Melatonin mediates selenium-induced tolerance to cadmium stress in tomato plants

Multi-walled carbon nanotubes affect yield, antioxidant response, and rhizosphere microbial community of scented rice under combined cadmium-lead (Cd-Pb) stress

Rice production is threatened by heavy metal stress. The use of multi-walled carbon nanotubes (MWCNTs) in agriculture has been reported in previous studies. We aimed to quantify the impact of MWCNTs on the growth and physiological characteristics of scented rice under cadmium (Cd) and lead (Pb) stresses. Therefore, a pot experiment was conducted, two scented rice varieties Yuxiangyouzhan and Xiangyaxiangzhan were used as materials grown under different concentrations of MWCNTs (0, 100, and 300 mg kg-1 recorded as CK, CNPs100, and CNPs300, respectively). The yield, antioxidant response, and rhizosphere microbial community of scented rice were studied. The results showed that compared with the CK treatment, the CNPs100 and CNPs300 treatments increased leaf dry weight by 17.95%-56.22% at the heading stage, and the H2O2 content in leaves decreased significantly by 36.64%-42.27% at the maturity stage. Under CNPs100 treatment, the grain yield of two scented rice varieties increased significantly by 17.54% and 27.40%, respectively. The MWCNTs regulated the distribution of the Cd and Pb in different plant tissues. The content of Cd (0.11-0.20 mg kg-1) and Pb (0.01-0.04 mg kg-1) in grain were at a safety level (<0.2 mg kg-1). Moreover, MWCNTs increased soil microbial community abundance and altered community composition structure under Cd-Pb stress, which in turn improved agronomic traits and quality of scented rice. Overall, this study suggested that the application of MWCNTs regulates the growth, yield, physiological response, and soil microbial community, the genotypes response effect of scented rice to MWCNTs is needed further studied.

Melatonin mitigates cadmium toxicity by promoting root development, delaying root senescence, and regulating cadmium transport in cotton

Cd ions are absorbed and transported from the soil by crop roots, which are the first organ to be exposed to Cd. This results in an increase in cadmium ions in crops, significantly affecting crop growth and yield. Exogenous melatonin (MT) can help reduce cadmium (Cd) stress in cotton, but the specific contribution of roots to this process remains unclear. In order to address this knowledge gap, an in-situ root phenotyping study was conducted to investigate the the phenotype and lifespan of roots under cadmium stress (Cd) and melatonin treatment (Cd + MT). The results showed that MT alleviated the decreases in plant height, leaf area, SPAD value, stem diameter, stomatal conductance and net photosynthetic rate under Cd stress, which further promoted the biomass accumulation in various cotton organs. What is more, the Cd + MT treatment increased root volume, surface area, and length under Cd stress by 25.63 %, 10.58 %, and 21.89 %, respectively, compared with Cd treatment. Interestingly, compared to Cd treatment, Cd + MT treatment also significantly extended the lifespan of roots and root hairs by 6.68 days and 2.18 days, respectively. In addition, Cd + MT treatment reduced the transport of Cd from roots to shoots, particularly to bolls, and decreased the Cd bioconcentration factor in bolls by 61.17 %, compared to Cd treatment. In conclusion, these findings show that applying MT externally helps reduce Cd stress by delaying root senescence, promoting root development and regulating Cd transport. This method can be an effective approach to managing Cd stress in cotton.

Transcriptomic analysis provides insights into the molecular mechanism of melatonin-mediated cadmium tolerance in Medicago sativa L

Cadmium (Cd), a toxic element, often makes a serious threat to plant growth and development. Previous studies found that melatonin (Mel) reduced Cd accumulation and reestablished the redox balance to alleviate Cd stress in Medicago sativa L., however, the complex molecular mechanisms are still elusive. Here, comparative transcriptome analysis and biochemical experiments were conducted to explore the molecular mechanisms of Mel in enhancing Cd tolerance. Results showed that 7237 differentially expressed genes (DEGs) were regulated by Mel pretreatment to Cd stress compared to the control condition in roots of Medicago sativa L. Besides, in comparison with Cd stress alone, Mel upregulated 1081 DEGs, and downregulated 1085 DEGs. These DEGs were mainly involved in the transcription and translation of genes and folding, sorting and degradation of proteins, carbohydrate metabolism, and hormone signal network. Application of Mel regulated the expression of several genes encoding ribosomal protein and E3 ubiquitin-protein ligase involved in folding, sorting and degradation of proteins. Moreover, transcriptomic analyse suggested that Mel might regulate the expression of genes encoding pectin lyase, UDP-glucose dehydrogenase, sucrose-phosphate synthase, hexokinase-1, and protein phosphorylation in the sugar metabolism. Therefore, these could promote sucrose accumulation and subsequently alleviate the Cd damage. In conclusion, above findings provided the mining of important genes and molecular basis of Mel in mitigating Cd tolerance and genetic cultivation of Medicago sativa L.

Different effects of selenium speciation on selenium absorption, selenium transformation and cadmium antagonism in garlic

Currently, planting selenium-rich crops using inorganic selenium such as selenate and selenite is used to address human selenium deficiency problems. In this paper, besides the above two traditional inorganic selenium speciation, we chose a new organic selenium speciation of potassium selenocyanoacetate to investigate the different effects of selenium speciation on selenium absorption, selenium transformation and cadmium antagonism via foliar application. Plantingexperiments showed that the selenium content of garlic bulbs treated with organic selenium was 1.8-3.9 times higher than that of inorganic selenium. Additionally, the absorption and transformation efficiency of organic selenium in garlic was also the highest, reaching over 95 %. Importantly, it was noteworthy that the cadmium content in bulbs treated with organic selenium was significantly lower than the Chinese food safety standard (0.2 mg/kg). Hence, this study provides an efficient organic selenium speciation which is beneficial to meet human selenium requirements and ensure safe utilization of cadmium-contaminated soils.

Selenium in plants: A nexus of growth, antioxidants, and phytohormones

Selenium (Se) is an essential micronutrient for both human and animals. Plants serve as the primary source of Se in the food chain. Se concentration and availability in plants is influenced by soil properties and environmental conditions. Optimal Se levels promote plant growth and enhance stress tolerance, while excessive Se concentration can result in toxicity. Se enhances plants ROS scavenging ability by promoting antioxidant compound synthesis. The ability of Se to maintain redox balance depends upon ROS compounds, stress conditions and Se application rate. Furthermore, Se-dependent antioxidant compound synthesis is critically reliant on plant macro and micro nutritional status. As these nutrients are fundamental for different co-factors and amino acid synthesis. Additionally, phytohormones also interact with Se to promote plant growth. Hence, utilization of phytohormones and modified crop nutrition can improve Se-dependent crop growth and plant stress tolerance. This review aims to explore the assimilation of Se into plant proteins, its intricate effect on plant redox status, and the specific interactions between Se and phytohormones. Furthermore, we highlight the proposed physiological and genetic mechanisms underlying Se-mediated phytohormone-dependent plant growth modulation and identified research opportunities that could contribute to sustainable agricultural production in the future.

Novel melatonin-producing Bacillus safensis EH143 mitigates salt and cadmium stress in soybean

