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

Melatonin confers thermotolerance and antioxidant capacity in Chinese cabbage

Due to the damaging effect of high temperatures on plant development, global warming is predicted to increase agricultural risks. Chinese cabbage holds considerable importance as a leafy vegetable that is extensively consumed and cultivated worldwide. Its year-round production also encounters severe challenges in the face of high temperatures. In this study, melatonin (MT), a pivotal multifunctional signaling molecule that coordinates responses to diverse environmental stressors was used to mitigate the harmful effects of high temperatures on Chinese cabbage. Through the utilization of growth indices, cytological morphology, physiological and biochemical responses, and RNA-Seq analysis, alongside an examination of the influence of crucial enzymes in the endogenous MT synthesis pathway on the thermotolerance of Chinese cabbage, we revealed that MT pretreatment enhanced photosynthetic activity, maintained signaling pathways associated with endoplasmic reticulum protein processing, and preserved circadian rhythm in Chinese cabbage under high temperatures. Furthermore, pretreatment with MT resulted in increased levels of soluble sugar, vitamin C, proteins, and antioxidant enzyme activity, along with decreased levels of malondialdehyde, nitrate, flavonoids, and bitter glucosinolates, ultimately enhancing the capacity of the organism to mitigate oxidative stress. The knockdown of the tryptophan decarboxylase gene, which encodes a key enzyme responsible for MT biosynthesis, resulted in a significant decline in the ability of transgenic Chinese cabbage to alleviate oxidative damage under high temperatures, further indicating an important role of MT in establishing the thermotolerance. Taken together, these results provide a mechanism for MT to improve the antioxidant capacity of Chinese cabbage under high temperatures and suggest beneficial implications for the management of other plants subjected to global warming.

Health Benefits of High Voltage Electrostatic Field Processing of Fruits and Vegetables

High voltage electrostatic field processing (HVEF) is a food preservation procedure frequently used to produce healthy minimally processed fruits and vegetables (F&V) as it reduces the growth of microorganisms and activates or inhibits various enzymes, thus retarding their natural ripening while preserving and even enhancing native nutritional quality and sensory characteristics. HVEF is one of the various nonthermal processing technology (NTPT) regarded as abiotic stress that can activate the antioxidant system of F&V and can also inhibith spoilage enzymes as, polyphenol oxidase (PPO), lipoxygenase (LOX), pectin methylesterase (PME), polygalacturonase (PG), cellulase (Cel), β-xylosidase, xyloglucan and endotransglycosylase/hydrolase, bringing positive effect on hardness, firmness, colour attributes, electric conductivity, antioxidant compounds, microstructure and decreasing electrolyte leakage (EL), malondialdehyde (MDA) contents and browning degree. This technique can also increase the contents of fructose, glucose, and sucrose and decrease the production of CO2 and H2O2. Additionally, it has been reported that HVEF could be used with other treatments, such as modified atmosphere packaging (MAP) and acidic electrolyzed water (AEW) treatment, to enhance its effects. Future works should deepen on elucidating the activation of the antioxidant systems by applying HVEF of critical enzymes related to the synthesis pathways of phenolic compounds (PC) and carotenoids (Car). Holistic approaches to the effects of HVEF on metabolism based on systems biology also need to be studied by considering the overall biochemical, physical, and process engineering related aspects of this technique.

Dual sources of melatonin and evidence for different primary functions

This article discusses data showing that mammals, including humans, have two sources of melatonin that exhibit different functions. The best-known source of melatonin, herein referred to as Source #1, is the pineal gland. In this organ, melatonin production is circadian with maximal synthesis and release into the blood and cerebrospinal fluid occurring during the night. Of the total amount of melatonin produced in mammals, we speculate that less than 5% is synthesized by the pineal gland. The melatonin rhythm has the primary function of influencing the circadian clock at the level of the suprachiasmatic nucleus (the CSF melatonin) and the clockwork in all peripheral organs (the blood melatonin) via receptor-mediated actions. A second source of melatonin (Source # 2) is from multiple tissues throughout the body, probably being synthesized in the mitochondria of these cells. This constitutes the bulk of the melatonin produced in mammals and is concerned with metabolic regulation. This review emphasizes the action of melatonin from peripheral sources in determining re-dox homeostasis, but it has other critical metabolic effects as well. Extrapineal melatonin synthesis does not exhibit a circadian rhythm and it is not released into the blood but acts locally in its cell of origin and possibly in a paracrine matter on adjacent cells. The factors that control/influence melatonin synthesis at extrapineal sites are unknown. We propose that the concentration of melatonin in these cells is determined by the subcellular redox state and that melatonin synthesis may be inducible under stressful conditions as in plant cells.

Drought and heat stress: insights into tolerance mechanisms and breeding strategies for pigeonpea improvement

MAIN CONCLUSION

Pigeonpea has potential to foster sustainable agriculture and resilience in evolving climate change; understanding bio-physiological and molecular mechanisms of heat and drought stress tolerance is imperative to developing resilience cultivars. Pigeonpea is an important legume crop that has potential resilience in the face of evolving climate scenarios. However, compared to other legumes, there has been limited research on abiotic stress tolerance in pigeonpea, particularly towards drought stress (DS) and heat stress (HS). To address this gap, this review delves into the genetic, physiological, and molecular mechanisms that govern pigeonpea's response to DS and HS. It emphasizes the need to understand how this crop combats these stresses and exhibits different types of tolerance and adaptation mechanisms through component traits. The current article provides a comprehensive overview of the complex interplay of factors contributing to the resilience of pigeonpea under adverse environmental conditions. Furthermore, the review synthesizes information on major breeding techniques, encompassing both conventional methods and modern molecular omics-assisted tools and techniques. It highlights the potential of genomics and phenomics tools and their pivotal role in enhancing adaptability and resilience in pigeonpea. Despite the progress made in genomics, phenomics and big data analytics, the complexity of drought and heat tolerance in pigeonpea necessitate continuous exploration at multi-omic levels. High-throughput phenotyping (HTP) is crucial for gaining insights into perplexed interactions among genotype, environment, and management practices (GxExM). Thus, integration of advanced technologies in breeding programs is critical for developing pigeonpea varieties that can withstand the challenges posed by climate change. This review is expected to serve as a valuable resource for researchers, providing a deeper understanding of the mechanisms underlying abiotic stress tolerance in pigeonpea and offering insights into modern breeding strategies that can contribute to the development of resilient varieties suited for changing environmental conditions.

