Exogenous melatonin mitigates boron toxicity in wheat

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.

Enhancing cadmium stress resilience in chickpea (Cicer arietinum L.) via exogenous melatonin application

Chickpea (Cicer arietinum L.) productivity is hindered by biotic and abiotic stresses, particularly heavy metal toxicity. The pot experiment was carried out at the botanical garden of The Islamia University of Bahawalpur, Bahawalpur-Pakistan. The experimental treatments comprised of following details: T0 = Control + 0 µM MT, T1 = Control + 15 µM MT, T2= Control + 30 µM MT, T3 = 100 µM Cd + 0 µM MT, T4 = 100 µM Cd + 15 µM MT and T5 = 100 µM Cd + 30 µM MT. A completely randomized design (CRD) with three replicates was used. Cd stress significantly reduced shoot fresh (51.3%) and dry weight (50.4%), total chlorophyll (53.6%), and shoot Ca2+ (56.6%). However, it increased proline (38.3%), total phenolics (74.2%), glycine betaine (46.4%), TSS (67.7%), TSP (50%), SOD (49.5%), POD (107%), and CAT (74.2%). Conversely, 30 µM MT improved shoot fresh (78.5%) and dry weight (76%), total chlorophyll (47%), SOD (26.5%), POD (35.8%), CAT (27.8%), proline (19%), TSS (24.5%), TSP (25.8%), and shoot Ca2+ (56.6%). Results indicated that MT enhanced photosynthetic pigments and antioxidant activities, maintained ion homeostasis, and reduces reactive oxygen species. Desi variety performed better than Kabuli, and 30 µM MT application effectively mitigated Cd toxicity.

Effects of Melatonin Pre- and Post-Drought Treatment on Oxidative Stress Markers and Expression of Proline-Related Transcripts in Young Wheat Plants

Wheat can tolerate a mild water deficit, but prolonged drought causes a number of detrimental physiological changes resulting in a substantial decrease in productivity. The present study evaluates the potential of the natural plant growth regulator melatonin to alleviate the negative effects of moderate drought in two Bulgarian winter wheat cultivars at the early vegetative stage. Melatonin doses of 75 µM were root-supplemented 24 h before or after the stress period. The levels of several biometric parameters, osmolyte content and stress indicators as well as the expression of genes coding for key enzymes of the proline biosynthesis pathway were analyzed in leaves at the end of the drought stress and after two and four days of recovery. Applied alone, melatonin did not exert significant effects on most of the monitored parameters. Water deprivation negatively affected seedlings' fresh weight and water content and increased the stress markers and osmolyte levels. These were accompanied by a high accumulation of TaP5CS and TaP5CR transcripts coding for the enzymes Δ-pyrroline-5-carboxylate synthase and Δ-pyrroline-5-carboxylate reductase, respectively. The effect of melatonin in reducing drought stress was similar whether applied before or after exposure, though slightly more effective when used as a pre-treatment.

Salicylic Acid and Melatonin Synergy Enhances Boron Toxicity Tolerance via AsA-GSH Cycle and Glyoxalase System Regulation in Fragrant Rice

Background: Boron is an essential micronutrient for plant growth and productivity, yet excessive boron leads to toxicity, posing significant challenges for agriculture. Fragrant rice is popular among consumers, but the impact of boron toxicity on qualitative traits of fragrant rice, especially aroma, remains largely unexplored. The individual potentials of melatonin and salicylic acid in reducing boron toxicity are less known, while their synergistic effects and mechanisms in fragrant rice remain unclear. Methods: Thus, this study investigates the combined application of melatonin and salicylic acid on fragrant rice affected by boron toxicity. One-week-old seedlings were subjected to boron (0 and 800 µM) and then treated with melatonin and salicylic acid (0 and 100 µM, for 3 weeks). Results: Boron toxicity significantly impaired photosynthetic pigments, plant growth, and chloroplast integrity while increasing oxidative stress markers such as hydrogen peroxide, malondialdehyde, methylglyoxal, and betaine aldehyde dehydrogenase. Likewise, boron toxicity abridged the precursors involved in the 2-acetyl-1-pyrroline (2-AP) biosynthesis pathway. However, individual as well as combined application of melatonin and salicylic acid ameliorated boron toxicity by strengthening the antioxidant defense mechanisms-including the enzymes involved during the ascorbate-glutathione (AsA-GSH) cycle and glyoxalase system-and substantially improved 2-AP precursors including proline, P5C, Δ1-pyrroline, and GABA levels, thereby restoring the 2-AP content and aroma. These findings deduce that melatonin and salicylic acid synergistically alleviate boron toxicity-induced disruptions on the 2-AP biosynthesis pathway by improving the 2-AP precursors and enzymatic activities, as well as modulating the physio-biochemical processes and antioxidant defense system of fragrant rice plants. Conclusions: The findings of this study have the potential to enhance rice productivity and stress tolerance, offering solutions to improve food security and sustainability in agricultural practices, particularly in regions affected by environmental stressors.

