Local melatonin application induces cold tolerance in distant organs of Citrullus lanatus L. via long distance transport

Cyclic nucleotide-gated ion channel 20 regulates melatonin-induced calcium signaling and cold tolerance in watermelon

Melatonin plays a crucial role in regulating plant cold tolerance, but the mechanisms underlying signal transduction remain elusive. In this study, we discovered that overexpression of the melatonin biosynthetic gene caffeic acid O-methyltransferase1 (COMT1) enhanced watermelon (Citrullus lanatus) cold tolerance, accompanied by the accumulation of cytosolic free calcium ([Ca2+]cyt), a stimulation of Ca2+ influx, and upregulation of 4 Ca2+-permeable channel genes (CNGC2/10/17/20). Conversely, the knockout of COMT1 exhibited contrasting effects compared with its overexpression. Knocking out the 4 CNGC genes revealed that only cyclic nucleotide-gated ion channel 20 (CNGC20) mediates melatonin-induced Ca2+ influx in response to cold stimuli. CNGC20 deletion impeded watermelon callus redifferentiation, prompting us to employ a virus-induced gene silencing strategy to suppress its expression. Silencing CNGC20 compromised COMT1 overexpression-induced [Ca2+]cyt accumulation, Ca2+ influx, and watermelon cold tolerance. Yeast 2-hybrid, bimolecular fluorescence complementation, firefly luciferase complementation imaging, and pull-down assays revealed an interaction between CNGC20 and calmodulin7 (CaM7). Overexpressing CaM7 inhibited melatonin-induced [Ca2+]cyt accumulation, Ca2+ influx, and watermelon cold tolerance. Conversely, silencing CaM7 increased [Ca2+]cyt accumulation, Ca2+ influx, and cold tolerance, whereas COMT1 overexpression failed to further enhance these responses in CaM7-silenced plants, indicating the negative regulation role of CaM7 in melatonin-mediated cold responses. Overall, these findings provide insights into the molecular mechanisms underlying melatonin-enhanced plant cold tolerance via Ca2+ signaling, holding potential for breeding/engineering cold-tolerant cucurbit varieties.

A self-amplifying NO-H2S loop mediates melatonin-induced CBF-responsive pathway and cold tolerance in watermelon

Melatonin is a pivotal bioactive molecule that enhances plant cold stress tolerance, but the precise mechanisms remain enigmatic. Here, we have discovered that overexpressing melatonin biosynthetic gene ClCOMT1 or applying exogenous melatonin activates the C-repeat binding factor (CBF)-responsive pathway and enhances watermelon cold tolerance. This enhancement is accompanied by elevated levels of nitric oxide (NO) and hydrogen sulfide (H2S), along with upregulation of nitrate reductase 1 (ClNR1) and L-cysteine desulfhydrase (ClLCD) genes involved in NO and H2S generation respectively. Conversely, knockout of ClCOMT1 exhibits contrasting effects compared to its overexpression. Furthermore, application of sodium nitroprusside (SNP, a NO donor) and NaHS (a H2S donor) promotes the accumulation of H2S and NO, respectively, activating the CBF pathway and enhancing cold tolerance. However, knockout of ClNR1 or ClLCD abolished melatonin-induced H2S or NO production respectively and abrogated melatonin-induced CBF pathway and cold tolerance. Conversely, supplementation with SNP and NaHS restored the diminished cold response caused by ClCOMT1 deletion. Additionally, deletion of either ClNR1 or ClLCD eliminated NaHS- or SNP-induced cold response, respectively. Overall, these findings suggest a reciprocal positive-regulatory loop between ClNR1-mediated NO and ClLCD-mediated H2S, which plays a crucial role in mediating the melatonin-induced enhancement of cold tolerance.

Melatonin Enhances the Low-Calcium Stress Tolerance by Regulating Brassinosteroids and Auxin Signals in Wax Gourd

In plants, calcium (Ca) serves as an essential nutrient and signaling molecule. Melatonin is a biologically active and multi-functional hormone that plays an important role in improving nutrient use efficiency. However, its involvement in plant responses to Ca deficiency remains largely unexplored. This study aimed to assess the effects of melatonin on Ca absorption, the antioxidant system, and root morphology under low-Ca (LCa) stress conditions, as well as to identify key regulatory factors and signaling pathways involved in these processes using transcriptome analysis. Under LCa conditions, wax gourd seedling exhibited significant decreases in Ca accumulation, showed inhibition of root growth, and demonstrated the occurrence of oxidative damage. However, melatonin application significantly enhanced Ca content in wax gourd seedlings, and it enhanced the absorption of Ca2+ in roots by upregulating Ca2+ channels and transport genes, including BhiCNGC17, BhiCNGC20, BhiECA1, BhiACA1, and BhiCAX1. Furthermore, the application of exogenous melatonin mitigated the root growth inhibition and oxidative damage caused by LCa stress. This was evidenced by increases in the root branch numbers, root tips, root surface area, and root volume, as well as enhanced root vitality and antioxidant enzyme activities, as well as decreases in the reactive oxygen species content in melatonin treated plants. Transcriptome results revealed that melatonin mainly modulated the brassinosteroids (BRs) and auxin signaling pathway, which play essential roles in root differentiation, elongation, and stress adaptation. Specifically, melatonin increased the active BR levels by upregulating BR6ox (a BR biosynthesis gene) and downregulating BAS1 (BR degradation genes), thereby affecting the BR signaling pathway. Additionally, melatonin reduced IAA levels but activated the auxin signaling pathway, indicating that melatonin could directly stimulate the auxin signaling pathway via an IAA-independent mechanism. This study provides new insights into the role of melatonin in nutrient stress adaptation, offering a promising and sustainable approach to improve nutrient use efficiency in wax gourd and other crops.

Melatonin Interaction with Other Phytohormones in the Regulation of Abiotic Stresses in Horticultural Plants

Horticultural crops play a vital role in global food production, nutrition, and the economy. Horticultural crops are highly vulnerable to abiotic stresses. These abiotic stresses hinder plant growth and development by affecting seed germination, impairing photosynthetic activity, and damaging root development, thus leading to a decrease in fruit yield, quality, and productivity. Scientists have conducted extensive research to investigate the mechanisms of resilience and the ability to cope with environmental stresses. In contrast, the use of phytohormones to alleviate the detrimental impacts of abiotic stresses on horticulture plants has been generally recognized as an effective method. Among phytohormones, melatonin (MT) is a novel plant hormone that regulates various plants' physiological functions such as seedling development, root system architecture, photosynthetic efficiency, balanced redox homeostasis, secondary metabolites production, accumulation of mineral nutrient uptake, and activated antioxidant defense system. Importantly, MT application significantly restricted heavy metals (HMs) uptake and increased mineral nutrient accumulation by modifying the root architecture system. In addition, MT is a naturally occurring, multifunctional, nontoxic biomolecule having antioxidant properties. Furthermore, this review described the hormonal interaction between MT and other signaling molecules in order to enhance abiotic stress tolerance in horticulture crops. This review focuses on current research advancements and prospective approaches for enhancing crop tolerance to abiotic stress.

