Exogenous Application Of Melatonin Research Articles

Exogenous Application of Melatonin Improves Drought Tolerance in Coffee by Regulating Photosynthetic Efficiency and Oxidative Damage

The protective role of melatonin in plants under abiotic stress has been reported, but little information is available on its mitigation effect on coffee (Coffea arabica) plants. The objective of this study was to determine the effect of exogenous application of 100 µM melatonin in coffee leaves under 3 months of drought stress treatment. Melatonin was found to alleviate the drought-induced damage in coffee through reducing the rate of chlorophyll degradation, electrolyte leakage, malonaldehyde content, and activating various antioxidant enzymes, such as catalase, guaiacol peroxidase, and superoxide dismutase. Melatonin application suppressed the expression of chlorophyll degradation gene PAO encoding pheophorbide a oxygenase, and upregulated the expression of photosynthetic gene RBCS2 encoding ribulose-1,5-bisphosphate oxygenase (Rubisco) protein, and a drought-related gene AREB encoding abscisic acid-responsive element binding protein. The photosynthetic efficiency of photosystem II under dark adaptation was also improved upon melatonin application in drought-stressed plants. Our results showed that both foliar spray and direct soil application of melatonin could improve drought tolerance by regulating photosynthetic efficiency and oxidative damage in C. arabica seedlings. This study provides insights in application of melatonin as a protective agent against drought stress in improvement of crop yields.

Melatonin Enhances the Waterlogging Tolerance of Prunus persica by Modulating Antioxidant Metabolism and Anaerobic Respiration

Prunus persica is considered one of the most hypoxia-intolerant stone fruits. However, waterlogging, which causes hypoxia, occurs frequently in southern China where peaches are commercially important. As useful waterlogging-tolerant rootstock for peaches is limited, effective emergency strategies are urgent to identify solutions for this problem. In this study, we assessed the effect of exogenous melatonin application at different concentrations on the physiological, metabolic, and molecular properties of peach seedlings during waterlogging. The results revealed that melatonin markedly enhanced the waterlogging tolerance of peach seedlings, especially using a 200-μM solution of melatonin, as evidenced by the tolerant phenotype including new leaves and root activity, as well as by the higher levels of relative chlorophyll content and stomatal aperture, compared with waterlogging-stressed plants. In addition, higher levels of superoxide dismutase (SOD, EC 1.15.1.1) and peroxidase (POD, EC 1.11.1.7) activity and lower levels of lipid peroxidation, hydrogen peroxide (H2O2), as well as ethylene content in melatonin-treated plants suggest that melatonin acts as an antioxidant in the context of waterlogging stress. Anaerobic respiration was controlled by melatonin through the enhanced aerenchyma and the suppressed regulation of metabolic enzymes (ADH: EC 1.1.1.1, PDC: EC 4.1.1.1, and LDH: EC 1.1.1.27) and the enzymatic and/or transcript level. Moreover, the expression levels of Ca2+ signalling and hypoxia-related ERF VII transcription factor genes were elevated by melatonin against waterlogging stress. Taken together, these results highlight that melatonin was an effective molecule for enhancement of hypoxia tolerance, particularly in peach plants, through the regulation of antioxidant enzymes and anaerobic respiration.

Melatonin-mediated regulation of anthocyanin biosynthesis and antioxidant defense confer tolerance to arsenic stress in Camellia sinensis L

Arsenic is a toxic metalloid for both animals and plants. The signaling molecule melatonin can enhance abiotic stress tolerance, but the effects of As and melatonin on tea plants and the mechanisms of resilience remain unclear. Here we report that excess As causes severe oxidative stress in tea leaves as revealed by significantly reduced maximal photochemical efficiency of photosystem-II, and increased reactive oxygen species accumulation and lipid peroxidation. However, exogenous melatonin application alleviated the As phytotoxicity and increased the anthocyanin content upto 69.4 % by selectively upregulating the expression of its biosynthetic genes such as CsCHS and CsANS. Comparison of As tolerance between two tea genotypes differing in basal levels of anthocyanin revealed that a tea cultivar with increased anthocyanin content, Zijuan (ZJ), showed enhanced tolerance to As stress compared with Longjing 43 (LJ43) that contained relatively low levels of anthocyanin. Interestingly, exogenous anthocyanin also enhanced As tolerance in LJ43, but exogenous melatonin did not improve As tolerance in ZJ genotype. Analysis of As content in tea leaves revealed that melatonin significantly reduced As content in LJ43 but not in ZJ, suggesting that melatonin-enhanced tolerance to As stress is largely dependent on the basal levels of anthocyanin in tea plants.