Recently, microorganism and exogenous melatonin application has been recognized as an efficient biological tool for enhancing salt tolerance and heavy metal detoxification in agriculture crops. Thus, the goal of this study was to isolate and evaluate a novel melatonin-producing plant growth promoting bacterium. With high-throughput whole genome sequencing, phytohormone measurements, expression profiling, and biochemical analysis, we can identify a novel PGPB that produces melatonin and unravel how it promotes soybean growth and development and protects against salt and Cd stress. We identify the melatonin synthesis pathway (tryptophan→tryptamine→serotonin melatonin) of the halotolerant (NaCl > 800 mM) and heavy metal-resistant (Cd >3 mM) rhizobacterium Bacillus safensis EH143 and use it to treat soybean plants subjected to Cd and NaCl stresses. Results show that EH143 will highly bioaccumulate heavy metals and significantly improve P and Ca2+ uptake and the K+/Na+ (93%↑under salt stress) ratio while reducing Cd uptake (49% under Cd stress) in shoots. This activity was supported by the expression of the ion regulator HKT1, MYPB67, and the calcium sensors CDPK5 and CaMK1 which ultimately led to increased plant growth. EH143 significantly decreased ABA content in shoots by 13%, 20%, and 34% and increased SA biosynthesis in shoots by 14.8%, 31%, and 48.2% in control, salt, and Cd-treated plants, upregulating CYP707A1 and CYP707A2 and PAL1 and ICS, respectively. The melatonin content significantly decreased along with a reduced expression of ASMT3 following treatment with EH143; moreover, reduced expression of peroxidase (POD) and superoxide dismutase (SOD) by 134.5% and 39% under salt+Cd stress, respectively and increased level of total amino acids were observed. Whole-genome sequencing and annotation of EH143 revealed the presence of the melatonin precursor tryptophan synthase (trpA, trpB, trpS), metal and other ion regulators (Cd: cadA, potassium: KtrA and KtrB, phosphate: glpT, calcium: yloB, the sodium/glucose cotransporter: sgIT, and the magnesium transporter: mgtE), and enzyme activators (including the siderophore transport proteins yfiZ and yfhA, the SOD sodA, the catalase katA1, and the glutathione regulator KefG) that may be involved in programming the plant metabolic system. As a consequence, EH143 treatment significantly reduced the contents of lipid peroxidation (O2-, MDA, and H2O2) up to 69%, 46%, and 29% in plants under salt+Cd stress, respectively. These findings suggest that EH143 could be a potent biofertilizer to alleviate NaCl and Cd toxicity in crops and serve as an alternative substitute for exogenous melatonin application.

Melatonin enhances cadmium tolerance in rice via long non-coding RNA-mediated modulation of cell wall and photosynthesis

In plants, melatonin (MLT) is a versatile signaling molecule involved in promoting plant development and mitigating the damage caused by heavy metal exposure. Long non-coding RNAs (lncRNAs) are essential components in the plant's response to various abiotic stress, functioning within the gene regulatory network. Here, a hydroponic experiment was performed to explore the involvement of lncRNAs in MLT-mediated amelioration of cadmium (Cd) toxicity in rice plants. The results demonstrated that applying 250 mg L-1 MLT in a solution containing 10 μM Cd leads to an effective reduction of 30.0% in shoot Cd concentration. Remarkably, the treatment resulted in a 21.2% improvement in potassium and calcium uptake, a 164.5% enhancement in net photosynthetic rate, and a 33.2% decrease in malondialdehyde accumulation, resulting increases in plant height, root length, and biomass accumulation. Moreover, a transcriptome analysis revealed 2510 differentially expressed transcripts, including the Cd transporters (-3.82-fold downregulated) and the Cd tolerance-associated genes (1.24-fold upregulated). Notably, regulatory network prediction uncovered 6 differentially expressed lncRNAs that act as competitive endogenous RNA or in RNA complex interactions. These key lncRNAs regulate the expression of target genes that are involved in pectin and cellulose metabolism, scavenging of reactive oxygen species, salicylic acid-mediated defense response, and biosynthesis of brassinosteroids, which ultimately modify the cell wall for Cd adsorption, safeguard photosynthesis, and control hormone signaling to reduce Cd toxicity. Our results unveiled a crucial lncRNA-mediated mechanism underlying MLT's role in Cd detoxification in rice plants, providing potential applications in agricultural practices and environmental remediation.

LC-MS and GC-MS Metabolomics Analyses Revealed That Different Exogenous Substances Improved the Quality of Blueberry Fruits under Soil Cadmium Toxicity

Exogenous substances (ESs) can regulate plant growth and respond to environmental stress, but the effects of different ESs on blueberry fruit quality under soil cadmium (Cd) toxicity and related metabolic mechanisms are still unclear. In this study, four ES treatments [salicylic acid (SA), spermidine (Spd), 2,4-epibrassinolide (EBR), and melatonin (MT)] significantly increased blueberry fruit size, single-fruit weight, sweetness, and anthocyanin content under soil Cd toxicity and effectively reduced fruit Cd content to safe consumption levels by promoting mineral uptake (Ca, Mg, Mn, Cu and Zn). Furthermore, a total of 445, 360, 429, and 554 differentially abundant metabolites (DAMs) (LC-MS) and 63, 48, 79, and 73 DAMs (GC-MS) were identified from four comparison groups (SA/CK, Spd/CK, EBR/CK and MT/CK), respectively. The analyses revealed that ESs improved blueberry fruit quality and tolerance to Cd toxicity mainly by regulating the changes in metabolites related to ABC transporters, the TCA cycle, flavonoid biosynthesis, and phenylpropanoid biosynthesis.

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.

Silicon-Selenium Interplay Imparts Cadmium Resistance in Wheat through an Up-Regulating Antioxidant System

Cadmium (Cd), being a highly toxic heavy metal, significantly impacts plant growth and development by altering nutrient uptake and causing oxidative and structural damage, resulting in reduced yield. To combat Cd toxicity and accumulation in wheat, it was hypothesized that co-application of Selenium (Se) and Silicon (Si) can reduce the adverse effect of Cd and regulate Cd resistance while improving Se fortification in wheat. Therefore, this study evaluated the comparative effect of Se and Si on the growth and antioxidant defense systems of wheat plants grown in a hydroponic setup. Briefly, the plants were acclimatized to the hydroponic solution for 1 week and then exposed to 10 µmol Cd. Afterwards, the treatments, including 0.2 mmol Si and 1.5 µmol Se, were applied as a root and foliar application, respectively. Plants supplemented with both Se and Si showed improved biomass and other physiological growth attributes, and this response was associated with improved activity/contents of antioxidants, including glutathione (GSH) content, glutathione reductase (GR), ascorbate peroxidase (APX), and catalase (CAT), with related lowering of hydrogen peroxide, malondialdehyde content, and structural damages. Moreover, by Se + Si supplementation, a decrease in total S levels in plant tissues was observed, whereas an increase in total protein concentration and GSH indicated a different and novel mechanism of Cd tolerance and S homeostasis in the plant. It was observed that Si was more involved in significantly reducing Cd translocation by stabilizing Cd in the root and reducing its content in the soluble fraction in both the root and shoot. Whereas Se was found to play the main role in reducing the oxidative damage caused by Cd, and the effect was more profound in the shoot. In addition, this study also observed a positive correlation between Si and Se for relative uptake, which had not been reported earlier. Our findings show that the Se and Si doses together benefit growth regulation and nutrient uptake; additionally, their combinations support the Cd resistance mechanism in wheat through upregulation of the antioxidant system and control of Cd translocation and subcellular distribution, ultimately contributing to the nutritional quality of wheat produced. Thus, it is concluded that the co-application of Se and Si has improved the nutritional quality while reducing the Cd risk in wheat and therefore needs to be employed as a potential strategy to ensure food safety in a Cd-contaminated environment.

The Roles of Mepiquate Chloride and Melatonin in the Morpho-Physiological Activity of Cotton under Abiotic Stress

Cotton growth and yield are severely affected by abiotic stress worldwide. Mepiquate chloride (MC) and melatonin (MT) enhance crop growth and yield by reducing the negative effects of abiotic stress on various crops. Numerous studies have shown the pivotal role of MC and MT in regulating agricultural growth and yield. Nevertheless, an in-depth review of the prominent performance of these two hormones in controlling plant morpho-physiological activity and yield in cotton under abiotic stress still needs to be documented. This review highlights the effects of MC and MT on cotton morpho-physiological and biochemical activities; their biosynthetic, signaling, and transduction pathways; and yield under abiotic stress. Furthermore, we also describe some genes whose expressions are affected by these hormones when cotton plants are exposed to abiotic stress. The present review demonstrates that MC and MT alleviate the negative effects of abiotic stress in cotton and increase yield by improving its morpho-physiological and biochemical activities, such as cell enlargement; net photosynthesis activity; cytokinin contents; and the expression of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase. MT delays the expression of NCED1 and NCED2 genes involved in leaf senescence by decreasing the expression of ABA-biosynthesis genes and increasing the expression of the GhYUC5, GhGA3ox2, and GhIPT2 genes involved in indole-3-acetic acid, gibberellin, and cytokinin biosynthesis. Likewise, MC promotes lateral root formation by activating GA20x genes involved in gibberellin catabolism. Overall, MC and MT improve cotton's physiological activity and antioxidant capacity and, as a result, improve the ability of the plant to resist abiotic stress. The main purpose of this review is to present an in-depth analysis of the performance of MC and MT under abiotic stress, which might help to better understand how these two hormones regulate cotton growth and productivity.