Silicon ameliorates cadmium (Cd) toxicity in pearl millet by inducing antioxidant defense system

Cadmium (Cd) stress is a significant environmental pollutant that can negatively impact crop yield and growth, and is a serious global issue. However, silicon (Si) has been shown to have a potential function in alleviating the effects of several abiotic stress conditions on crops, including Cd stress. This study investigated the effectiveness of applying silicon to soil as a method for reducing cadmium toxicity in pearl millet (IP14599) seedlings. Seeds of IP14599 were treated with Si + Cd element which cumulated to a combination of 9 treatments. Different Cd concentration of (0, 200, and 300 mg/kg-1) was taken and manually mixed into a sieved soil prior to planting and Si (0, 100 and 200 mg/kg-1) was selectively introduced till after attaining 12 days of seedling emergence. The physiochemical parameters of Cd stressed plants investigated includes chlorophyll, gas exchange attributes, proline, relative water contents, malondialdehyde (MDA) content and antioxidant enzymes (superoxide dismutase (SOD),catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD). Our result revealed that the metal (Cd) caused serious oxidative impairment thereby reducing photosynthetic performance, increased activity of MDA and Cd content in the roots and leaves of IP14599.In addition, Si increased the growth pattern and antioxidant defense action thereby mitigating the Cd toxicity. The results revealed that at Si 200, Si significantly increased the chlorophyll, carotenoids and plant height at 122 %, 69 % and 128 % under the Cd 200 and Cd 300 mg/kg-1 treatment, respectively. The single treatment at Si100 and Si 200 decreased ROS by 29 %, and 37 % respectively and MDA decreased by 33 % and 43 % in contrast to Cd 200 and 300 treatments, respectively. However, Si200 showed significant increase in the activities of APX 97 %, SOD by 89 %, CAT 35 % and POD 86 % as compared to single Si, Cd or combine Cd + Si treatment. Also, a gradual decline in Cd level in both the leaf and root was present when exposed to high concentrations of Si at Si200 and 300 mg/kg-1. Our findings revealed that Si might significantly increase the capacity to tolerate Cd stress in crop plants. Therefore, the study revealed that Si has the potential to alleviate Cd-induced toxicity by reducing Cd assimilation and enhancing the growth attributes of IP14599 plants.

Organic amendment-mediated reclamation and build-up of soil microbial diversity in salt-affected soils: fostering soil biota for shaping rhizosphere to enhance soil health and crop productivity

Soil salinization is a serious environmental problem that affects agricultural productivity and sustainability worldwide. Organic amendments have been considered a practical approach for reclaiming salt-affected soils. In addition to improving soil physical and chemical properties, organic amendments have been found to promote the build-up of new halotolerant bacterial species and microbial diversity, which plays a critical role in maintaining soil health, carbon dynamics, crop productivity, and ecosystem functioning. Many reported studies have indicated the development of soil microbial diversity in organic amendments amended soil. But they have reported only the development of microbial diversity and their identification. This review article provides a comprehensive summary of the current knowledge on the use of different organic amendments for the reclamation of salt-affected soils, focusing on their effects on soil properties, microbial processes and species, development of soil microbial diversity, and microbial processes to tolerate salinity levels and their strategies to cope with it. It also discusses the factors affecting the microbial species developments, adaptation and survival, and carbon dynamics. This review is based on the concept of whether addition of specific organic amendment can promote specific halotolerant microbe species, and if it is, then which amendment is responsible for each microbial species' development and factors responsible for their survival in saline environments.

Plant growth and stress-regulating metabolite response to biochar utilization boost crop traits and soil health

Introduction

The utilization of biochar (BC) as a soil amendment in agriculture has gained significant traction among many farmers and researchers, primarily due to its eco-friendly role in boosting crop output. However, the performance of specific metabolites (e.g., zeatin, melatonin, sucrose, and phenyllactic acid) in the various tissues of sugarcane plant (leaf, stem, and root) and rhizosphere soil-deemed plant growth and stress regulators in a long-term BC-amended field remains poorly understood. Additionally, literature on the shift in soil attributes and crop growth triggered by the strong response of these bioactive compounds to longterm BC utilization remains undocumented.

Methods

Metabolome integrated with highthroughput sequencing analyses were conducted to identify and quantify the performance of plant growth and stress-regulating metabolites in a long-term BC-amended field. Additionally, we investigated how the response of these compounds to BC-treated soil influences crop traits and soil biochemical properties.

Results

We also identified and quantified the performance of pathogenic bacteria and unraveled the association between these compounds and potential plant growth-promoting bacteria. The BC-supplemented soil significantly boosted the crop traits, including brix, sucrose content, and chlorophyll, as well as soil nutrients, such as soil total nitrogen (TN), ammonium (NH4 +-N), and nitrate (NO3 --N). We also noticed that metabolite-deemed plant growth and stress regulators, including melatonin and phenyllactic acid, were enriched considerably in the stem and root tissues of the BC-amended soil. Zeatin in the leaf, stem, and root tissues exhibited the same trend, followed by sucrose in the leaf tissue of the BC-treated soil, implying that the strong response of these compounds to BC utilization contributed to the promotion of crop traits and soil quality. Pathogenic bacteria belonging to Proteobacteria and Acidobacteria were suppressed under the BC-supplemented soil, especially in the root tissue and rhizosphere soil, whereas plant growth-regulating bacteria, mainly Bradyrhizobium, responded strongly and positively to several metabolites.

Discussion

Our finding provides valuable information for agronomists, farmers, and environmentalists to make informed decisions about crop production, land use, and soil management practices. Proper soil assessment and understanding of the interaction between the attributes of soil, BC, and metabolites are essential for promoting sustainable agriculture practices and land conservation.

Impact and mitigation of lead, cadmium and micro/nano plastics in fragrant rice

Heavy metals (HMs) and micro(nano)plastics (MNPs), represent a significant risk to global food supply as well as a potential risk to humankind. Over 50% of the worldwide population eat rice every day, and rice aroma is a significant qualitative trait that is highly valued by consumers and fetches premium prices in the global market. Despite the huge commercial importance of fragrant rice, limited studies were directed to investigate the influence of HMs and MNPs on yield related traits and 2-Acetyl-1-pyrroline (2-AP) compound, mainly responsible for aroma production in fragrant rice. In this review, we found that the interaction of HMs and MNPs in fragrant rice is complex and accumulation of HMs and MNPs was higher in root as compared to the grains. Nutrients and phytohormones mediated mitigation of HMs and MNPs were most effective sustainable strategies. In addition, monitoring the checkpoints of 2-AP biosynthesis and its interaction with HMs and MNPs is challenging. Finally, we explained the potential challenges that fragrant rice faces considering the continuous rise in environmental pollutants and discussed the future avenues of research to improve fragrant rice's yield and qualitative traits.

Review on Pesticide Abiotic Stress over Crop Health and Intervention by Various Biostimulants

Plants are essential for life on earth, and agricultural crops are a primary food source for humans. For the One Health future, crop health is crucial for safe, high-quality agricultural products and the development of future green commodities. However, the overuse of pesticides in modern agriculture raises concerns about their adverse effects on crop resistance and product quality. Recently, biostimulants, including microecological bacteria agents and nanoparticles, have garnered worldwide interest for their ability to sustain plant health and enhance crop resistance. This review analyzed the effects and mechanisms of pesticide stress on crop health. It also investigated the regulation of biostimulants on crop health and the multiomics mechanism, combining research on nanoselenium activating various crop health aspects conducted by the authors' research group. The paper helps readers understand the impact of pesticides on crop health and the positive influence of various biostimulants, especially nanomaterials and small molecules, on crop health.

Sustained organic amendments utilization enhances ratoon crop growth and soil quality by enriching beneficial metabolites and suppressing pathogenic bacteria

Introduction

Organic soil amendments such as filter mud (FM) and biochar (BC) can potentially influence the abundance and composition of metabolites. However, our current understanding of the stimulatory effects of FM and BC's long-term impact on stress-regulating metabolites, such as abscisic acid (ABA), jasmonic acid (JA), melatonin, and phenyllactic acid (PLA), and these substrates regulatory effects on disease-causing bacteria in sugarcane ratooning field, which is susceptible to nutrients depletion, diseases, etc., remain poorly understood. Additionally, little is known about how the long-term interaction of these substrates and compounds influences sugarcane ratooning soil enzyme activities, nutrient cycling, and crop growth performance.