Exogenous Melatonin Improves Drought Tolerance in Maize Seedlings by Regulating Photosynthesis and the Ascorbate–Glutathione Cycle

Melatonin is known to exert protective effects in maize against drought stress, but the knowledge regarding interaction among the melatonin, photosynthetic efficiency, ascorbate–glutathione cycle still remains elusive. Two contrasting maize (Zea mays L.) genotypes, SD609 (drought-tolerant) and SD902 (drought-sensitive), were subjected to moderate and severe drought stress along with 100 μM melatonin treatment. Exogenous melatonin increased fluorescence curve levels for both genotypes under drought stress, and higher in SD609 than SD902. In both genotypes, melatonin application also significantly increased the photochemical efficiency of photosystem II (PSII), the effective quantum yield of PSII and photosystem I (PSI), and the electron transport rate between PSII and PSI under drought stress, but it decreased the quantum yield of energy dissipation and the donor side and acceptor side impairment of PSI under drought stress, regardless of stress severity. Exogenous melatonin also contributed to the reduction of membrane damage under drought stress, as reflected by significantly decreased levels of MDA, superoxide anions and H2O2 of both genotypes exposed to drought. Furthermore, melatonin treatment increased enzyme activity in AsA–GSH cycle, and upregulated the genes expression of related enzymes in AsA–GSH cycle. The effects of melatonin were more pronounced in SD609 than SD902. Collectively, these results indicated that exogenous melatonin application increased photosynthetic electron transport rate and accelerated the AsA–GSH cycle, which are factors that play an important role in drought tolerance in maize. The beneficial effect of melatonin on the drought-tolerance SD609 was more obvious.

Does Exogenous Application of Melatonin Ameliorate Lead Toxicity in Eruca vesicaria Plants?

MELATONIN (N-acetyl-5-methoxytryptamine) is receiving a considerable interest due to its prospected role in alleviating the heavy metals stress in plants. Thus a step toward the profitable use of melatonin (ME) in agriculture, the present study investigated the exogenous application of ME in Eurica vesicaria grown in contaminated soil amended with lead nitrate Pb(NO3)2 to produce Pb levels of 350mg/kg. Melatonin was applied by two methods: priming of seeds for 12hrs [0.0μM (P0); 25μM (P1) and 50μM (P2)] and by foliar spray at seedling foliage [0.0μM (F0); 25μM (F1) and 50μM (F2)].Rocket plants affected by Pb toxicity exhibited significant reduction in the growth characteristics (fresh and dry weights of shoot and root, and leaf area/plant), photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids), photosynthetic activity (14CO2-assimilation), and photosynthetic enzymes (phosphoenolpyruvate carboxylase and ribulose-1,5-bisphosphate-carboxylase/oxygenase). Meanwhile, exogenous application of ME appeared to ameliorate toxic effects of lead by improving the above criteria as compared to the control or Pb treated plants. These results showed that ME can promote the growth of Eurica vesicaria growing in Pb-contaminated soil by stimulating the photosynthetic activity, and the foliar spray method is the optimum at a dose of ME (50μM).

Exogenous Application of Melatonin Induces Tolerance to Salt Stress by Improving the Photosynthetic Efficiency and Antioxidant Defense System of Maize Seedling

Melatonin is a ubiquitous signaling plant hormone that plays a crucial role in regulating the growth and development of plants under stress conditions. Since a few species have been investigated to unveil the effect of exogenous melatonin on salt stress, in the current research study, we investigated the effects of melatonin by measuring the photosynthetic characteristics and antioxidant defense system of maize seedling under salt stress (200 mM NaCl), along with different concentrations of melatonin (MT1—30, MT2–60, and MT3–90 µM) treatments. Salt stress reduced the plant growth characteristics and photosynthetic efficiency by increasing the ROS accumulation and reducing the antioxidant enzyme activities of maize seedling. However, pretreatment with melatonin on roots alleviated NaCl-induced decrease in photosynthetic rate and oxidative stress in a dose-dependent manner. Our results revealed that exogenous application of melatonin at an optimum concentration (60 µM) under salt stress conditions significantly increased the growth of plants, chlorophyll content, photosynthetic efficiency, antioxidant enzymes activities, i.e., superoxide dismutase (SOD), peroxide (POD), catalase (CAT) and ascorbate peroxidase (APX), and reduced the reactive oxygen species (ROS). This study unraveled the crucial role of melatonin in salt stress mitigation and thus can be implicated in the management of salinity in maize seedling.

