The Effect of Foliar Application of Melatonin on Changes in Secondary Metabolite Contents in Two Citrus Species Under Drought Stress Conditions

Synergistic effects of melatonin and glycine betaine on seed germination, seedling growth, and biochemical attributes of maize under salinity stress

Salinity stress represents a major threat to crop production by inhibiting seed germination, growth of seedlings, and final yield and, therefore, to the social and economic prosperity of developing countries. Recently, plant growth-promoting substances have been widely used as a chemical strategy for improving plant resilience towards abiotic stresses. This study aimed to determine whether melatonin (MT) and glycine betaine (GB) alone or in combination could alleviate the salinity-induced impacts on seed germination and growth of maize seedlings. Increasing NaCl concentration from 100 to 200 mM declined seed germination rate (4.6-37.7%), germination potential (24.5-46.7%), radical length (7.7-40.0%), plumule length (2.2-35.6%), seedling fresh (1.7-41.3%) and dry weight (23.0-56.1%) compared to control (CN) plants. However, MT and GB treatments lessened the adverse effects of 100 and 150 mM NaCl and enhanced germination comparable to control plants. In addition, results from the pot experiments show that 200 mM NaCl stress disrupted the osmotic balance and persuaded oxidative stress, presented by higher electrolyte leakage, hydrogen peroxide, superoxide radicals, and malondialdehyde compared to control plants. However, compared to the NaCl treatment, NaCl+MT+GB treatment decreased the accumulation of malondialdehyde (24.2-42.1%), hydrogen peroxide (36.2-44.0%), and superoxide radicals (20.1-50.9%) by up-regulating the activity of superoxide dismutase (28.4-51.2%), catalase (82.2-111.5%), ascorbate peroxidase (40.3-59.2%), and peroxidase (62.2-117.9%), and by enhancing osmolytes accumulation, thereby reducing NaCl-induced oxidative damages. Based on these findings, the application of MT+GB is an efficient chemical strategy for improving seed germination and growth of seedlings by improving the physiological and biochemical attributes of maize under 200 mM NaCl stress.

Metabolite and Transcriptome Profiling Analysis Provides New Insights into the Distinctive Effects of Exogenous Melatonin on Flavonoids Biosynthesis in Rosa rugosa

Although the petals of Rosa rugosa are rich in flavonoids and their bioactivity has a significant impact on human health, the flavonoid content decreases during flower development. In this study, R. rugosa 'Feng hua' was used to investigate the effects of the melatonin foliar spray on enhancing the quality of rose by focusing on major flavonoids. The results showed that the contents of total flavonoids in rose petals at the full bloom stage induced by melatonin obeyed a bell-shaped curve, with a maximum at 0.3 mM, indicating the concentration-dependent up-regulation of flavonoid biosynthesis. In the treatment with 0.3 mM melatonin, metabolomic analyses showed that the concentrations of ten main flavonoids were identified to be increased by melatonin induction, with high levels and increases observed in three flavonols and two anthocyanins. KEGG enrichment of transcriptomic analysis revealed a remarkable enrichment of DEGs in flavonoid and flavonol biosynthesis, such as Rr4CL, RrF3H, and RrANS. Furthermore, functional validation using virus-induced gene silencing technology demonstrated that Rr4CL3 is the crucial gene regulating flavonoid biosynthesis in response to the stimulant of melatonin. This study provides insights into the exogenous melatonin regulation mechanism of biosynthesis of flavonoids, thereby offering potential industrial applications.

