Melatonin confers plant tolerance against cadmium stress via the decrease of cadmium accumulation and reestablishment of microRNA-mediated redox homeostasis

Repair effects of melatonin application on physiological damages induced by UV-B radiation in alfalfa seedlings

Melatonin is one of the important endogenous signaling molecules for regulating plant growth and development, and various abiotic stress responses. UV-B is an intrinsic component of sunlight. Plants are particularly vulnerable to UV-B radiation compared to other abiotic stressors due to their sessile growth patterns. Previous studies reported that melatonin played a key role in Arabidopsis UV-B signaling responses and UV-B resistance. However, there are still few reports on the biological effects of melatonin in leguminous plants responses to UV-B. We have selected alfalfa pretreated with exogenous melatonin, coumarin, or 5-hydroxytryptamine as biological materials, and the interactive effects between melatonin metabolisms in plant and UV-B radiation will be investigated by testing hypocotyl elongation, intracellular reactive oxygen species (ROS) accumulation, physiochemical reaction, antioxidant defense, and its related gene expression. The results have showed that UV-B radiation significantly inhibited hypocotyl elongation, altered leaf phenotype and photosynthetic parameters, induced ROS levels and cell membrane damage, decreased antioxidant enzyme activities except for POD (Peroxidase), and downregulated related gene expression. However, exogenous melatonin efficiently repaired detrimental effects by UV-B radiation through activating antioxidant defense and enhancing photosynthetic efficiency. We have also found that 5-hydroxytryptamine treatment displayed the same biological effects via increasing endogenous melatonin. However, coumarin application had no obvious improvements. These data will provide experimental foundation for further understanding legume responses to UV-B and its molecular mechanisms, and also fill the research gaps in this field.

Integrated transcriptomics, metabolomics and physiological analyses reveal the regulatory mechanism of dopamine in Nicotiana tabacum response to cadmium stress

Cadmium (Cd) pollution is a serious threat to many plants. Dopamine (DOPA) can potentially alleviate abiotic stress in plants, however, the effects of DOPA on Cd biotoxicity remain largely elusive. This study explored the beneficial effects of DOPA on Cd tolerance and detoxification in tobacco using transcriptomics and metabolomics combined with physiological and biochemical analyses. The results showed that applying DOPA reduced Cd absorption in tobacco plants, altered its subcellular distribution, decreased the soluble Cd proportion within cells and organelles, and increased the soluble Cd proportion associated with the cell wall. Analysis of Cd chemical forms revealed that DOPA decreased the concentrations of inorganic Cd and water-soluble Cd (with limited mobility), while increased the Cd complexed with pectin, proteins, and phosphate in the cell walls and vacuoles. Furthermore, DOPA enhanced CAT, SOD, POD, APX activity, decreased Cd-induced O2·-, H2O2, and MDA accumulation and damage, and preserved cell wall structural integrity, thereby improved the plant photosynthetic capacity (Fv/Fm, Gs, Tr, Pn). Comprehensive transcriptome and metabolome analyses revealed that DOPA modulated amino acid and lipid metabolism, and upregulated the expression of genes involved in cell wall synthesis (PME, XTH et al.), Cd uptake and detoxification (FROs, HIPPs et al.), as well as light-harvesting protein (Psa, Psb, LHC et al.). This study establishes a theoretical foundation for understanding how DOPA enhances plant tolerance to Cd stress and elucidates the fundamental regulatory mechanisms involved.

The teamwork of melatonin, ethylene and H2S in abiotic stress adaptation in plants

Abiotic stresses significantly reduce plant growth and productivity, challenging agricultural sustainability. Plants have evolved adaptive mechanisms to counter these stresses, including antioxidant defences, biochemical changes, and hormonal signaling. Among these, the hormone melatonin (MT) and signaling molecules, ethylene (ET) and hydrogen sulfide (H2S), play pivotal roles, interacting in complex ways that modulate stress responses. Melatonin, known for its antioxidant properties, interacts with ET pathways to regulate its production. While ET is essential for stress signaling, its overproduction can exacerbate oxidative damage, and MT helps modulate ET levels to prevent such detrimental effects. Moreover, MT regulates H2S synthesis by activating L-cysteine desulfhydrase (LCD) and D-cysteine desulfhydrase (DCD), enhancing its protective effects under stress. Hydrogen sulfide supports MT synthesis, indicating a bidirectional relationship. Evidence suggests that H2S plays a role in fine-tuning ET levels under stress conditions, supporting optimal signaling for resilience. This review explores the intricate interactions among MT, ET, and H2S, shedding light on potential crosstalk mechanisms that strengthen plant stress tolerance, aiming to enhance crop resilience through targeted manipulation of these pathways.

Uncovering miRNA-mRNA regulatory modules of cotton in response to cadmium stress

Cadmium (Cd2+), a non-essential heavy metal for plant, adversely effects on crop productivity and food safety. Cotton, predominantly cultivated as a non-food crop, offers the advantage of not transferring Cd2+ into the food chain, making it an effective option for remediating Cd2+contaminated soils. While previous researches have extensively examined the gene expression responses of cotton to Cd2+ stress, insights at the post-transcriptional level remain limited. In this study, a comprehensive methodology was employed, incorporating miRNA sequencing, degradomics, and RNA sequencing, to investigate the responses of the Cd2+-tolerant cotton cultivar XM and the Cd2+-sensitive cotton cultivar ZM24 under Cd2+ exposure. The analysis revealed that these the identified miRNA-target gene pairs predominantly influence various biological processes, including light signaling, cell wall biogenesis, abiotic stress responses, transportation, and hormone signaling pathways in response to Cd2+ stress. Overall, our findings suggest that newly identified miRNAs and their corresponding target genes in cotton may contribute to enhance tolerance to Cd2+ stress through multiple mechanisms, facilitating the breeding of superior cotton cultivars with enhanced tolerance to Cd2+ toxicity.