Melatonin improves the germination rate of cotton seeds under drought stress by opening pores in the seed coat.

The germination of cotton (Gossypium hirsutum L.) seeds is affected by drought stress; however, little is known about the physiological mechanism affecting germination and the effect of melatonin (MT) on cotton seed germination under drought stress. Therefore, we studied the effects of exogenous MT on the antioxidant capacity and epidermal microstructure of cotton under drought stress. The results demonstrated a retarded water absorption capacity of testa under drought stress, significantly inhibiting germination and growth in cotton seeds. Drought stress led to the accumulation of reactive oxygen species (ROS), malondialdehyde (MDA), and osmoregulatory substances (e.g., proline, soluble protein, and soluble sugars); it also decreased the activity of antioxidant enzymes and α-amylase. Drought stress inhibited gibberellin acid (GA3) synthesis and increased abscisic acid (ABA) content, seriously affecting seed germination. However, seeds pre-soaked with MT (100 µM) showed a positive regulation in the number and opening of stomata in cotton testa. The exogenous application of MT increased the germination rate, germination potential, radical length, and fresh weight, as well as the activities of superoxide dismutase (SOD), peroxidase (POD), and α-amylase. In addition, MT application increased the contents of organic osmotic substances by decreasing the hydrogen peroxide (H2O2), superoxide anion (O2-), and MDA levels under drought stress. Further analysis demonstrated that seeds pre-soaked with MT alleviated drought stress by affecting the ABA and GA3 contents. Our findings show that MT plays a positive role in protecting cotton seeds from drought stress.

Melatonin Regulatory Mechanisms and Phylogenetic Analyses of Melatonin Biosynthesis Related Genes Extracted from Peanut under Salinity Stress.

Melatonin improves the tolerance of plants to various environmental stresses by protecting plant cells against oxidative stress damage. The objective of the current study was to determine whether exogenous melatonin (MT) treatments could help protecting peanut (Arachis hypogaea) seedlings against salinity stress. This was achieved by investigating enzymatic and non-enzymatic antioxidant systems and the expression of melatonin biosynthesis related genes in response to salinity stress with or without exogenous MT. The results showed a significant increase in the concentrations of reactive oxygen species (ROS) in peanut seedlings under salinity stress. The exogenous application of melatonin decreased the levels of ROS through the activation of antioxidant enzymes in peanut seedlings under salinity stress. Transcription levels of melatonin biosynthesis related genes such as N-acetylserotonin methyltransferase (ASMT1, ASMT2, ASMT3), tryptophan decarboxylase (TDC), and tryptamine 5-hydroxylase (T5H) were up-regulated with a 150 µM melatonin treatment under salinity stress. The results indicated that melatonin regulated the redox homeostasis by its ability to induce either enzymatic or non-enzymatic antioxidant systems. In addition, phylogenetic analysis of melatonin biosynthesis genes (ASMT1, ASMT2, ASMT3, TDC, T5H) were performed on a total of 56 sequences belonging to various plant species including five new sequences extracted from Arachis hypogaea (A. hypogaea). This was based on pairwise comparison among aligned nucleotides and predicted amino acids as well as on substitution rates, and phylogenetic inference. The analyzed sequences were heterogeneous and the A. hypogaea accessions were primarily closest to those of Manihot esculenta, but this needs further clarification.

Melatonin induced changes in photosynthetic efficiency as probed by OJIP associated with improved chromium stress tolerance in canola (Brassica napus L.)

