and development were reviewed. Three integrated mechanisms of melatonin on plant cells were found: (1) serving as an antioxidant, (2) modulating plant hormone crosstalk, and (3) signaling through a putative plant melatonin receptor. Reliable approaches for characterizing the function of barley melatonin biosynthetic genes and to modulate the melatonin contents in barley grains are discussed. The present paper should be helpful for the improvement of barley production under hostile environments and for the reduction of pesticide and fungicide usage in barley cultivation. This study is also beneficial for the enhancement of the nutritional values and healthcare functions of barley in the food industry.

Exogenous Gibberellin Treatment Enhances Melatonin Synthesis for Melatonin-Enriched Rice Production

Melatonin production is induced by many abiotic and biotic stressors; it modulates the levels of many plant hormones and their signaling pathways. This study investigated the effects of plant hormones on melatonin synthesis. Melatonin synthesis in rice seedlings was significantly induced upon exogenous gibberellin 3 (GA₃) treatment, while it was severely decreased by GA synthesis inhibitor paclobutrazol. In contrast, abscisic acid (ABA) strongly inhibited melatonin synthesis, whereas its inhibitor norflurazon (NF) induced melatonin synthesis. The observed GA-mediated increase in melatonin was closely associated with elevated expression levels of melatonin biosynthetic genes such as TDC3, T5H, and ASMT1; it was also associated with reduced expression levels of catabolic genes ASDAC and M2H. In a paddy field, the treatment of immature rice seeds with exogenous GA led to enhanced melatonin production in rice seeds; various transgenic rice plants downregulating a GA biosynthesis gene (GA3ox2) and a signaling gene (Gα) showed severely decreased melatonin levels, providing in vivo genetic evidence that GA has a positive effect on melatonin synthesis. This is the first study to report that GA is positively involved in melatonin synthesis in plants; GA treatment can be used to produce melatonin-rich seeds, vegetables, and fruits, which are beneficial for human health.

Jasmonate and Melatonin Act Synergistically to Potentiate Cold Tolerance in Tomato Plants

Both jasmonic acid (JA) and melatonin (MT) have been demonstrated to play positive roles in cold tolerance, however, whether and how they crosstalk in the cold responses in plants remain elusive. Here, we report that JA and MT act synergistically in the cold tolerance in tomato plants (Solanum lycopersicum). It was found that JA and MT were both substantially accumulated in response to cold stress and foliar applications of methyl jasmonate (MeJA) and MT promoted cold tolerance as evidenced by increased Fv/Fm, decreased relative electrolyte leakage (EL) and declined H₂O₂ accumulation in tomato plants. Inhibition of MT biosynthesis attenuated MeJA-induced cold tolerance, while inhibition of JA biosynthesis reduced MT accumulation in tomato plants under cold conditions. Furthermore, qRT-PCR analysis showed that the expressions of two MT biosynthetic genes, SlSNAT and SlAMST, were strongly induced by MeJA, whereas suppression of SlMYC2, a master JA signaling regulator, abated the expressions of SlSNAT and SlAMST under cold stress. Additionally, suppression of SlMYC2 reduced MT accumulation, decreased Fv/Fm and increased EL in cold-stressed tomato plants. Interestingly, exogenous MT promoted JA accumulation, while inhibition of MT biosynthesis significantly reduced JA accumulation in tomato plants under the cold condition. Taken together, these results suggest that JA and MT act cooperatively in cold tolerance and form a positive feedback loop, amplifying the cold responses of tomato plants. Our findings might be translated into the development of cold-resistant tomato cultivars by genetically manipulating JA and MT pathways.