Comprehensive evaluation of drought stress on medicinal plants: a meta-analysis

Drought stress significantly affects plants by altering their physiological and biochemical processes, which can severely limit their growth and development. Similarly, drought has severe negative effects on medicinal plants, which are essential for healthcare. The effects are particularly significant in areas that rely mostly on traditional medicine, which might potentially jeopardize both global health and local economies. Understanding effects of droughts on medicinal plants is essential for developing strategies to enhance plant adaptability to drought stress, which is vital for sustaining agricultural productivity under changing climatic conditions. In this study, a meta-analysis was conducted on 27 studies examining various parameters such as plant yield, chlorophyll content, relative water content, essential oil content, essential oil yield, non-enzymatic antioxidants, enzymatic antioxidants, phenols, flavonoids, and proline content. The analysis explored the effects of drought across different stress conditions (control, moderate, and severe) to gain deeper insights into the drought's impact. The categorization of these stress conditions was based on field or soil capacity: control (100-80%), moderate (80-50%), and severe (below 50%). This classification was guided by the authors' descriptions in their studies. According to meta-analysis results, enzymatic antioxidants emerge as the most responsive parameters to stress. Other parameters such as relative water content (RWC) and yield also exhibit considerable negative mean effect sizes under all three stress conditions. Therefore, when evaluating the impacts of drought stress on medicinal plants, it is beneficial to include these three parameters (enzymatic antioxidants, RWC, and yield) in an evaluation of drought stress. The chlorophyll content has been determined not to be a reliable indicator for measuring impact of drought stress. Also, measuring antioxidants such as flavonoids and phenols could be a better option than using radical scavenging methods like DPPH (2, 2-difenil-1-pikrilhidrazil), FRAP (ferric reducing antioxidant power), and ABTS (2, 2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid)).

Synergistic effects of melatonin and 24-epibrassinolide on chickpea water deficit tolerance

BACKGROUND

Water deficiency stress reduces yield in grain legumes, primarily due to a decrease in the pods number. Melatonin (ML) and 24-epibrassinolide (EBL) are recognized for their hormone-like properties that improve plant tolerance to abiotic stresses. This study aimed to assess the impact of different concentrations of ML (0, 100, and 200 µM) and EBL (0, 3, and 6 µM) on the growth, biochemical, and physiological characteristics of chickpea plants under water-stressed conditions.

RESULTS

The study's findings indicated that under water-stressed conditions, a decrease in seed (30%) and pod numbers (31%), 100-seed weight (17%), total chlorophyll content (46%), stomatal conductance (33%), as well as an increase in H2O2 (62%), malondialdehyde content (40%), and electrolyte leakage index (40%), resulted in a 40% reduction in chickpea plants grain yield. Our findings confirmed that under water-stressed conditions, seed oil, seed oil yield, and seed protein yield dropped by 20%, 55%, and 36%, respectively. The concurrent exogenous application of ML and EBL significantly reduces oxidative stress, plasma membrane damage, and reactive oxygen species (ROS) content. This treatment also leads to increased yield and its components, higher pigment content, enhanced oil and protein yield, and improved enzymatic and non-enzymatic antioxidant activities such as catalase, superoxide dismutase, polyphenol oxidase, ascorbate peroxidase, guaiacol peroxidase, flavonoid, and carotenoid. Furthermore, it promotes the accumulation of osmoprotectants such as proline, total soluble protein, and sugars.

CONCLUSIONS

Our study found that ML and EBL act synergistically to regulate plant growth, photosynthesis, osmoprotectants accumulation, antioxidant defense systems, and maintain ROS homeostasis, thereby mitigating the adverse effects of water deficit conditions. ML and EBL are key regulatory network components in stressful conditions, with significant potential for future research and practical applications. The regulation metabolic pathways of ML and EBL in water-stressed remains unknown. As a result, future research should aim to elucidate the molecular mechanisms by employing genome editing, RNA sequencing, microarray, transcriptomic, proteomic, and metabolomic analyses to identify the mechanisms involved in plant responses to exogenous ML and EBL under water deficit conditions. Furthermore, the economical applications of synthetic ML and EBL could be an interesting strategy for improving plant tolerance.