Methods

To answer these questions, we adopted metabolomics tools combined with high-throughput sequencing to explore the stimulatory effects of the long-term addition of FM and BC on metabolites (e.g., PLA and abscisic aldehyde) and quantify these substrates' regulatory effects on disease-causing bacteria, soil enzyme activities, nutrient cycling, and crop growth performance.

Results

The result revealed that ratoon crop weight, stem diameter, sugar content, as well as soil physico-chemical properties, including soil nitrate (NH3 +-N), organic matter (OM), total nitrogen (TN), total carbon (TC), and β-glucosidase, marked a significant increase under the BC and FM-amended soils. Whereas soil available potassium (AK), NO3 -N, cellulase activity, and phosphatase peaked under the BC-amended soil, primarily due to the enduring effects of these substrates and metabolites. Furthermore, BC and FM-amended soils enriched specific stress-regulating metabolites, including JA, melatonin, abscisic aldehyde, etc. The sustained effects of both BC and FM-amended soils suppressed disease-causing bacteria, eventually promoting ratooning soil growth conditions. A number of key bioactive compounds had distinct associations with several beneficial bacteria and soil physico-chemical properties.

Discussion

This study proves that long-term BC and FM application is one of the eco-friendly strategies to promote ratoon crop growth and soil quality through the enrichment of stress-regulating metabolites and the suppression of disease-causing bacteria.

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

Deciphering the melatonin-mediated response and signalling in the regulation of heavy metal stress in plants

MAIN CONCLUSION

Melatonin has a protective effect against heavy metal stress in plants by immobilizing HM in cell walls and sequestering them in root cell vacuoles, reducing HM's translocation from roots to shoots. It enhances osmolyte production, increases antioxidant enzyme activity, and improves photosynthesis, thereby improving cellular functions. Understanding the melatonin-mediated response and signalling can sustain crop production in heavy metal-stressed soils. Melatonin is a pleiotropic signal molecule that plays a critical role in plant growth and stress tolerance, particularly against heavy metals in soil. Heavy metals (HMs) are ubiquitously found in the soil-water environment and readily taken up by plants, thereby disrupting mineral nutrient homeostasis, osmotic balance, oxidative stress, and altered primary and secondary metabolism. Plants combat HM stress through inbuilt defensive mechanisms, such as metal exclusion, restricted foliar translocation, metal sequestration and compartmentalization, chelation, and scavenging of free radicals by antioxidant enzymes. Melatonin has a protective effect against the damaging effects of HM stress in plants. It achieves this by immobilizing HM in cell walls and sequestering them in root cell vacuoles, reducing HM's translocation from roots to shoots. This mechanism improves the uptake of macronutrients and micronutrients in plants. Additionally, melatonin enhances osmolyte production, improving the plant's water relations, and increasing the activity of antioxidant enzymes to limit lipid peroxidation and reactive oxygen species (ROS) levels. Melatonin also decreases chlorophyll degradation while increasing its synthesis, and enhances RuBisCO activity for better photosynthesis. All these functions contribute to improving the cellular functions of plants exposed to HM stress. This review aims to gain better insight into the melatonin-mediated response and signalling under HM stress in plants, which may be useful in sustaining crop production in heavy metal-stressed soils.

Soil Amendments and Foliar Melatonin Reduced Pb Uptake, and Oxidative Stress, and Improved Spinach Quality in Pb-Contaminated Soil

Amending Pb-affected soil with biochar (BH) and magnesium potassium phosphate cement (MKC) reduces Pb uptake in plants. Moreover, foliar applications of melatonin and proline are also known to reduce plant oxidative stress and Pb uptake. However, little is known about combining both techniques, i.e., adding a combo immobilizing dose (CIA = mixture of BH and MKC at 50:50 ratio) in Pb-polluted soil and foliar application of proline and melatonin for reducing Pb uptake and oxidative stress in spinach. Control, proline, melatonin, CIA, CIA+proline, and CIA+melatonin were the treatments utilized in this pot study to see their effects on reducing plant oxidative stress, Pb uptake, and improving spinach quality in Pb-polluted soil. Moreover, Pb bioavailability, enzymatic activities, and numbers of bacteria, fungi, and actinomycetes in the soil were also evaluated. The effect of CIA on reducing Pb in the soil-plant system and improving soil enzymes and microbial numbers was more pronounced than melatonin alone. The most effective treatment was CIA+melatonin reducing Pb availability in soil (77%), shoots (95%), and roots (84%), alleviating oxidative stress, and improving plant biomass (98%) and nutrients. Soil enzymatic activities and the number of microorganisms in the rhizosphere were also highest with CIA+melatonin. Results highlight the significance of CIA+melatonin, as an inexpensive approach, in remediating Pb-polluted soil and improving spinach quality. However, further research is needed to understand the significance of CIA+melatonin on different crops and various soil Pb concentrations before employing this technique commercially in agriculture and environment sectors.

Reduction in the cadmium (Cd) accumulation and toxicity in pearl millet (Pennisetum glaucum L.) by regulating physio-biochemical and antioxidant defense system via soil and foliar application of melatonin

Cadmium (Cd) is among the toxic pollutants that harms the both animals and plants. The natural antioxidant, melatonin can improve Cd-stress tolerance but its potential role in reducing Cd stress and resilience mechanisms in pearl millet (Pennisetum glaucum L.) is remain unclear. The present study suggests that Cd causes severe oxidative damage by decreasing photosynthesis, and increasing reactive oxygen species (ROS), malondialdehyde content (MDA), and Cd content in different parts of pearl millet. However, exogenous melatonin (soil application and foliar treatment) mitigated the Cd toxicity and enhanced the growth, antioxidant defense system, and differentially regulated the expression of antioxidant-responsive genes i. e superoxide dismutase SOD-[Fe] 2, Fe-superoxide dismutase, Peroxiredoxin 2C, and L-ascorbate peroxidase-6. The results showed that foliar melatonin at F-200/50 significantly increased the plant height, chlorophyll a, b, a+b and carotenoids by 128%, 121%, 150%, 122%, and 69% over the Cd treatment, respectively. The soil and foliar melatonin at S-100/50 and F-100/50 reduced the ROS by 36%, and 44%, and MDA by 42% and 51% over the Cd treatment, respectively. Moreover, F200/50 significantly boosted the activities of antioxidant enzymes i. e SOD by 141%, CAT 298%, POD 117%, and APX 155% over the Cd treatment. Similarly, a significant reduction in Cd content in root, stem, and leaf was found on exposure to higher concentrations of exogenous melatonin. These findings suggest that exogenous melatonin may significantly and differentially improve the tolerance to Cd stress in crop plants. However, field applications, type of plant species, concentration of dose, and type of stress may vary with the degree of tolerance in crop plants.