2-Hydroxymelatonin, Rather Than Melatonin, Is Responsible for RBOH-Dependent Reactive Oxygen Species Production Leading to Premature Senescence in Plants

Unlike animals, plants amply convert melatonin into 2-hydroxymelatonin (2-OHM) and cyclic 3-hydroxymelatonin (3-OHM) through the action of melatonin 2-hydroxylase (M2H) and melatonin 3-hydroxylase (M3H), respectively. Thus, the effects of exogenous melatonin treatment in plants may be caused by melatonin, 2-OHM, or 3-OHM, or some combination of these compounds. Indeed, studies of melatonin's effects on reactive oxygen species (ROS) production have reported conflicting results. In this study, we demonstrated that 2-OHM treatment induced ROS production, whereas melatonin did not. ROS production from 2-OHM treatment occurred in old arabidopsis leaves in darkness, consistent with an ethylene-mediated senescence mechanism. Transgenic tobacco plants containing overexpressed rice M2H exhibited dwarfism and leaf necrosis of the upper leaves and early senescence of the lower leaves. We also demonstrated that 2-OHM-mediated ROS production is respiratory burst NADPH oxidase (RBOH)-dependent and that 2-OHM-induced senescence genes require ethylene and the abscisic acid (ABA) signaling pathway in arabidopsis. In contrast to melatonin, 2-OHM treatment induced senescence symptoms such as leaf chlorosis and increased ion leakage in arabidopsis. Senescence induction is known to begin with decreased levels of proteins involved in chloroplast maintenance, including Lhcb1 and ClpR1. Together, these results show that 2-OHM acts as a senescence-inducing factor by inducing ROS production in plants.

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.

Metabolite profiling and transcriptome analyses reveal novel regulatory mechanisms of melatonin biosynthesis in hickory

Studies have shown that melatonin regulates the expression of various elements in the biosynthesis and catabolism of plant hormones. In contrast, the effects of these different plant hormones on the biosynthesis and metabolism of melatonin and their underlying molecular mechanisms are still unclear. In this study, the melatonin biosynthesis pathway was proposed from constructed metabolomic and transcriptomic libraries from hickory (Carya cathayensis Sarg.) nuts. The candidate pathway genes were further identified by phylogenetic analysis, amino-acid sequence alignment, and subcellular localization. Notably, most of the transcription factor-related genes coexpressed with melatonin pathway genes were hormone-responsive genes. Furthermore, dual-luciferase and yeast one-hybrid assays revealed that CcEIN3 (response to ethylene) and CcAZF2 (response to abscisic acid) could activate melatonin biosynthesis pathway genes, a tryptophan decarboxylase coding gene (CcTDC1) and an N-acetylserotonin methyltransferase coding gene (CcASMT1), by directly binding to their promoters, respectively. Our results provide a molecular basis for the characterization of novel melatonin biosynthesis regulatory mechanisms and demonstrate for the first time that abscisic acid and ethylene can regulate melatonin biosynthesis.

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.

Melatonin Improves Cotton Salt Tolerance by Regulating ROS Scavenging System and Ca2 + Signal Transduction

As one of the cash crops, cotton is facing the threat of abiotic stress during its growth and development. It has been reported that melatonin is involved in plant defense against salt stress, but whether melatonin can improve cotton salt tolerance and its molecular mechanism remain unclear. We investigated the role of melatonin in cotton salt tolerance by silencing melatonin synthesis gene and exogenous melatonin application in upland cotton. In this study, applicating of melatonin can improve salt tolerance of cotton seedlings. The content of endogenous melatonin was different in cotton varieties with different salt tolerance. The inhibition of melatonin biosynthesis related genes and endogenous melatonin content in cotton resulted in the decrease of antioxidant enzyme activity, Ca²⁺ content and salt tolerance of cotton. To explore the protective mechanism of exogenous melatonin against salt stress by RNA-seq analysis. Melatonin played an important role in the resistance of cotton to salt stress, improved the salt tolerance of cotton by regulating antioxidant enzymes, transcription factors, plant hormones, signal molecules and Ca²⁺ signal transduction. This study proposed a regulatory network for melatonin to regulate cotton's response to salt stress, which provided a theoretical basis for improving cotton's salt tolerance.