Melatonin mediates phenolic acids accumulation in barley sprouts under MeJA stress

Phenolic acids are secondary metabolites in higher plants, with antioxidant, anticancer, and anti-aging effects on the human body. Therefore, foods rich in phenolic acids are popular. Methyl jasmonate (MeJA) promoted phenolic acids accumulation but also inhibited sprout growth. Melatonin (MT) was a new type of plant hormone that not only alleviated plants' abiotic stress, but also promoted the synthesis of plant-stimulating metabolism. This study aimed to elucidate the mechanism of exogenous MT on the growth and development, and phenolic acids metabolism of barley sprouts under MeJA treatment. The results showed that MT increased the phenolic acids content in sprouts by increasing the activities of phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase, and up-regulating the gene expression of phenylalanine ammonia-lyase, cinnamic acid 4-hydroxylase, 4-coumarate: coenzyme a ligase, and ferulic acid-5-hydroxylase. MT attenuated the growth inhibition of barley sprouts under MeJA stress by increasing the activities of regulated antioxidant enzymes and the expression of their corresponding genes. Furthermore, MT increased the NO content and induced Ca2+ burst in barley sprouts under MeJA stress. These events were inhibited by DL-4-Chlorophenylalanine. These results suggested that MT ameliorated growth inhibition and promoted the biosynthesis of phenolic acids in barley sprouts under MeJA stress.

Melatonin alleviates Hg toxicity by modulating redox homeostasis and the urea cycle in moss

Mercury (Hg) is a highly toxic metal and can cause severe damage to many organisms under natural conditions. As an effective free radical scavenger and antioxidant, Melatonin (MT) has played important protective roles in alleviating oxidative damage caused by environmental cues including heavy metal stress in plants. However, the detailed mechanisms of melatonin in alleviating Hg toxicity still remain unclear in plants. Our results showed that the application of melatonin greatly reduced the concentrations of total and intracellular Hg in Taxiphyllum taxirameum. Meanwhile, melatonin significantly improved the antioxidant capacity and thus alleviated oxidative damage to the chloroplasts of T. taxirameum under Hg stress. Metabolic pathway analysis further revealed that melatonin-treated plants exhibited higher levels of 48 metabolites, including sugars, amino acids, and lipids, than non-melatonin-treated plants under Hg stress. Additionally, we further found that melatonin addition greatly improved the concentrations of four organic acids and three amino acids (Orn, Cit and Arg) related to the urea cycle, and thereby changed the levels of putrescine (Put) and spermidine (Spd) in T. taxirameum exposed to Hg stress. Further experiments showed that the high concentration of Put dramatically caused oxidative damage under Hg stress, while Spd effectively alleviated Hg toxicity in T. taxirameum. Taken together, this study provides new insight into the underlying mechanisms of melatonin in alleviating heavy metal toxicity in plants.

Fruit crops combating drought: Physiological responses and regulatory pathways

Drought is a common stress in agricultural production. Thus, it is imperative to understand how fruit crops respond to drought and to develop drought-tolerant varieties. This paper provides an overview of the effects of drought on the vegetative and reproductive growth of fruits. We summarize the empirical studies that have assessed the physiological and molecular mechanisms of the drought response in fruit crops. This review focuses on the roles of calcium (Ca2+) signaling, abscisic acid (ABA), reactive oxygen species signaling, and protein phosphorylation underlying the early drought response in plants. We review the resulting downstream ABA-dependent and ABA-independent transcriptional regulation in fruit crops under drought stress. Moreover, we highlight the positive and negative regulatory mechanisms of microRNAs in the drought response of fruit crops. Lastly, strategies (including breeding and agricultural practices) to improve the drought resistance of fruit crops are outlined.