Melatonin improves the lead tolerance in Plantago ovata by modulating ROS homeostasis, phytohormone status and expression of stress-responsive genes

KEY MESSAGE

Melatonin increases Pb tolerance in P. ovata seedlings via the regulation of growth and stress-related phytohormones, ROS scavenging and genes responsible for melatonin synthesis, metal chelation, and stress defense. Lead (Pb) is a highly toxic heavy metal that accumulates in plants through soil and air contamination and impairs its plant growth and development. Because of its pharmaceutical importance, improvements in Plantago ovata yield against abiotic stresses are necessary. Melatonin (MEL) is a stress-alleviating biostimulator and our results showed a reduction in Pb induced phytotoxicity by enhancing plant growth attributes and balancing protective osmolytes. Pb-induced reactive oxygen species accumulation, including superoxide and peroxide free radicals and their mitigation through enzymatic antioxidants, was demonstrated in presence of MEL. Cell viability and Pb bioaccumulation were determined to understand the extent of cellular damage. Moreover, MEL increased secondary metabolite (flavonoids and anthocyanins) contents by 2-3-fold at the lowest Pb concentrations. Similar increases in the relative expression of genes (PoPAL and PoPPO), which are responsible for the production of non-enzymatic antioxidants, were observed. Notably, the upregulation of the PoCOMT gene up to 4-fold indicates increased melatonin production, as manifested in the phytomelatonin level. MEL supplementation also increased the auxin (IAA) level by 3-fold in the 100 µM Pb treatment group, while the abscisic acid (ABA) level decreased (1.4-fold) and the expression of PoMYB (a stress-related transcription factor) increased (up to 2.66-fold). Additionally, we found extreme downregulation (up to 18-fold) in the relative expression of PoMT 2 (a metal binding thiol compound) with melatonin treatment, which is otherwise upregulated (by 6-fold) during Pb stress. In the current study, these effects collectively revealed that MEL contribute to enhanced plant growth and Pb stress tolerance.

Comparative Efficacy of Melatonin and Brassinolide in Mitigating the Adverse Effects of Cadmium on Wolffia arrhiza

Melatonin (MT) and brassinolide (BL) are phytohormones that regulate various physiological processes in plants. This study investigates their effects on Wolffia arrhiza when exposed to cadmium (Cd). Plant hormones were quantified using liquid chromatography-mass spectrometry, while photosynthetic pigments and phytochelatins (PCs) were analyzed through high-performance liquid chromatography. Protein, monosaccharide levels, and antioxidant activities were also spectrophotometrically measured. The findings reveal that MT and BL treatment decreased Cd accumulation in W. arrhiza compared to plants only exposed to Cd. MT was particularly effective in reversing Cd-induced growth inhibition and reducing stress markers more significantly than BL. It also enhanced antioxidant activity and maintained higher levels of photosynthetic pigments, proteins, and sugars. Although BL was less effective in these aspects, it promoted greater synthesis of glutathione and PCs in Cd-exposed duckweed. Overall, both MT and BL alleviate the negative impact of Cd on W. arrhiza, confirming their crucial role in supporting plant health under metal stress conditions.

Integrative application of licorice root extract and melatonin improves faba bean growth and production in Cd-contaminated saline soil

BACKGROUND

Globally, salinity poses a threat to crop productivity by hindering plant growth and development via osmotic stress and ionic cytotoxicity. Plant extracts have lately been employed as exogenous adjuvants to improve endogenous plant defense mechanisms when grown under various environmental stresses, such as salinity. This study investigated the potential of melatonin (Mt; 0, 50, and 100 mM) as an antioxidant and licorice root extract (LRE; 0.0 and 3%) as an organic biostimulant applied sequentially as a foliar spray on faba bean (Vicia faba L.) grown in cadmium (Cd)-contaminated saline soil conditions [Cd = 4.71 (mg kg- 1 soil) and ECe = 7.84 (dS m- 1)]. Plants not receive any treatment and sprayed with H2O were considered controls. The experimental treatments were laid out in strip plot in a randomized complete block design replicated thrice, where the LRE and Mt were considered as vertical and horizontal strips, respectively. Growth characteristics, photosynthetic pigments, nutrient uptake, physiology and metabolic responses, anatomical features, and yield were assessed.

RESULTS

Cadmium (Cd) and salinity-induced stress significantly altered leaf integrity, photosynthetic efficiency, total soluble sugars (TSS), free proline (FPro), total phenolic, DPPH, and total soluble proteins (TSP), non-enzymatic and enzymatic antioxidants, growth characteristics and yield-related traits. However, the application of LRE + Mt considerably improved these negative effects, with higher improvements were observed due to application of LRE + Mt100. Application of LRE + Mt significantly reduced hydrogen peroxide (H2O2) accumulation, lipid peroxidation and Cd content in leaves and seeds, all of which had increased due to Cd stress. Application of LRE + Mt significantly mitigated the Cd-induced oxidative damage by increasing the activity of reactive oxygen species (ROS) scavenging enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, in parallel with enhanced ascorbate and reducing glutathione content. Exogenous application of LRE + Mt significantly increased osmolyte content, including FPro, TSS, and total phenols and mitigated Cd-induced reduction to considerable levels.

CONCLUSIONS

Our findings showed that LRE + Mt increased V. faba plants' morphological, physiological, and biochemical properties, reducing Cd stress toxicity, and promoting sustainable agricultural practices.

CLINICAL TRIAL NUMBER

Not applicable.

Cadmium translocation combined with metabolomics analysis revealed potential mechanisms of MT@MSN-CS and GSH@MSN-CS in reducing cadmium accumulation in rice (Oryza sativa L.) grains

Applying nano-delivery systems for phytohormones via foliar application has proven effective in reducing grain cadmium (Cd) levels in crops. However, the mechanisms underlying this reduction remain inadequately understood. This study integrated the determination of leaf photosynthetic parameters, Cd translocation analysis, and metabolomics to elucidate the effects of reduced glutathione (GSH) and melatonin (MT), delivered with or without chitosan-encapsulated mesoporous silica nanoparticles (MSN-CS), on grain Cd levels in rice. Our findings revealed that the foliar application of MT@MSN-CS significantly outperformed MT alone in reducing grain Cd levels and enhancing leaf photosynthesis under Cd stress. Conversely, GSH@MSN-CS showed comparable effects to GSH alone. Foliar-applied GSH@MSN-CS and MT@MSN-CS both decreased the Cd transport coefficients from panicle nodes to brown rice by 26.2-53.3%, with MT@MSN-CS demonstrating superior efficiency in reducing Cd concentrations across roots, stems, leaves, panicle nodes, and grains. Metabolomic analysis revealed substantial shifts in rice metabolite profiles following GSH@MSN-CS and MT@MSN-CS treatments. Foliar application of MT@MSN-CS or GSH@MSN-CS may rapidly and effectively activate the primary antioxidant defense system and alleviate membrane lipid peroxidation in rice grown on low-to-moderately Cd-contaminated soils by upregulating amino acid metabolism. The secondary defense mechanism, phenylpropanoid biosynthesis, was reprogrammed to reduce energy expenditure and decrease Cd translocation.

Regulation of Plant Growth and Development by Melatonin

Melatonin is a naturally occurring chemical with pleiotropic effects in various species. In plants, melatonin is associated with a variety of plant physiological processes, including plant growth and development, stress responses, etc. Thus, melatonin may hold promise for improving crop yields and agricultural sustainability. This review describes the biosynthetic mode of melatonin and its properties and summarizes its functions in growth, development, and reproduction. In addition, the role of melatonin in plants facing various stressful environments is elaborated upon, and its relationship with other phytohormones is summarized. Through this review, we recognize the problems and challenges facing melatonin research and propose some feasible solutions.