Priming Potato Plants with Melatonin Protects Stolon Formation under Delayed Salt Stress by Maintaining the Photochemical Function of Photosystem II, Ionic Homeostasis and Activating the Antioxidant System

Melatonin is among one of the promising agents able to protect agricultural plants from the adverse action of different stressors, including salinity. We aimed to investigate the effects of melatonin priming (0.1, 1.0 and 10 µM) on salt-stressed potato plants (125 mM NaCl), by studying the growth parameters, photochemical activity of photosystem II, water status, ion content and antioxidant system activity. Melatonin as a pleiotropic signaling molecule was found to decrease the negative effect of salt stress on stolon formation, tissue water content and ion status without a significant effect on the expression of Na⁺/H⁺-antiporter genes localized on the vacuolar (NHX1 to NHX3) and plasma membrane (SOS1). Melatonin effectively decreases the accumulation of lipid peroxidation products in potato leaves in the whole range of concentrations studied. A melatonin-induced dose-dependent increase in Fv/Fm together with a decrease in uncontrolled non-photochemical dissipation Y(NO) also indicates decreased oxidative damage. The observed protective ability of melatonin was unlikely due to its influence on antioxidant enzymes, since neither SOD nor peroxidase were activated by melatonin. Melatonin exerted positive effects on the accumulation of water-soluble low-molecular-weight antioxidants, proline and flavonoids, which could aid in decreasing oxidative stress. The most consistent positive effect was observed on the accumulation of carotenoids, which are well-known lipophilic antioxidants playing an important role in the protection of photosynthesis from oxidative damage. Finally, it is possible that melatonin accumulated during pretreatment could exert direct antioxidative effects due to the ROS scavenging activity of melatonin molecules.

Exogenous melatonin mediates radish (Raphanus sativus) and Alternaria brassicae interaction in a dose-dependent manner

Radish (Raphanus sativus L.) is an economically important vegetable worldwide, but its sustainable production and breeding are highly threatened by blight disease caused by Alternaria brassicae. Melatonin is an important growth regulator that can influence physiological activities in both plants and microbes and stimulate biotic stress resistance in plants. In this study, 0-1500 μM melatonin was exogenously applied to healthy radish seedlings, in vitro incubated A. brassicae, and diseased radish seedlings to determine the effects of melatonin on host, pathogen, and host-pathogen interaction. At sufficient concentrations (0-500 μM), melatonin enhanced growth and immunity of healthy radish seedlings by improving the function of organelles and promoting the biosynthesis of antioxidant enzymes, chitin, organic acid, and defense proteins. Interestingly, melatonin also improved colony growth, development, and virulence of A. brassicae. A strong dosage-dependent effect of melatonin was observed: 50-500 μM promoted host and pathogen vitality and resistance (500 μM was optimal) and 1500 μM inhibited these processes. Significantly less blight was observed on diseased seedlings treated with 500 μM melatonin, indicating that melatonin more strongly enhanced the growth and immunity of radish than it promoted the development and virulence of A. brassicae at this treatment concentration. These effects of MT were mediated by transcriptional changes of key genes as identified by RNA-seq, Dual RNA-seq, and qRT-PCR. The results from this work provide a theoretical basis for the application of melatonin to protect vegetable crops against pathogens.

Melatonin as a Possible Natural Anti-Viral Compound in Plant Biocontrol

Melatonin is a multifunctional and ubiquitous molecule. In animals, melatonin is a hormone that is involved in a wide range of physiological activities and is also an excellent antioxidant. In plants, it has been considered a master regulator of multiple physiological processes as well as of hormonal homeostasis. Likewise, it is known for its role as a protective biomolecule and activator of tolerance and resistance against biotic and abiotic stress in plants. Since infections by pathogens such as bacteria, fungi and viruses in crops result in large economic losses, interest has been aroused in determining whether melatonin plays a relevant role in plant defense systems against pathogens in general, and against viruses in particular. Currently, several strategies have been applied to combat infection by pathogens, one of them is the use of eco-friendly chemical compounds that induce systemic resistance. Few studies have addressed the use of melatonin as a biocontrol agent for plant diseases caused by viruses. Exogenous melatonin treatments have been used to reduce the incidence of several virus diseases, reducing symptoms, virus titer, and even eradicating the proliferation of viruses such as Tobacco Mosaic Virus, Apple Stem Grooving Virus, Rice Stripe Virus and Alfalfa Mosaic Virus in tomato, apple, rice and eggplant, respectively. The possibilities of using melatonin as a possible natural virus biocontrol agent are discussed.

Melatonin from Microorganisms, Algae, and Plants as Possible Alternatives to Synthetic Melatonin

Melatonin dietary supplements are widely consumed worldwide, with developed countries as the largest consumers, with an estimated annual growth rate of approximately 10% until 2027, mainly in developing countries. The wide use of melatonin against sleep disorders and particular problems, such as jet lag, has been added to other applications, such as anti-aging, anti-stress, immune system activation, anticancer, and others, which have triggered its use, normally without a prescription. The chemical industry currently covers 100% of the needs of the melatonin market. Motivated by sectors with more natural consumption habits, a few years ago, the possibility of obtaining melatonin from plants, called phytomelatonin, arose. More recently, the pharmaceutical industry has developed genetically modified microorganisms whose ability to produce biological melatonin in bioreactors has been enhanced. This paper reviews the aspects of the chemical and biological synthesis of melatonin for human consumption, mainly as dietary supplements. The pros and cons of obtaining melatonin from microorganisms and phytomelatonin from plants and algae are analyzed, as well as the advantages of natural melatonin, avoiding unwanted chemical by-products from the chemical synthesis of melatonin. Finally, the economic and quality aspects of these new products, some of which are already marketed, are analyzed.

Exogenous application of melatonin to mitigate drought stress-induced oxidative damage in Phoebe sheareri seedlings

Background

Drought stress is a major prevalent environmental factor impairing growth. Melatonin mitigates the impacts of drought stress on plants. However, melatonin's role in Phoebe sheareri (Hemsl.) Gamble (P. sheareri) is unknown. We aimed to reveal the protective effects of melatonin on P. sheareri seedlings under drought conditions.

Methods

Melatonin was sprayed under drought or normal water conditions. The parameters, including growth, physiological factors, and phytohormones of P. sheareri, were examined.

Results

Compared to the normal control group, drought stress inhibited the growth of seedlings and significantly reduced the content of carotenoids, SOD, POD, APX, PPO, CAT, GR, and soluble sugars, and increased the contents of MDA, O₂ ^•-, proline, soluble proteins, ABA, and JA-Me in P. sheareri seedlings. However, melatonin treatment significantly reversed the adverse drought-induced responses and promoted the P. sheareri seedling's growth. Moreover, the heatmap and principal component analysis suggested a high similarity in the behavior patterns of the six measured antioxidant enzymes in P. sheareri seedlings.

Conclusion

Our study reported for the first time that melatonin has a protective role in P. sheareri seedlings under drought-stress conditions. This role is related to ROS scavenging, activation of antioxidant enzymes, and crosstalk of phytohormones. This study provided a theoretical basis for improving the ability of P. sheareri adapted to arid environments.

Role of Melatonin and Nitrogen Metabolism in Plants: Implications under Nitrogen-Excess or Nitrogen-Low

Melatonin is a new plant hormone involved in multiple physiological functions in plants such as germination, photosynthesis, plant growth, flowering, fruiting, and senescence, among others. Its protective role in different stress situations, both biotic and abiotic, has been widely demonstrated. Melatonin regulates several routes in primary and secondary plant metabolism through the up/down-regulation of many enzyme/factor genes. Many of the steps of nitrogen metabolism in plants are also regulated by melatonin and are presented in this review. In addition, the ability of melatonin to enhance nitrogen uptake under nitrogen-excess or nitrogen-low conditions is analyzed. A model that summarizes the distribution of nitrogen compounds, and the osmoregulation and redox network responses mediated by melatonin, are presented. The possibilities of using melatonin in crops for more efficient uptake, the assimilation and metabolization of nitrogen from soil, and the implications for Nitrogen Use Efficiency strategies to improve crop yield are also discussed.