Chromium toxicity is considered as a major problem for agricultural soil that reduced crop productivity by affecting photosynthetic tissues. Exogenous application of melatonin can alleviate the adverse effects of chromium toxicity on plant growth. However, little is known about its effect on thylakoidal protein complexes responsible for conversion of solar energy to biochemical energy. Chlorophyll fluorescence a transients considered one of the best non-invasive and rapid method for the evaluation of photosynthetic (Photosystem II) efficiency of plants and plant health under environmental stress conditions. In the present study, three-week old plants of two canola cultivars AC-Excel and DGL were applied to melatonin (0, 1, 5, 10 μM) when grown under chromium stress (0, 50 and 100 μM) for further two weeks. Chromium stress reduced the growth (fresh and dry weights of shoots and roots) of both canola cultivars and exogenous application of 5 and 10 μM melatonin improved the growth of canola at 50 or 100 μM chromium stress. This improvement was greater in cv DGL than in AC-Excel. Increasing chromium decreased the photosynthetic pigments (chlorophyll a and chlorophyll b). However, 5 and 10 μM melatonin application improved chlorophyll a at 50 μM chromium stress. Structural stability and efficiency of photosystem II (PSII) measured as performance index (PIABS) and ratios of fluorescence (Fv/Fm, Fv/Fo) Fv decreased due to chromium stress. JIP-test parameters showed that chromium stress increased the absorption and trapping fluxes with decrease in electron transport fluxes which caused the damage to reaction centers (RC), detachment of oxygen evolving complex (OEC) from RC or inefficiency of electron transfer from OEC to RC. Such adverse effects were greater in cv AC-Excel. However exogenous application of melatonin improved PIABS, electron transport per reaction center (ET/RC), reduced variable fluorescence at J step (VJ) reflecting melatonin protected PSII from chromium stress induced damage by protecting OEC. Thus, OJIP fluorescence transients are quite helpful for understanding the intersystem electron transport beyond photosystem II in canola cultivars due to melatonin application under chromium stress. FindingsExogenous application of melatonin alleviated toxic effects of chromium on plant growth of canola by modulating photosynthesis, enhanced photosystem II efficiency and regulation of electron transport flux to protect photo-inhibition of PSII from oxidative damage.

Transcriptomic analysis of short-term heat stress response in Pinellia ternata provided novel insights into the improved thermotolerance by spermidine and melatonin

Heat stress has been a major environmental factor limiting the growth and development of Pinellia ternata which is an important Chinese traditional medicine. It has been reported that spermidine (SPD) and melatonin (MLT) play pivotal roles in modulating heat stress response (HSR). However, the roles of SPD and MLT in HSR of P. ternata, and the potential mechanism is still unknown. Here, exogenous SPD and MLT treatments alleviated heat-induced damages in P. ternata, which was supported by the increased chlorophyll content, OJIP curve, and relative water content, and the decreased malondialdehyde and electrolyte leakage. Then, RNA sequencing between CK (control) and Heat (1 h of heat treatment) was conducted to analyze how genes were in response to short-term heat stress in P. ternata. A total of 14,243 (7870 up- and 6373 down-regulated) unigenes were differentially expressed after 1 h of heat treatment. Bioinformatics analysis revealed heat-responsive genes mainly included heat shock proteins (HSPs), ribosomal proteins, ROS-scavenging enzymes, genes involved in calcium signaling, hormone signaling transduction, photosynthesis, pathogen resistance, and transcription factors such as heat stress transcription factors (HSFs), NACs, WRKYs, and bZIPs. Among them, PtABI5, PtNAC042, PtZIP17, PtSOD1, PtHSF30, PtHSFB2b, PtERF095, PtWRKY75, PtGST1, PtHSP23.2, PtHSP70, and PtLHC1 were significantly regulated by SPD or MLT treatment with same or different trends under heat stress condition, indicating that exogenous application of MLT and SPD might enhance heat tolerance in P. ternata through regulating these genes but may with different regulatory patterns. These findings contributed to the identification of potential genes involved in short-term HSR and the improved thermotolerance by MLT and SPD in P. ternata, which provided important clues for improving thermotolerance of P. ternata.