GhCOMT33D modulates melatonin synthesis, impacting plant response to Cd2+ in cotton via ROS

Caffeic acid-3-O-methyltransferase (COMT) serves as the final pivotal enzyme in melatonin biosynthesis and plays a crucial role in governing the synthesis of melatonin in plants. This research used bioinformatics to analyze the phylogenetic relationships, gene structure, and promoter cis-acting elements of the upland cotton COMT gene family members， which it identified as the key gene GhCOMT33D to promote melatonin synthesis and responding to Cd2+ stress. After silencing GhCOMT33D through virus-induced gene silencing (VIGS), cotton seedlings showed less resistance to Cd2+ stress. Under Cd2+ stress, the melatonin content in the silenced plants significantly decreased, while ROS, MDA, and proline accumulated in the plant cells. The stomatal aperture of the leaves was reduced, hindering normal photosynthesis, leading to cotton leaves withering and yellowing, and epidermal cells becoming twisted and deformed, with a large number of gaps appearing. The non-silenced plants had a significantly higher melatonin content and were in better condition, providing important evidence for further research on how plant melatonin enhances the Cd2+ resistance of cotton and its regulatory mechanisms.

Protective effects of the exogenous application of salicylic acid and chitosan on chromium-induced photosynthetic capacity and osmotic adjustment in Aconitum napellus

Chitosan (CTS) is recognized for enhancing a plant's resilience to various environmental stresses, such as salinity and drought. Moreover, salicylic acid (SA) is acknowledged as a growth regulator involved in addressing metal toxicity. However, the effectiveness of both compounds in mitigating Cr-induced stress has remained relatively unexplored, especially in the case of Aconitum napellus, a medicinally and floricultural important plant. Therefore, the primary objective of this study was to investigate the potential of CTS and SA in alleviating chromium (Cr)-induced stress in A. napellus. To address these research questions, we conducted a controlled experiment using potted plants to evaluate the individual and combined impacts of CTS and SA on plants exposed to Cr stress. Foliar application of CTS (0.4 g/L) or SA (0.25 mmol/L) led to significant improvements in the growth, chlorophyll content, fluorescence, and photosynthetic traits of A. napellus plants under Cr stress. The most notable effects were observed with the combined application of CTS and SA, resulting in increases in various morphological parameters, such as shoot length (2.89% and 7.02%) and root length (27.75% and 3.36%) under the Cr 1 and Cr 2 treatments, respectively. Additionally, several physiological parameters, such as chlorophyll a (762.5% and 145.56%), chlorophyll b (762.5% and 145.56%), carotenoid (17.03% and 28.57%), and anthocyanin (112.01% and 47.96%) contents, were notably improved under the Cr 1 and Cr 2 treatments, respectively. Moreover, the combined treatment of CTS and SA improved the fluorescence parameters while decreasing the levels of enzymatic antioxidants such as catalase (27.59% and 43.79%, respectively). The application also notably increased osmoprotectant parameters, such as the total protein content (54.11% and 20.07%) and the total soluble sugar content (78.17% and 49.82%) in the leaves of A. napellus in the Cr 1 and 2 treatments, respectively. In summary, these results strongly suggest that the simultaneous use of exogenous CTS and SA is an effective strategy for alleviating the detrimental effects of Cr stress on A. napellus. This integrated approach opens promising avenues for further exploration and potential implementation within agricultural production systems.

The effect of cadmium on soil and plants, and the influence of Serendipita indica (Piriformospora indica) in mitigating cadmium stress

Due to environmental pollution, the risk of cadmium stress for crops is soaring, so researchers are exploring inexpensive solutions to enhance cultivated crops in contaminated soil. Using microorganisms to reduce cadmium risk has been one of the most effective strategies in recent decades. Serendipita indica (Piriformospora indica) is one of the best endophyte fungi that, in addition to reducing heavy metal stress for crops, can significantly reduce the threat of other abiotic stresses. As part of this research, cadmium in soil has been investigated, as well as its effects on plants' morphophysiological and biochemical characteristics. The present review has also attempted to identify the role of Serendipita indica in improving the growth and performance of crops, as well as its possible effect on reducing the risk of cadmium. The results showed that Serendipita indica enhance the growth and productivity of plants in contaminated environments by improving soil quality, reducing cadmium absorption, improving the activity of antioxidant enzymes and secondary metabolites, raising water and mineral absorption, and altering morphophysiological structures.

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

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

NtGCN2 confers cadmium tolerance in Nicotiana tabacum L. by regulating cadmium uptake, efflux, and subcellular distribution

General control non-derepressible-2 (GCN2) is widely expressed in eukaryotes and responds to biotic and abiotic stressors. However, the precise function and mechanism of action of GCN2 in response to cadmium (Cd) stress in Nicotiana tabacum L. (tobacco) remains unclear. We investigated the role of NtGCN2 in Cd tolerance and explored the mechanism by which NtGCN2 responds to Cd stress in tobacco by exposing NtGCN2 transgenic tobacco lines to different concentrations of CdCl2. NtGCN2 was activated under 50 μmol·L-1 CdCl2 stress and enhanced the Cd tolerance and photosynthetic capacities of tobacco by increasing chlorophyll content and antioxidant capacity by upregulating NtSOD, NtPOD, and NtCAT expression and corresponding enzyme activities and decreasing malondialdehyde and O2·- contents. NtGCN2 enhanced the osmoregulatory capacity of tobacco by elevating proline (Pro) and soluble sugar contents and maintaining low levels of relative conductivity. Finally, NtGCN2 enhanced Cd tolerance in tobacco by reducing Cd uptake and translocation, promoting Cd efflux, and regulating Cd subcellular distribution. In conclusion, NtGCN2 improves the tolerance of tobacco to Cd through a series of mechanisms, namely, increasing antioxidant, photosynthetic, and osmoregulation capacities and regulating Cd uptake, translocation, efflux, and subcellular distribution. This study provides a scientific basis for further exploration of the role of NtGCN2 in plant responses to Cd stress and enhancement of the Cd stress signaling network in tobacco.

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.

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.