Exogenous melatonin improved the growth and development of naked oat seedlings under cadmium stress

Heavy metal pollution has become a global problem, which affect more and more crop yields. Melatonin (MT) is widely used in plant stress resistance to alleviate the toxicity caused by heavy metals and other stresses. In this paper, the effects of exogenous 50 μM and 100 μM MT on the growth and development of naked oat seedlings under cadmium stress (25 mg L^-1) were studied. The results showed that different concentrations of MT could promote the growth of naked oat seedlings under 25-mg L^-1 cadmium stress. The application of exogenous melatonin could significantly increase the plant height, fresh weight, dry weight, chlorophyll, and proline contents of naked oats. MT could also reduce the contents of hydrogen peroxide, superoxide anion, and malondialdehyde in the cells of naked oat seedlings, and increase the activities of SOD, POD, and CAT. In addition, exogenous melatonin could affect the gene expression of LOX, POX, and Asmap1 in MAPK family and NAC and WRKY1 in TFS family in naked oat seedlings, thus promoting the growth and development of naked oat seedlings. In conclusion, this study is the first to demonstrate that MT is able to alleviate the negative effects to treat naked oat seedlings with cadmium stress. Therefore, melatonin has the potential to be applied in crops threatened by heavy metals.

The involvement of hydrogen sulphide in melatonin-induced tolerance to arsenic toxicity in pepper (Capsicum annuum L.) plants by regulating sequestration and subcellular distribution of arsenic, and antioxidant defense system

Melatonin (MT) and hydrogen sulphide (H₂S) are recognised as vital biomolecules actively taking part in plant defence systems as free radical scavengers and antioxidants against a myriad of biotic and abiotic stressors. However, it has been yet unknown in plants subjected to arsenic (As) toxicity whether or not H₂S interacts with MT to regulate endogenous antioxidant defence system. Prior to beginning As stress (As-S) treatments, MT (0.10 mM) was applied externally to plants daily for three days. AsS was then started for two weeks with As(V) (0.1 mM as Na₂HAsO₄·7H₂O). The treatment of As reduced plant biomass (24.4%) and chlorophyll a (51.7%), chlorophyll b (25.9%), while it increased subcellular As in roots and leaves, levels of glutathione (GSH), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), methylglyoxal (MG), H₂S and phytochelatins (PCs) in pepper plants. In As-stressed pepper plants, the application of MT increased plant biomass (16.3%), chlorophyll a (52.7%), chlorophyll b (28.2%), antioxidant enzymes' activities, and H₂S accumulation, while it lowered the concentrations of MDA and H₂O₂. In As-treated plants, GSH and phytochelatins (PCs) were increased by MT by regulating As sequestration to make it harmless. The addition of MT increased As accumulation in the vacuoles of roots and caused the soluble fraction of As in vacuoles to become less toxic to vital organelles. MT-induced tolerance to As stress was further enhanced using NaHS, a source of H₂S. Hypotaurine (0.1 mM HT), a H₂S scavenger, was applied to the control and As-stressed plants together with MT and MT + NaHS to determine whether H₂S was implicated in MT-induced increased As-S tolerance. By reducing H₂S generation in pepper plants, HT counteracted the beneficial effects of MT, whereas the addition of NaHS to MT + HT restored the negative effects of HT, proving that H₂S is necessary for the pepper plants As-stress tolerance caused by MT.

Using Exogenous Melatonin, Glutathione, Proline, and Glycine Betaine Treatments to Combat Abiotic Stresses in Crops

Abiotic stresses, such as drought, salinity, heat, cold, and heavy metals, are associated with global climate change and hamper plant growth and development, affecting crop yields and quality. However, the negative effects of abiotic stresses can be mitigated through exogenous treatments using small biomolecules. For example, the foliar application of melatonin provides the following: it protects the photosynthetic apparatus; it increases the antioxidant defenses, osmoprotectant, and soluble sugar levels; it prevents tissue damage and reduces electrolyte leakage; it improves reactive oxygen species (ROS) scavenging; and it increases biomass, maintains the redox and ion homeostasis, and improves gaseous exchange. Glutathione spray upregulates the glyoxalase system, reduces methylglyoxal (MG) toxicity and oxidative stress, decreases hydrogen peroxide and malondialdehyde accumulation, improves the defense mechanisms, tissue repairs, and nitrogen fixation, and upregulates the phytochelatins. The exogenous application of proline enhances growth and other physiological characteristics, upregulates osmoprotection, protects the integrity of the plasma lemma, reduces lipid peroxidation, increases photosynthetic pigments, phenolic acids, flavonoids, and amino acids, and enhances stress tolerance, carbon fixation, and leaf nitrogen content. The foliar application of glycine betaine improves growth, upregulates osmoprotection and osmoregulation, increases relative water content, net photosynthetic rate, and catalase activity, decreases photorespiration, ion leakage, and lipid peroxidation, protects the oxygen-evolving complex, and prevents chlorosis. Chemical priming has various important advantages over transgenic technology as it is typically more affordable for farmers and safe for plants, people, and animals, while being considered environmentally acceptable. Chemical priming helps to improve the quality and quantity of the yield. This review summarizes and discusses how exogenous melatonin, glutathione, proline, and glycine betaine can help crops combat abiotic stresses.

OsSGT1 promotes melatonin-ameliorated seed tolerance to chromium stress by affecting the OsABI5-OsAPX1 transcriptional module in rice

Chromium (Cr) pollution threatens plant development and growth. Application of melatonin (Mel) is emerging as an effective ally to resist stress, but how Mel ameliorates seed germination upon exposure to heavy metals is poorly understood. Here, we found (i) that seed priming with Mel considerably alleviated Cr stress during rice (Oryza sativa) seed germination and (ii) that germination performance was significantly improved in suppressor of the G2 allele of skp1 (OsSGT1) overexpression lines, while mutations of OsSGT1 and/or abscisic acid-insensitive 5 (OsABI5) noticeably abrogated such Mel-induced tolerance to Cr. Complementation assays suggested that the restored expression of OsSGT1 could not rescue the weak germination of sgt1-1abi5 under Cr stress, even upon Mel priming, but the expression of OsABI5 driven by the promoter of OsSGT1 significantly restored the Mel-ameliorated germination and the expression of ascorbate peroxidase 1 (OsAPX1) in sgt1-1abi5. Further analysis indicated that OsABI5 directly regulated the transcriptional expression of OsAPX1, whose encoding products promoted H₂ O₂ scavenging to maintain redox homeostasis, which is essential for germination. Collectively, this work demonstrates that OsSGT1 regulates OsABI5 to target OsAPX1, mediating the stimulatory effects of Mel on germination of Cr-stressed seeds, which provides a guide for the application of Mel in rice production.

Melatonin enhances stress tolerance in pigeon pea by promoting flavonoid enrichment, particularly luteolin in response to salt stress

Melatonin improves plant resistance to multiple stresses by participating in the biosynthesis of metabolites. Flavonoids are an important family of plant secondary metabolites and are widely recognized to be involved in resistance; however, the crosstalk between melatonin and flavonoid is largely unknown. We found that the resistance of pigeon pea (Cajanus cajan) to salt, drought, and heat stresses were significantly enhanced by pre-treatment with melatonin. Combined transcriptome and LC-ESI-MS/MS metabolomics analyses showed that melatonin significantly induced the enrichment of flavonoids and mediated the reprogramming of biosynthetic pathway genes. The highest fold-increase in expression in response to melatonin treatment was observed for the CcF3´H family, which encodes an enzyme that catalyses the biosynthesis of luteolin, and the transcription factor CcPCL1 directly bonded to the CcF3´H-5 promoter to enhance its expression. In addition, salt stress also induced the expression of CcPCL1 and CcF3´H-5, and their overexpression in transgenic plants greatly enhanced salt tolerance by promoting the biosynthesis of luteolin. Overall, our results indicated that pre-treatment of pigeon pea with melatonin promoted luteolin biosynthesis through the CcPCL1 and CcF3´H-5 pathways, resulting in salt tolerance. Our study shows that melatonin enhances plant tolerance to multiple stresses by mediating flavonoid biosynthesis, providing new avenues for studying the crosstalk between melatonin and flavonoids.