Exogenous Melatonin Regulates Physiological Responses and Active Ingredient Levels in Polygonum cuspidatum under Drought Stress

Polygonum cuspidatum, an important medicinal plant, is rich in resveratrol and polydatin, but it frequently suffers from drought stress in the nursery stage, which inhibits the plant's growth, active components concentrations, and the price of rhizome in the later stage. The purpose of this study was to analyze how exogenous 100 mM melatonin (MT) (an indole heterocyclic compound) affected biomass production, water potential, gas exchange, antioxidant enzyme activities, active components levels, and resveratrol synthase (RS) gene expression of P. cuspidatum seedlings growing under well-watered and drought stress conditions. The 12-week drought treatment negatively affected the shoot and root biomass, leaf water potential, and leaf gas exchange parameters (photosynthetic rate, stomatal conductance, and transpiration rate), whereas the application of exogenous MT significantly increased these variables of stressed and non-stressed seedlings, accompanied by higher increases in the biomass, photosynthetic rate, and stomatal conductance under drought versus well-watered conditions. Drought treatment raised the activities of superoxide dismutase, peroxidase, and catalase in the leaves, while the MT application increased the activities of the three antioxidant enzymes regardless of soil moistures. Drought treatment reduced root chrysophanol, emodin, physcion, and resveratrol levels, while it dramatically promoted root polydatin levels. At the same time, the application of exogenous MT significantly increased the levels of the five active components, regardless of soil moistures, with the exception of no change in the emodin under well-watered conditions. The MT treatment also up-regulated the relative expression of PcRS under both soil moistures, along with a significantly positive correlation between the relative expression of PcRS and resveratrol levels. In conclusion, exogenous MT can be employed as a biostimulant to enhance plant growth, leaf gas exchange, antioxidant enzyme activities, and active components of P. cuspidatum under drought stress conditions, which provides a reference for drought-resistant cultivation of P. cuspidatum.

Melatonin Role in Plant Growth and Physiology under Abiotic Stress

Phyto-melatonin improves crop yield by mitigating the negative effects of abiotic stresses on plant growth. Numerous studies are currently being conducted to investigate the significant performance of melatonin in crops in regulating agricultural growth and productivity. However, a comprehensive review of the pivotal performance of phyto-melatonin in regulating plant morpho-physiological and biochemical activities under abiotic stresses needs to be clarified. This review focused on the research on morpho-physiological activities, plant growth regulation, redox status, and signal transduction in plants under abiotic stresses. Furthermore, it also highlighted the role of phyto-melatonin in plant defense systems and as biostimulants under abiotic stress conditions. The study revealed that phyto-melatonin enhances some leaf senescence proteins, and that protein further interacts with the plant's photosynthesis activity, macromolecules, and changes in redox and response to abiotic stress. Our goal is to thoroughly evaluate phyto-melatonin performance under abiotic stress, which will help us better understand the mechanism by which phyto-melatonin regulates crop growth and yield.

Comparative analysis of different artificial neural networks for predicting and optimizing in vitro seed germination and sterilization of petunia

The process of optimizing in vitro seed sterilization and germination is a complicated task since this process is influenced by interactions of many factors (e.g., genotype, disinfectants, pH of the media, temperature, light, immersion time). This study investigated the role of various types and concentrations of disinfectants (i.e., NaOCl, Ca(ClO)2, HgCl2, H2O2, NWCN-Fe, MWCNT) as well as immersion time in successful in vitro seed sterilization and germination of petunia. Also, the utility of three artificial neural networks (ANNs) (e.g., multilayer perceptron (MLP), radial basis function (RBF), and generalized regression neural network (GRNN)) as modeling tools were evaluated to analyze the effect of disinfectants and immersion time on in vitro seed sterilization and germination. Moreover, non‑dominated sorting genetic algorithm‑II (NSGA‑II) was employed for optimizing the selected prediction model. The GRNN algorithm displayed superior predictive accuracy in comparison to MLP and RBF models. Also, the results showed that NSGA‑II can be considered as a reliable multi-objective optimization algorithm for finding the optimal level of disinfectants and immersion time to simultaneously minimize contamination rate and maximize germination percentage. Generally, GRNN-NSGA-II as an up-to-date and reliable computational tool can be applied in future plant in vitro culture studies.