Exogenous Melatonin Regulates Plant-Disease Interaction by Inducing Maize Resistance and Decreasing the Pathogenicity of Fusarium graminearum

Plants cannot avoid environmental challenges and are constantly threatened by diverse biotic and abiotic stresses. However, plants have developed a unique immune system to defend themselves against the invasion of various pathogens. Melatonin, N-acetyl-5-methoxytryptamine has positive physiological effects in plants that are involved in disease resistance. The processes underlying melatonin-induced pathogen resistance in plants are still unknown. The current study explores how melatonin regulates the plant-disease interaction in maize. The results showed that 400 μM melatonin strongly reduced the disease lesion on maize stalks by 1.5 cm and corn by 4.0 cm caused by Fusarium graminearum PH-1. Furthermore, after treatment with melatonin, the plant defense enzymes like SOD significantly increased, while POD and APX significantly decreased compared to the control. In addition, melatonin can also improve maize's innate immunity, which is mediated by melatonin treatments through the salicylic acid signaling pathway, and up-regulate the defense-associated expression of PR1, LOX1, OXR, serPIN, and WIPI genes in maize. Melatonin not only inhibits the disease in the maize stalks and corn, but also down-regulates the deoxynivalenol (DON) production-related expression of genes Tri1, Tri4, Tri5, and Tri6 in maize. Overall, this study sheds new light on the mechanisms by which melatonin regulates antioxidant enzymes and defense-related genes involved in plant immunity to effectively suppress plant diseases.

Unraveling the regulatory role of MYC2 on ASMT gene expression in wheat: Implications for melatonin biosynthesis and drought tolerance

Recognized for its multifaceted functions, melatonin is a hormone found in both animals and plants. In the plant kingdom, it plays diverse roles, regulating growth, development, and stress responses. Notably, melatonin demonstrates its significance by mitigating the effects of abiotic stresses like drought. However, understanding the precise regulatory mechanisms controlling melatonin biosynthesis genes, especially during monocots' response to stresses, requires further exploration. Seeking to understand the molecular basis of drought stress tolerance in wheat, we analyzed RNA-Seq libraries of wheat exposed to drought stress using bioinformatics methods. In light of our findings, we identified that the Myelocytomatosis oncogenes 2 (MYC2) transcription factor is a hub gene upstream of a main melatonin biosynthesis gene, N-acetylserotonin methyltransferase (ASMT), in the wheat drought response-gene network. Promoter analysis of the ASMT gene suggested that it might be a target gene of MYC2. We conducted a set of molecular and physiochemical assays along with robust machine learning approaches to elevate those findings further. MYC2 and ASMT were co-regulated under Jasmonate, drought, and a combination of them in the leaf tissues of wheat was detected. A meaningful correlation was observed among gene expression profiles, melatonin contents, photosynthetic activities, antioxidant enzyme activities, H2 O2 levels, and plasma membrane damage. The results indicated an evident relationship between jasmonic acid and the melatonin biosynthesis pathway. Moreover, it seems that the MYC2-ASMT module might contribute to wheat drought tolerance by regulating melatonin contents.

Experimental approaches in studying active biomolecules modulating fruit ripening: Melatonin as a case study

Phytohormones are naturally occurring small organic molecules found at low concentrations in plants. They perform essential functions in growth and developmental processes, from organ initiation to senescence, including fruit ripening. These regulatory molecules are studied using different experimental approaches, such as performing exogenous applications, evaluating endogenous levels, and/or obtaining genetically modified lines. Here, we discuss the advantages and limitations of current experimental approaches used to study active biomolecules modulating fruit ripening, focusing on melatonin. Although melatonin has been implicated in fruit ripening in several model fruit crops, current knowledge is affected by the different experimental approaches used, which have given different and sometimes even contradictory results. The methods of application and the doses used have produced different results in studies based on exogenous applications, while different measurement methods and ways of expressing results explain most of the variability in studies using correlative analyses. Furthermore, studies on genetically modified crops have focused on tomato (Solanum lycopersicum L.) plants only. However, TILLING and CRISPR methodologies are becoming essential tools to complement the results from the experimental approaches described above. This will not only help the scientific community better understand the role of melatonin in modulating fruit ripening, but it will also help develop technological advances to improve fruit yield and quality in major crops. The combination of various experimental approaches will undoubtedly lead to a complete understanding of the function of melatonin in fruit ripening in the near future, so that this knowledge can be effectively transferred to the field.

Melatonin-induced plant adaptation to cadmium stress involves enhanced phytochelatin synthesis and nutrient homeostasis in Solanum lycopersicum L

Cadmium (Cd) pollution is an increasingly serious problem in crop production. Although significant progress has been made to comprehend the molecular mechanism of phytochelatins (PCs)-mediated Cd detoxification, the information on the hormonal regulation of PCs is very fragmentary. In the present study, we constructed TRV-COMT, TRV-PCS, and TRV-COMT-PCS plants to further assess the function of CAFFEIC ACID O-METHYLTRANSFERASE (COMT) and PHYTOCHELATIN SYNTHASE (PCS) in melatonin-induced regulation of plant resistance to Cd stress in tomato. Cd stress significantly decreased chlorophyll content and CO₂ assimilation rate, but increased Cd, H₂O₂ and MDA accumulation in the shoot, most profoundly in PCs deficient TRV-PCS and TRV-COMT-PCS plants. Notably, Cd stress and exogenous melatonin treatment significantly increased endogenous melatonin and PC contents in non-silenced plants. Results also explored that melatonin could alleviate oxidative stress and enhance antioxidant capacity and redox homeostasis by conserving improved GSH:GSSG and ASA:DHA ratios. Moreover, melatonin improves osmotic balance and nutrient absorption by regulating the synthesis of PCs. This study unveiled a crucial mechanism of melatonin-regulated PC synthesis, persuaded Cd stress tolerance and nutrient balance in tomato, which may have potential implications for the enhancement of plant resistance to toxic heavy metal stress.

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.

Melatonin mediates elevated carbon dioxide-induced photosynthesis and thermotolerance in tomato

Increasing carbon dioxide (CO₂ ) promotes photosynthesis and mitigates heat stress-induced deleterious effects on plants, but the regulatory mechanisms remain largely unknown. Here, we found that tomato (Solanum lycopersicum L.) plants treated with high atmospheric CO₂ concentrations (600, 800, and 1000 µmol mol^-1 ) accumulated increased levels of melatonin (N-acetyl-5-methoxy tryptamine) in their leaves and this response is conserved across many plant species, including Arabidopsis, rice, wheat, mustard, cucumber, watermelon, melon, and hot pepper. Elevated CO₂ (eCO₂ ; 800 µmol mol^-1 ) caused a 6.8-fold increase in leaf melatonin content, and eCO₂ -induced melatonin biosynthesis preferentially occurred through chloroplast biosynthetic pathways in tomato plants. Crucially, manipulation of endogenous melatonin levels by genetic means affected the eCO₂ -induced accumulation of sugar and starch in tomato leaves. Furthermore, net photosynthetic rate, maximum photochemical efficiency of photosystem II, and transcript levels of chloroplast- and nuclear-encoded photosynthetic genes, such as rbcL, rbcS, rbcA, psaD, petB, and atpA, significantly increased in COMT1 overexpressing (COMT1-OE) tomato plants, but not in melatonin-deficient comt1 mutants at eCO₂ conditions. While eCO₂ enhanced plant tolerance to heat stress (42°C) in wild-type and COMT1-OE, melatonin deficiency compromised eCO₂ -induced thermotolerance in comt1 plants. The expression of heat shock proteins genes increased in COMT1-OE but not in comt1 plants in response to eCO₂ under heat stress. Further analysis revealed that eCO₂ -induced thermotolerance was closely linked to the melatonin-dependent regulation of reactive oxygen species, redox homeostasis, cellular protein protection, and phytohormone metabolism. This study unveiled a crucial mechanism of elevated CO₂ -induced thermotolerance in which melatonin acts as an essential endogenous signaling molecule in tomato plants.