Boron toxicity in plants: understanding mechanisms and developing coping strategies; a review

KEY MESSAGE

Boron is essential for plants, but excess can induce toxicity. Boron (B) is a vital micronutrient for plants, but excess B can induce toxicity symptoms and reduce crop yields. B bioavailability depends on soil properties, including clay type, pH, and organic matter content. Symptoms of B toxicity include reduced shoot and root growth, leaf chlorosis and necrosis, impaired photosynthesis, and disrupted pollen development. This review paper examines the current knowledge on B toxicity mechanisms, tolerance strategies, and management approaches in plants. This review covers (1) factors affecting B bioavailability; (2) toxicity symptoms in plants; (3) uptake, transport, and detoxification mechanisms; and (4) strategies. To mitigate toxicity, plants reduce B uptake, activate efflux transporters, compartmentalize B, and enhance antioxidant systems. On the basis of this review, future research should focus on identifying novel tolerance mechanisms, exploring genetic strategies for improved B management, and developing innovative agronomic interventions. These insights will facilitate the breeding and management of crops for enhanced productivity under B toxicity stress.

A meta-analysis of photosynthetic efficiency and stress mitigation by melatonin in enhancing wheat tolerance

BACKGROUND

Our meta-analysis examines the effects of melatonin on wheat under varying abiotic stress conditions, focusing on photosynthetic parameters, chlorophyll fluorescence, leaf water status, and photosynthetic pigments. We initially collected 177 publications addressing the impact of melatonin on wheat. After meticulous screening, 31 published studies were selected, encompassing 170 observations on photosynthetic parameters, 73 on chlorophyll fluorescence, 65 on leaf water status, 240 on photosynthetic pigments.

RESULTS

The analysis revealed significant heterogeneity across studies (I² > 99.90%) for the aforementioned parameters and evidence of publication bias, emphasizing the complex interaction between melatonin application and plant physiological responses. Melatonin enhanced the overall response ratio (lnRR) for photosynthetic rates, stomatal conductance, transpiration rates, and fluorescence yields by 20.49, 22.39, 30.96, and 1.09%, respectively, compared to the control (no melatonin). The most notable effects were under controlled environmental conditions. Moreover, melatonin significantly improved leaf water content and reduced water potential, particularly under hydroponic conditions and varied abiotic stresses, highlighting its role in mitigating water stress. The analysis also revealed increases in chlorophyll pigments with soil drenching and foliar spray, and these were considered the effective application methods. Furthermore, melatonin influenced chlorophyll SPAD and intercellular CO2 concentrations, suggesting its capacity to optimize photosynthetic efficiency.

CONCLUSIONS

This synthesis of meta-analysis confirms that melatonin significantly enhances wheat's resilience to abiotic stress by improving photosynthetic parameters, chlorophyll fluorescence, leaf water status, and photosynthetic pigments. Despite observed heterogeneity and publication bias, the consistent beneficial effects of melatonin, particularly under controlled conditions with specific application methods e.g. soil drenching and foliar spray, demonstrate its utility as a plant growth regulator for stress management. These findings encourage focused research and application strategies to maximize the benefits of melatonin in wheat farming, and thus contributing to sustainable agricultural practices.