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 Regulates Chloroplast Protein Quality Control via a Mitogen-Activated Protein Kinase Signaling Pathway

Serotonin N-acetyltransferase 1 (SNAT1), the penultimate enzyme for melatonin biosynthesis has shown N-acetyltransferase activity toward multiple substrates, including histones, serotonin, and plastid proteins. Under two different light conditions such as 50 or 100 μmol m⁻² s⁻¹, a SNAT1-knockout (snat1) mutant of Arabidopsis thaliana ecotype Columbia (Col-0) exhibited small size phenotypes relative over wild-type (WT) Arabidopsis Col-0. Of note, the small phenotype is stronger when growing at the 50 μmol m⁻² s⁻¹, exhibiting a dwarfism phenotype and delayed flowering. The snat1 Arabidopsis Col-0 accumulated less starch than the WT Col-0. Moreover, snat1 exhibited lower Lhcb1, Lhcb4, and RBCL protein levels, compared with the WT Col-0, but no changes in the corresponding transcripts, suggesting the involvement of melatonin in chloroplast protein quality control (CPQC). Accordingly, caseinolytic protease (Clp) and chloroplast heat shock proteins (CpHSPs), two key proteins involved in CPQC, as well as ROS defense were suppressed in snat1. In contrast, exogenous melatonin treatment induced expression of Clp, CpHSP, APX1, and GST, but not other growth-related genes such as DWF4, KS, and IAA1. Finally, the induction of ClpR1, APX1, and GST1 in response to melatonin was inhibited in the mitogen-activated protein kinase (MAPK) knockdown Arabidopsis (mpk3/6), suggesting that melatonin-mediated CPQC was mediated, in part, by the MAPK signaling cascade. These results suggest that melatonin is involved in CPQC, which plays a pivotal role in starch synthesis in plants.

A Systematic Review of Melatonin in Plants: An Example of Evolution of Literature

Melatonin (N-acetyl-5-methoxy-tryptamine) is a mammalian neurohormone, antioxidant and signaling molecule that was first discovered in plants in 1995. The first studies investigated plant melatonin from a human perspective quantifying melatonin in foods and medicinal plants and questioning whether its presence could explain the activity of some plants as medicines. Starting with these first handful of studies in the late 1990s, plant melatonin research has blossomed into a vibrant and active area of investigation and melatonin has been found to play critical roles in mediating plant responses and development at every stage of the plant life cycle from pollen and embryo development through seed germination, vegetative growth and stress response. Here we have utilized a systematic approach in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) protocols to reduce bias in our assessment of the literature and provide an overview of the current state of melatonin research in plants, covering 1995-2021. This review provides an overview of the biosynthesis and metabolism of melatonin as well as identifying key themes including: abiotic stress responses, root development, light responses, interkingdom communication, phytohormone and plant signaling. Additionally, potential biases in the literature are investigated and a birefringence in the literature between researchers from plant and medical based which has helped to shape the current state of melatonin research. Several exciting new opportunities for future areas of melatonin research are also identified including investigation of non-crop and non-medicinal species as well as characterization of melatonin signaling networks in plants.

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.

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

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

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.

New insights on neurotransmitters signaling mechanisms in plants

Neurotransmitters (NTs) such as acetylcholine, biogenic amines (dopamine, noradrenaline, adrenaline, histamine), indoleamines [(melatonin (MEL) & serotonin (SER)] have been found not only in mammalians, but also in diverse living organisms-microorganisms to plants. These NTs have emerged as potential signaling molecules in the last decade of investigations in various plant systems. NTs have been found to play important roles in plant life including-organogenesis, flowering, ion permeability, photosynthesis, circadian rhythm, reproduction, fruit ripening, photomorphogenesis, adaptation to environmental changes. This review will provide an overview of recent advancements on the physiological and molecular mechanism of NTs in plants. Moreover, molecular crosstalk of SER and MEL with various biomolecules is also discussed. The study of these NTs may serve as new understanding of the mechanisms of signal transmission and cell sensing in plants subjected to various environmental stimulus.