Study on peanut protein oxidation and metabolomics/proteomics analysis of peanut response under hypoxic/re-aeration storage

To better understand the effect of oxygen levels in the storage environment on peanut protein oxidation and explore the mechanism, the functional properties and the oxidation degree of peanut proteins extracted from peanuts under conventional storage (CS), nitrogen modified atmosphere storage (NS, hypoxic) and re-aeration storage (RS) were investigated. Metabolomics and proteomics were employed to analyze peanut's response to hypoxic/re-aeration storage environment. The results showed that NS retarded the decline of the functional properties and the oxidation of peanut proteins, while the process were accelerated after re-aeration. That was the result of the metabolic changes of peanuts under different storage environments. The omics results presented the decreased (NS)/increased (RS) levels of the antioxidant-related proteins acetaldehyde dehydrogenase and glutathione S-transferase, and the inhibition (NS)/activation (RS) of metabolic pathways such as the TCA cycle and the pentose phosphate pathway. This study provided a reference for the re-aeration storage of other agricultural products.

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

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

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.

Melatonin: Current status and future perspectives in horticultural plants

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. Different phytohormones are produced by plants to mitigate the adverse effects of these stresses. One such phytohormone is melatonin (MEL), which, being a potential bio-stimulator, helps to govern a wide array of functions in horticultural crops. Recent advancements have determined the role of MEL in plants' responses to abiotic stresses. MEL enhances physiological functions such as seed germination, growth and development, seedling growth, root system architecture, and photosynthetic efficiency. The potential function of MEL in stressful environments is to regulate the enzymatic and non-enzymatic antioxidant activity, thus playing a role in the substantial scavenging of reactive oxygen species (ROS). Additionally, MEL, as a plant growth regulator and bio-stimulator, aids in promoting plant tolerance to abiotic stress, mainly through improvements in nutrient uptake, osmolyte production, and cellular membrane stability. This review, therefore, focuses on the possible functions of MEL in the induction of different abiotic stresses in horticultural crops. Therefore, this review would help readers learn more about MEL in altered environments and provide new suggestions on how this knowledge could be used to develop stress tolerance.

Comparative bioinformatics analysis and abiotic stress responses of expansin proteins in Cucurbitaceae members: watermelon and melon

Watermelon and melon are members of the Cucurbitaceae family including economically significant crops in the world. The expansin protein family, which is one of the members of the cell wall, breaks down the non-covalent bonds between cell wall polysaccharides, causing pressure-dependent cell expansion. Comparative bioinformatics and molecular characterization analysis of the expansin protein family were carried out in the watermelon (Citrullus lanatus) and melon (Cucumis melo) plants in the study. Gene expression levels of expansin family members were analyzed in leaf and root tissues of watermelon and melon under ABA, drought, heat, cold, and salt stress conditions by quantitative real-time PCR analysis. After comprehensive searches, 40 expansin proteins (22 ClaEXPA, 14 ClaEXPLA, and 4 ClaEXPB) in watermelon and 43 expansin proteins (19 CmEXPA, 15 CmEXPLA, 3 CmEXPB, and 6 CmEXPLB) in melon were identified. The greatest orthologous genes were identified with soybean expansin genes for watermelon and melon. However, the latest divergence time between orthologous genes was determined with poplar expansin genes for watermelon and melon expansin genes. ClaEXPA-04, ClaEXPA-09, ClaEXPB-01, ClaEXPB-03, and ClaEXPLA-13 genes in watermelon and CmEXPA-12, CmEXPA-10, and CmEXPLA-01 genes in melon can be involved in tissue development and abiotic stress response of the plant. The current study combining bioinformatics and experimental analysis can provide a detailed characterization of the expansin superfamily which has roles in growth and reaction to the stress of the plant. The study ensures detailed data for future studies examining gene functions including the roles in plant growth and stress conditions.

Melatonin delays ABA-induced leaf senescence via H2 O2 -dependent calcium signalling

Precocious leaf senescence can reduce crop yield and quality by limiting the growth stage. Melatonin has been shown to delay leaf senescence; however, the underlying mechanism remains obscure. Here, we show that melatonin offsets abscisic acid (ABA) to protect photosystem II and delay the senescence of attached old leaves under the light. Melatonin induced H₂ O₂ accumulation accompanied by an upregulation of melon respiratory burst oxidase homolog D (CmRBOHD) under ABA-induced stress. Both melatonin and H₂ O₂ induced the accumulation of cytoplasmic-free Ca²⁺ ([Ca²⁺ ]_cyt ) in response to ABA, while blocking of Ca²⁺ influx channels attenuated melatonin- and H₂ O₂ -induced ABA tolerance. CmRBOHD overexpression induced [Ca²⁺ ]_cyt accumulation and delayed leaf senescence, whereas deletion of Arabidopsis AtRBOHD, a homologous gene of CmRBOHD, compromised the melatonin-induced [Ca²⁺ ]_cyt accumulation and delay of leaf senescence in Arabidopsis under ABA stress. Furthermore, melatonin, H₂ O₂  and Ca²⁺ attenuated ABA-induced K⁺ efflux and subsequent cell death. CmRBOHD overexpression and AtRBOHD deletion alleviated and aggravated the ABA-induced K⁺ efflux, respectively. Taken together, our study unveils a new mechanism by which melatonin offsets ABA action to delay leaf senescence via RBOHD-dependent H₂ O₂ production that triggers [Ca²⁺ ]_cyt accumulation and subsequently inhibits K⁺ efflux and delays cell death/leaf senescence in response to ABA.

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.

Exogenous melatonin improves salt tolerance mainly by regulating the antioxidant system in cyanobacterium Nostoc flagelliforme

Melatonin is a multifunctional nontoxic bio-stimulant or signaling molecule, generally distributing in different animal and plant organs for invigorating numerous physiological processes against abiotic stresses. In this study, we investigated the potential impact of melatonin on the cyanobacterium Nostoc flagelliforme when exposed to salt stress according to some biochemical and physiological parameters, such as relative electrolyte leakage, PSII activity, and photosynthetic pigments including chlorophyll a, phycocyanobilin, and phycoerythrobilin. We found that melatonin could also maintain K⁺ homeostasis in salt-stressed N. flagelliforme. These above results confirmed melatonin had multiple functions in hyperosmotic stress and ion stress caused by salinity. Notably, we observed melatonin could regulate the reactive oxygen species (ROS) signal and distinctly decrease the content of hydrogen peroxide and superoxide anion in salt-stressed cells, which were largely attributed to the increased antioxidant enzymes activities including catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase. Finally, qRT-PCR analysis showed that melatonin stimulated the expression of antioxidant genes (NfCAT, NfSOD, and NfGR). In general, our findings demonstrate melatonin has beneficial effects on N. flagelliforme under salt stress by intensively regulating antioxidant system.