Zinc oxide nanoparticles and Klebsiella sp. SBP-8 alleviates chromium toxicity in Brassica juncea by regulation of antioxidant capacity, osmolyte production, nutritional content and reduction in chromium adsorption

Heavy metals are one of the most damaging environmental toxins that hamper growth of plants. These noxious chemicals include lead (Pb), arsenic (As), nickel (Ni), cadmium (Cd) and chromium (Cr). Chromium is one of the toxic metal which induces various oxidative processes in plants. The emerging role of nanoparticles as pesticides, fertilizers and growth regulators have attracted the attention of various scientists. Current study was conducted to explore the potential of zinc oxide nanoparticles (ZnONPs) alone and in combination with plant growth promoting rhizobacteria (PGPR) Klebsiella sp. SBP-8 in Cr stress alleviation in Brassica juncea (L.). Chromium stress reduced shoot fresh weight (40%), root fresh weight (28%), shoot dry weight (28%) and root dry weight (34%) in B. juncea seedlings. Chromium stressed B. juncea plants showed enhanced levels of malondialdehyde (MDA), electrolyte leakage (EL), hydrogen peroxide (H2O2) and superoxide ion (O2• -). However, co-supplementation of ZnONPs and Klebsiella sp. SBP-8 escalated the activity of antioxidant enzymes i.e., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) in B. juncea grown in normal and Cr-toxic soil. It is further proposed that combined treatment of ZnONPs and Klebsiella sp. SBP-8 may be useful for alleviation of other abiotic stresses in plants.

Melatonin-Mediated Molecular Responses in Plants: Enhancing Stress Tolerance and Mitigating Environmental Challenges in Cereal Crop Production

Cereal crops are crucial for global food security; however, they are susceptible to various environmental stresses that significantly hamper their productivity. In response, melatonin has emerged as a promising regulator, offering potential benefits for stress tolerance and crop growth. This review explores the effects of melatonin on maize, sorghum, millet, rice, barley, and wheat, aiming to enhance their resilience to stress. The application of melatonin has shown promising outcomes, improving water use efficiency and reducing transpiration rates in millet under drought stress conditions. Furthermore, it enhances the salinity and heavy metal tolerance of millet by regulating the activity of stress-responsive genes. Similarly, melatonin application in sorghum enhances its resistance to high temperatures, low humidity, and nutrient deficiency, potentially involving the modulation of antioxidant defense and aspects related to photosynthetic genes. Melatonin also exerts protective effects against drought, salinity, heavy metal, extreme temperatures, and waterlogging stresses in maize, wheat, rice, and barley crops by decreasing reactive oxygen species (ROS) production through regulating the antioxidant defense system. The molecular reactions of melatonin upregulated photosynthesis, antioxidant defense mechanisms, the metabolic pathway, and genes and downregulated stress susceptibility genes. In conclusion, melatonin serves as a versatile tool in cereal crops, bolstering stress resistance and promoting sustainable development. Further investigations are warranted to elucidate the underlying molecular mechanisms and refine application techniques to fully harness the potential role of melatonin in cereal crop production systems.

Exogenous melatonin alleviates nicosulfuron toxicity by regulating the growth, photosynthetic capacity, and antioxidative defense of sweet corn seedlings

Improper use of nicosulfuron (NSF) may induce harmful effects on plants during weed control. Melatonin (MT) regulates photosynthetic and physiological processes in plants. This study aimed to explore the effects of MT on alleviating NSF toxicity by measuring the growth parameters, photosynthetic capacity, and antioxidative responses in sweet corn seedlings. Compared to NSF alone, exogenous MT increased chlorophyll content, transpiration rate, net photosynthetic rate, stomatal conductance, and maximum efficiency of PSII photochemistry, while reduced malondialdehyde, hydrogen peroxide, superoxide anion radical, and proline contents. Moreover, MT also increased the activity of ascorbate peroxidase and the expression levels of ZmAPX1, ZmAPX2, ZmALS1, and ZmCYP81A9. The inhibition of p-chlorophenylalanine inhibited the positive effects of MT on photosynthetic and physiological indexes. The results indicated that pretreatment with MT might effectively mitigate NSF toxicity in sweet corn seedlings.

Boron-resistant Alternaria alternata (OG14) mitigates boron stress by improving physiological and antioxidative response in wheat (Triticum aestivum L.)