Synergistic effects of calcium and melatonin on physiological and phytochemical attributes of Dracocephalum kotschyi genotypes under salinity stress

Since regulatory roles of calcium (Ca) and melatonin (MT) in physiological responses of plants to salinity stress are lacking, various Dracocephalum kotschyi genotypes (Bojnord, Urmia, Fereydunshahr, and Semirom) were pretreated with exogenous Ca (5 mM), MT (100 μM), and Ca + MT in the presence of salt (75 mM NaCl). In addition measuring the concentration of phenolic compounds by high performance liquid chromatography (HPLC), histochemical evaluations of essential oils and phenolic compounds in glandular trichomes of leaf samples were performed by light microscope. Salt stress reduced shoot fresh (SFW) and dry weight (SDW), leaf area (LA), relative water content (RWC), and maximum efficiency of photosystem II (F_v /F_m ), but enhanced total phenolic content (TPC) and total flavonoids content (TFC), phenolic compounds concentrations, DPPH radical scavenging capacity, electrolyte leakage (EL), proline and hydrogen peroxide (H₂ O₂ ) concentrations, and Na⁺ /K⁺ and essential oils and TPC of the glandular trichomes of leaves in all D. kotschyi genotypes. Foliar spraying of Ca, MT, and particularly Ca + MT on D. kotschyi seedlings improved SFW, SDW, RWC, TPC, TFC, proline and phenolic compounds concentrations, F_v /F_m , and DPPH radical scavenging capacity, but reduced H₂ O₂ , EL, and Na⁺ /K⁺ in the leaves and essential oils and TPC in the glandular trichomes of all genotypes under both non-stress and salt stress conditions. These findings indicate that the crosstalk between MT and Ca synergistically improves salt tolerance, TPC and TFC, phenolic compounds concentration, and essential oils accumulation in glandular trichomes of different D. kotschyi genotypes.

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

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

Exogenous melatonin ameliorates drought stress in Agropyron mongolicum by regulating flavonoid biosynthesis and carbohydrate metabolism

Drought is one of the most common abiotic stressors in plants. Melatonin (MT) is a high-efficiency and low-toxicity growth regulator that plays an important role in plant responses to drought stress. As a wild relative of wheat, Agropyron mongolicum has become an important species for the improvement of degraded grasslands and the replanting of sandy grasslands. However, the physiological and molecular mechanisms by which exogenous MT regulates drought stress in A. mongolicum remain unclear. To assess the effectiveness of MT intervention (100 mg·L^-1), polyethylene glycol 6000 was used to simulate drought stress, and its ameliorating effects on drought stress in A. mongolicum seedlings were investigated through physiology, transcriptomics, and metabolomics. Physiological analysis indicated that MT treatment increased the relative water content and chlorophyll content and decreased the relative conductivity of A. mongolicum seedlings. Additionally, MT decreased malondialdehyde (MDA) and reactive oxygen species (ROS) accumulation by enhancing antioxidant enzyme activities. The transcriptome and metabolite profiling analysis of A. mongolicum seedlings treated with and without MT under drought stress identified the presence of 13,466 differentially expressed genes (DEGs) and 271 differentially expressed metabolites (DEMs). The integrated analysis of transcriptomics and metabolomics showed that DEGs and DEMs participated in diverse biological processes, such as flavonoid biosynthesis and carbohydrate metabolism. Moreover, MT may be involved in regulating the correlation of DEGs and DEMs in flavonoid biosynthesis and carbohydrate metabolism during drought stress. In summary, this study revealed the physiological and molecular regulatory mechanisms of exogenous MT in alleviating drought stress in A. mongolicum seedlings, and it provides a reference for the development and utilization of MT and the genetic improvement of drought tolerance in plants from arid habitats.