Melatonin alleviates chromium toxicity by altering chromium subcellular distribution and enhancing antioxidant metabolism in wheat seedlings

The endogenous stimulating molecule melatonin (N-acetyl-5-methoxytryptamine, MT) has an important function in mitigating the impact of multiple abiotic stressors. However, the ameliorating effect of MT on chromium (Cr) stress and its mechanisms remains unclear. Therefore, the present study aimed to clarify the mitigating effect of exogenous MT (0 μM and 100 μM) on wheat seedlings under Cr (0 μM and 50 μM) stress stemming from the growth and physiological characteristics, phytochelatin (PC) biosynthesis, Cr subcellular distribution, and antioxidant system of the plants in these treatments. The results showed that endogenous MT application significantly promoted plant growth and improved root morphology of wheat seedlings under Cr stress due to decreased Cr and reactive oxygen species (ROS) accumulation in both roots and leaves. Accumulation and transport of Cr from roots to leaves were reduced by MT, because enhanced vacuolar sequestration via upregulated PC accumulation, took place, derived from the fact that MT upregulated the expression of key genes for PC synthesis (TaPCS and Taγ-ECS). Furthermore, MT pre-treatment alleviated Cr-induced oxidative damage by diminishing lipid peroxidation and cell apoptosis, profiting from the enhanced scavenging ability of ROS as a result of the MT-induced increase in the activities of superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, and the related encoding gene expression levels of TaSOD2, TaCAT, TaAPX, and TaGR. In conclusion, endogenous MT application improved the growth traits, antioxidant system, and decreased Cr accumulation especially at the leaf level in wheat seedlings under Cr stress mainly through enhancing antioxidant enzyme activities and altering Cr subcellular distribution via strengthening PC biosynthesis. The mechanisms of MT-induced plant tolerance to Cr stress could help develop new strategies for secure crop production in Cr-polluted soils.

Melatonin reduces cadmium accumulation via mediating the nitric oxide accumulation and increasing the cell wall fixation capacity of cadmium in rice

Melatonin (MT) is participated in plants' response to cadmium (Cd) tolerance, although its work model remains elusive. Here, the function of MT in adjusting Cd accumulation in rice was investigated. 'Nipponbare' (Nip) was cultured in the -Cd (1/2 Kimura B), -Cd + MT (1/2 Kimura B with 1 μM MT), +Cd (1/2 Kimura B plus 1 μM Cd) and +Cd + MT (1/2 Kimura B with 1 μM Cd and 1 μM MT) nutrient solutions for 7 d. Cd markedly induced the endogenous MT accumulation in rice roots and shoots, even within 1 h. MT applied exogenously elevated the hemicelluloses level, which in turn increased the cell wall's binding capacity to Cd. Furthermore, MT applied exogenously down-regulated the transcription level of Natural Resistance-Associated Macrophage Protein 1 (OsNRAMP1), OsNRAMP5, a major facilitator superfamily gene (OsCd1), and IRON-REGULATED TRANSPORTER 1 (OsIRT1), all of which were responsible for Cd intake, thus less Cd was entered into roots. Moreover, MT applied exogenously also up-regulated transcription level of Cadmium accumulation in Leaf 1 (OsCAL1) and Heavy Metal ATPase 3 (OsHMA3), two genes both attributed to the decreased Cd accumulation in shoots through expelling Cd out of cells and chelating Cd in the vacuoles, respectively. In addition, MT applied exogenously further aggravated the production of nitric oxide (NO) that induced by Cd, while application of a NO donor-SNP mimicked this alleviatory effect of the MT, indicating MT decreased rice Cd accumulation relied on the accumulation of NO.

Genetic and evolutionary dissection of melatonin response signaling facilitates the regulation of plant growth and stress responses

The expansion of gene families during evolution could generate functional diversity among their members to regulate plant growth and development. Melatonin, a phylogenetically ancient molecule, is vital for many aspects of a plant's life. Understanding the functional diversity of the molecular players involved in melatonin biosynthesis, signaling, and metabolism will facilitate the regulation of plant phenotypes. However, the molecular mechanism of melatonin response signaling elements in regulating this network still has many challenges. Here, we provide an in-depth analysis of the functional diversity and evolution of molecular components in melatonin signaling pathway. Genetic analysis of multiple mutants in plant species will shed light on the role of gene families in melatonin regulatory pathways. Phylogenetic analysis of these genes was performed, which will facilitate the identification of melatonin-related genes for future study. Based on the abovementioned signal networks, the mechanism of these genes was summarized to provide reference for studying the regulatory mechanism of melatonin in plant phenotypes. We hope that this work will facilitate melatonin research in higher plants and finely tuned spatio-temporal regulation of melatonin signaling.

Transcriptional Regulatory Network of Plant Cadmium Stress Response

Cadmium (Cd) is a non-essential heavy metal with high toxicity to plants. Plants have acquired specialized mechanisms to sense, transport, and detoxify Cd. Recent studies have identified many transporters involved in Cd uptake, transport, and detoxification. However, the complex transcriptional regulatory networks involved in Cd response remain to be elucidated. Here, we provide an overview of current knowledge regarding transcriptional regulatory networks and post-translational regulation of the transcription factors involved in Cd response. An increasing number of reports indicate that epigenetic regulation and long non-coding and small RNAs are important in Cd-induced transcriptional responses. Several kinases play important roles in Cd signaling that activate transcriptional cascades. We also discuss the perspectives to reduce grain Cd content and improve crop tolerance to Cd stress, which provides a theoretical reference for food safety and the future research of plant varieties with low Cd accumulation.

Advances in the Role and Mechanisms of Essential Oils and Plant Extracts as Natural Preservatives to Extend the Postharvest Shelf Life of Edible Mushrooms

China has a large variety of edible mushrooms and ranks first in the world in terms of production and variety. Nevertheless, due to their high moisture content and rapid respiration rate, they experience constant quality deterioration, browning of color, loss of moisture, changes in texture, increases in microbial populations, and loss of nutrition and flavor during postharvest storage. Therefore, this paper reviews the effects of essential oils and plant extracts on the preservation of edible mushrooms and summarizes their mechanisms of action to better understand their effects during the storage of mushrooms. The quality degradation process of edible mushrooms is complex and influenced by internal and external factors. Essential oils and plant extracts are considered environmentally friendly preservation methods for better postharvest quality. This review aims to provide a reference for the development of new green and safe preservation and provides research directions for the postharvest processing and product development of edible mushrooms.

Reactive oxygen species signaling is involved in melatonin-induced reduction of chlorothalonil residue in tomato leaves

Pesticide overuse has led to serious global concerns regarding food safety and environmental pollution. Although the reduction of pesticide residue is critical, our knowledge about induced pesticide metabolism in plants remains fragmentary. Melatonin (N-acetyl-5-methoxytryptamine) is an effective stress-relieving agent in both animals and plants, but little is known about the melatonin signaling mechanism and its effect on pesticide metabolism in plants. Here, we found that exogenous melatonin treatment significantly reduced chlorothalonil residue by 41 % but suppression of endogenous melatonin accumulation increased chlorothalonil residue in tomato leaves. Moreover, melatonin increased photosynthesis, Fv/Fm, Calvin cycle enzyme activity, antioxidant enzyme activity, glutathione pool, and RESPIRATORY BURST HOMOLOG1 (RBOH1) expression in tomato leaves. However, the upregulation of RBOH1, CYP724B2, GST1, GST2, GSH and ABC, the increased glutathione concentrations and the activity of detoxification enzymes due to melatonin treatment were all significantly attenuated by the treatment with an NADPH oxidase inhibitor and a ROS scavenger, indicating a clear relationship between the reduction of pesticide residue and induction in detoxifying enzymes and genes upon melatonin treatment in an apoplastic H₂O₂-dependent manner. These results reveal that melatonin-induced reduction in chlorothalonil residue is mediated by H₂O₂ signaling in tomato leaves.

Melatonin Involved in Protective Effects against Cadmium Stress in Wolffia arrhiza

Melatonin (MT) is a new plant hormone that protects against adverse environmental conditions. In the present study, the responses of Wolffia arrhiza exposed to cadmium (Cd) and MT were analyzed. Quantitative analysis of MT and precursors of its biosynthesis was performed using LC-MS-MS. The photosynthetic pigments and phytochelatins (PCs) contents were determined using HPLC, while protein and monosaccharides, stress markers, and antioxidant levels were determined using spectrophotometric methods. Interestingly, the endogenous level of MT and its substrates in W. arrhiza exposed to 1-100 µM Cd was significantly higher compared to the control. Additionally, the application of 25 µM MT and Cd intensified the biosynthesis of these compounds. The most stimulatory effect on the growth and content of pigments, protein, and sugars was observed in plants treated with 25 µM MT. In contrast, Cd treatment caused a decrease in plant weight and level of these compounds, while the application of 25 µM MT mitigated the inhibitory effect of Cd. Additionally, Cd enhanced the level of stress markers; simultaneously, MT reduced their content in duckweed exposed to Cd. In plants treated with Cd, PC levels were increased by Cd treatment and by 25 µM MT. These results confirmed that MT mitigated the adverse effect of Cd. Furthermore, MT presence was reported for the first time in W. arrhiza. In summary, MT is an essential phytohormone for plant growth and development, especially during heavy metal stress.