Nano zero-valent iron and melatonin synergistically alters uptake and translocation of Cd and As in soil-rice system and mechanism in soil chemistry and microbiology

Nanoscale zero-valent iron (Fe) is a promising nanomaterial for remediating heavy metal-contaminated soils. Melatonin (MT) is essential to alleviate environmental stress in plants. However, the conjunction effects of Fe and MT (FeMT) on rice Cd, As accumulation and the mechanism of soil chemical and microbial factors interaction are unclear. Here, a pot experiment was conducted to evaluated the effects of the FeMT for rice Cd, As accumulation and underlying mechanisms. The findings showed that FeMT significantly reduced grains Cd by 92%-87% and As by over 90%, whereas improving grains Fe by over 213%. Soil available-Cd and iron plaques-Cd (extracted by dithionite-citrate-bicarbonate solution, DCB-Cd) significantly regulated roots Cd, thus affected Cd transport to grains. Soil pH significantly affected soil As and DCB-As, which further influenced roots As uptake and the transport to shoots and grains. The interactions between the soil bacterial community and soil Fe, available Fe, and DCB-Fe together affected root Fe absorption and transportation in rice. FeMT significantly influenced rhizosphere soil bacterial α- and β-diversity. Firmicutes as the dominant phylum exhibited a significant positive response to FeMT measure, and acted a key role in reducing soil Cd and As availability mainly by improving iron-manganese plaques. The increase of soil pH caused by FeMT was beneficial only for Actinobacteriota growth, which reduced Cd, As availability probably through complexation and adsorption. FeMT also showed greater potential in reducing human health and ecological risks by rice consumption and straw returning. These results showed the important role of both soil chemical and microbial factors in FeMT-mediated rice Cd, As reduction efficiency. This study opens a novel strategy for safe rice production and improvement of rice iron nutrition level in heavy-metals polluted soil, but also provides new insights into the intricate regulatory relationships among soil biochemistry, toxic elements, microorganism, and plants.

Melatonin as a key regulator in seed germination under abiotic stress

Seed germination (SG) is the first stage in a plant's life and has an immense importance in sustaining crop production. Abiotic stresses reduce SG by increasing the deterioration of seed quality, and reducing germination potential, and seed vigor. Thus, to achieve a sustainable level of crop yield, it is important to improve SG under abiotic stress conditions. Melatonin (MEL) is an important biomolecule that interplays in developmental processes and regulates many adaptive responses in plants, especially under abiotic stresses. Thus, this review specifically summarizes and discusses the mechanistic basis of MEL-mediated SG under abiotic stresses. MEL regulates SG by regulating some stress-specific responses and some common responses. For instance, MEL induced stress specific responses include the regulation of ionic homeostasis, and hydrolysis of storage proteins under salinity stress, regulation of C-repeat binding factors signaling under cold stress, starch metabolism under high temperature and heavy metal stress, and activation of aquaporins and accumulation of osmolytes under drought stress. On other hand, MEL mediated regulation of gibberellins biosynthesis and abscisic acid catabolism, redox homeostasis, and Ca2+ signaling are amongst the common responses. Nonetheless factors such as endogenous MEL contents, plant species, and growth conditions also influence above-mentioned responses. In conclusion, MEL regulates SG under abiotic stress conditions by interacting with different physiological mechanisms.

NPs-Ca promotes Cd accumulation and enhances Cd tolerance of rapeseed shoots by affecting Cd transfer and Cd fixation in pectin

The use of rapeseed (Brassica napus) as a hyperaccumulator plant has shown great promise for the remediation of cadmium (Cd) contaminated soils. Nanosized materials (NPs) have been shown to mitigate heavy metal toxicity in plants, but it is unknown how l-aspartate nano-calcium (NPs-Ca) affects Cd uptake, transport, and tolerance in rapeseed. A soil pot experiment was conducted with two treatments: a control treatment (CK) with 2.16 g CaCl2 and NPs-Ca treatment with 6.00 g NPs-Ca, to evaluate the effects and mechanisms of NPs-Ca on Cd tolerance in rapeseed. Compared to CaCl2, NPs-Ca promoted Cd transportation from roots to shoots by up-regulating the expression of Cd transport genes (ABCC12, HMA8, NRAM6, ZIP6, CAX4, PCR2, and HIP6). Therefore, NPs-Ca increased Cd accumulation in rapeseed shoots by 39.4%. Interestingly, NPs-Ca also enhanced Cd tolerance in the shoots, resulting in lower hydrogen peroxide (H2O2) accumulation and proline content, as well as higher antioxidant enzyme activities (POD, CAT). Moreover, NPs-Ca reduced the activity of pectin-degrading enzymes (polygalacturonase: PG, β-galactosidase: β-GAL), promoted the activity of pectin methyl esterase (PME), and changed transcription levels of related genes (PME, PMEI, PG, PGIP, and β-GAL). NPs-Ca treatment also significantly increased the Cd content in cell walls by 59.8%, that is, more Cd was immobilized in cell walls, and less Cd entered organelles in shoots of NPs-Ca treatment due to increased pectin content and degree of pectin demethylation. Overall, NPs-Ca increased Cd accumulation in rapeseed shoots by promoting Cd transport from roots to shoots. And meantime, NPs-Ca enhanced Cd tolerance of shoots by inhibiting pectin degradation, promoting pectin demethylation and increasing Cd fixation in pectin. These findings suggest that NPs-Ca can improve the potential of rapeseed as a hyperaccumulator for the remediation of Cd-contaminated soil and the protection of the environment. Furthermore, the study provides a theoretical basis for the application of NPs-Ca in the phytoremediation of Cd-contaminated soils with hyperaccumulating plants.

Phytomelatonin: A key regulator of redox and phytohormones signaling against biotic/abiotic stresses

Plants being sessile in nature, are exposed to unwarranted threats as a result of constantly changing environmental conditions. These adverse factors can have negative impacts on their growth, development, and yield. Hormones are key signaling molecules enabling cells to respond rapidly to different external and internal stimuli. In plants, melatonin (MT) plays a critical role in the integration of various environmental signals and activation of stress-response networks to develop defense mechanisms and plant resilience. Additionally, melatonin can tackle the stress-induced alteration of cellular redox equilibrium by regulating the expression of redox hemostasis-related genes and proteins. The purpose of this article is to compile and summarize the scientific research pertaining to MT's effects on plants' resilience to biotic and abiotic stresses. Here, we have summarized that MT exerts a synergistic effect with other phytohormones, for instance, ethylene, jasmonic acid, and salicylic acid, and activates plant defense-related genes against phytopathogens. Furthermore, MT interacts with secondary messengers like Ca²⁺, nitric oxide, and reactive oxygen species to regulate the redox network. This interaction triggers different transcription factors to alleviate stress-related responses in plants. Hence, the critical synergic role of MT with diverse plant hormones and secondary messengers demonstrates phytomelatonin's importance in influencing multiple mechanisms to contribute to plant resilience against harsh environmental factors.