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.

Exogenous Melatonin Reprograms the Rhizosphere Microbial Community to Modulate the Responses of Barley to Drought Stress

The rhizospheric melatonin application-induced drought tolerance has been illuminated in various plant species, while the roles of the rhizosphere microbial community in this process are still unclear. Here, the diversity and functions of the rhizosphere microbial community and related physiological parameters were tested in barley under the rhizospheric melatonin application and drought. Exogenous melatonin improved plant performance under drought via increasing the activities of non-structural carbohydrate metabolism enzymes and activating the antioxidant enzyme systems in barley roots under drought. The 16S/ITS rRNA gene sequencing revealed that drought and melatonin altered the compositions of the microbiome. Exogenous melatonin increased the relative abundance of the bacterial community in carbohydrate and carboxylate degradation, while decreasing the relative abundance in the pathways of fatty acid and lipid degradation and inorganic nutrient metabolism under drought. These results suggest that the effects of melatonin on rhizosphere microbes and nutrient condition need to be considered in its application for crop drought-resistant cultivation.

Exogenous melatonin enhances soybean (Glycine max (L.) Merr.) seedling tolerance to saline-alkali stress by regulating antioxidant response and DNA damage repair

Saline-alkali (SA) stress induces excessive reactive oxygen species (ROS) accumulation in plant cells, resulting in oxidative damages of membranes, lipids, proteins, and nucleic acids. Melatonin has antioxidant protection effects in living organisms and thus has received a lot of attention. This study aimed to investigate the effect and regulating mechanism of melatonin treatment on soybean tolerance to SA stress. In this study, cultivars Heihe 49 (HH49, SA-tolerant) and Henong 95 (HN95, SA-sensitive) were pot-cultured in SA soil, then treated with MT (0-300 μM) at V1 stage. SA stress induced ROS accumulation and DNA damage in the seedling roots of both cultivars, causing G1/S arrest in HN95 and G2/M arrest in HH49. Melatonin treatment enhanced the activity of antioxidant enzymes in soybean seedling roots and reduced ROS accumulation. Additionally, melatonin treatment upregulated DNA damage repair genes, thus enhancing the reduction of DNA oxidative damage under SA stress. The effects of melatonin treatment were manifested as decreased RAPD polymorphism, 8-hydroxy-2'-deoxyguanine (8-OH-dG) level, and relative density of apurinic sites (AP-sites). Meanwhile, melatonin treatment partially alleviated the SA-induced G1/S arrest in HN95 and G2/M arrest in HH49, thus enhancing soybean seedling tolerance to SA stress.

Mammalian Melatonin Agonist Pharmaceuticals Stimulate Rhomboid Proteins in Plants

Melatonin is a human neurotransmitter and plant signalling metabolite that perceives and directs plant metabolism. The mechanisms of melatonin action in plants remain undefined. We hypothesized that roots have a melatonin-specific receptor and/or transporter that can respond to melatonin-mediating pharmaceuticals. To test this hypothesis Arabidopsis seedlings were grown with melatonin pharmaceutical receptor agonists: ramelteon and tasimelteon, and/or antagonists: luzindole and 4-P-PDOT. Ramelteon was found both to mimic and competitively inhibit melatonin metabolism in plants. Due to the higher selectivity of ramelteon for the MT1 receptor type in humans, a sequence homology search for MT1 in Arabidopsis identified the rhomboid-like protein 7 (RBL7). In physiological studies, Arabidopsis rbl7 mutants were less responsive to ramelteon and melatonin. Quantum dot visualizations of the effects of ramelteon on melatonin binding to root cell membranes revealed a potential mechanism. We propose that RBL7 is a melatonin-interacting protein that directs root architecture and growth in a mechanism that is responsive to environmental factors.

Interaction between Melatonin and NO: Action Mechanisms, Main Targets, and Putative Roles of the Emerging Molecule NOmela

Melatonin (MEL), a ubiquitous indolamine molecule, has gained interest in the last few decades due to its regulatory role in plant metabolism. Likewise, nitric oxide (NO), a gasotransmitter, can also affect plant molecular pathways due to its function as a signaling molecule. Both MEL and NO can interact at multiple levels under abiotic stress, starting with their own biosynthetic pathways and inducing a particular signaling response in plants. Moreover, their interaction can result in the formation of NOmela, a very recently discovered nitrosated form of MEL with promising roles in plant physiology. This review summarizes the role of NO and MEL molecules during plant development and fruit ripening, as well as their interactions. Due to the impact of climate-change-related abiotic stresses on agriculture, this review also focuses on the role of these molecules in mediating abiotic stress tolerance and the main mechanisms by which they operate, from the upregulation of the entire antioxidant defense system to the post-translational modifications (PTMs) of important molecules. Their individual interaction and crosstalk with phytohormones and H₂S are also discussed. Finally, we introduce and summarize the little information available about NOmela, an emerging and still very unknown molecule, but that seems to have a stronger potential than MEL and NO separately in mediating plant stress response.

12-Deoxyphorbol-13-Hexadecanoate Abrogates OVX-Induced Bone Loss in Mice and Osteoclastogenesis via Inhibiting ROS Level and Regulating RANKL-Mediated NFATc1 Activation

Osteoporosis is a major health problem in the elderly. Almost every bone can fracture due to the increased bone fragility in osteoporosis, posing a major challenge to public health. 12-Deoxyphorbol-13-hexadecanoate (DHD), one of the main bioactive components of Stellera chamaejasme L. (Lang Du), is considered to have antitumor, antibacterial, and antifungal properties. However, the role of DHD in osteoporosis is still elusive. In this study, we demonstrated for the first time that DHD inhibits the receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis and bone resorption in a dose- and time-dependent manner without exhibiting cytotoxicity in vitro. Mechanistically, we found that DHD not only represses the expression of osteoclasts marker genes by suppressing RANKL-induced mitogen-activated protein kinase (MAPK) and calcium signaling pathways but also scavenges reactive oxygen species (ROS) through enhancing cytoprotective enzymes expression. Furthermore, DHD inhibits the activation of nuclear factor of activated T cells 1 (NFATc1) during RANKL-induced osteoclasts formation. Preclinical studies revealed that DHD protects against bone loss in ovariectomy (OVX) mice. In sum, our data confirmed that DHD could potentially inhibit osteoclastogenesis by abrogating RANKL-induced MAPK, calcium, and NFATc1 signaling pathways and promoting the expression of ROS scavenging enzymes, thereby preventing OVX-induced bone loss. Thus, DHD may act as a novel therapeutic agent to manage osteoporosis.