Applications of Metabolomics for the Elucidation of Abiotic Stress Tolerance in Plants: A Special Focus on Osmotic Stress and Heavy Metal Toxicity

Plants undergo metabolic perturbations under various abiotic stress conditions; due to their sessile nature, the metabolic network of plants requires continuous reconfigurations in response to environmental stimuli to maintain homeostasis and combat stress. The comprehensive analysis of these metabolic features will thus give an overview of plant metabolic responses and strategies applied to mitigate the deleterious effects of stress conditions at a biochemical level. In recent years, the adoption of metabolomics studies has gained significant attention due to the growing technological advances in analytical biochemistry (plant metabolomics). The complexity of the plant biochemical landscape requires sophisticated, advanced analytical methods. As such, technological advancements in the field of metabolomics have been realized, aided much by the development and refinement of separatory techniques, including liquid and gas chromatography (LC and GC), often hyphenated to state-of-the-art detection instruments such as mass spectrometry (MS) or nuclear resonance magnetic (NMR) spectroscopy. Significant advances and developments in these techniques are briefly highlighted in this review. The enormous progress made thus far also comes with the dawn of the Internet of Things (IoT) and technology housed in machine learning (ML)-based computational tools for data acquisition, mining, and analysis in the 4IR era allowing for broader metabolic coverage and biological interpretation of the cellular status of plants under varying environmental conditions. Thus, scientists can paint a holistic and comprehensive roadmap and predictive models for metabolite-guided crop improvement. The current review outlines the application of metabolomics and related technological advances in elucidating plant responses to abiotic stress, mainly focusing on heavy metal toxicity and subsequent osmotic stress tolerance.

Crosstalk between melatonin and reactive oxygen species in fruits and vegetables post-harvest preservation: An update

Fruits and vegetables contain numerous nutrients, such as vitamins, minerals, phenolic compounds, and dietary fibers. They reduce the incidence of cardiovascular diseases and the risk of certain chronic diseases, and improve the antioxidant and anti-inflammatory capacity. Moreover, melatonin was found in various fruits and vegetables species. Melatonin acts as a multifunctional compound to participate in various physiological processes. In recent years, many advances have been found that melatonin is also appraised as a key modulator on the fruits and vegetables post-harvest preservation. Fruits and vegetables post-harvest usually elicit reactive oxygen species (ROS) generation and accumulation. Excess ROS stimulate cell damage, protein structure destruction, and tissue aging, and thereby reducing their quality. Numerous studies find that exogenous application of melatonin modulates ROS homeostasis by regulating the antioxidant enzymes and non-enzymatic antioxidants systems. Further evidences reveal that melatonin often interacts with hormones and other signaling molecules, such as ROS, nitric oxide (NO), hydrogen sulfide (H₂S), and etc. Among these 'new' molecules, crosstalks of melatonin and ROS, especially the H₂O₂ produced by RBOHs, are provided in fruits and vegetables post-harvest preservation in this review. It will provide reference for complicated integration of both melatonin and ROS as signal molecules in future study.

Interaction between selenium and essential micronutrient elements in plants: A systematic review

Nutrient imbalance (i.e., deficiency and toxicity) of microelements is an outstanding environmental issue that influences each aspect of ecosystems. Although the crucial roles of microelements in entire lifecycle of plants have been widely acknowledged, the effective control of microelements is still neglected due to the narrow safe margins. Selenium (Se) is an essential element for humans and animals. Although it is not believed to be indispensable for plants, many literatures have reported the significance of Se in terms of the uptake, accumulation, and detoxification of essential microelements in plants. However, most papers only concerned on the antagonistic effect of Se on metal elements in plants and ignored the underlying mechanisms. There is still a lack of systematic review articles to summarize the comprehensive knowledge on the connections between Se and microelements in plants. In this review, we conclude the bidirectional effects of Se on micronutrients in plants, including iron, zinc, copper, manganese, nickel, molybdenum, sodium, chlorine, and boron. The regulatory mechanisms of Se on these micronutrients are also analyzed. Moreover, we further emphasize the role of Se in alleviating element toxicity and adjusting the concentration of micronutrients in plants by altering the soil conditions (e.g., adsorption, pH, and organic matter), promoting microbial activity, participating in vital physiological and metabolic processes, generating element competition, stimulating metal chelation, organelle compartmentalization, and sequestration, improving the antioxidant defense system, and controlling related genes involved in transportation and tolerance. Based on the current understanding of the interaction between Se and these essential elements, future directions for research are suggested.

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.

Phosphorus Regulates the Level of Signaling Molecules in Rice to Reduce Cadmium Toxicity

Phosphorus treatment can reduce Cd accumulation and Cd toxicity in rice, but alterations in the internal regulatory network of rice during this process have rarely been reported. We have removed the effect of cadmium phosphate precipitation from the hydroponic system, treated a pair of different Cd-response rice varieties with different levels of phosphorus and cadmium and examined the changes in physiological indicators and regulatory networks. The results demonstrated that phosphorus treatment significantly reduced Cd accumulation in both types of rice, although the antioxidant systems within the two types of rice produced opposite responses. Overall, 3 mM phosphorus treatment to Cd-N decreased the expression of OsIAA17 and OsACO1 by 32% and 37%, respectively, while increasing the expression of OsNR2 by 83%; these three genes regulate the synthesis of auxin, ethylene, and nitric oxide in rice. IAA and NO levels in rice shoots increased by 24% and 96%, respectively, and these changes contribute to Cd detoxification. The cadmium transporter genes OsHMA2, OsIRT1, and OsABCC1 were significantly down-regulated in Cd-N roots after triple phosphorus treatment. These data suggest that phosphorus treatment can reduce Cd accumulation and enhance Cd resistance in rice by affecting the expression of signaling molecules.

Melatonin enhanced oilseed rape growth and mitigated Cd stress risk: A novel trial for reducing Cd accumulation by bioenergy crops

Melatonin (M) is a pleiotropic molecule that improves plant growth and increases heavy metal tolerance. The role of M for improving plant growth and tolerance under cadmium (Cd) stress, and mitigation of Cd-induced toxicity has not yet been sufficiently examined. Therefore, here we conducted a glasshouse experiment to explore the influence of various M dosages on Cd detoxification and stress-tolerance responses of Brassica napus under high Cd content (30 mg kg^-1). The effects of M on the modulation of Cd tolerance in B. napus plants have been investigated using various growth attributes, Cd accumulation and tolerance indices, and secondary metabolic parameters. We found that Cd stress inhibited root growth (by 11.9%) as well as triggered reactive oxygen species accumulation (by 31.2%) and MDA levels (by 18.7%); however, exogenous M substantially alleviated the adverse effect of oxidative stress by decreasing levels of H₂O₂ (by 38.7%), MDA (by 13.8%) and EL (by 1.8%) in the Cd-stressed plants, as compared to the M-untreated plants (control). Interestingly, exogenous M reduced Cd accumulation in roots (∼48.2-58.3-fold), stem (∼2.9-5.0-fold) and leaves (∼4.7-6.6-fold) compared to control plants, which might be due to an M-induced defense and/or detoxification response involving a battery of antioxidants. Overall, addition of the exogenous M to the Cd-stressed plants profoundly enhanced Cd tolerance in B. napus relative to control plants. These results suggested the biostimulatory role (at the physiological and molecular level) of M in improving growth, Cd tolerance, and Cd detoxification in B. napus, which indicate the potentiality of M for green remediation of Cd contaminated soils. This green trial would provide a reference for producing renewable bioenergy crops under Cd stress in contaminated soils. However, these recommendations should be verified under field conditions and the potential mechanisms for the interaction between Cd and M should be explicitly explored.