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.

Effects of melatonin on growth and antioxidant capacity of naked oat (Avena nuda L) seedlings under lead stress

Melatonin (MT) plays an important role in plant response to abiotic stress. In recent years, lead (Pb) pollution has seriously affected the living environment of plants. In this study, we applied two different concentrations of MT to naked oat seedlings under Pb stress to explore the effect of MT on naked oat seedlings under Pb pollution. The results showed that Pb stress seriously inhibited the growth and development of naked oat seedlings, which was alleviated by MT. MT could increase the soluble protein content and decrease the proline content of naked oat seedlings to maintain the osmotic balance of naked oat seedlings. The application of MT could accelerate the removal of reactive oxygen species (ROS) and improve the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), so as to maintain the redox balance in naked oat seedlings. Exogenous melatonin could significantly increase the chlorophyll content of naked oat seedlings under Pb treatment, so as to improve the photosynthesis efficiency of naked oat seedlings. MT could also remarkably up regulate the expression of the genes of LOX, POX and Asmap1, and affect the expression of transcription factors NAC and WRKY1. It might regulate the expression of downstream genes through MAPKs pathways and TFs to improve the Pb tolerance of naked oat seedlings. These results proved that MT could significantly promote the growth and development of naked oats seedlings under Pb stress, which is expected to be applied in agricultural production practice.

Effect of Melatonin in Broccoli Postharvest and Possible Melatonin Ingestion Level

The post-harvest stage of broccoli production requires cold storage to obtain enough days of shelf life. It has been proved that melatonin is useful as a post-harvest agent in fruits and vegetables, including broccoli. In this study, the broccoli heads treated with melatonin have a longer shelf life than the control samples, which was reflected in parameters such as fresh weight, hue angle (expresses color quality), and chlorophyll and carotenoid contents. Treatments with 100 μM melatonin for 15 or 30 min seem to be the most appropriate, extending the broccoli’s shelf life to almost 42 days, when it is normally around 4 weeks. In addition, a study on the possible impact that melatonin treatments in broccoli could have on melatonin intake in humans is presented. The levels of superficial melatonin, called washing or residual melatonin, are measured, showing the possible incidence in estimated blood melatonin levels. Our results suggest that post-harvest treatments with melatonin do not have to be a handicap from a nutritional point of view, but more research is needed.

The Antioxidant Cyclic 3-Hydroxymelatonin Promotes the Growth and Flowering of Arabidopsis thaliana

In plants, melatonin is metabolized into several compounds, including the potent antioxidant cyclic 3-hydroxymelatonin (3-OHM). Melatonin 3-hydroxylase (M3H), a member of the 2-oxo-glutarate-dependent enzyme family, is responsible for 3-OHM biosynthesis. Although rice M3H has been cloned, its roles are unclear, and no homologs in other plant species have been characterized. Here, we cloned and characterized Arabidopsis thaliana M3H (AtM3H). The purified recombinant AtM3H exhibited K_m and V_max values of 100 μM and 20.7 nmol/min/mg protein, respectively. M3H was localized to the cytoplasm, and its expression peaked at night. Based on a 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, 3-OHM exhibited 15-fold higher antioxidant activity than melatonin. An Arabidopsis M3H knockout mutant (m3h) produced less 3-OHM than the wildtype (WT), thus reducing antioxidant activity and biomass and delaying flowering. These defects were caused by reduced expression of FLOWERING LOCUS T (FT) and gibberellin-related genes, which are responsible for flowering and growth. Exogenous 3-OHM, but not exogenous melatonin, induced FT expression. The peak of M3H expression at night matched the FT expression pattern. The WT and m3h exhibited similar responses to salt stress and pathogens. Collectively, our findings indicate that 3-OHM promotes growth and flowering in Arabidopsis.

Melatonin alleviates copper stress to promote rice seed germination and seedling growth via crosstalk among various defensive response pathways

Copper (Cu) contamination dramatically affects crop growth and thus threatens crop production; while applications of melatonin (MT) serve as an effective way to tolerate Cu stress for plant development, the underlying mechanism remains largely unknown in rice. Here, we found that Cu toxicity remarkably decreased germination rates and seedling growth compared to the untreated control (CK), while seed priming with a solution of 100 μM MT significantly alleviated the adverse effects on Cu-stressed seeds. In addition, the MT treatment decreased the accumulation of Cu in seedlings at 7 days after imbibition (DAI), possibly through enhanced Cu sequestration, and improved reserve mobilization through the promoted activity of α-amylase and protease in seeds under Cu stress. Interestingly, gibberellin (GA) synthesis was restored to or even exceeded the CK levels in the MT presoaking treatment, while the abscisic acid (ABA) content decreased compared to those of the Cu-stressed seeds, suggesting crosstalk between MT and other phytohormones, e.g., GA and ABA. More importantly, MT pretreatment also significantly promoted the growth of postgermination seedlings. This was largely ascribed to the MT-ameliorated antioxidant system, which consequently reduced the accumulation of Cu stress-induced oxidative products, e.g., hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and superoxide (O₂·^_). Collectively, these results demonstrate that seed priming with MT could greatly mitigate the adverse effects of Cu stress on seed germination and subsequent postgermination growth through crosstalk among various defensive response pathways. This study provides vital guidance for applications of MT in agronomic production.

Direct and indirect effects of soil salinization on soil seed banks in salinizing wetlands in the Songnen Plain, China

Soil salinization has become a widespread threat to the structure and ecological functioning of inland wetlands globally. Soil seed banks can be important for plant regeneration in salinizing wetlands. To explore the effects of soil salinization on soil seed banks and their potential role in revegetation, we studied the structure and composition of plant communities and soil seed banks along a soil salinization gradient, and analyzed the responses of Carex-dominated and Phragmites-dominated communities to saline-alkaline stress in the Songnen Plain, China. We found that the dominant species of aboveground vegetation were different along the soil salinization gradient. Carex spp. dominated in the non-salinized and mild salinity wetlands, and Phragmites australis dominated in wetlands with moderate and high levels of salinity. The species richness of aboveground vegetation, and the density and richness of soil seed banks were higher in wetlands with lower salinity. The structural equation model indicated that the difference in soil salinization was directly associated with the aboveground species richness, and density and richness of the soil seed banks, while it was indirectly associated with the density and richness of the soil seed banks by directly affecting the composition and the species richness of the aboveground vegetation. Soil seed banks in Phragmites communities were more tolerant of saline-alkaline stress than Carex communities. This study indicates that soil salinization affects the size and composition of soil seed banks and limits their role in plant regeneration in wetlands of the Songnen Plain. In addition to hydrological regulation, the reduction of soil salinity is necessary to protect and restore biodiversity in salinizing wetlands.

