Aluminum (Al) toxicity is a major limiting factor for plant growth in acidic soils. Melatonin (MT) is involved in plant responses to various environmental stresses. In this study, the role of exogenous MT in alleviating Al toxicity was investigated in soybean (Glycine max L.). The results demonstrated that MT application alleviated Al-induced inhibition of root elongation, reduced Al accumulation in root tips, and mitigated oxidative damage and cell death in root tips. Under Al stress, MT treatment increased the activities of antioxidant enzymes (SOD, CAT, APX, and POD) and the contents of antioxidants (ASA and GSH) in root tips. Furthermore, Al stress significantly enhanced citrate secretion from soybean roots, while MT application further promoted citrate efflux under Al exposure. Under Al stress, MT treatment significantly increased citric acid levels in root tips by upregulating the expression of citrate synthase gene and downregulating the expression of aconitase gene. In addition, MT application significantly increased the expression of citrate transporter genes (GmMATE13 and GmMATE47) in root tips. Taken together, these findings suggest that MT enhances soybean tolerance to Al stress by activating the antioxidant defense system and promoting citrate secretion. This study provides a theoretical foundation for the application of MT to mitigate Al toxicity in plants.

Soil salinity is a severe abiotic stress that damages plant growth and development. As an antioxidant and free radical scavenger, melatonin is well known for helping plants survive abiotic conditions, including salinity stress. Here, we report that the salt-related gene MsSNAT1, encoding a rate-limiting melatonin biosynthesis enzyme, is located in the chloroplast and contributes to salinity stress tolerance in alfalfa. We found that the MsSNAT1 overexpressing alfalfa lines exhibited higher endogenous melatonin levels and increased tolerance to salt stress by promoting antioxidant systems and improving ion homeostasis. Furthermore, through a combination of transcriptome sequencing, dual-luciferase assays and transgenic analysis, we identified that the basic leucine zipper (bZIP) transcription factor, MsbZIP55, is associated with salt response and MsSNAT1 expression. EMSA analysis and ChIP-qPCR uncovered that MsbZIP55 can recognize and directly bind to the MsSNAT1 promoter in vitro and in vivo. MsbZIP55 acts as a negative regulator of MsSNAT1 expression, thereby reducing melatonin biosynthesis. Morphological analysis revealed that overexpressing MsbZIP55 conferred salt sensitivity to transgenic alfalfa through a higher Na+/K+ ratio and lower antioxidant activities, which could be alleviated by applying exogenous melatonin. Silencing of MsbZIP55 by RNA interference in alfalfa resulted in higher expression of MsSNAT1 and promoted salt tolerance by enhancing the antioxidant system enzyme activities and ion homeostasis. Our findings indicate that the MsbZIP55-MsSNAT1 module plays a crucial role in regulating melatonin biosynthesis in alfalfa while facilitating protection against salinity stress. These results shed light on the regulatory mechanism of melatonin biosynthesis related to the salinity stress response in alfalfa.

Fusarium wilt, caused by the soil-borne fungal pathogen Fusarium oxysporum (Fo), is widely recognized as one of the most devastating fungal diseases, inflicting significant damage on a wide range of agricultural and horticultural crops. Despite melatonin has recently emerged as a potential enhancer of plant resistance against Fo, the underlying mechanisms remain elusive. In this study, our results demonstrate that exogenous melatonin and MeJA enhance watermelon resistance against Fusarium oxysporum f. sp. Niveum race 2 (FON2) in a dose-dependent manner. The optimal concentration for melatonin and MeJA was determined to be 10 mu M and 1 mu M, respectively. Both melatonin and MeJA inhibited FON2 mycelial growth on PDA medium in a dose-dependent manner. Furthermore, exogenous melatonin significantly stimulated upregulation of MeJA synthesis genes and increased MeJA content upon FON2 infection. However, pretreatment with a MeJA synthesis inhibitor (DIECA) suppressed the induction of melatonin-induced resistance against FON2. Furthermore, MeJA also induced the upregulation of melatonin biosynthetic gene caffeic acid O-methyltransferase 1 (ClCOMT1) and increased melatonin accumulation in response to FON2. Notably, the reduction in FON2 resistance caused by ClCOMT1 deletion was completely restored through exogenous application of MeJA. These results suggest that melatonin facilitates MeJA accumulation, which provides feedback to promote melatonin accumulation, forming a reciprocal positive regulatory loop in response to FON2 infection. Additionally, polyphenol oxidase, phenylalanine ammonia lyase, and lignin are involved in the MeJA-induced resistance against FON2. The growing concern over minimizing pesticide usage and transitioning to sustainable and natural control strategies underscores the significant potential of such a mechanism in combating Fo.