Melatonin: First-line soldier in tomato under abiotic stress current and future perspective

Melatonin is a natural, multifunctional, nontoxic, regulatory, and ubiquitous biomolecule, having low molecular weight and pleiotropic effects in the plant kingdom. It is a recently discovered plant master regulator which has a crucial role under abiotic stress conditions (salinity, drought, heat, cold, alkalinity, acid rain, ozone, and metals stress). In the solanaceous family, the tomato is highly sensitive to abiotic stresses that affect its growth and development, ultimately hampering production and productivity. Melatonin acts as a strong antioxidant, bio-stimulator, and growth regulator, facilitating photosynthesis, delaying leaf senescence, and increasing the antioxidant enzymes system through direct scavenging of reactive oxygen species (ROS) under abiotic stresses. In addition, melatonin also boosts morphological traits such as vegetative growth, leaf photosynthesis, root architecture system, mineral nutrient elements, and antioxidant activities in tomato plants, confirming their tolerances against salinity, drought, heat, cold, alkalinity, acid rain, chemical, pathogen, and metals stress. In this review, an attempt has been made to summarize the potential role of melatonin for tomato plant endurance towards abiotic stresses, along with the known relationship between the two.

Exogenous melatonin improved photosynthetic efficiency of photosystem II by reversible phosphorylation of thylakoid proteins in wheat under osmotic stress

It has been well demonstrated that melatonin plays an important protective role in photosynthesis of plants under various environmental stresses, while the detailed mechanisms by which melatonin protects photosystem II (PSII) under environmental stress are still unclear. In the study, the effects of melatonin on photosynthetic efficiency, energy dissipation, PSII protein composition, and reversible phosphorylation of thylakoid proteins were investigated in wheat plants under osmotic stress. The results showed that osmotic stress significantly reduced pigment content, photochemical efficiency of PSII, oxygen-evolving activity, and dissipation of excess excitation energy, while 25 μM melatonin applications greatly alleviated their decline under osmotic stress. Western blot data of PSII proteins revealed that melatonin upregulated the levels of D1, Lhcb5, Lhcb6, PsbQ, and PsbS proteins in wheat exposed to osmotic stress. In addition, thylakoid membrane proteins were strongly phosphorylated in wheat under osmotic stress with or without melatonin. Furthermore, the results from PSII protein dephosphorylation showed that exogenous melatonin promoted the dephosphorylation of LCHII, CP43, and D1 under osmotic stress. Therefore, our findings suggest that melatonin can provide an effective protection for the photosynthetic apparatus by the regulation of PSII proteins and the reversible phosphorylation of thylakoid proteins under drought stress.

The role of melatonin in tomato stress response, growth and development

Melatonin has attracted widespread attention after its discovery in higher plants. Tomato is a key model economic crop for studying fleshy fruits. Many studies have shown that melatonin plays important role in plant stress resistance, growth, and development. However, the research progress on the role of melatonin and related mechanisms in tomatoes have not been systematically summarized. This paper summarizes the detection methods and anabolism of melatonin in tomatoes, including (1) the role of melatonin in combating abiotic stresses, e.g., drought, heavy metals, pH, temperature, salt, salt and heat, cold and drought, peroxidation hydrogen and carbendazim, etc., (2) the role of melatonin in combating biotic stresses, such as tobacco mosaic virus and foodborne bacillus, and (3) the role of melatonin in tomato growth and development, such as fruit ripening, postharvest shelf life, leaf senescence and root development. In addition, the future research directions of melatonin in tomatoes are explored in combination with the role of melatonin in other plants. This review can provide a theoretical basis for enhancing the scientific understanding of the role of melatonin in tomatoes and the improved breeding of fruit crops.

Synergistic Effect of Melatonin and Selenium Improves Resistance to Postharvest Gray Mold Disease of Tomato Fruit

Melatonin (MT) is a ubiquitous hormone molecule that is commonly distributed in nature. MT not only plays an important role in animals and humans but also has extensive functions in plants. Selenium (Se) is an essential micronutrient for animals and humans, and is a beneficial element in higher plants at low concentrations. Postharvest diseases caused by fungal pathogens lead to huge economic losses worldwide. In this study, tomato fruits were treated with an optimal sodium selenite (20 mg/L) and melatonin (10 μmol/L) 2 h and were stored for 7 days at room temperature simulating shelf life, and the synergistic effects of Se and MT collectively called Se-Mel on gray mold decay in tomato fruits by Botrytis cinerea was investigated. MT did not have antifungal activity against B. cinerea in vitro, while Se significantly inhibited gray mold development caused by B. cinerea in tomatoes. However, the interaction of MT and Se showed significant inhibition of the spread and growth of the disease, showing the highest control effect of 74.05%. The combination of MT with Se treatment enhanced the disease resistance of fruits by improving the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as increasing the gene expression level of pathogenesis-related (PR) proteins. Altogether, our results indicate that the combination of MT and Se would induce the activation of antioxidant enzymes and increase the expression of PR proteins genes that might directly enhance the resistance in tomato fruit against postharvest pathogenic fungus B. cinerea.

Melatonin-Induced Protection Against Plant Abiotic Stress: Mechanisms and Prospects

Global warming in this century increases incidences of various abiotic stresses restricting plant growth and productivity and posing a severe threat to global food production and security. The plant produces different osmolytes and hormones to combat the harmful effects of these abiotic stresses. Melatonin (MT) is a plant hormone that possesses excellent properties to improve plant performance under different abiotic stresses. It is associated with improved physiological and molecular processes linked with seed germination, growth and development, photosynthesis, carbon fixation, and plant defence against other abiotic stresses. In parallel, MT also increased the accumulation of multiple osmolytes, sugars and endogenous hormones (auxin, gibberellic acid, and cytokinins) to mediate resistance to stress. Stress condition in plants often produces reactive oxygen species. MT has excellent antioxidant properties and substantially scavenges reactive oxygen species by increasing the activity of enzymatic and non-enzymatic antioxidants under stress conditions. Moreover, the upregulation of stress-responsive and antioxidant enzyme genes makes it an excellent stress-inducing molecule. However, MT produced in plants is not sufficient to induce stress tolerance. Therefore, the development of transgenic plants with improved MT biosynthesis could be a promising approach to enhancing stress tolerance. This review, therefore, focuses on the possible role of MT in the induction of various abiotic stresses in plants. We further discussed MT biosynthesis and the critical role of MT as a potential antioxidant for improving abiotic stress tolerance. In addition, we also addressed MT biosynthesis and shed light on future research directions. Therefore, this review would help readers learn more about MT in a changing environment and provide new suggestions on how this knowledge could be used to develop stress tolerance.

Comparative Analysis of Italian Lettuce (Lactuca sativa L. var. ramose) Transcriptome Profiles Reveals the Molecular Mechanism on Exogenous Melatonin Preventing Cadmium Toxicity

Cadmium (Cd) accumulation in lettuce causes a large amount of yield loss during industry. Although many studies report that exogenous melatonin helps to alleviate the Cd stress of lettuce, the molecular mechanism for how plant tissue responds to Cd treatment is unclear. Herein, we applied both PacBio and Illumina techniques for Italian lettuce under different designed treatments of Cd and melatonin, aiming to reveal the potential molecular pathway of the response to Cd stress as well as the how the pre-application of exogenous melatonin affect this process. This result reveals that the root has the biggest expression pattern shift and is a more essential tissue to respond to both Cd and melatonin treatments than leaves. We reveal the molecular background of the Cd stress response in prospects of antioxidant and hormone signal transduction pathways, and we found that their functions are diverged and specifically expressed in tissues. We also found that candidate genes related to melatonin detoxify during Cd stress. Our study sheds new light for future research on how melatonin improves the cadmium resistance of lettuce and also provide valuable data for lettuce breeding.

Cadmium in Cereal Crops: Uptake and Transport Mechanisms and Minimizing Strategies

Cadmium (Cd) contamination in soils and accumulation in cereal grains have posed food security risks and serious health concerns worldwide. Understanding the Cd transport process and its management for minimizing Cd accumulation in cereals may help to improve crop growth and grain quality. In this review, we summarize Cd uptake, translocation, and accumulation mechanisms in cereal crops and discuss efficient measures to reduce Cd uptake as well as potential remediation strategies, including the applications of plant growth regulators, microbes, nanoparticles, and cropping systems and developing low-Cd grain cultivars by CRISPR/Cas9. In addition, miRNAs modulate Cd translocation, and accumulation in crops through the regulation of their target genes was revealed. Combined use of multiple remediation methods may successfully decrease Cd concentrations in cereals. The findings in this review provide some insights into innovative and applicable approaches for reducing Cd accumulation in cereal grains and sustainable management of Cd-contaminated paddy fields.