Melatonin regulates the functional components of photosynthesis, antioxidant system, gene expression, and metabolic pathways to induce drought resistance in grafted Carya cathayensis plants

The Chinese hickory (Carya cathayensis) is an economically important tree species popular for its nuts. However, the tree requires a long time to reach the nut-producing phase. To overcome this problem, grafting is widely used to reduce the time from the vegetative to the reproductive phase. This tree species also faces many environmental challenges due to climate change; drought is an important factor affecting growth and development. Here, we designed an experiment to assess the protective efficiency of melatonin in grafted Chinese hickory plants under drought stress. The results revealed that exogenously applied melatonin successfully recovered the growth of grafted Chinese hickory plants and improved photosynthetic efficiency. Exogenously applied melatonin also boosted the antioxidative defense system of the plants under drought stress, resulting in enhanced reactive oxygen species (ROS) scavenging. The accumulation of compatible solutes such as total soluble sugars and proline was also triggered by melatonin. Moreover, the analyses using metabolomics revealed that drought-stressed plants treated with melatonin regulated key metabolic pathways such as phenylpropanoid, chlorophyll and carotenoid biosynthesis, carbon fixation, and sugar metabolism. To further validate the physiological, biochemical, and metabolomic factors, we studied the molecular mechanisms by analyzing the expression of key genes involved in chlorophyll metabolism (chlorophyllase, CHLASE), antioxidative defense (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; peroxidase, POD), and phenylalanine ammonia-lyase (PAL). Exogenously applied melatonin significantly regulated the transcript levels of key genes involved in the biological processes mentioned above. Melatonin also showed crosstalk with other hormones (zeatin, gibberellin A14, 24-epibrassinolide, jasmonic acid, and abscisic acid) to regulate the physiological processes. The results of this study show that melatonin regulates biological processes at the metabolic and molecular levels to resist drought stress.

Effects of Light Quality and Phytochrome Form on Melatonin Biosynthesis in Rice

Light is an important factor influencing melatonin synthesis in response to cadmium treatment in rice. However, the effects of light quality on, and the involvement of phytochrome light receptors in, melatonin production have not been explored. In this study, we used light-emitting diodes (LEDs) to investigate the effect of light wavelength on melatonin synthesis, and the role of phytochromes in light-dependent melatonin induction in rice. Upon cadmium treatment, peak melatonin production was observed under combined red and blue (R + B) light, followed by red (R) and blue light (B). However, both far-red (FR) LED light and dark treatment (D) failed to induce melatonin production. Similarly, rice seedlings grown under the R + B treatment showed the highest melatonin synthesis, followed by those grown under B and R. These findings were consistent with the results of our cadmium treatment experiment. To further confirm the effects of light quality on melatonin synthesis, we employed rice photoreceptor mutants lacking functional phytochrome genes. Melatonin induction was most inhibited in the phytochrome A mutant (phyA) followed by the phyB mutant under R + B treatment, whereas phyB produced the least amount of melatonin under R treatment. These results indicate that PhyB is an R light receptor. Expression analyses of genes involved in melatonin biosynthesis clearly demonstrated that tryptophan decarboxylase (TDC) played a key role in phytochrome-mediated melatonin induction when rice seedlings were challenged with cadmium.

Simultaneous Suppression of Two Distinct Serotonin N-Acetyltransferase Isogenes by RNA Interference Leads to Severe Decreases in Melatonin and Accelerated Seed Deterioration in Rice

Serotonin N-acetyltransferase (SNAT) is the penultimate enzyme in the melatonin biosynthetic pathway, in which serotonin is converted into N-acetylserotonin (NAS) in plants. To date, two SNAT isogenes with low amino acid sequence homologies have been identified. Their single suppression in rice has been reported, but their double suppression in rice has not yet been attempted. Here, we generated double-suppression transgenic rice (snat1+2) using the RNA interference technique. The snat1+2 exhibited retarded seedling growths in conjunction with severe decreases in melatonin compared to wild-types and single-suppression rice plants (snat1 or snat2). The laminar angle was decreased in the snat1+2 rice compared to that of the wild-types and snat1, but was comparable to that of snat2. The reduced germination speed in the snat1+2 was comparable to that of snat2. Seed-aging testing revealed that snat1 was the most severely deteriorated, followed by snat1+2 and snat2, suggesting that melatonin is positively involved in seed longevity.