Involvement of Nitric Oxide and Melatonin Enhances Cadmium Resistance of Tomato Seedlings through Regulation of the Ascorbate-Glutathione Cycle and ROS Metabolism

Melatonin (MT) and nitric oxide (NO) act as signaling molecules that can enhance cadmium (Cd) stress resistance in plants. However, little information is available about the relationship between MT and NO during seedling growth under Cd stress. We hypothesize that NO may be involved in how MT responds to Cd stress during seedling growth. The aim of this study is to evaluate the relationship and mechanism of response. The results indicate that different concentrations of Cd inhibit the growth of tomato seedlings. Exogenous MT or NO promotes seedling growth under Cd stress, with a maximal biological response at 100 μM MT or NO. The promotive effects of MT-induced seedling growth under Cd stress are suppressed by NO scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (cPTIO), suggesting that NO may be involved in MT-induced seedling growth under Cd stress. MT or NO decreases the content of hydrogen peroxide (H₂O₂), malonaldehyde (MDA), dehydroascorbic acid (DHA), and oxidized glutathione (GSSG); improves the content of ascorbic acid (AsA) and glutathione (GSH) and the ratios of AsA/DHA and GSH/GSSG; and enhances the activities of glutathione reductase (GR), monodehydroascorbic acid reductase (MDHAR), dehydroascorbic acid reductase (DHAR), ascorbic acid oxidase (AAO), and ascorbate peroxidase (APX) to alleviate oxidative damage. Moreover, the expression of genes associated with the ascorbate-glutathione (AsA-GSH) cycle and reactive oxygen species (ROS) are up-regulated by MT or NO under Cd conditions, including AAO, AAOH, APX1, APX6, DHAR1, DHAR2, MDHAR, and GR. However, NO scavenger cPTIO reverses the positive effects regulated by MT. The results indicate that MT-mediated NO enhances Cd tolerance by regulating AsA-GSH cycle and ROS metabolism.

Seed pretreatment with melatonin confers cadmium tolerance to chickpea seedlings through cellular redox homeostasis and antioxidant gene expression improvement

Several phytoremediation strategies have been undertaken to alleviate cadmium (Cd)-mediated injury to crop yield resulting from agricultural land pollution. In the present study, the potentially beneficial effect of melatonin (Me) was appraised. Therefore, chickpea (Cicer arietinum L.) seeds were imbibed for 12 H in distilled water or Me (10 µM) solution. Then, the seeds germinated in the presence or the absence of 200 µM CdCl₂ for 6 days. Seedlings obtained from Me-pretreated seeds exhibited enhanced growth traits, reflected by fresh biomass and length increase. This beneficial effect was associated with a decreased Cd accumulation in seedling tissues (by 46 and 89% in roots and shoots, respectively). Besides, Me efficiently protected the cell membrane integrity of Cd-subjected seedlings. This protective effect was manifested by the decreased lipoxygenase activity and the subsequently reduced accumulation of 4-hydroxy-2-nonenal. Melatonin counteracted the Cd-mediated stimulation of the pro-oxidant NADPH-oxidase (90 and 45% decrease compared to non-pretreated Cd-stressed roots and shoots, respectively) and NADH-oxidase activities (almost 40% decrease compared to non-pretreated roots and shoots), preventing, thus, hydrogen peroxide overaccumulation (50 and 35% lesser than non-pretreated roots and shoots, respectively). Furthermore, Me enhanced the cellular content of pyridine nicotinamide reduced forms [NAD(P)H] and their redox state. This effect was associated with the Me-mediated stimulation of the glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase activities, concomitantly with the inhibition of NAD(P)H-consuming activities. These effects were accompanied by the up-regulation of G6PDH gene expression (45% increase in roots) and the down-regulation of the respiratory burst oxidase homolog protein F (RBOHF) gene expression (53% decrease in roots and shoots). Likewise, Me induced an increased activity and gene transcription of the Asada-Halliwell cycle, namely ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase, concomitantly with a reduction of the glutathione peroxidase activity. This modulating effect led to the restoration of the redox homeostasis of the ascorbate and the glutathione pools. Overall, current results attest that seed pretreatment with Me is effective in Cd stress relief and can be a beneficial crop-protective approach.

Melatonin and Abiotic Stress Tolerance in Crop Plants

Increasing food demand by the growing human population and declining crop productivity due to climate change affect global food security. To meet the challenges, developing improved crops that can tolerate abiotic stresses is a priority. Melatonin in plants, also known as phytomelatonin, is an active component of the various cellular mechanisms that alleviates oxidative damage in plants, hence supporting the plant to survive abiotic stress conditions. Exogenous melatonin strengthens this defence mechanism by enhancing the detoxification of reactive by-products, promoting physiological activities, and upregulating stress-responsive genes to alleviate damage during abiotic stress. In addition to its well-known antioxidant activity, melatonin protects against abiotic stress by regulating plant hormones, activating ER stress-responsive genes, and increasing protein homoeostasis, heat shock transcription factors and heat shock proteins. Under abiotic stress, melatonin enhances the unfolded protein response, endoplasmic reticulum-associated protein degradation, and autophagy, which ultimately protect cells from programmed cell death and promotes cell repair resulting in increased plant survival.

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.

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.

Pre treatment of melatonin rescues cotton seedlings from cadmium toxicity by regulating key physio-biochemical and molecular pathways

Melatonin, a plant/animal origin hormone, regulates plant response to abiotic stresses by protecting them from oxidative damage. This study identified physiochemical and molecular mechanism of melatonin-induced cadmium (Cd) stress tolerance and detoxification in cotton seedlings. Cotton seedlings, with or without melatonin (15 µM) pretreatment, were subjected to Cd (100 µM) stress in a hydroponic medium for eight days. We found that higher cellular Cd accumulation in leaf tissues significantly inhibited the growth and physiology of cotton seedlings. In contrast, melatonin-treated seedlings maintained leaf photosynthetic capacity, producing relatively higher fresh (17.4%) and dry (19.3%) weights than non-melatonin-treated plants under Cd-contaminated environments. The improved growth and leaf functioning were strongly linked with the melatonin-induced repression of Cd transporter genes (LOC107894197, LOC107955631, LOC107899273) in roots. Thus, melatonin induced downregulation of the Cd transporter genes further inhibited Cd ion transport towards leaf tissues. This suggests that the differentially expressed transporter genes (DEG) are key drivers of the melatonin-mediated regulation of Cd transportation and sequestration in cotton. Melatonin also protected cotton seedlings from Cd-induced oxidative injury by reducing tissues malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) levels and increasing the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) enzymes. Transcriptomic analysis revealed that melatonin activated mitogen-activated protein kinase (MAPK) signaling pathways to simulate stomatal adjustment and photosynthesis in Cd-stressed leaves. Further, melatonin protects intercellular organs, particularly ribosomes, from Cd-induced oxidative damage by promoting ribosomal biosynthesis and improving translational efficiency. The findings elucidated the molecular basis of melatonin-mediated Cd stress tolerance in plants and provided a key for the effective strategy of Cd accumulation in cotton.