Role of melatonin in promoting plant growth by regulating carbon assimilation and ATP accumulation

Melatonin (MT) is a phytohormone important in mediating diverse plant growth processes. In this study, we performed transcriptomic, qRT-PCR, physiological and biochemical analyses of Brassica rapa seedlings in order to understand how MT promotes plant growth. The results showed that exogenous MT increased the rate of cyclic electron flow around photosystem (PS) I, fluorescence quantum yield, and electron transport efficiency between PSII and PSI to promote the vegetative growth of B. rapa seedlings without affecting oxidative stress level, as compared to control. However, MT treatment significantly reduced photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) by 2.25-, 1.23- and 3.50-fold at 0.05 level, respectively. This occurred in parallel with the down-regulation of the genes for carbon fixation in photosynthetic organisms in a KEGG pathway enrichment. More accelerated plant growth despite the reduced photosynthesis rate and the enhanced electron transport rate suggested that NADPH and adenosine triphosphate (ATP) were preferentially diverted into other anabolic reactions than the Calvin cycle upon MT application. MT treatment increased ATP level and facilitated carbon assimilation into primary metabolism that led to a significant enhancement of soluble protein, sucrose, and fructose, but a significant decrease in glucose content. MT-induced carbon assimilation into primary metabolism was driven by up-regulation of the genes for glutathione metabolism, Krebs cycle, ribosome, and DNA replication in a KEGG pathway enrichment, as well as down-regulation of the genes for secondary metabolites. Our results provide an insight into MT-mediated metabolic adjustments triggered by coordinate changes in a wide range of gene expression profiles to help improve the plant functionality.

Interactions of Gibberellins with Phytohormones and Their Role in Stress Responses

Gibberellins are amongst the main plant growth regulators. Discovered over a century ago, the interest in gibberellins research is growing due to their current and potential applications in crop production and their role in the responses to environmental stresses. In the present review, the current knowledge on gibberellins’ homeostasis and modes of action is outlined. Besides this, the complex interrelations between gibberellins and other plant growth regulators are also described, providing an intricate network of interactions that ultimately drives towards precise and specific gene expression. Thus, genes and proteins identified as being involved in gibberellin responses in model and non-model species are highlighted. Furthermore, the molecular mechanisms governing the gibberellins’ relation to stress responses are also depicted. This review aims to provide a comprehensive picture of the state-of-the-art of the current perceptions of the interactions of gibberellins with other phytohormones, and their responses to plant stresses, thus allowing for the identification of the specific mechanisms involved. This knowledge will help us to improve our understanding of gibberellins’ biology, and might help increase the biotechnological toolbox needed to refine plant resilience, particularly under a climate change scenario.

Exogenous Melatonin Improves Cold Tolerance of Strawberry (Fragaria × ananassa Duch.) through Modulation of DREB/CBF-COR Pathway and Antioxidant Defense System

The strawberry (Fragaria × ananassa Duch.) is an important fruit crop cultivated worldwide for its unique taste and nutritional properties. One of the major risks associated with strawberry production is cold damage. Recently, melatonin has emerged as a multifunctional signaling molecule that influences plant growth and development and reduces adverse consequences of cold stress. The present study was conducted to investigate the defensive role of melatonin and its potential interrelation with abscisic acid (ABA) in strawberry plants under cold stress. The results demonstrate that melatonin application conferred improved cold tolerance on strawberry seedlings by reducing malondialdehyde and hydrogen peroxide contents under cold stress. Conversely, pretreatment of strawberry plants with 100 μM melatonin increased soluble sugar contents and different antioxidant enzyme activities (ascorbate peroxidase, catalase, and peroxidase) and non-enzymatic antioxidant (ascorbate and glutathione) activities under cold stress. Furthermore, exogenous melatonin treatment stimulated the expression of the DREB/CBF—COR pathways’ downstream genes. Interestingly, ABA treatment did not change the expression of the DREB/CBF—COR pathway. These findings imply that the DREB/CBF-COR pathway confers cold tolerance on strawberry seedlings through exogenous melatonin application. Taken together, our results reveal that melatonin (100 μM) pretreatment protects strawberry plants from the damages induced by cold stress through enhanced antioxidant defense potential and modulating the DREB/CBF—COR pathway.

H2S aids osmotic stress resistance by S-sulfhydration of melatonin production-related enzymes in Arabidopsis thaliana

Hydrogen sulfide closed Arabidopsis thaliana stomata by increasing the transcription of melatonin-producing enzymes and the post-translational modification levels to combat osmotic stress. Hydrogen sulfide (H₂S) and melatonin (MEL) reportedly have similar functions in many aspects of plant growth, development and stress response. They regulate stomatal movement and enhance drought resistance. However, their physiological relationship is not well understood. Here, their crosstalk involved in osmotic stress resistance in Arabidopsis thaliana was studied. Exogenous H₂S and MEL closed stomata under normal or osmotic stress conditions and increased the relative water contents of plants under osmotic stress conditions. At the same time, exogenous H₂S and MEL responded to osmotic stress by increasing the content of proline and soluble sugar, and reducing malondialdehyde (MDA) content and relative conductivity. Using mutants in the MEL-associated production of serotonin N-acetyltransferase (snat), caffeic acid O-methyltransferase (comt1) and N-acetylserotonin methyltransferase (asmt), we determined that H₂S was partially dependent on MEL to close stomata. Additionally, the overexpression of ASMT promoted stomatal closure. Exogenous H₂S increased the transcription levels of SNAT, ASMT and COMT1. Furthermore, exogenous H₂S treatments increased the endogenous MEL content significantly. At the post-translational level, H₂S sulfhydrated the SNAT and ASMT, but not COMT1, enzymes associated with MEL production. Thus, H₂S appeared to promote stomatal closure in response to osmotic stress by increasing the transcription levels of MEL synthesis-related genes and the sulfhydryl modification of the encoded enzymes. These results increased our understanding of H₂S and MEL functions and interactions under osmotic stress conditions.

Exogenous Melatonin Promotes the Salt Tolerance by Removing Active Oxygen and Maintaining Ion Balance in Wheat (Triticum aestivum L.)