Melatonin as an inducer of arecoline and their coordinated roles in anti-oxidative activity and immune responses

Melatonin and it induced arecoline in arecoline play coordinated roles in immune responses.

Regulation of Brassinosteroid Homeostasis in Higher Plants

Brassinosteroids (BRs) are known as one of the major classes of phytohormones essential for various processes during normal plant growth, development, and adaptations to biotic and abiotic stresses. Significant progress has been achieved on revealing mechanisms regulating BR biosynthesis, catabolism, and signaling in many crops and in model plant Arabidopsis. It is known that BRs control plant growth and development in a dosage-dependent manner. Maintenance of BR homeostasis is therefore critical for optimal functions of BRs. In this review, updated discoveries on mechanisms controlling BR homeostasis in higher plants in response to internal and external cues are discussed.

Knockout of Arabidopsis Serotonin N-Acetyltransferase-2 Reduces Melatonin Levels and Delays Flowering

Melatonin plays roles in both plant growth and defense. Serotonin N-acetyltransferase (SNAT) catalyzes formation of N-acetylserotonin (NAS) from serotonin. Plants contain two SNAT isogenes, which exhibit low-level amino acid homology. We studied the ArabidopsisthalianaSNAT2 (AtSNAT2) gene; we prepared recombinant SNAT2 protein and characterized a snat2 knockout mutant. The SNAT2 protein exhibited 27% amino acid homology with SNAT1; the Km was 232 μM and the Vmax was 2160 pmol/min/mg protein. Melatonin inhibited SNAT enzyme activity in vitro. SNAT2 mRNA was abundantly expressed in flowers; the melatonin content of flowers of the snat2 mutant was significantly less than that of wild-type flowers. The mutant exhibited delayed flowering and reductions in leaf area and biomass compared to the wild type. Delayed flowering was attributable to reductions in the expression levels of the gibberellin biosynthetic genes ent-kaurene synthase (KS) and FLOWERING LOCUS T (FT).

Adaptation of the metabolomics profile of rice after Pyricularia oryzae infection

Pyricularia oryzae (P. oryzae), one of the most devastating fungal pathogens, is the cause of blast disease in rice. Infection with a blast fungus induces biological responses in the host plant that lead to its survival through the termination or suppression of pathogen growth, and metabolite compounds play vital roles in plant interactions with a wide variety of other organisms. Numerous studies have indicated that rice has a multi-layered plant immune system that includes pre-developed (e.g., cell wall and phytoanticipins), constitutive and inducible (phytoalexins) defence barriers against stresses. Significant progress towards understanding the basis of the molecular mechanisms underlying the defence responses of rice to P. oryzae has been achieved. Nonetheless, even though the important metabolites in the responses of rice to pathogens have been identified, their exact mechanisms and their contributions to plant immunity against blast fungi have not been elucidated. The purpose of this review is to summarize and discuss recent advances towards the understanding of the integrated metabolite variations in rice after P. oryzae invasion.

Melatonin Deficiency Confers Tolerance to Multiple Abiotic Stresses in Rice via Decreased Brassinosteroid Levels

Melatonin has long been recognized as a positive signaling molecule and potent antioxidant in plants, which alleviates damage caused by adverse conditions such as salt, cold, and heat stress. In this study, we found a paradoxical role for melatonin in abiotic stress responses. Suppression of the serotonin N-acetyltransferase 2 (snat2) gene encoding the penultimate enzyme in melatonin biosynthesis led to simultaneous decreases in both melatonin and brassinosteroid (BR) levels, causing a semi-dwarf with erect leaf phenotype, typical of BR deficiency. Here, we further characterized snat2 rice in terms of grain morphology and abiotic stress tolerance, to determine whether snat2 rice exhibited characteristics similar to those of BR-deficient rice. As expected, the snat2 rice exhibited tolerance to multiple stress conditions including cadmium, salt, cold, and heat, as evidenced by decreased malondialdehyde (MDA) levels and increased chlorophyll levels, in contrast with SNAT2 overexpression lines, which were less tolerant to stress than wild type plants. In addition, the length and width of grain from snat2 plants were reduced relative to the wild type, which is reminiscent of BR deficiency in rice. Other melatonin-deficient mutant rice lines with suppressed BR synthesis (i.e., comt and t5h) also showed tolerance to salt and heat stress, whereas melatonin-deficient rice seedlings without decreased BR levels (i.e., tdc) failed to exhibit increased stress tolerance, suggesting that stress tolerance was increased not by melatonin deficiency alone, but by a melatonin deficiency-mediated decrease in BR.