A state-of-the-art review on cadmium uptake, toxicity, and tolerance in rice: From physiological response to remediation process

Cadmium (Cd), a major contaminant of concern, has been extensively reviewed and debated for its anthropogenic global shifts. Cadmium levels in rice grains raise wide food safety concerns. The aim of this review is therefore to capture the dynamics of Cd in paddy soil, translocation pathways of Cd from soil to consumption rice, and assess its bio-accessibility in human consumption. In crop plants, Cd reduces absorption of nutrients and water, triggers oxidative stress, and inhibits plant metabolism. Understanding the mechanisms and behaviour of Cd in paddy soil and rice allows to explain, predict and intervene in Cd transferability from soil to grains and human exposure. Factors affecting Cd movement in soil, and further to rice grain, are elucidated. Recently, physiological and molecular understanding of Cd transport in rice plants have been advanced. Morphological-biochemical characteristics and Cd transporters of plants in such a movement were also highlighted. Ecologically viable remediation approaches, including low input cost agronomic methods, phytoremediation and microbial bioremediation methods, are emerging.

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.

Exogenous melatonin enhances Cd stress tolerance in Platycladus orientalis seedlings by improving mineral nutrient uptake and oxidative stress

The development of agriculture and industry has led to a gradual increase in the levels of cadmium (Cd) in the soil, which, due to its high mobility in soil, makes Cd deposition in plants a serious threat to the health of animals and humans. The important role of melatonin (MT) in regulating plant growth and adaptation to environmental stress has become a pertinent research topic, but the mechanisms of action of MT in Cd-stressed Platycladus orientalis seedlings are unclear. Here, we investigated the mitigation mechanism of exogenous MT application on P. orientalis seedlings under Cd stress. Cd stress significantly inhibited the growth of P. orientalis seedlings by disrupting photosynthetic pigments, mineral balance, osmotic balance, and oxidative balance. In contrast, the application of exogenous MT significantly increased the growth parameters of P. orientalis seedlings, reduced Cd accumulation and transfer in the seedlings, increased the content of iron, manganese, zinc, copper, chlorophyll, soluble protein, soluble sugar, and proline, reduced the content of glutathione, increased the activities of superoxide dismutase and peroxidase, and significantly enhanced the expression of antioxidant-related genes (POD, GST, and APX). It also effectively reduced the content of hydrogen peroxide and malondialdehyde to inhibit the production of reactive oxygen species, thus alleviating Cd-induced oxidative stress. In addition, MT significantly upregulated the expression of the ethanol dehydrogenase (ADH) gene, which is effective in removing the acetaldehyde produced by anaerobic respiration in seedlings under stress, thereby reducing the toxic effects on P. orientalis. The results showed that exogenous MT enhanced the tolerance of P. orientalis seedlings to Cd stress by regulating photosynthesis, mineral balance, osmotic balance, and the antioxidant system and that the optimal concentration of MT was 200 μmol·L^-1.

An miR156-regulated nucleobase-ascorbate transporter 2 confers cadmium tolerance via enhanced anti-oxidative capacity in barley

INTRODUCTION

Cadmium (Cd) is one of the most detrimental heavy metal pollutants, seriously affecting crop production and human health. Nucleobase-ascorbic acid transporters (NAT) are widely present in many living organisms including plants, animals and microbes; however, the role of NAT in plant Cd tolerance remains unknown.

OBJECTIVES

To identify Cd-induced miRNAs that target HvNAT2 and to determine the role of this gene and its product in Cd tolerance.

METHODS

High-throughput-sequencing was used to identify the miRNA expression profile of barley roots in response to Cd stress. Overexpression (OX) and RNAi lines were then constructed for HvNAT2 and comparative transcriptomic analysis was performed to determine the function of this transporter examining its effects on traits such as Cd uptake/flux and translocation, morphology and antioxidant capacity in relation to Cd tolerance. In addition, phylogenetic analysis was performed to obtain insights into the evolution of HvNAT2.

RESULTS

Cd stress-induced genome-wide expression profiles of miRNAs identified a Cd-induced miRNA, miR156g-3p_3, that had HvNAT2 as its target. HvNAT2 was negatively regulated in the high-Cd-accumulating and Cd-tolerant genotype Zhenong8. Evolutionary analysis indicated that orthologues of the plasma membrane localized, HvNAT2, can be traced back to the sister group of land plants, the streptophyte algae. Overexpression of HvNAT2 increases Cd tolerance with higher tissue Cd accumulation but less oxidative damage in transgenic barley plants. RNAi of HvNAT2 leads to a significant reduction of Cd tolerance. The higher Cd accumulation in roots of the OX3 line was also demonstrated by confocal microscopy and electrophysiology. Transcriptome analysis showed that the enhancement of antioxidant capacity by HvNAT2 was related to stress signaling pathways. Furthermore, oxidative stress tolerance in HvNAT2-OX plants was regulated by the synthesis of phytochelatins and the glutathione metabolism cycle.

CONCLUSION

Our study reveals a key molecular mechanism of NAT in Cd tolerance in plants that is useful for sustainable agricultural production and management of hazardous this heavy metal for better environment management and ecosystem function.

Attenuation of negative effects caused by a combination of heat and cadmium stress in Arabidopsis thaliana deficient in jasmonic acid synthesis

Due to recent global warming, heat stress can simultaneously occur with cadmium (Cd) stress in regions suffering from metal pollution. In this study, we investigated the effects of heat, Cd and their combination on the growth and physiological characteristics of Arabidopsis thaliana. Arabidopsis plants were more susceptible to a combination of heat and Cd stress than to each stress applied individually, although the accumulation of Cd in shoots was comparable between plants subjected to Cd stress and the combined stress. Plants subjected to this stress combination showed a dramatic reduction in the accumulation of the photosynthetic reaction center proteins in photosystem II as well as a tendency toward enhanced lipid peroxidation, suggesting that the negative effects of a combination of heat and Cd stresses might be caused by oxidative damage accompanied by damage to the photosynthetic apparatus. Interestingly, aos and lox3 mutants deficient in jasmonic acid (JA) synthesis showed attenuation of the negative effects caused by a combination of heat and Cd stresses on the growth and maximum quantum efficiency of photosystem II. The roles of JA might be altered when heat stress is combined with Cd stress, despite its significance in the tolerance of plants to Cd stress when individually applied, which has been shown in previous studies.