Effect of Exogenous Melatonin Application on the Grain Yield and Antioxidant Capacity in Aromatic Rice under Combined Lead–Cadmium Stress

This study aimed to determine the mechanism of exogenous melatonin application in alleviating the combined Pb and Cd (Pb-Cd) toxicity on aromatic rice (Oryza sativa L.). In this study, a pot experiment was conducted; two aromatic rice varieties, Yuxiangyouzhan and Xiangyaxiangzhan, were selected, and sprays using 50, 100, 200, and 400 μmol L−1 melatonin (denoted as S50, S100, S200, and S400) and irrigation using 100, 300, and 500 μmol L−1 melatonin (denoted as R100, R300, and R500) were also selected. The results showed that, under the S50, S100, and S200 treatments, the Pb content of aromatic rice grain decreased, and the grain yield increased significantly. Moreover, the application of exogenous melatonin significantly reduced the accumulation of H2O2 in rice leaves at maturity under Cd–Pb stress and reduced the MDA content in Xiangyaxiangzhan leaves. In addition, the microbial community structure changed significantly under S50 and R300 treatments. Some pathways, such as the synthesis of various amino acids and alanine, aspartate, and glutamate metabolism, were regulated by S50 treatment. Overall, melatonin application improved aromatic rice grain yield while reducing heavy metal accumulation by regulating the antioxidant capacity and metabolites in aromatic rice plants and altering the physicochemical properties and microbial community structures of the soil.

Melatonin as a Possible Natural Safener in Crops

Melatonin is a well-known animal hormone with relevant and multiple cellular and hormonal roles. Its discovery in plants in 1995 has led to a great diversity of molecular and physiological studies that have been showing its multiple actions also in plants. Its roles as a biostimulator and modulator agent of responses to abiotic and biotic stresses have been widely studied. This review raises the possible use of melatonin as a natural safener in herbicide treatments. Existing studies have shown excellent co-acting qualities between both the following agents: herbicide and melatonin. The presence of melatonin reduces the damage caused by the herbicide in the crop and enhances the stress antioxidant response of plants. In this area, a similar role is suggested in the co-action between fungicides and melatonin, where a synergistic response has been demonstrated in some cases. The possible reduction in the fungicide doses is proposed as an eco-friendly advance in the use of these pesticides in certain crops. Finally, future research and applied actions of melatonin on these pest control agents are suggested.

Melatonin in Brassicaceae: Role in Postharvest and Interesting Phytochemicals

Brassicaceae plants are of great interest for human consumption due to their wide variety and nutritional qualities. Of the more than 4000 species that make up this family, about a hundred varieties of 6–8 genera are extensively cultivated. One of the most interesting aspects is its high content of glucosinolates, which are plant secondary metabolites with widely demonstrated anti-oncogenic properties that make them healthy. The most relevant Brassicaceae studies related to food and melatonin are examined in this paper. The role of melatonin as a beneficial agent in seedling grown mainly in cabbage and rapeseed and in the postharvest preservation of broccoli is especially analyzed. The beneficial effect of melatonin treatments on the organoleptic properties of these commonly consumed vegetables can be of great interest in the agri-food industry. Melatonin application extends the shelf life of fresh-cut broccoli while maintaining optimal visual and nutritional parameters. In addition, an integrated model indicating the role of melatonin on the organoleptic properties, the biosynthesis of glucosinolates and the regulatory action of these health-relevant compounds with anti-oncogenic activity is presented.

Role of exogenous melatonin involved in phenolic acid metabolism of germinated hulless barley under NaCl stress

In this study, the effects of exogenous MT on phenolic acids biosynthesis and the response to NaCl stress in germinating barley were investigated to explicate the role and molecular mechanism of MT in the regulation of phenolic acids and biomass under salt stress. Results showed that exogenous MT increased the gene expression and activities of phenylalanine ammonia lyase and cinnamate 4-hydroxylase involved in phenols biosynthesis. As a result, phenolic acids contents significantly increased, and ferulic acid, p-coumaric acid and p-hydroxybenzoic acid were mostly induced by exogenous MT treatment. Meanwhile, exogenous MT application reduced the damage of NaCl stress, including promotion sprout growth, biomass and Ca²⁺ influs, malonaldehyde and H₂O₂ content reduction, increases of peroxidase, superoxide dismutase and catalase activities in barley seedlings. These results indicated that exogenous MT was essential for inducing phenolic acids accumulation and alleviated the inhibition of NaCl stress on barley seedlings.

Effect of Salt Stress and Foliar Application of Salicylic Acid on Morphological, Biochemical, Anatomical, and Productivity Characteristics of Cowpea (Vigna unguiculata L.) Plants

The present study aimed to investigate the impact of salinity on vegetative growth, chemical constituents, and yields of cowpeas (Vigna unguiculata) and the possible benefits of salicylic acid (SA) on these plants after damage from salinity. To achieve these objectives, two pot experiments were carried out at the Faculty of Agriculture, Al-Azhar University, Egypt, during the two growing seasons of 2019 and 2020. The results revealed that salinity significantly decreased, and SA treatment substantially increased the plant height, number of compound leaves, number of internodes per plant, fresh weights of leaves and stems, productivity, photosynthetic pigments content, and concentrations of nitrogen (N), phosphorus (P), and potassium (K) of the cowpea plants compared with the control. The anatomical structure of stems and leaves of the plants were also investigated, and it was found that positive variations in the anatomical structure of the median portion of the main stems and blades of mature foliage leaves were detected in the stressed and SA-treated plants. In conclusion, SA treatment increased the salt stress tolerance of cowpea plants by improving the morphological and physiological attributes of the plants.

Melatonin functions in priming of stomatal immunity in Panax notoginseng and Arabidopsis thaliana

Melatonin (MT) plays important roles in plant disease response, but the mechanisms are largely unknown. Here, we show that MT functions in stomatal immunity in Panax notoginseng and Arabidopsis thaliana. Biochemical analyses showed that MT-induced stomatal closure plays a prominent role in preventing invasion of bacteria Pseudomonas syringe pv. tomato (Pst) DC3000 via activation of mitogen-activated protein kinase (MAPK) and NADPH oxidase-mediated reactive oxygen species production in P. notoginseng. The first putative phytomelatonin receptor 1 (PMTR1) is a plasma membrane protein required for perceiving MT signaling in stomatal closure and activation of MAPK. Biochemical and genetic tests found PMTR1 is essential for flg22- and MT-induced MAPK activation in a heterotrimeric GTP-binding protein Gα subunit GPA1-independent manner. GPA1 functions in the same genetic pathways of FLS2/BAK1 (Flagellin Sensing 2/Brassinosteroid Insensitive 1-associated kinase 1)- as well as PMTR1-mediated flg22 and MT signaling in stomatal closure. The stomata in pmtr1 are insensitive to MT and flg22, but the application of MT induces stomatal closure and reduces the bacterial growth in fls2 and bak1 plants, indicating that PMTR1 might be a downstream signaling component in FLS2- and BAK1-mediated stomatal immunity. In summary, our results (i) demonstrate that phytomelatonin functions in the priming of stomatal immunity and (ii) provide insights into the phytomelatonin signaling transduction pathway.

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.