Iron (Fe) deficiency is a critical constraint on global food security, particularly affecting high-value horticultural crops such as strawberries (Fragaria x ananassa). This study examines the roles of melatonin and hydrogen sulfide (H2S) signaling in mitigating Fe deficiency stress by improving Fe bioavailability and enhancing plant resilience. Strawberry plants were cultivated under Fe-sufficient and Fe-deficient conditions and treated with 100 mu M melatonin and 3 mM dl-propargylglycine (PAG), an inhibitor of L-cysteine desulfhydrase (L-DES), which regulates H2S production. Fe deficiency significantly reduced chlorophyll content and photosynthetic efficiency while elevating oxidative stress markers such as hydrogen peroxide (H2O2), malondialdehyde (MDA), and electrolyte leakage (EL). Melatonin application alleviated Fe deficiency effects by enhancing Fe utilization, stimulating L-DES activity, and promoting H2S production. Melatonin also improved antioxidant defenses by boosting the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), as well as maintaining ascorbate-glutathione (AsA-GSH) redox dynamics. The addition of 3 mM PAG inhibited L-DES activity, resulting in reduced H2S levels and diminished melatonin-induced benefits, underscoring the essential role of L-DES-mediated H2S synthesis. Despite the presence of PAG, the co-application of 0.2 mM sodium hydrosulfide (NaHS) and melatonin restored Fe bioavailability, growth, and antioxidant capacity, suggesting a synergistic interaction between melatonin and H2S. This study highlights the potential of melatonin and H2S signaling to improve Fe homeostasis and mitigate oxidative stress in Fe-deficient plants. The findings offer strategies to enhance crop resilience and productivity in nutrient-deficient soils, thereby promoting sustainable agriculture and global food security.

Cucumbers, cultivated globally on 3.7 million hectares, face yield losses due to salinity, highlighting the need for effective mitigation strategies for degraded soils. Melatonin (MT) has gained significant interest for its ability to relieve plant stress. To explore the regulatory role of exogenous MT in maintaining redox homeostasis in cucumber seedlings under saline-alkali stress (SA), this study employed the cucumber cultivar 'Xinchun No. 4 '. Simulated saline-alkali conditions were applied, and the effects of exogenous MT on seedling growth, reactive oxygen species (ROS) production, the ascorbate-glutathione (AsA-GSH) cycle, and changes in leaf anatomy were systematically assessed. The findings reveal that exposure to 40 mmol center dot L-1 saline-alkali stress significantly impaired cucumber seedling growth, reduced biomass, and led to excessive accumulation of hydrogen peroxide (H2O2) and superoxide anions (O2 center dot ) in the leaves. This, resulted in increased lipid peroxidation (indicated by elevated malondialdehyde (MDA) levels), whichi further compromised the cell membrane. Application of 10 mu mol center dot L-1 MT effectively reduced ROS levels, lowered MDA content, and mitigated electrolyte leakage. MT also enhanced AsA and GSH levels, improved AsA/DHA and GSH/GSSG ratios, and upregulated key AsA-GSH cycle genes (CsAPX, CsAAO, CsMDAR, CsDHAR, CsGR), leading to a significant increase in enzymatic activity. In addition, MT alleviated stress-induced stomatal closure, thereby restoring normal stomatal function. These findings suggest that MT enhances saline-alkali tolerance by mitigating oxidative damage, promoting antioxidant defenses, and effectively preserving stomatal function. Thus, our study points to a sustainable strategy to improve crop resilience in salinized environments via MT application.