Melatonin (MT) is a small molecule indole hormone that plays an important role in the regulation of biological processes and abiotic stress resistance. Previous studies have confirmed that MT promotes the normal development of plants under stress by mediating physiological regulation mechanisms. However, the physiological mechanism of exogenous MT regulating seed germination and seedling growth of wheat under salt stress is still unclear. In this study, NaCl stress decreased germination rate and inhibited seedling growth of wheat, but shoot length, root length, and plant weight of SM15 did not change significantly. The addition of 300 μM MT in the cultivation solution directly promoted the germination rate of SM15 and ZM18, and lateral root production, but decreased the germination rate of JM22 and inhibited the length of germ and radicle of three varieties under salt stress. For wheat seedling, application of MT could increase proline content, soluble protein, soluble sugar, Ca2+ content, and vital amino acid content in leaves to keep high water content, low level of H2O2 content, and low [K+]/[Na+] ratio. MT increased root vigor and [K+]/[Na+] ratio and decreased H2O2 content in root induced by salt stress. In conclusion, MT enhanced salt tolerance in wheat seeds and seedlings by regulating the synthesis of soluble protein and sugar, ion compartmentation in roots and leaves, enhancement of enzymatic systems, and changes in amino acid levels. Salt resistance varied with different varieties under the same environmental condition. SM15 was a higher salt-resistant variety and JM22 was a salt-sensitive one. In wheat production, the application of exogenous MT should consider the differences among varieties of wheat during the sowing and seedling stages.

Melatonin-Mediated Abiotic Stress Tolerance in Plants

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

Abscisic Acid Mediates Grafting-Induced Cold Tolerance of Watermelon via Interaction With Melatonin and Methyl Jasmonate

Grafting is widely used to increase plant defense responses to various stresses. Grafting-induced cold tolerance is associated with the increase of the antioxidant potential of plants; however, the underlying mechanisms remain unclear. Here, we found that pumpkin rootstocks promote antioxidant enzyme activities and alleviate cold-induced oxidative damage, accompanied by increased abscisic acid (ABA), melatonin, and methyl jasmonate (MeJA) levels in leaves. Increased ABA accumulation in leaves was attributed partly to the increased ABA levels in rootstocks. ABA induced antioxidant enzymes activities and the accumulation of melatonin and MeJA, while inhibition of ABA synthesis blocked the rootstock-induced antioxidant activity and the accumulation of melatonin and MeJA under cold stress. Melatonin and MeJA application also enhanced ABA accumulation in leaves after cold exposure, whereas inhibition of melatonin or MeJA synthesis attenuated the rootstock-induced increase of ABA. Moreover, melatonin and MeJA application alleviated cold-induced oxidative stress, but inhibition of melatonin or MeJA synthesis lowered the rootstock- or ABA-induced antioxidant potential and tolerance to cold. These findings indicate that ABA plays an important role in the grafting-induced cold tolerance by promoting the accumulation of melatonin and MeJA, which in turn, promote ABA accumulation, forming a positive feedback loop.

The role of watermelon caffeic acid O-methyltransferase (ClCOMT1) in melatonin biosynthesis and abiotic stress tolerance

Melatonin is a pleiotropic signaling molecule that regulates plant growth and responses to various abiotic stresses. The last step of melatonin synthesis in plants can be catalyzed by caffeic acid O-methyltransferase (COMT), a multifunctional enzyme reported to have N-acetylserotonin O-methyltransferase (ASMT) activity; however, the ASMT activity of COMT has not yet been characterized in nonmodel plants such as watermelon (Citrullus lanatus). Here, a total of 16 putative O-methyltransferase (ClOMT) genes were identified in watermelon. Among them, ClOMT03 (Cla97C07G144540) was considered a potential COMT gene (renamed ClCOMT1) based on its high identities (60.00-74.93%) to known COMT genes involved in melatonin biosynthesis, expression in almost all tissues, and upregulation under abiotic stresses. The ClCOMT1 protein was localized in the cytoplasm. Overexpression of ClCOMT1 significantly increased melatonin contents, while ClCOMT1 knockout using the CRISPR/Cas-9 system decreased melatonin contents in watermelon calli. These results suggest that ClCOMT1 plays an essential role in melatonin biosynthesis in watermelon. In addition, ClCOMT1 expression in watermelon was upregulated by cold, drought, and salt stress, accompanied by increases in melatonin contents. Overexpression of ClCOMT1 enhanced transgenic Arabidopsis tolerance against such abiotic stresses, indicating that ClCOMT1 is a positive regulator of plant tolerance to abiotic stresses.

The Plant-Stress Metabolites, Hexanoic Aacid and Melatonin, Are Potential "Vaccines" for Plant Health Promotion

A plethora of compounds stimulate protective mechanisms in plants against microbial pathogens and abiotic stresses. Some defense activators are synthetic compounds and trigger responses only in certain protective pathways, such as activation of defenses under regulation by the plant regulator, salicylic acid (SA). This review discusses the potential of naturally occurring plant metabolites as primers for defense responses in the plant. The production of the metabolites, hexanoic acid and melatonin, in plants means they are consumed when plants are eaten as foods. Both metabolites prime stronger and more rapid activation of plant defense upon subsequent stress. Because these metabolites trigger protective measures in the plant they can be considered as "vaccines" to promote plant vigor. Hexanoic acid and melatonin instigate systemic changes in plant metabolism associated with both of the major defense pathways, those regulated by SA- and jasmonic acid (JA). These two pathways are well studied because of their induction by different microbial triggers: necrosis-causing microbial pathogens induce the SA pathway whereas colonization by beneficial microbes stimulates the JA pathway. The plant's responses to the two metabolites, however, are not identical with a major difference being a characterized growth response with melatonin but not hexanoic acid. As primers for plant defense, hexanoic acid and melatonin have the potential to be successfully integrated into vaccination-like strategies to protect plants against diseases and abiotic stresses that do not involve man-made chemicals.

Melatonin as a plant biostimulant in crops and during post-harvest: a new approach is needed

A great amount of data covering a wide variety of plant species and experimental conditions has demonstrated the beneficial actions that melatonin exerts on many aspects of plant development, including germination, photosynthesis and water economy. Melatonin behaves especially well as a plant biostimulator against biotic and abiotic stressors, increasing stress tolerance. The present contribution sets out possible future multidisciplinary studies, in which the impact of using melatonin with respect to agriculture, food technology, human nutrition and the environment needs to be clearly established. In crops, the effective dose and best formulations for individual plant species and cultivation conditions should be studied. As regards post-harvest, the focus should be on the half-life time of melatonin in fruits and water-residue treatments. Detailed studies are lacking on the human intake of phytomelatonin in different diets. Studies on the metabolization of phytomelatonin and the combined effect with other phytonutrients such as carotenoids, chlorophylls, flavonoids, fibers, etc., would also be of interest. In soils, the possible interaction between melatonin and microbiome and non-vertebrate animals is of primordial interest. In terms of the environment, although melatonin is classified as a non-hazardous agent, its limitations as a possible animal hormone disruptor have been suggested. Specific studies on the permanence of melatonin in plant tissues, plant by-products, soil, freshwater and honeybees, amongst others, are proposed to obtain crucial information. © 2021 Society of Chemical Industry.