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

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

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

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

Melatonin: A New Plant Hormone and/or a Plant Master Regulator?

Melatonin is a pleiotropic molecule with many diverse actions in plants. It is considered primarily an antioxidant with important actions in the control of reactive oxygen and nitrogen species (ROS and RNS), among other free radicals, and harmful oxidative molecules present in plant cells. In addition, plant melatonin is involved in multiple physiological actions, such as growth, rooting, seed germination, photosynthesis, and protection against abiotic and/or biotic stressors. The recent identification of the first plant melatonin receptor opened the door to this regulatory molecule being considered a new plant hormone. However, due to the diversity of its actions, melatonin has also been proposed as a plant master regulator. Here, we discuss the most recent data in respect to both perspectives.

Hypocotyl Elongation Inhibition of Melatonin Is Involved in Repressing Brassinosteroid Biosynthesis in Arabidopsis

Melatonin functions as a plant hormone/regulator in the regulation of growth and development. However, the underlying mechanisms are still unclear. In this study, we found that a high dose of melatonin inhibited hypocotyl elongation in a dose-dependent manner in Arabidopsis. An expression profile analysis showed that hypocotyl growth inhibition by melatonin was involved in reprograming the expression of cell elongation genes and brassinosteroid (BRs) biosynthetic genes. Furthermore, similar to BR biosynthetic inhibitor brassinazole (BRZ), a high concentration of melatonin upregulated BR-biosynthetic genes and downregulated BR-induced genes involved in cell elongation, while melatonin was inefficient in brassinazole-resistant mutants like the bzr1-1D and bes1-D in hypocotyl inhibition. The comparative expression profile analysis showed an opposite expression mode in the co-regulated genes between melatonin and BZR1 or melatonin and brassinolide (BL). Additionally, exogenous BL rescued the repressive phenotype of BR biosynthesis-deficient mutant like det2-1 even in the presence of high-dose melatonin, but not BR receptor mutant bri1-5 or signal transduction mutant bin2-1. A biochemical analysis further confirmed that melatonin reduced endogenous BR levels in a dose-dependent manner in Arabidopsis. Taken together, these results indicate that melatonin inhibits BR biosynthesis but does not block BR signaling in the inhibition of hypocotyl elongation and extends insights on the role of melatonin in cross-talking with plant hormone signaling.

Phytomelatonin: Recent advances and future prospects

Melatonin (MEL) has been revealed as a phylogenetically conserved molecule with a ubiquitous distribution from primitive photosynthetic bacteria to higher plants, including algae and fungi. Since MEL is implicated in numerous plant developmental processes and stress responses, the exploration of its functions in plant has become a rapidly progressing field with the new paradigm of involvement in plants growth and development. The pleiotropic involvement of MEL in regulating the transcripts of numerous genes confirms its vital involvement as a multi-regulatory molecule that architects many aspects of plant development. However, the cumulative research in plants is still preliminary and fragmentary in terms of its established functions compared to what is known about MEL physiology in animals. This supports the need for a comprehensive review that summarizes the new aspects pertaining to its functional role in photosynthesis, phytohormonal interactions under stress, cellular redox signaling, along with other regulatory roles in plant immunity, phytoremediation, and plant microbial interactions. The present review covers the latest advances on the mechanistic roles of phytomelatonin. While phytomelatonin is a sovereign plant growth regulator that can interact with the functions of other plant growth regulators or hormones, its qualifications as a complete phytohormone are still to be established. This review also showcases the yet to be identified potentials of phytomelatonin that will surely encourage the plant scientists to uncover new functional aspects of phytomelatonin in plant growth and development, subsequently improving its status as a potential new phytohormone.

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

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