Human Naa50 Shows Serotonin N-Acetyltransferase Activity, and Its Overexpression Enhances Melatonin Biosynthesis, Resulting in Osmotic Stress Tolerance in Rice

A new clade of serotonin N-acetyltransferase (SNAT), the penultimate enzyme in the melatonin biosynthetic pathway, has been reported in the archaeon Thermoplasma volcanium. The closest homolog of archaea SNAT in human was an N-alpha-acetyltransferase50 (Naa50). To determine whether human Naa50 (hNaa50) shows SNAT enzyme activity, we chemically synthesized and expressed the hNaa50 gene in Escherichia coli, followed by Ni²⁺ affinity purification. Purified recombinant hNaa50 showed SNAT activity (K_m and V_max values of 986 μM and 1800 pmol/min/mg protein, respectively). To assess its in vivo function, hNaa50 was overexpressed in rice (hNaa50-OE). The transgenic rice plants produced more melatonin than nontransgenic wild-type rice, indicating that hNaa50 is functionally coupled with melatonin biosynthesis. Due to its overproduction of melatonin, hNaa50-OE had a higher tolerance against osmotic stress than the wild type. Enhanced expression of the chaperone genes BIP1 and CNX in hNaa50-OE plants was responsible for the increased tolerance. It is concluded that hNaa50 harbors serotonin N-acetyltransferase enzyme activity in addition to its initial N-alpha-acetyltransferase, suggesting the bifunctionality of the hNaa50 enzyme toward serotonin and protein substrates. Consequently, ectopic overexpression of hNaa50 in rice enhanced melatonin synthesis, indicating that hNaa50 is in fact involved in melatonin biosynthesis.

Functions of Melatonin during Postharvest of Horticultural Crops

Melatonin, a tryptophan-derived molecule, is endogenously generated in animal, plant, fungal and prokaryotic cells. Given its antioxidant properties, it is involved in a myriad of signaling functions associated with various aspects of plant growth and development. In higher plants, melatonin (Mel) interacts with plant regulators such as phytohormones, as well as reactive oxygen and nitrogen species including hydrogen peroxide (H2O2), nitric oxide (NO) and hydrogen sulfide (H2S). It shows great potential as a biotechnological tool to alleviate biotic and abiotic stress, to delay senescence and to conserve the sensory and nutritional quality of postharvest horticultural products which are of considerable economic importance worldwide. This review provides a comprehensive overview of the biochemistry of Mel, whose endogenous induction and exogenous application can play an important biotechnological role in enhancing the marketability and hence earnings from postharvest horticultural crops.

Sly-miR398 Participates in Cadmium Stress Acclimation by Regulating Antioxidant System and Cadmium Transport in Tomato (Solanum lycopersicum)

Cadmium (Cd) pollution is one of the major threats in agricultural production, and can cause oxidative damage and growth limitation in plants. MicroRNA398 (miR398) is involved in plant resistance to different stresses, and the post-transcriptional regulation of miR398 on CSDs plays a key role. Here, we report that miR398 was down-regulated in tomato in response to Cd stress. Simultaneously, CSD1 and SOD were up-regulated, with CSD2 unchanged, suggesting CSD1 is involved in miR398-induced regulation under Cd stress. In addition, the role of miR398 in Cd tolerance in tomato was evaluated using a transgenic line overexpressing MIR398 (miR398#OE) in which the down-expression of miR398 was disrupted. The results showed that Cd stress induced more significant growth inhibition, oxidative damage, and antioxidant enzymes disorder in miR398#OE than that in wild type (WT). Moreover, higher Cd concentration in the shoot and xylem sap, and net Cd influx rate, were observed in miR398#OE, which could be due to the increased Cd uptake genes (IRT1, IRT2, and NRAMP2) and decreased Cd compartmentalization gene HMA3. Overall, our results indicate that down-regulated miR398 plays a protective role in tomato against Cd stress by modulating the activity of antioxidant enzymes and Cd uptake and translocation.

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.

Melatonin alleviates arsenite toxicity by decreasing the arsenic accumulation in cell protoplasts and increasing the antioxidant capacity in rice

Arsenic (As) is a common environmental pollutant that seriously interferes with the normal growth of organisms. There is an urgent need to take environment-safe and efficient strategies to mitigate As toxicity. Melatonin (MT) is a pleiotropic molecule that regulates plant growth and organ development and alleviates heavy metal stresses. The experiment aims to explore the mechanism of MT in reducing arsenite toxicity by hydroponic rice seedlings. The results showed that MT application reduced the As content in rice roots and shoots by 26.4% and 37.5%, respectively, and mainly decreased As content in the soluble fractions of the rice root cell. MT application also increased the As content of chelated-soluble pectin and alkali-soluble pectin in the cell wall by 14.7% and 74.4%, respectively. It promoted the generation of the functional group of the root cell walls by the FTIR analysis, indicating that MT may promote the fixation of As on the cell wall. Meanwhile, MT contributed to scavenging excess H2O2, reducing MDA content, and maintaining normal morphology of root cells by stimulating SOD, POD and CAT activities and increasing the level of GSH. The research deepens our understanding of how MT participates in maintaining redox homeostasis in rice cells, reducing As toxicity, and decreasing As concentration in rice seedlings, thereby providing more possibilities for reducing As accumulation in rice.

Melatonin and nitric oxide: Dual players inhibiting hazardous metal toxicity in soybean plants via molecular and antioxidant signaling cascades

Melatonin (MT), a ubiquitous signaling molecule, is known to improve plant growth. Its regulatory function alongside nitric oxide (NO) is known to induce heavy metal (Cd and Pb) stress tolerance, although the underlying mechanisms remain unknown. Here, we observed that the combined application of MT and NO remarkably enhanced plant biomass by reducing oxidative stress. Both MT and NO minimized metal toxicity by significantly lowering the levels of endogenous abscisic acid and jasmonic acid via downregulating NCED3 and upregulating catabolic genes (CYP707A1 and CYP707A2). MT/NO-induced mitigation of Cd and Pb stress was associated with increased endo-melatonin and variable endo-S-nitrosothiol levels caused by enhanced expression of gmNR and gmGSNOR mRNAs. Remarkably, the combined application of MT/NO reduced soil Cd and Pb mobilization by increasing the uptake of Ca²⁺ and K⁺ and increasing the exudation of organic acids into the rhizosphere. These results correlated with the upregulation of MTF-1 and WARKY27 during metal translocation. MT/NO regulates the MAPK and CDPK cascades to promote plant cell survival and Ca²⁺ signaling, thereby imparting resistance to heavy metal toxicity. In conclusion, MT/NO modulates the stress-resistance machinery to mitigate Cd and Pb toxicity by regulating the activation of antioxidant and molecular transcription factors.

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.