Positive Interaction between H2O2 and Ca2+ Mediates Melatonin-Induced CBF Pathway and Cold Tolerance in Watermelon (Citrullus lanatus L.)

Cold stress is a major environmental factor that detrimentally affects plant growth and development. Melatonin has been shown to confer plant tolerance to cold stress through activating the C-REPEAT BINDING FACTOR (CBF) pathway; however, the underlying modes that enable this function remain obscure. In this study, we investigated the role of H₂O₂ and Ca²⁺ signaling in the melatonin-induced CBF pathway and cold tolerance in watermelon (Citrullus lanatus L.) through pharmacological, physiological, and genetic approaches. According to the results, melatonin induced H₂O₂ accumulation, which was associated with the upregulation of respiratory burst oxidase homolog D (ClRBOHD) during the early response to cold stress in watermelon. Besides, melatonin and H₂O₂ induced the accumulation of cytoplasmic free Ca²⁺ ([Ca²⁺]_cyt) in response to cold. This was associated with the upregulation of cyclic nucleotide-gated ion channel 2 (ClCNGC2) in watermelon. However, blocking of Ca²⁺ influx channels abolished melatonin- or H₂O₂-induced CBF pathway and cold tolerance. Ca²⁺ also induced ClRBOHD expression and H₂O₂ accumulation in early response to cold stress in watermelon. Inhibition of H₂O₂ production in watermelon by RBOH inhibitor or in Arabidopsis by AtRBOHD knockout compromised melatonin-induced [Ca²⁺]_cyt accumulation and melatonin- or Ca²⁺-induced CBF pathway and cold tolerance. Overall, these findings indicate that melatonin induces RBOHD-dependent H₂O₂ generation in early response to cold stress. Increased H₂O₂ promotes [Ca²⁺]_cyt accumulation, which in turn induces H₂O₂ accumulation via RBOHD, forming a reciprocal positive-regulatory loop that mediates melatonin-induced CBF pathway and subsequent cold tolerance.

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.

Alternative oxidase gene induced by nitric oxide is involved in the regulation of ROS and enhances the resistance of Pleurotus ostreatus to heat stress

Background

In China, during the cultivation process of Pleurotus ostreatus, the yield and quality of fruiting bodies are easily affected by high temperatures in summer. Nitric oxide (NO) plays an important regulatory role in the response to abiotic stress, and previous studies have found that NO can induce alternative oxidase (aox) experssion in response to heat stress (HS) by regulating aconitase. However, the regulatory pathway of NO is complex, and the function and regulation of the aox gene in the response to HS remain unclear.

Results

In this study, we found that NO affected nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP) levels, reduced hydrogen peroxide (H₂O₂) and superoxide anion (O₂⁻) contents, and slowed O₂⁻ production. Further RNA-Seq results showed that NO regulated the oxidation-reduction process and oxidoreductase activity, affected the cellular respiration pathway and activated aox gene expression. The function of aox was determined by constructing overexpression (OE) and RNA interference (RNAi) strains. The results showed that the OE-aox strains exhibited obviously improved growth recovery after exposure to HS. During exposure to HS, the OE-aox strains exhibited reduced levels of NADH, the product of the tricarboxylic acid (TCA) cycle, and decreased synthesis of ATP, which reduced the production and accumulation of reactive oxygen species (ROS), whereas the RNAi-aox strains exhibited the opposite result. In addition, aox mediated the expression of antioxidant enzyme genes in the mycelia of P. ostreatus under HS through the retrograde signaling pathway.

Conclusions

This study shows that the expression of the aox gene in P. ostreatus mycelia can be induced by NO under HS, that it regulates the TCA cycle and cell respiration to reduce the production of ROS, and that it can mediate the retrograde signaling pathway involved in the mycelial response to HS.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01626-y.

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.

Methyl jasmonate mediates melatonin-induced cold tolerance of grafted watermelon plants

Root-shoot communication has a critical role in plant adaptation to environmental stress. Grafting is widely applied to enhance the abiotic stress tolerance of many horticultural crop species; however, the signal transduction mechanism involved in this tolerance remains unknown. Here, we show that pumpkin- or figleaf gourd rootstock-enhanced cold tolerance of watermelon shoots is accompanied by increases in the accumulation of melatonin, methyl jasmonate (MeJA), and hydrogen peroxide (H₂O₂). Increased melatonin levels in leaves were associated with both increased melatonin in rootstocks and MeJA-induced melatonin biosynthesis in leaves of plants under cold stress. Exogenous melatonin increased the accumulation of MeJA and H₂O₂ and enhanced cold tolerance, while inhibition of melatonin accumulation attenuated rootstock-induced MeJA and H₂O₂ accumulation and cold tolerance. MeJA application induced H₂O₂ accumulation and cold tolerance, but inhibition of JA biosynthesis abolished rootstock- or melatonin-induced H₂O₂ accumulation and cold tolerance. Additionally, inhibition of H₂O₂ production attenuated MeJA-induced tolerance to cold stress. Taken together, our results suggest that melatonin is involved in grafting-induced cold tolerance by inducing the accumulation of MeJA and H₂O₂. MeJA subsequently increases melatonin accumulation, forming a self-amplifying feedback loop that leads to increased H₂O₂ accumulation and cold tolerance. This study reveals a novel regulatory mechanism of rootstock-induced cold tolerance.

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.

New tools for characterizing early brown stem rot disease resistance signaling in soybean

Brown stem rot (BSR) reduces soybean [Glycine max (L.) Merr.] yield by up to 38%. The BSR causal agent is Phialophora gregata f. sp. sojae, a slow-growing, necrotrophic fungus whose life cycle includes latent and pathogenic phases, each lasting several weeks. Brown stem rot foliar symptoms are often misdiagnosed as other soybean diseases or nutrient stress, making BSR resistance especially difficult to phenotype. To shed light on the genes and networks contributing to P. gregata resistance, we conducted RNA sequencing (RNA-seq) of a resistant genotype (PI 437970, Rbs3). Leaf, stem, and root tissues were collected 12, 24, and 36 h after stab inoculation with P. gregata, or mock infection, in the plant stem. By using multiple tissues and time points, we could see that leaves, stems, and roots use the same defense pathways. Our analyses suggest that P. gregata induces a biphasic defense response, with pathogen-associated molecular pattern (PAMP) triggered immunity observed in leaves at 12 and 24 h after infection (HAI) and effector triggered immunity detected at 36 h after infection in the stems. Gene networks associated with defense, photosynthesis, nutrient homeostasis, DNA replication, and growth are the hallmarks of resistance to P. gregata. While P. gregata is a slow-growing pathogen, our results demonstrate that pathogen recognition occurs hours after infection. By exploiting the genes and networks described here, we will be able to develop novel diagnostic tools to facilitate breeding and screening for BSR resistance.