Nitric oxide mediates melatonin-induced isoflavone accumulation and growth improvement in germinating soybeans under NaCl stress

The involvement of nitric oxide (NO) in exogenous melatonin (MT)-induced isoflavone accumulation and growth improvement in NaCl-stressed soybeans was investigated in this study. The results demonstrated that MT increased the activity of nitrate reductase (NR) and upregulated the relative expression of NR1, NR2, and nitric oxide synthase1, which subsequently led to an increase in NO content. MT and sodium nitroprusside (SNP, as an NO donor) markedly increased isoflavone content by enhancing the activities of cinnamic acid 4-hydroxylase (C4H) and phenylalanine ammonia lyase (PAL), and by upregulating gene expression of C4H, Isoflavone synthase, PAL, and Chalcone isomerase 1A, which are involved in isoflavone biosynthesis. Moreover, MT, as well as SNP, improved the growth and biomass of NaCl-treated soybeans by increasing the activities of superoxide dismutase, catalase, and peroxidase, and reducing the accumulation of H₂O₂ and O₂^•- in soybeans under NaCl stress. These MT-induced responses were entirely reversed by the supply of 4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, a specific scavenger of NO), which in turn considerably decreased endogenous NO content. These results suggest that NO acts as an important downstream signal molecule, mediating MT-induced isoflavone accumulation and growth improvement in NaCl-stressed soybeans.

Mechanism of calcium in melatonin enhancement of functional substance-phenolic acid in germinated hulless barley

Phenolic acid is a physiologically active substance that has a variety of effects on humans. Barley sprouts are often used as food ingredients to enrich phenolic acids and to further produce functional foods rich in phenolic acids. In this study, the mechanism of Ca²⁺ involvement in regulating phenolic acid biosynthesis and plant growth in barley by melatonin (MT) under NaCl stress was investigated. According to the studies, MT (25 μM) increased total calcium content, induced Ca²⁺ burst, and up-regulated the gene expression of calcium-regulated protein-dependent protein kinase and calcium-binding protein transcription-activating protease in NaCl-stressed (60 mM) barley. Exogenous MT and its combined CaCl₂ (0.4 mM) significantly promoted phenolic acid biosynthesis by increasing the activity of C4H and PAL, and induced gene expression of PAL and F5H. The addition of exogenous CaCl₂ and MT caused systemic tolerance in NaCl-stressed barley, as determined by a decrease in the fluorescence intensity of hydrogen peroxide and oxygen radical anions as well as an enhancement in the antioxidant enzyme, thus significantly increasing sprout length and fresh weight. In addition, combined use of MT with Ca²⁺ antagonists (lanthanum chloride or ethylene glycol tetraacetic acid), impaired all impacts as mentioned above. These findings imply that Ca²⁺ participated in MT-induced phenolic acid biosynthesis and growth improvement in NaCl-stressed barley.

Phytomelatonin: an unexpected molecule with amazing performances in plants

Phytomelatonin, a multifunctional molecule that has been found to be present in all plants examined to date, has an important role in plants as a modulatory agent (a biostimulator) that improves plant tolerance to both biotic and abiotic stress. We present a review of phytomelatonin that considers its roles in plant metabolism and in particular its interactions with plant hormone network. In the primary metabolism of plants, melatonin improves the rate and efficiency of photosynthesis, as well related factors such as stomatal conductance, intercellular CO2, and Rubisco activity. It has also been shown to down-regulate some senescence transcription factors. Melatonin up-regulates many enzyme transcripts related to carbohydrates (including sucrose and starch), amino acids, and lipid metabolism, optimizing N, P, and S uptake. With respect to the secondary metabolism, clear increases in polyphenol, glucosinolate, terpenoid, and alkaloid contents have been described in numerous melatonin-treated plants. Generally, the most important genes of these secondary biosynthesis pathways have been found to be up-regulated by melatonin. The great regulatory capacity of melatonin is a result of its control of the redox and plant hormone networks. Melatonin acts as a plant master regulator, up-/down-regulating different plant hormone levels and signalling, and is a key player in redox homeostasis. It has the capacity to counteract diverse critical situations such as pathogen infections and abiotic stresses, and provide plants with varying degrees of tolerance. We propose possible future applications of melatonin for crop improvement and post-harvest product preservation.

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.