Combined Application of Arbuscular Mycorrhizal Fungi and Exogenous Melatonin Alleviates Drought Stress and Improves Plant Growth in Tobacco Seedlings

Studies have shown that the application of arbuscular mycorrhizal (AM) fungi or exogenous melatonin (MT) can alleviate drought stress and improve plant growth, but the additive effects of both treatments on plants grown under drought stress are largely unknown. In this study, we conducted a pot experiment to investigate the effects of AM inoculation (Funneliformis mosseae BGC XJ01) and/or MT application on tobacco (Nicotiana tabacum L. cv. Yuyan No. 6) seedling growth, photosynthetic and chlorophyll fluorescence parameters, antioxidant enzymatic activity, osmotic adjustment substance accumulation, and nutrient uptake under three water conditions (75–80%, 50–55%, and 30–35% of the maximum moisture retention capacity). The results show that applying either the AM inoculant or MT alone significantly increased tobacco seedling growth and decreased the negative effects of drought stress. Furthermore, AM inoculation alone promoted root function (root biomass, root/shoot ratio, root system architecture), facilitated the capture and conversion of solar energy (photosynthetic rate, ΦPSII), and increased nutrient uptake more effectively than MT. In contrast, exogenous MT application alone was more effective at increasing peroxidase and catalase activity and decreasing H2O2 and MDA accumulation, which in turn enhanced the adaptation of seedlings to drought stress by improving their antioxidant capacity and reducing oxidative damage. Nevertheless, applying exogenous MT significantly enhanced the AM colonization rate under AM inoculation conditions but had no obvious effect on AM colonization under noninoculated conditions. The combined application of AM and MT had an additive effect and produced the largest increases in tobacco seedling growth, photosynthetic ability, antioxidant enzymatic activity, and N, P, and K uptake and the largest decreases in H2O2 and MDA contents of all the treatments. The results suggest that AM inoculation in combination with exogenous MT application may render plants more productive and more tolerant of drought stress.

Melatonin and plant growth-promoting rhizobacteria alleviate the cadmium and arsenic stresses and increase the growth of Spinacia oleracea L.

Melatonin (N-acetyl-5-methoxytryptamine) is a recently discovered natural product that helps the plant to cope with environmental stresses. In the same way, plant growth-promoting rhizobacteria colonise plant roots and enhance plant stress tolerance. To study the impact of exogenous melatonin and Bacillus licheniformis on the growth of Spinacia oleracea L. seedlings were treated with 100 µmol exogenous melatonin and B. licheniformis under cadmium (Cd) and arsenic (As) stresses by a pot experiment. Different plant growth parameters, antioxidant enzymes, and lipid peroxidation were studied. The results showed that melatonin application and B. licheniformis inoculation alleviated As and Cd toxicity by significantly reducing the negative impacts of stresses and increasing the fresh and dry weight as well as preventing the damage to the chlorophyll content of S. oleracea L. Moreover, supplementation of melatonin, and B. licheniformis, enhanced activities of antioxidant enzymes superoxide dismutase, peroxidase, catalase, thus acting as a line of defense against As and Cd stresses. Similarly, lipid peroxidation was also inhibited by exogenous melatonin and B. licheniformis inoculation. Exogenous application of melatonin and inoculating roots of S. oleracea L. with B. licheniformis found to ameliorate the harmful effects of As and Cd contamination.

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.

Exogenous application of melatonin maintains storage quality of jujubes by enhancing anti-oxidative ability and suppressing the activity of cell wall-degrading enzymes

Jujube fruit was treated with melatonin to evaluate the changes of quality parameters, anti-oxidative ability and cell wall-degrading enzymes activities during room temperature storage. The results showed that melatonin treatment significantly inhibited respiratory rate and ethylene release of jujubes. Melatonin treatment also delayed the changes of color and sustained titratable acidity. Moreover, melatonin treatment increased the activities of glutathione reductase, ascorbate peroxidase and monodehydroascorbate reductase, and enhanced ascorbic acid, dehydroascorbic acid and reduced glutathione contents in jujubes. In addition, application of melatonin delayed fruit softening by inhibiting the activities of pectin methylesterase, polygalacturonase, cellulose and β-glucosidase and production of water soluble pectin and maintaining water insoluble pectin content. All these results indicate that application of melatonin after harvest could delay fruit softening, improve anti-oxidative capacity, and thus delay fruit senescence of jujubes.

Role of Melatonin in Arbuscular Mycorrhizal Fungi-Induced Resistance to Fusarium Wilt in Cucumber.