The tomato is among the crops with the most extensive cultivated area and greatest consumption in our nation; nonetheless, secondary salinization of facility soil significantly hinders the sustainable growth of facility agriculture. Melatonin (MT), as an innovative plant growth regulator, is essential in stress responses. This research used a hydroponic setup to replicate saline stress conditions. Different endogenous levels of melatonin (MT) were established by foliar spraying of 100 mu molL-1 MT, the MT synthesis inhibitor p-CPA (100 mu molL-1), and a combination of p-CPA and MT, to investigate the mechanism by which MT mitigates the effects of salt stress on the photosynthetic efficiency of tomato seedlings. Results indicated that after six days of salt stress, the endogenous MT content in tomato seedlings drastically decreased, with declines in the net photosynthetic rate and photosystem performance indices (PItotal and PIabs). The OJIP fluorescence curve exhibited distortion, characterized by anomalous K-band and L-band manifestations. Exogenous MT dramatically enhanced the gene (TrpDC, T5H, SNAcT, and AcSNMT) expression of critical enzymes in MT synthesis, therefore boosting the level of endogenous MT. The application of MT enhanced the photosynthetic parameters. MT treatment decreased the fluorescence intensities of the J-phase and I-phase in the OJIP curve under salt stress, attenuated the irregularities in the K-band and L-band performance, and concurrently enhanced quantum yield and energy partitioning ratios. It specifically elevated phi Po, phi Eo, and psi o, while decreasing phi Do. The therapy enhanced parameters of both the membrane model (ABS/RC, DIo/RC, ETo/RC, and TRo/RC) and leaf model (ABS/CSm, TRo/CSm, ETo/CSm, and DIo/CSm). Conversely, the injection of exogenous p-CPA exacerbated salt stress-related damage to the photosystem of tomato seedlings and diminished the beneficial effects of MT. The findings suggest that exogenous MT mitigates salt stress-induced photoinhibition by (1) modulating endogenous MT concentrations, (2) augmenting PSII reaction center functionality, (3) safeguarding the oxygen-evolving complex (OEC), (4) reinstating PSI redox potential, (5) facilitating photosynthetic electron transport, and (6) optimizing energy absorption and dissipation. As a result, MT markedly enhanced photochemical performance and facilitated development and salt stress resilience in tomato seedlings.

Drought stress negatively affects cotton pollen fertility, which in turn leads to a decrease in seed number per boll and boll weight. Exogenous melatonin application significantly enhances pollen fertility under drought conditions, while the specific underlying mechanisms remain unclear. A pot experiment was conducted using a cultivar Yuzaomian 9110 under two moisture treatments (soil relative water content at 75 +/- 5 % and 45 +/- 5 %) with two melatonin concentration (0 and 200 mu M) to investigate the effects of exogenous melatonin on the structural traits and physiological metabolism of cotton anthers and its' relationships with pollen fertility. Results demonstrated the significant impact of drought on anthers development and metabolism, with damage to the anther tapetum and decreased starch and adenosine triphosphate (ATP) contents, subsequently resulting in reduced pollen germination rate, seed number per boll and boll weight. Melatonin application in water-deficit anthers up-regulated the expression of sucrose transporter protein (GhSWEET55) and phosphate sucrose synthetase, promoting sucrose import and synthesis, respectively. However, it also increased sucrose synthase and acid convertase, accelerating sucrose decomposition and reducing its content. Additionally, melatonin application promoted starch accumulation in water-deficit anthers by enhancing activities of adenosine diphosphate glucose pyrophosphorylase and soluble starch synthase, meaning that potential energy storage was increased, which facilitated the formation of pollen fertility. Although melatonin application reduced the expression of pyruvate kinase (GhPK) and glucose 6-phosphogluconate dehydrogenase (GhG6PD) genes in water-deficit anthers, it upregulated hexokinase (GhHXK) and citrate synthase (GhCIT) expression, enhancing ATP content, and ultimately pollen fertility, seed number and boll weight under drought. In summary, exogenous melatonin preserved cotton pollen fertility under drought stress by regulating carbohydrate and energy metabolism, especially enhancing starch and ATP accumulation in anthers.

Key messageMelatonin 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.AbstractLead (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 mu M 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.

Fodder soybean (Glycine max L.) with high protein and yield is a popular forage grass in northeast China. Seasonal drought inhibits its growth and development during seedling stage. The objective of this study was to observe morpho-physiological changes in fodder soybean seedlings under melatonin (MT) treatments and identify appropriate concentration to alleviate the drought damage. Two varieties commonly used in northeast China were treated with 0, 50, 100, and 150 mu M melatonin at soil water content of 30%. The results indicated that applying melatonin enhanced height, biomass and altered root morphology of fodder soybean seedlings under water-deficient conditions. The treatments with melatonin at different concentrations significantly reduced the contents of H2O2, O2- and MDA, while boosting the capacity of the antioxidant defense system and the content of osmotic adjustment substances. Meanwhile, increases in light energy capture and transmission efficiency were observed. Furthermore, treatment with melatonin regulated the expression levels of genes associated with photosynthesis and the antioxidant defense system. Notably, 100 mu M melatonin treatment produced the most favorable effect in all treatments under drought conditions. These research results provide new information for enhancing the drought tolerance of fodder soybean using chemical measures.

BackgroundGlobally, 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.ResultsCadmium (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.ConclusionsOur 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 numberNot applicable.