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 enhances drought stress tolerance in maize through coordinated regulation of carbon and nitrogen assimilation

Melatonin is a pleiotropic regulatory molecule in plants and is involved in regulating plant tolerance to drought stress. Here, we conducted transcriptomic and physiological analyses to identify metabolic processes associated with the enhanced tolerance of the melatonin-treated maize (Zea mays L.) seedlings to water deficit. Maize seedlings were foliar sprayed with either 50 μM melatonin or water and exposed to drought stress for 12 d in growth chambers. Drought stress significantly suppressed seedling growth, and melatonin application partially alleviated this growth inhibition. RNA-Seq analysis revealed that genes whose expression was significantly altered by melatonin were mainly related to carbon (C) and nitrogen (N) metabolism. Analysis of transcriptomics, enzyme activity, and metabolite content data, melatonin-treated plants exhibited a higher level of relatively stable C and N metabolism than untreated plants; this phenotype of melatonin-treated plants was associated with their higher photosynthesis, sucrose biosynthesis, N assimilation, and protein biosynthesis capacities under drought stress. Overall, our results suggest that melatonin enhances drought stress tolerance in maize through coordinated regulation of C and N metabolism.

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.

Melatonin and Carbohydrate Metabolism in Plant Cells

Melatonin, a multifunctional molecule that is present in all living organisms studied, is synthesized in plant cells in several intercellular organelles including in the chloroplasts and in mitochondria. In plants, melatonin has a relevant role as a modulatory agent which improves their tolerance response to biotic and abiotic stress. The role of melatonin in stress conditions on the primary metabolism of plant carbohydrates is reviewed in the present work. Thus, the modulatory actions of melatonin on the various biosynthetic and degradation pathways involving simple carbohydrates (mono- and disaccharides), polymers (starch), and derivatives (polyalcohols) in plants are evaluated. The possible applications of the use of melatonin in crop improvement and postharvest products are examined.

Heavy metal stress in rice: Uptake, transport, signaling, and tolerance mechanisms

Heavy metal contamination of agricultural fields has become a global concern as it causes a direct impact on human health. Rice is the major food crop for almost half of the world population and is grown under diverse environmental conditions, including heavy metal-contaminated soil. In recent years, the impact of heavy metal contamination on rice yield and grain quality has been shown through multiple approaches. In this review article, different aspects of heavy metal stress, that is uptake, transport, signaling and tolerance mechanisms, are comprehensively discussed with special emphasis on rice. For uptake, some of the transporters have specificity to one or two metal ions, whereas many other transporters are able to transport many different ions. After uptake, the intercellular signaling is mediated through different signaling pathways involving the regulation of various hormones, alteration of calcium levels, and the activation of mitogen-activated protein kinases. Heavy metal stress signals from various intermediate molecules activate various transcription factors, which triggers the expression of various antioxidant enzymes. Activated antioxidant enzymes then scavenge various reactive oxygen species, which eventually leads to stress tolerance in plants. Non-enzymatic antioxidants, such as ascorbate, metalloids, and even metal-binding peptides (metallothionein and phytochelatin) can also help to reduce metal toxicity in plants. Genetic engineering has been successfully used in rice and many other crops to increase metal tolerance and reduce heavy metals accumulation. A comprehensive understanding of uptake, transport, signaling, and tolerance mechanisms will help to grow rice plants in agricultural fields with less heavy metal accumulation in grains.

Exogenic Melatonin Reduces the Toxic Effect of Polymetallic Stress on Barley Plants

The effect of melatonin on growth parameters, the photochemical activity of photosystem II (PS II), the content of the main photosynthetic pigments, and lipid peroxidation in barley plant leaves under polymetallic stress were studied. Melatonin reduces the toxic effect of polymetals on biomass accumulation, root growth, and maintenance of the assimilating surface. The protective action of the hormone is based on its ability to reduce the intensity of oxidative stress by maintaining the level of carotenoids and increasing the activity of superoxide dismutase, but not by regulating the photochemical activity of chloroplasts. The effectiveness of melatonin does not depend on the duration of exposure. The data obtained can be the basis for optimizing the use of melatonin as a plant priming inducer.

Ameliorative effect of melatonin improves drought tolerance by regulating growth, photosynthetic traits and leaf ultrastructure of maize seedlings

BACKGROUND

Melatonin is considered a potential plant growth regulator to enhance the growth of plants and increase tolerance to various abiotic stresses. Nevertheless, melatonin's role in mediating stress response in different plant species and growth cycles still needs to be explored. This study was conducted to understand the impact of different melatonin concentrations (0, 50, 100, and 150 μM) applied as a soil drench to maize seedling under drought stress conditions. A decreased irrigation approach based on watering was exposed to maize seedling after drought stress was applied at 40-45% of field capacity.

RESULTS

The results showed that drought stress negatively affected the growth behavior of maize seedlings, such as reduced biomass accumulation, decreased photosynthetic pigments, and enhanced the malondialdehyde and reactive oxygen species (ROS). However, melatonin application enhanced plant growth; alleviated ROS-induced oxidative damages by increasing the photosynthetic pigments, antioxidant enzyme activities, relative water content, and osmo-protectants of maize seedlings.

CONCLUSIONS

Melatonin treatment also enhanced the stomatal traits, such as stomatal length, width, area, and the number of pores under drought stress conditions. Our data suggested that 100 μM melatonin application as soil drenching could provide a valuable foundation for improving plant tolerance to drought stress conditions.

Melatonin regulates antioxidant strategy in response to continuous salt stress in rice seedlings

Melatonin mediates multiple physiological processes in plants and is involved in many reactions related to the protection of plants from abiotic stress. In this paper, the effect of melatonin on the antioxidant capacity of rice under salt stress was studied. Melatonin alleviated the inhibition of salt stress on the growth of rice seedlings, mainly by increasing the dry weight and fresh weight of shoots and roots. Melatonin alleviated the membrane damage caused by salt stress, which was mainly manifested by the decrease of TBARS content and the decrease of leaf and root damage. During the whole salt stress period, rice after melatonin pretreatment showed lower ROS (H₂O₂, O₂^•-,OH^-) accumulation. In the early stage (1-3 d) of stress, the rice after melatonin pretreatment showed a strong increase in antioxidant enzyme activity, while in the later stage (5,7 d), it showed a strong increase in antioxidant content. During the whole period of salt stress, melatonin had a weak regulatory effect on AsA-GSH cycle. Through the above regulation process, the decreasing effect of melatonin on ROS content of rice under salt stress did not decrease with prolonged stress time in a short time (1-7 d). In conclusion, melatonin improved the antioxidant capacity of rice under continuous salt stress, and rice showed variable antioxidant strategies after melatonin pretreatment.

Inhibition of Rice Serotonin N-Acetyltransferases by MG149 Decreased Melatonin Synthesis in Rice Seedlings

We examined the effects of two histone acetyltransferase (HAT) inhibitors on the activity of rice serotonin N-acetyltransferases (SNAT). Two rice recombinant SNAT isoenzymes (SNAT1 and SNAT2) were incubated in the presence of either MG149 or MB3, HAT inhibitors. MG149 significantly inhibited the SNAT enzymes in a dose-dependent manner, especially SNAT1, while SNAT2 was moderately inhibited. By contrast, MB3 had no effect on SNAT1 or SNAT2. The application of 100 μM MG149 to rice seedlings decreased melatonin by 1.6-fold compared to the control, whereas MB3 treatment did not alter the melatonin level. MG149 significantly decreased both melatonin and N-acetylserotonin when rice seedlings were challenged with cadmium, a potent elicitor of melatonin synthesis in rice. Although MG149 inhibited melatonin synthesis in rice seedlings, no melatonin deficiency-induced lamina angle decrease was observed due to the insufficient suppression of SNAT2, which is responsible for the lamina angle decrease in rice.