Functions and prospects of melatonin in plant growth, yield, and quality

Melatonin (N-acetyl-5-methoxytryptamine) is an indole molecule widely found in animals and plants. It is well known that melatonin improves plant resistance to various biotic and abiotic stresses due to its potent free radical scavenging ability while being able to modulate plant signaling and response pathways through mostly unknown mechanisms. In recent years, an increasing number of studies have shown that melatonin plays a crucial role in improving crop quality and yield by participating in the regulation of various aspects of plant growth and development. Here, we review the effects of melatonin on plant vegetative growth and reproductive development, and systematically summarize its molecular regulatory network. Moreover, the effective concentrations of exogenously applied melatonin in different crops or at different growth stages of the same crop are analysed. In addition, we compare endogenous phytomelatonin concentrations in various crops and different organs, and evaluate a potential function of phytomelatonin in plant circadian rhythms. The prospects of different approaches in regulating crop yield and quality through exogenous application of appropriate concentrations of melatonin, endogenous modification of phytomelatonin metabolism-related genes, and the use of nanomaterials and other technologies to improve melatonin utilization efficiency are also discussed.

Melatonin mediated activation of MAP kinase pathway may reduce DNA damage stress in plants: A review

Melatonin is an important biomolecule found in diverse groups of organisms. Under different abiotic stresses, the synthesis of melatonin is markedly increased suggesting pivotal roles of melatonin in plants enduring stresses. Being an endogenous signaling molecule with antioxidant activity, melatonin alters many physiological responses and is found to be involved in regulating DNA damage responses. However, the molecular mechanisms of melatonin in response to DNA damage have not yet been studied. The present review aims to provide insights into the molecular mechanisms of melatonin in response to DNA damage in plants. We propose that the MAP kinase pathway is involved in regulating melatonin dependent response of plants under DNA damage stress. Where melatonin might activate MAPK via H₂ O₂ or Ca²⁺ dependent pathways. The activated MAPK in turn might phosphorylate and activate SOG1 and repressor type MYBs to mitigate DNA damage under abiotic stress.

A strategy to promote carotenoids production in Dunaliella bardawil by melatonin combined with photoinduction

Microalgae are considered to be a very promising class of raw material for carotenoid production. In this study, melatonin (MLT), a widely used plant growth regulator, was added to the autotrophic medium of Dunaliella bardawil to explore its effects on the growth and pigment accumulation of Dunaliella bardawil. The results showed that the induction of exogenous MLT alone was not beneficial to the growth and pigment accumulation of Dunaliella bardawil, and the higher the concentration, the more obvious the inhibitory effect on the algal cells. Therefore, a strategy to promote carotenoid accumulation in Dunaliella bardawil by combining exogenous MLT and light induction was carried out. Under 4500 LUX light intensity, the content of zeaxanthin was significantly increased under exogenous MLT induction. In the 200 μg/mL, 300 μg/mL, and 400 μg/mL MLT-treated groups, the zeaxanthin single-cell content in the 300 μg/mL-treated group was as high as 0.38 ng/mL (0.17 ng/mL in the control group), which was 1.24-fold higher compared to the control. Under 9500 LUX light intensity, all carotenoids showed an increasing trend in all experimental groups, except for zeaxanthin, which showed a decreasing trend. The effect of 300 μg/mL showed the most obvious in the 200 μg/mL,300 μg/mL, and 400 μg/mL MLT treatment groups, where the lutein, α-carotene and β-carotene contents were 1.24, 1.14 and 1.31 times higher than those of the control group, respectively. Overall, exogenous MLT at high light intensities had a significant effect on pigment accumulation in Dunaliella bardawil.

Alleviation of cadmium phytotoxicity through melatonin modulated physiological functions, antioxidants, and metabolites in tomato (Solanum lycopersicum L.)

The rising concentration of cadmium (Cd) builds a harmful effect on human and plant health associated with food chain contagion. Melatonin (MT) is an indole compound. Hence, the experiment was conducted to understand the physiological and biochemical mechanism of Cd detoxification by exogenous MT in tomato. Pots were filled with 30 ppm of Cd spiked soil and different concentration of exogenous MT was given to the plants through seed treatment (250 ppm), foliar spray viz., 25, 50, and 100 ppm, and both, whereas the foliar spray was given at 30 days after transplanting (DAT) and 46 DAT. When the plants are exposed to Cd stress, it reduces the gas exchange characters. The results revealed that foliar spray of 25 ppm of exogenous MT recorded the highest photosynthetic rate, stomatal conductance, and osmotic potential. MT had a direct interaction with reactive oxygen species scavenging by elevating endogenous antioxidant enzymes as well as the metabolites in plants. The contribution of MT foliar spray of 25 ppm at 30 and 46 DAT can mitigate Cd stress and it has potential implications for ensuring food safety and food security in marginal agriculture.

Functional Characterization of Arylalkylamine N-Acetyltransferase, a Pivotal Gene in Antioxidant Melatonin Biosynthesis from Chlamydomonas reinhardtii

Arylalkylamine N-acetyltransferase (AANAT) is a pivotal enzyme in melatonin biosynthesis that catalyzes the conversion of serotonin to N-acetylserotonin. Homologs of animal AANAT genes are present in animals, but not in plants. An AANAT homolog was found in Chlamydomonas reinhardtii, but not other green algae. The characteristics of C. reinhardtii AANAT (CrAANAT) are unclear. Here, full-length CrAANAT was chemically synthesized and expressed in Escherichia coli. Recombinant CrAANAT exhibited AANAT activity with a Km of 247 μM and Vmax of 325 pmol/min/mg protein with serotonin as the substrate. CrAANAT was localized to the cytoplasm in tobacco leaf cells. Transgenic rice plants overexpressing CrAANAT (CrAANAT-OE) exhibited increased melatonin production. CrAANAT-OE plants showed a longer seed length and larger second leaf angle than wild-type plants, indicative of the involvement of brassinosteroids (BRs). As expected, BR biosynthesis- and signaling-related genes such as D2, DWARF4, DWARF11, and BZR1 were upregulated in CrAANAT-OE plants. Therefore, an increased endogenous melatonin level by ectopic overexpression of CrAANAT seems to be closely associated with BR biosynthesis, thereby influencing seed size.

Application of exogenous melatonin in vitro and in planta: a review of its effects and mechanisms of action

Melatonin is a natural indolamine that regulates many physiological functions in plants. The most prominent role of melatonin in plants has been its ability to work as an anti-stressor agent. Exogenous melatonin can prevent cell death and promote cell proliferation through its antioxidant properties, enhancement of polyamine biosynthesis, and the ability to shift cell metabolism in case of stressors like sugar starvation. Melatonin scavenges reactive oxygen species and thus preventing damage to cell membranes and other organelles. Its application in different plant culture systems reveals its important physiological and biochemical roles during the growth and development of these cultures. It has been observed that the exogenous melatonin protects callus culture, reduces cold-induced apoptosis in cell suspension, and stimulates adventitious and lateral roots formation. This review presents the physiological and biochemical effects of exogenous melatonin on in vitro culture systems, including its impact on biomass accumulation, growth, and development of plants.

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.