Crosstalk between melatonin and Ca2+/CaM evokes systemic salt tolerance in Dracocephalum kotschyi

In this study, the role of calcium/calmodulin (Ca²⁺/CaM) and melatonin (Mel) as two signal molecules in inducing systemic salt tolerance of Dracocephalum kotschyi Boiss. was investigated. Salinity stress (100 mM NaCl) reduced plant growth and induced ionic, osmotic, and oxidative damages in D. kotschyi leaves. Detection of cytosolic free Ca²⁺ ([Ca²⁺]_cyt) by the Fura-2 method and the measurement of endogenous Mel by GC-MS demonstrated that salinity induced Ca²⁺ burst and increased endogenous Mel content in D. kotschyi leaves. Root pretreatment with 5 mM Ca²⁺ or 100 μM Mel recovered plant growth, reduced leaf electrolytic leakage, H₂O_2, and MDA contents and improved membrane integrity not only at the application site (roots), but also at the untreated distal parts (leaves) under salt stress. Rhizospheric treatment with Mel and Ca²⁺ triggered systemic tolerance in D. kotschyi, as judged from improving RWC, increasing proline content, modulating Na⁺, K⁺, and Ca²⁺ homeostasis, and enhancing the activities of SOD, CAT, APX, and POD in the leaves of salt-stressed plants. Mel augmented [Ca²⁺]_cyt, but the rhizospheric application of Ca²⁺ antagonists impaired the latter responses. Furthermore, root pretreatment with Ca²⁺ increased Mel content, but the application of p-chlorophenylalanine (as an inhibitor of Mel biosynthesis) decreased the above attributes in the leaves of Ca²⁺-treated plants, leading to an arrest in the Ca²⁺-induced systemic salt tolerance. These novel results suggest that interaction of Ca²⁺/CaM and Mel is involved in overcoming salt-induced ionic, osmotic, and oxidative damages and Ca²⁺ and Mel may act as long-distance signals for inducing systemic salt tolerance in D. kotschyi.

Melatonin Represses Oil and Anthocyanin Accumulation in Seeds

Previous studies have clearly demonstrated that the putative phytohormone melatonin functions directly in many aspects of plant growth and development. In Arabidopsis (Arabidopsis thaliana), the role of melatonin in seed oil and anthocyanin accumulation, and corresponding underlying mechanisms, remain unclear. Here, we found that serotonin N-acetyltransferase1 (SNAT1) and caffeic acid O-methyltransferase (COMT) genes were ubiquitously and highly expressed and essential for melatonin biosynthesis in Arabidopsis developing seeds. We demonstrated that blocking endogenous melatonin biosynthesis by knocking out SNAT1 and/or COMT significantly increased oil and anthocyanin content of mature seeds. In contrast, enhancement of melatonin signaling by exogenous application of melatonin led to a significant decrease in levels of seed oil and anthocyanins. Further gene expression analysis through RNA sequencing and reverse-transcription quantitative PCR demonstrated that the expression of a series of important genes involved in fatty acid and anthocyanin accumulation was significantly altered in snat1-1 comt-1 developing seeds during seed maturation. We also discovered that SNAT1 and COMT significantly regulated the accumulation of both mucilage and proanthocyanidins in mature seeds. These results not only help us understand the function of melatonin and provide valuable insights into the complicated regulatory network controlling oil and anthocyanin accumulation in seeds, but also divulge promising gene targets for improvement of both oil and flavonoids in seeds of oil-producing crops and plants.

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.

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.

Insights into nitric oxide-melatonin crosstalk and N-nitrosomelatonin functioning in plants

Similar to animal systems, plants have been suggested to possess both positive and antagonistic interactions between nitric oxide (NO) and melatonin. This review summarizes the current understanding of NO-melatonin crosstalk in plants with regard to redox homoeostasis, regulation of gene expression, and developmental changes. It also addresses the possible role of N-nitrosomelatonin (NOmela), which is likely to be associated with redox signaling and long-distance communication. Localization and quantification of NOmela are expected to add new insights into its precise role in plants. Methodological advances in imaging, isolation, and quantification of such a transient molecule require further attention. The quest for the biological role of NOmela in plants should lure physiologists to pursue investigations to obtain solid experimental evidence.

Melatonin-Induced Transcriptome Variation of Rapeseed Seedlings under Salt Stress

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

The bZIP gene family in watermelon: genome-wide identification and expression analysis under cold stress and root-knot nematode infection

The basic leucine zipper (bZIP) family transcription factors play crucial roles in regulating plant development and stress response. In this study, we identified 62 ClabZIP genes from watermelon genome, which were unevenly distributed across the 11 chromosomes. These ClabZIP proteins could be classified into 13 groups based on the phylogenetic relationships, and members in the same group showed similar compositions of conserved motifs and gene structures. Transcriptome analysis revealed that a number of ClabZIP genes have important roles in the melatonin (MT) induction of cold tolerance. In addition, some ClabZIP genes were induced or repressed under red light (RL) or root-knot nematode infection according to the transcriptome data, and the expression patterns of several ClabZIP genes were further verified by quantitative real-time PCR, revealing their possible roles in RL induction of watermelon defense against nematode infection. Our results provide new insights into the functions of different ClabZIP genes in watermelon and their roles in response to cold stress and nematode infection.

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

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

Application of melatonin promotes anthocyanin accumulation in crabapple leaves

Anthocyanins are a class of compounds that are widespread in plants, where they provide protection against stresses, and are also beneficial to human health as dietary components. Melatonin application is known to affect anthocyanin production, but the relationship between anthocyanin and melatonin is still unclear. In this study, we analyzed anthocyanin contents and the expression levels of anthocyanin biosynthetic and regulatory genes in tissue cultured plantlets of two Malus crabapple cultivars following various exogenous melatonin treatments under light and dark conditions. The application of exogenous melatonin not only promoted anthocyanin accumulation in leaves, but also increased the contents of flavonols and proanthocyanins (PAs), via a process that was not dependent on light. Quantitative real time PCR (qRT-PCR) analyses indicated that the expression of flavonoid biosynthetic genes, flavonoid related transcription factors and melatonin biosynthetic genes was induced by melatonin. We propose that anthocyanin biosynthesis is regulated by melatonin in crabapple leaves via the expression of flavonoid related transcription factors. This study provides insight into the mechanism of melatonin induction of anthocyanin biosynthesis in woody plants, and suggests that pretreatment with melatonin may represent a cultivation strategy to increase the flavonoid contents of plants.