Red and blue light function antagonistically to regulate cadmium tolerance by modulating the photosynthesis,antioxidant defense system and Cd uptake in cucumber(Cucumis sativus L.)

Cadmium (Cd) is highly toxic to both plants and humans.Light plays crucial roles in plant growth, development and stress responses, but how light functions in plant Cd response remain unclear.Here,we found that Cd treatment significantly induced the expression of PHYB but not PHYA and CRY1 in leaves and roots of cucumber. Correspondingly,compared with white light (W) during Cd stress,red light(R) increased Cd sensitivity,whereas blue light (B) enhanced Cd tolerance as evidenced by decreased Cd-induced chlorosis, growth inhibition, photosynthesis inhibition and chloroplast ultrastructure damage.Furthermore,B markedly increased the transcripts and activities of the antioxidant enzymes including ascorbate peroxidase (APX),catalase (CAT),superoxide dismutase (SOD) and glutathione reductase (GR),as well as glutathione (GSH) content and GSH1 expression, resulting in hydrogen peroxide (H₂O₂) and superoxide (O₂^.-) reduction,but R treatment showed the opposite trend. Moreover, R and B markedly up-regulated and down-regulated the expression levels of Cd uptake and transport genes including IRT1, NRAMP1 and HMA3, leading to more and less Cd accumulation than the W-treated plants in both shoots and roots, respectively under Cd stress. Collectively, our data clearly demonstrate that R and B function antagonistically to regulate Cd tolerance in cucumber via modulating the photosynthesis, antioxidant defense system and Cd uptake, providing a novel light quality control strategy to enhance crop Cd tolerance and food safety.

He-Ne laser irradiation ameliorates cadmium toxicity in wheat by modulating cadmium accumulation, nutrient uptake and antioxidant defense system

Cadmium (Cd) is one of the most hazardous heavy metals that negatively affect the growth and yield of wheat. He-Ne laser irradiation is known to ameliorate cadmium (Cd) stress in wheat. However, the underlying mechanism of He-Ne laser irradiation on protecting wheat against Cd stress is not well recognized. In present study, Cd-treated wheat showed significant reduction in growth, root morphology and total chlorophyll content, but notably increase of Cd accumulation in both roots and shoots. However, He-Ne laser irradiation dramatically reduced concentrations of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and increased total chlorophyll content and activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) in roots of wheat plants under Cd stress. Further, He-Ne laser irradiation significantly upregulated the transcripts of TaGR (glutathione reductase) and TaGST (glutathione-S-transferase) genes along with the increased activities of GR and GST and glutathione (GSH) concentration in roots of wheat seedlings under Cd stress. In addition, He-Ne laser irradiation enhanced the uptake of mineral elements (N, P, Mg, Fe, Zn and Cu), and significantly decreased Cd uptake and transport mainly through down-regulating the expressions of Cd transport genes (TaHMA2 and TaHMA3) in roots of wheat seedlings under Cd stress. Overall, these findings suggested that He-Ne laser irradiation alleviated the adverse effects of Cd on wheat growth by enhancing antioxidant defense system, improving mineral nutrient status, and decreasing the Cd uptake and transport. This study provides new insights into the roles of He-Ne laser irradiation in the amelioration of Cd stress in wheat and indicates the potential application of this irradiation in crop breeding and growth under Cd stress conditions.

Exogenous melatonin mitigates chromium toxicity in maize seedlings by modulating antioxidant system and suppresses chromium uptake and oxidative stress

Melatonin, being an endogenous signaling molecule plays important role in plant growth and stress alleviation. The present study was conducted to evaluate the ameliorative role of melatonin against Cr toxicity in maize seedlings. The Cr toxicity (50, 100 and 200 µM) severely affected hydroponically grown seedlings growth in a dose-dependent manner; however, the melatonin (0.5 and 1.0 µM) application markedly restored toxicity-induced growth retardation. Higher dose of melatonin (1.0 µM) was more effective in case of lower Cr toxicity (50 and 100 µM). Exposure of 200 µM Cr caused 45% and 43% reduction in shoot and root lengths and more than 80% reduction in biomass. In case of 200 µM Cr toxicity, application of 1.0 µM MT effectively restored shoot and root lengths reduction (from 45 to 30%) and biomass decline (from 80 to around 60%). Biomass restoration by 1.0 µM melatonin under 50 and 100 µM Cr was even more pronounced bringing it near to control plants having no Cr exposure. Further, both melatonin levels also improved root tips, root diameter, root volume and root surface area that had been damaged by Cr exposure. The melatonin also alleviated Cr-induced chlorophyll and carotenoids inhibition, improved relative water content, and markedly lowered proline and MDA content in shoots. Lower accumulation of MDA and proline, and greater membrane stability indices indicate that the melatonin conferred better plant growth by playing the role of antioxidant and detoxifying oxidative stress creating substances. Although antioxidant enzymes viz. SOD, POD, CAT and APX activities were also elevated by MT, this increase was not significantly different in the most of cases. No significant difference in NPK contents of shoot was observed by Cr and melatonin application indicating the growth retardation being caused directly by Cr intrinsic toxicity and not by nutrients deficiency. The melatonin-based amelioration of Cr toxicity in maize seedlings seems as the result of its nature as antioxidant, and not by activation/elevation of antioxidative enzymatic system.

GABA-mediated inhibition of cadmium uptake and accumulation in apples

GABA, a four-carbon non-protein amino acid, plays an important role in animals and plants. We previously found GABA could alleviate alkali stress in apple seedlings. However, its physiological mechanism under heavy metal cadmium (Cd) stress need to be further studied. Thus, we explored its biological role in response to Cd stress. It was verified that 0.5 mM GABA could effectively alleviate Cd toxicity. Using NMT technique, we found that exogenous GABA could significantly reduce the net Cd²⁺ fluxes in apple roots, and Cd content was significantly lower than that in roots under Cd stress. Further analysis indicated exogenous GABA could significantly reduce the expression of genes related to the uptake and transport of Cd in apples under Cd stress. In addition, exogenous GABA could significantly increase the content of amino acids in apple roots under Cd stress. GAD is a key enzyme in GABA synthesis, we obtained transgenic apple roots of overexpression MdGAD1. Compared with the control, transgenic roots accumulated less Cd, maintained lower Cd uptake by roots, and lower expression of related transport genes. These results showed that GABA could effectively alleviate Cd toxicity in apple seedlings and provide a new perspective of GABA to alleviate Cd stress.

Melatonin Rescues Photosynthesis and Triggers Antioxidant Defense Response in Cucumis sativus Plants Challenged by Low Temperature and High Humidity

Environmental factors such as low temperature (LT) and high humidity (HH) hinder plant growth and development in plastic tunnels and solar greenhouses in the cold season. In this study, we examined the effect of melatonin (MT) on shoot-based tolerance to LT and HH conditions in cucumber (Cucumis sativus) seedlings and explored its underlying mechanism. LT and HH stress inhibited growth and biomass accumulation, produced leaf chlorosis, led to oxidative stress, lowered chlorophyll and carotenoid contents, reduced photosynthetic and photosystem II (PSII) activities, and increased the level of intercellular carbon dioxide and the non-photochemical quenching of photosystem I (PSI) and PSII. However, foliar application of MT significantly improved the morphological indices and photosynthetic efficiency of cucumber seedlings, which entailed the elevation of electrolyte leakage, lipid peroxidation, and reactive oxygen species accumulation by boosting the antioxidant enzyme defense system under LT and HH conditions. Additionally, the measurement of nitrogen (N), magnesium (Mg), and iron (Fe) contents in roots and leaves showed that MT significantly augmented the nutrient uptake of cucumber seedlings exposed to LT and HH stresses. Furthermore, MT application increased the transcripts levels of genes encoding antioxidant enzymes under LT and HH conditions, whereas treatment with LT and HH suppressed these genes, suggesting that MT application increases the LT and HH tolerance of cucumber seedlings. Overall, our results suggest that MT application increases the tolerance of cucumber seedlings to LT and HH stress by enhancing the plant morphometric parameters, regulating PSI and PSII, and activating the antioxidant defense mechanism. Thus, the exogenous application of MT could be potentially employed as a strategy to improve the LT and HH tolerance of cucumber.