Melatonin is a multifunctional molecule that confers tolerance to a number of biotic and abiotic stresses in plants. However, the role of melatonin in plant response to Fusarium oxysporum and the interaction with arbuscular mycorrhizal fungi (AMF) remain unclear. Here we show that exogenous melatonin application promoted the AMF colonization rate in cucumber roots, which potentially suppressed Fusarium wilt as evidenced by a decreased disease index and an increased control effect. Leaf gas exchange analysis revealed that Fusarium inoculation significantly decreased the net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentrations (Ci), and transpiration rate (Tr). Intriguingly, either melatonin application or AMF inoculation significantly increased the Pn, Gs, Tr, and dry biomass, and their combined treatment showed a more profound effect under Fusarium stress. Further analysis showed that Fusarium induced oxidative stress as evidenced by increased lipid peroxidation and electrolyte leakage. Conversely, either melatonin or AMF drastically attenuated the levels of malondialdehyde, H2O2, and electrolyte leakage in Fusarium-inoculated plants, and their combined treatment caused a further decrease. Fusarium inoculation decreased the activity and transcripts of superoxide dismutase and ascorbate peroxidase, and the content of glutathione and proline. Besides, the activity and transcripts of peroxidase and catalase, the content of phenols and flavonoids increased after Fusarium infection. Importantly, melatonin and/or AMF significantly increased those parameters with the greatest effect with their combined treatment under Fusarium stress. Our results suggest that a positive collaboration between melatonin and AMF enhances resistance to Fusarium wilt in cucumber plants.

Melatonin: Awakening the Defense Mechanisms during Plant Oxidative Stress.

Melatonin is a multifunctional signaling molecule that is ubiquitously distributed in different parts of a plant and responsible for stimulating several physio-chemical responses to adverse environmental conditions. In this review, we show that, although plants are able to biosynthesize melatonin, the exogenous application of melatonin to various crops can improve plant growth and development in response to various abiotic and biotic stresses (e.g., drought, unfavorable temperatures, high salinity, heavy metal contamination, acid rain, and combined stresses) by regulating antioxidant machinery of plants. Current knowledge suggests that exogenously applied melatonin can enhance the stress tolerance of plants by regulating both the enzymatic and non-enzymatic antioxidant defense systems. Enzymic antioxidants upregulated by exogenous melatonin include superoxide dismutase, catalase, glutathione peroxidase, and enzymes involved in the ascorbate–glutathione cycle (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase), whereas levels of non-enzymatic antioxidants such as ascorbate, reduced glutathione, carotenoids, tocopherols, and phenolics are also higher under stress conditions. The enhanced antioxidant system consequently exhibits lower lipid peroxidation and greater plasma membrane integrity when under stress. However, these responses vary greatly from crop to crop and depend on the intensity and type of stress, and most studies to date have been conducted under controlled conditions. This means that a wider range of crop field trials and detailed transcriptomic analysis are required to reveal the gene regulatory networks involved in the between melatonin, antioxidants, and abiotic stress.

Melatonin Mitigates Chilling-Induced Oxidative Stress and Photosynthesis Inhibition in Tomato Plants.

Melatonin has been demonstrated to play a variety of roles in plants. Of particular importance is its role as a potent antioxidative agent. In the present study, we generated melatonin-deficient tomato plants using virus-induced gene silencing (VIGS) approach and melatonin-rich tomato plants by foliar application of melatonin. These tomato plants were used to assess the effect of melatonin on chilling-induced oxidative stress and chilling-induced photosynthesis inhibition. We found that melatonin deficiency increased accumulation of reactive oxygen species (ROS) and aggravated lipid peroxidation in chilling-stressed tomato leaves, while exogenous application of melatonin had the opposite effect. Under chilling stress, melatonin-deficient tomato plants showed impaired antioxidant capacity as evidenced by lower activities of antioxidant enzymes and decreased rations of reduced glutathione (GSH)/oxidized glutathione (GSSG) and reduced ascorbate (AsA)/oxidized ascorbate (DHA), compared with melatonin-rich tomato plants. Furthermore, suppression of melatonin biosynthesis led to more photosynthesis inhibition under the chilling condition and compromised the capability of subsequent photosynthesis recovery in tomato plants. In addition, melatonin-deficient tomato plants displayed less activity of an important Calvin-Benson cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase) than melatonin-rich tomato plants under chilling stress. Collectively, our data indicate that melatonin is critical for antioxidant capacity and redox balance and is in favor of photosynthesis in tomato plants under chilling stress.