Melatonin-Induced Inhibition of Shiraia Hypocrellin A Biosynthesis Is Mediated by Hydrogen Peroxide and Nitric Oxide

Melatonin (MLT), an evolutionarily conserved pleiotropic molecule, is implicated in numerous physiological processes in plants and animals. However, the effects of MLT on microbes have seldom been reported. In this study, we examined the influence of exogenous MLT on the growth and hypocrellin biosynthesis of bambusicolous fungus Shiraia sp. S9. Hypocrellin A (HA) is a photoactivated and photoinduced perylenequinone (PQ) toxin in Shiraia. Exogenous MLT at 100.00 μM not only decreased fungal conidiation and spore germination but inhibited HA contents significantly in fungal cultures under a light/dark (24 h:24 h) shift. MLT treatment was associated with higher activity of antioxidant enzymes (superoxide dismutase, catalase and peroxidase) and a marked decline in reactive oxygen species (ROS) production in the mycelia. Moreover, MLT induced endogenous nitric oxide (NO) production during the culture. The NO donor sodium nitroprusside (SNP) potentiated MLT-induced inhibition of O₂⁻ production, but NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) enhanced O₂⁻ production, whereas MLT-induced NO level was increased by the ROS scavenger vitamin C (Vc). The changes in NO and H₂O₂ were proved to be involved in the MLT-induced downregulation of the expressions of HA biosynthetic genes, leading to the suppression of HA production. This study provides new insight into the regulatory roles of MLT on fungal secondary metabolism activities and a basis for understanding self-resistance in phototoxin-producing fungi.

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

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

Exogenous Melatonin Protects Lime Plants from Drought Stress-Induced Damage by Maintaining Cell Membrane Structure, Detoxifying ROS and Regulating Antioxidant Systems

Lime is an important commercial product in tropical and subtropical regions, where drought stress is becoming one of the most severe environmental challenges in the agricultural sector. Melatonin is an antioxidant molecule that helps plants regulate their development and respond to a variety of stresses. In this research, the effects of exogenous melatonin treatments were evaluated at different concentrations (0, 50, 100, and 150 μM) on biochemical aspects and gene expression in two species of lime plants (“Mexican lime” and “Persian lime”) under normal (100% field capacity (FC)) and drought stress conditions (75% and 40% FC). The experiments were factorial and based on a completely randomized design (CRD) with four replicates. Drought stress caused electrolyte leakage (EL) as well as accumulations of hydrogen peroxide (H2O2) and malondialdehyde (MDA), indicating the occurrence of damage to cellular membranes. In contrast, the melatonin pretreatment at various concentrations reduced the levels of EL, H2O2 and MDA while mitigating the negative effects of drought stress on the two lime species. The application of melatonin (100-μM) significantly increased the level of proline content and activity of antioxidant enzymes in plants under drought stress compared to control plants. According to real-time PCR analysis, drought stress and melatonin treatment enhanced the expression of genes involved in ROS scavenging, proline biosynthesis, and cell redox regulation in both species, as compared to their respective controls. According to these findings, melatonin is able to detoxify ROS and regulate antioxidant systems, thereby protecting lime plants from drought stress-induced damages.

Melatonin in Plant Defense against Abiotic Stress

Abiotic stress adversely affects plant growth and metabolism and as such reduces plant productivity. Recognized as a major contributor in the production of reactive oxygen species (ROS), it hinders the growth of plants through induction of oxidative stress. Biostimulants such as melatonin have a multifunctional role, acting as a defense strategy in minimizing the effects of oxidative stress. Melatonin plays important role in plant processes ranging from seed germination to senescence, besides performing the function of a biostimulant in improving the plant’s productivity. In addition to its important role in the signaling cascade, melatonin acts as an antioxidant that helps in scavenging ROS, generated as part of different stresses among plants. The current study was undertaken to elaborate the synthesis and regulation of melatonin in plants, besides emphasizing its function under various abiotic stress namely, salt, temperature, herbicides, heavy metals, and drought. Additionally, a special consideration was put on the crosstalk of melatonin with phytohormones to overcome plant abiotic stress.