Walnut N-Acetylserotonin Methyltransferase Gene Family Genome-Wide Identification and Diverse Functions Characterization During Flower Bud Development

Melatonin widely mediates multiple developmental dynamics in plants as a vital growth stimulator, stress protector, and developmental regulator. N-acetylserotonin methyltransferase (ASMT) is the key enzyme that catalyzes the final step of melatonin biosynthesis in plants and plays an essential role in the plant melatonin regulatory network. Studies of ASMT have contributed to understanding the mechanism of melatonin biosynthesis in plants. However, AMST gene is currently uncharacterized in most plants. In this study, we characterized the JrASMT gene family using bioinformatics in a melatonin-rich plant, walnut. Phylogenetic, gene structure, conserved motifs, promoter elements, interacting proteins and miRNA analyses were also performed. The expansion and differentiation of the ASMT family occurred before the onset of the plant terrestrialization. ASMT genes were more differentiated in dicotyledonous plants. Forty-six ASMT genes were distributed in clusters on 10 chromosomes of walnut. Four JrASMT genes had homologous relationships both within walnut and between species. Cis-regulatory elements showed that JrASMT was mainly induced by light and hormones, and targeted cleavage of miRNA172 and miR399 may be an important pathway to suppress JrASMT expression. Transcriptome data showed that 13 JrASMT were differentially expressed at different periods of walnut bud development. WGCNA showed that JrASMT1/10/13/23 were coexpressed with genes regulating cell fate and epigenetic modifications during early physiological differentiation of walnut female flower buds. JrASMT12/28/37/40 were highly expressed during morphological differentiation of flower buds, associated with altered stress capacity of walnut flower buds, and predicted to be involved in the regulatory network of abscisic acid, salicylic acid, and cytokinin in walnut. The qRT-PCR validated the results of differential expression analysis and further provided three JrASMT genes with different expression profiles in walnut flower bud development. Our study explored the evolutionary relationships of the plant ASMT gene family and the functional characteristics of walnut JrASMT. It provides a valuable perspective for further understanding the complex melatonin mechanisms in plant developmental regulation.

Melatonin Positively Regulates Both Dark- and Age-Induced Leaf Senescence by Reducing ROS Accumulation and Modulating Abscisic Acid and Auxin Biosynthesis in Cucumber Plants

Melatonin (MT), as a signaling molecule, plays a vital role in regulating leaf senescence in plants. This study aimed to verify the antioxidant roles of MT in delaying dark- or age-induced leaf senescence of cucumber plants. The results showed that endogenous MT responds to darkness and overexpression of CsASMT, the key gene of MT synthesis, and delays leaf senescence stimulated by darkness, as manifested by significantly lower malonaldehyde (MDA) and reactive oxygen species (ROS) contents as well as higher activities and gene expression of antioxidant enzymes compared to the control. Moreover, MT suppressed both age- or dark-induced leaf senescence of cucumber, as evidenced by a decrease in senescence-related gene SAG20 and cell-death-related gene PDCD expression and ROS content and an increase in antioxidant capacity and chlorophyll biosynthesis compared with the H2O-treated seedlings. Meanwhile, the suppression of age-induced leaf senescence by melatonin was also reflected by the reduction in abscisic acid (ABA) biosynthesis and signaling pathways as well as the promotion of auxin (IAA) biosynthesis and signaling pathways in cucumber plants in the solar greenhouse. Combining the results of the two separate experiments, we demonstrated that MT acts as a powerful antioxidant to alleviate leaf senescence by activating the antioxidant system and IAA synthesis and signaling while inhibiting ABA synthesis and signaling in cucumber plants.

The E3 Ubiquitin Ligase Gene Sl1 Is Critical for Cadmium Tolerance in Solanum lycopersicum L

Heavy metal cadmium (Cd) at high concentrations severely disturbs plant growth and development. The E3 ubiquitin ligase involved in protein degradation is critical for plant tolerance to abiotic stress, but the role of E3 ubiquitin ligases in Cd tolerance is largely unknown in tomato. Here, we characterized an E3 ubiquitin ligase gene Sl1, which was highly expressed in roots under Cd stress in our previous study. The subcellular localization of Sl1 revealed that it was located in plasma membranes. In vitro ubiquitination assays confirmed that Sl1 had E3 ubiquitin ligase activity. Knockout of the Sl1 gene by CRISPR/Cas9 genome editing technology reduced while its overexpression increased Cd tolerance as reflected by the changes in the actual quantum efficiency of PSII photochemistry (Φ_PSII) and hydrogen peroxide (H₂O₂) accumulation. Cd-induced increased activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) were compromised in sl1 mutants but were enhanced in Sl1 overexpressing lines. Furthermore, the content of Cd in both shoots and roots increased in sl1 mutants while reduced in Sl1 overexpressing plants. Gene expression assays revealed that Sl1 regulated the transcript levels of heavy metal transport-related genes to inhibit Cd accumulation. These findings demonstrate that Sl1 plays a critical role in regulating Cd tolerance by relieving oxidative stress and resisting heavy metal transportation in tomato. The study provides a new understanding of the mechanism of plant tolerance to heavy metal stress.

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

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

Mitigation effects of exogenous melatonin-selenium nanoparticles on arsenic-induced stress in Brassica napus

Melatonin (MT) and selenium (Se) application known to decrease heavy metal uptake and toxicity in plants. By mixing the Se in MT medium a new complex MT-Se nanoparticles (MT-Se NPs) was synthesized and we investigated the role of MT-Se NPs on B. napus growth and tolerance against As stress. The MT-Se particles significantly enhanced the plant growth and other associated physiological attributes under As stress. The As treatment at 80 μM was more phytotoxic, however MT-Se NPs application resulted in a substantial increase in leaf chlorophyll fluorescence, biomass accumulation, and decreased ROS relative to As stressed plants. The use of MT-Se NPs to As stressed plants reduced photosynthetic inhibition and oxidative stress and attenuated the increase in MDA and H₂O₂ contents. The application of MT-Se NPs also boosted the antioxidant enzymes activities such as SOD, POD and CAT as well as the APX, GR and GSH activates under As stress. The results also showed MT-Se NPs treatments alleviated the growth inhibition induced by As and reduced the accumulation of As in leaves and roots of B. napus seedlings. Moreover, treatment with MT-Se NPs improved the plant growth more successfully than treatment of MT and Se alone. This study explored the mechanism of melatonin and selenium efficiency in the composition can be jointly encouraged to exert synergistic effects and boost plant enzymatic activities.

Melatonin Promotes the Chilling Tolerance of Cucumber Seedlings by Regulating Antioxidant System and Relieving Photoinhibition

Chilling adversely affects the photosynthesis of thermophilic plants, which further leads to a decline in growth and yield. The role of melatonin (MT) in the stress response of plants has been investigated, while the mechanisms by which MT regulates the chilling tolerance of chilling-sensitive cucumber remain unclear. This study demonstrated that MT positively regulated the chilling tolerance of cucumber seedlings and that 1.0 μmol⋅L-1 was the optimum concentration, of which the chilling injury index, electrolyte leakage (EL), and malondialdehyde (MDA) were the lowest, while growth was the highest among all treatments. MT triggered the activity and expression of antioxidant enzymes, which in turn decreased hydrogen peroxide (H2O2) and superoxide anion (O2 ⋅-) accumulation caused by chilling stress. Meanwhile, MT attenuated the chilling-induced decrease, in the net photosynthetic rate (Pn) and promoted photoprotection for both photosystem II (PSII) and photosystem I (PSI), regarding the higher maximum quantum efficiency of PSII (Fv/Fm), actual photochemical efficiency (ΦPSII), the content of active P700 (ΔI/I0), and photosynthetic electron transport. The proteome analysis and western blot data revealed that MT upregulated the protein levels of PSI reaction center subunits (PsaD, PsaE, PsaF, PsaH, and PsaN), PSII-associated protein PsbA (D1), and ribulose-1,5-bisphosphate carboxylase or oxygenase large subunit (RBCL) and Rubisco activase (RCA). These results suggest that MT enhances the chilling tolerance of cucumber through the activation of antioxidant enzymes and the induction of key PSI-, PSII-related and carbon assimilation genes, which finally alleviates damage to the photosynthetic apparatus and decreases oxidative damage to cucumber seedlings under chilling stress.