Exogenous melatonin mitigates the salinity damages and improves the growth of pistachio under salinity stress

This study investigated the ability of exogenous melatonin (MT) to alleviate salinity stress in pistachio seedlings (Pistacia vera cv. Badami-Zarand) in a controlled greenhouse. Seedlings were treated with different salinity levels (25, 50, and 100 and 150 mM). Foliar application of melatonin with 0, 25, 50, 75, 100, 125 and 150 μmol/L concentrations was applied two times during the seeding treated with the various salinity levels. The results indicated that the deleterious effects of MT application were observed when they were applied under non-stressful conditions (control). The greatest protective effect against simulated salinity stress was observed in 100 μmol/L of MT in 150 mM salinity levels. Melatonins markedly alleviated the inhibitory effects of salinity on root and shoot growth, chlorophyll level and ion leakage. Activities of antioxidant enzymes, proline content, endogenous polyamine and melatonin increased with exogenous application of MT in salinity stress condition. Exogenous melatonin was also associated with significant increases in the nutrient uptake. Therefore, exogenous melatonin can attenuate the salinity damage via enhancing anti-oxidation ability, osmotic activity-adjusting, nutrient uptake and polyamines biosynthesis.

Interactive effects of exogenous melatonin and Rhizophagus intraradices on saline-alkaline stress tolerance in Leymus chinensis.

Melatonin, a ubiquitous molecule found in almost all organisms, is considered an important regulator in plant growth. However, little is known about the interactive effect of melatonin and arbuscular mycorrhizal (AM) fungi on plant resistance against soil salinity and alkalinity. To fill in such a gap in knowledge, we conducted three experiments to explore (1) whether exogenous melatonin and an AM fungus had interactive effects on plant response to saline-alkaline stress, (2) whether the influence of melatonin on mycorrhizal plant stress tolerance was attributable to effect on the AM fungus, and (3) whether the effect of melatonin application was due to changes in soil salinity and alkalinity. We found interactive effects between melatonin and the AM fungus on alleviating ROS burst, decreasing malondialdehyde content and protecting Leymus chinensis photosynthetic activity through activation of antioxidant enzyme and gene expression (superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) in plant shoots and roots. Our results showed that exogenous melatonin promoted spore germination and hyphal length of the AM fungus under Petri-dish conditions. However, exogenous melatonin application did not exhibit significant effects on soil salinity and alkalinity. This study provides an insight into the beneficial effects of exogenous melatonin on saline-alkaline stress tolerance in mycorrhizal L. chinensis through regulating antioxidant systems, protecting photosynthetic activity, and promoting associated AM fungal growth without changing soil salinity and alkalinity. It also reveals potential applications of exogenous melatonin and AM fungi forthe restoration of saline-alkaline degraded grassland.

Melatonin alleviates iron stress by improving iron homeostasis, antioxidant defense and secondary metabolism in cucumber

Iron is an essential element for plants and animals; however, deficit or excess iron can result in stress on plants. In this study, we unveiled the mechanisms of shoot-based tolerance to iron toxicity by melatonin (N-acetyl-5-methoxytryptamine) in cucumber plants. Both low and high iron (Fe) supply in hydroponics decreased growth and biomass accumulation, induced chlorosis and oxidative stress, and reduced chlorophyll content, photosynthesis rate and transpiration rate in cucumber leaves. Notably, the negative effect of low-Fe was more profound than that of high-Fe treatment. However, exogenous melatonin application alleviated those inhibitions in growth, biochemical and physiological parameters, which entailed an elevation of endogenous melatonin content and a reduction of electrolyte leakage, reactive oxygen species accumulation and lipid peroxidation by improving the activity and transcripts of antioxidant enzymes and secondary metabolism-related enzymes, and concentrations of phenols and flavonoids under low and high iron conditions after melatonin treatment. Analysis of iron content in leaves and roots revealed that melatonin significantly increased the iron content under low-Fe conditions, but it decreased the same under high-Fe conditions. Moreover, melatonin increased the transcript levels of FRO2 and IRT1 under low-Fe, but it decreased those transcripts under high-Fe, suggesting that melatonin plays a dual role in iron uptake under low and high iron conditions. This study expands the stress ameliorative role of melatonin in plants and may have potential implications in agronomic management of crops in low and high iron prone soils.