Evolution of Portulacineae Marked by Gene Tree Conflict and Gene Family Expansion Associated with Adaptation to Harsh Environments

Several plant lineages have evolved adaptations that allow survival in extreme and harsh environments including many families within the plant clade Portulacineae (Caryophyllales) such as the Cactaceae, Didiereaceae, and Montiaceae. Here, using newly generated transcriptomic data, we reconstructed the phylogeny of Portulacineae and examined potential correlates between molecular evolution and adaptation to harsh environments. Our phylogenetic results were largely congruent with previous analyses, but we identified several early diverging nodes characterized by extensive gene tree conflict. For particularly contentious nodes, we present detailed information about the phylogenetic signal for alternative relationships. We also analyzed the frequency of gene duplications, confirmed previously identified whole genome duplications (WGD), and proposed a previously unidentified WGD event within the Didiereaceae. We found that the WGD events were typically associated with shifts in climatic niche but did not find a direct association with WGDs and diversification rate shifts. Diversification shifts occurred within the Portulacaceae, Cactaceae, and Anacampserotaceae, and whereas these did not experience WGDs, the Cactaceae experienced extensive gene duplications. We examined gene family expansion and molecular evolutionary patterns with a focus on genes associated with environmental stress responses and found evidence for significant gene family expansion in genes with stress adaptation and clades found in extreme environments. These results provide important directions for further and deeper examination of the potential links between molecular evolutionary patterns and adaptation to harsh environments.

Melatonin Mediates Enhancement of Stress Tolerance in Plants

Melatonin is a multifunctional signaling molecule, ubiquitously distributed in different parts of plants and responsible for stimulating several physiological responses to adverse environmental conditions. In the current review, we showed that the biosynthesis of melatonin occurred in plants by themselves, and accumulation of melatonin fluctuated sharply by modulating its biosynthesis and metabolic pathways under stress conditions. Melatonin, with its precursors and derivatives, acted as a powerful growth regulator, bio-stimulator, and antioxidant, which delayed leaf senescence, lessened photosynthesis inhibition, and improved redox homeostasis and the antioxidant system through a direct scavenging of reactive oxygen species (ROS) and reactive nitrogen species (RNS) under abiotic and biotic stress conditions. In addition, exogenous melatonin boosted the growth, photosynthetic, and antioxidant activities in plants, confirming their tolerances against drought, unfavorable temperatures, salinity, heavy metals, acid rain, and pathogens. However, future research, together with recent advancements, would support emerging new approaches to adopt strategies in overcoming the effect of hazardous environments on crops and may have potential implications in expanding crop cultivation against harsh conditions. Thus, farming communities and consumers will benefit from elucidating food safety concerns.

Foliar Accumulation of Melatonin Applied to the Roots of Maize (Zea mays) Seedlings

Plants absorb melatonin from the environments as well as they synthesize the regulatory molecule. We applied melatonin to the roots of maize (Zea mays) seedlings and examined its accumulation in the leaves. Melatonin accumulation in the leaves was proportional to the exogenously applied concentrations up to 5 mM, without saturation. Time-course analysis of the accumulated melatonin content did not show an adaptable (or desensitizable) uptake system over a 24-h period. Melatonin accumulation in the leaves was reduced significantly by the plant hormones abscisic acid (ABA) and salicylic acid (SA), which commonly cause stomatal closure. The application of ABA and benzo-18-crown-6 (18-CR, a stomata-closing agent) induced stomatal closure and simultaneously decreased melatonin content in the leaves. When plants were shielded from airflow in the growth chamber, melatonin accumulation in the leaves decreased, indicating the influence of reduced transpiration. We conclude that melatonin applied exogenously to the root system is absorbed, mobilized upward according to the transpirational flow, and finally accumulated in the leaves.

Melatonin enhances salt tolerance by promoting MYB108A-mediated ethylene biosynthesis in grapevines

The signal molecules melatonin and ethylene play key roles in abiotic stress tolerance. The interplay between melatonin and ethylene in regulating salt tolerance and the underlying molecular mechanism of this interplay remain unclear. Here, we found that both melatonin and 1-aminocyclopropane-1-carboxylic acid (ACC, a precursor of ethylene) enhanced the tolerance of grapevine to NaCl; additionally, ethylene participated in melatonin-induced salt tolerance. Further experiments indicated that exogenous treatment and endogenous induction of melatonin increased the ACC content and ethylene production in grapevine and tobacco plants, respectively. The expression of MYB108A and ACS1, which function as a transcription factor and a key gene involved in ethylene production, respectively, was strongly induced by melatonin treatment. Additionally, MYB108A directly bound to the promoter of ACS1 and activated its transcription. MYB108A expression promoted ACC synthesis and ethylene production by activating ACS1 expression in response to melatonin treatment. The suppression of MYB108A expression partially limited the effect of melatonin on the induction of ethylene production and reduced melatonin-induced salt tolerance. Collectively, melatonin promotes ethylene biosynthesis and salt tolerance through the regulation of ACS1 by MYB108A.

Genome-Wide Analysis of Watermelon HSP20s and Their Expression Profiles and Subcellular Locations under Stresses

Watermelon (Citrullus lanatus L.), which is an economically important cucurbit crop that is cultivated worldwide, is vulnerable to various adverse environmental conditions. Small heat shock protein 20s (HSP20s) are the most abundant plant HSPs and they play important roles in various biotic and abiotic stress responses. However, they have not been systematically investigated in watermelon. In this study, we identified 44 watermelon HSP20 genes and analyzed their gene structures, conserved domains, phylogenetic relationships, chromosomal distributions, and expression profiles. All of the watermelon HSP20 proteins have a conserved the α-crystallin (ACD) domain. Half of the ClHSP20s arose through gene duplication events. Plant HSP20s were grouped into 18 subfamiles and a new subfamily, nucleo-cytoplasmic XIII (CXIII), was identified in this study. Numerous stress- and hormone-responsive cis-elements were detected in the putative promoter regions of the watermelon HSP20 genes. Different from that in other species, half of the watermelon HSP20s were repressed by heat stress. Plant HSP20s displayed diverse responses to different virus infections and most of the ClHSP20s were generally repressed by Cucumber green mottle mosaic virus (CGMMV). Some ClHSP20s exhibited similar transcriptional responses to abscisic acid, melatonin, and CGMMV. Subcellular localization analyses of six selected HSP20- green fluorescence protein fusion proteins revealed diverse subcellular targeting. Some ClHSP20 proteins were affected by CGMMV, as reflected by changes in the size, number, and distribution of fluorescent granules. These systematic analyses provide a foundation for elucidating the physiological functions and biological roles of the watermelon HSP20 gene family.