Effect of Alternaria alternata (OG14) isolated from a rock lichen (Xanthoria sp.) was investigated on the relief of boron stress in wheat. To determine the tolerance level to B stress, the fungus was grown at increasing boric acid (BA) concentrations in the range of 0.0-2.5 M. No significant change in colony development of the fungus was observed up to 1 M BA application compared to the control but after this dose, it decreased depending on the increase in the BA dose. When the element content of wheat seedlings was evaluated by ICP-MS, BA application increased B content together with Mg, P, K, Fe contents of the seedlings to very high levels compared to the control. However, fungus + BA treatments decreased the content of B and the other elements in the seedlings. The BA applications resulted in an increase in the levels of reactive oxygen species, including H2O2 and O2.-as well as lipid peroxidation in the seedlings. However, when the fungal inoculation was performed under the same BA conditions, the levels of these parameters decreased. The fungus inoculation stimulated the activity of all studied enzymes compared to BA applications. BA applications alone increased non - enyzmatic the oxidized ascorbate level more than the reducing ascorbate, leading to a decrease in the AsA/DHA ratio. The results show that A. alternata treatment can mitigate the negative effects of B stress on wheat seedlings by reducing ROS, LPO, B content, increasing the capacity of enzymatic and non-enzymatic antioxidants, and improving root and shoot length.

The Fungal Root Endophyte Serendipita indica (Piriformospora indica) Enhances Bread and Durum Wheat Performance under Boron Toxicity at Both Vegetative and Generative Stages of Development through Mechanisms Unrelated to Mineral Homeostasis

While the importance of beneficial soil microorganisms for soil health and crop performance has been receiving ever-increasing attention, Serendipita indica has been widely studied as a fungal root endophyte with significant potential for increasing the stress tolerance of host plants. Boron (B) toxicity as an adverse soil condition is particularly prevalent in arid and semi-arid regions and threatens crop production. Studies on S. indica-wheat symbiosis are limited, and effects of S. indica on crops have never been reported in the context of B toxicity. Here, two pot experiments were conducted under greenhouse conditions to investigate the effects of S. indica on the growth and yield parameters of bread (Triticum aestivum) and durum wheat (Triticum durum) grown at different levels of B toxicity in native vs. sterilized soil, and parameters related to root colonization, membrane damage, oxidative stress, chlorophyll, and mineral nutrition were measured to elucidate the physiological mechanisms of damage and benefit. Boron toxicity decreased early vegetative growth and grain yield, but it did not affect the straw dry weight of mature plants, whereas S. indica significantly enhanced the vegetative growth, straw dry weight, and the grain number of both wheat species. Membrane damage as demonstrated by increased lipid peroxidation and relative electrolyte leakage was caused by B toxicity and alleviated by S. indica. The benefits provided by S. indica could not be attributed to any significant changes in tissue concentrations of B or other minerals such as phosphorus. Soil sterilization generally improved plant performance but it did not consistently strengthen or weaken the effects of S. indica. The presented results suggest that S. indica may be used as an effective microbial inoculant to enhance wheat growth under adverse soil conditions such as B toxicity through mechanisms that are possibly unrelated to mineral homeostasis.

Melatonin and strigolactone mitigate chromium toxicity through modulation of ascorbate-glutathione pathway and gene expression in tomato

Chromium (Cr) is considered one of the most hazardous metal contaminant reducing crop production and putting human health at risk. Phytohormones are known to regulate chromium stress, however, the function of melatonin and strigolactones in Chromium stress tolerance in tomato is rarely investigated. Here we investigated the potential role of melatonin (ML) and strigolactone (SL) on mitigating Chromium toxicity in tomato. With exposure to 300 μM Cr stress a remarkable decline in growth (63.01%), biomass yield (50.25)%, Pigment content (24.32%), photosynthesis, gas exchange and Physico-biochemical attributes of tomato was observed. Cr treatment also resulted in oxidative stress closely associated with higher H2O2 generation (215.66%), Lipid peroxidation (50.29%), electrolyte leakage (440.01%) and accumulation of osmolytes like proline and glycine betine. Moreover, Cr toxicity up-regulated the transcriptional expression profiles of antioxidant, stress related and metal transporter genes and down-regulated the genes related to photosynthesis. The application of ML and SL alleviated the Cr induced phytotoxic effects on photosynthetic pigments, gas exchange parameters and restored growth of tomato plants. ML and SL supplementation induced plant defense system via enhanced regulation of antioxidant enzymes, ascorbate and glutathione pool and transcriptional regulation of several genes. The coordinated regulation of antioxidant and glyoxalase systems expressively suppressed the oxidative stress. Hence, ML and SL application might be considered as an effective approach for minimizing Cr uptake and its detrimental effects in tomato plants grown in contaminated soils. The study may also provide new insights into the role of transcriptional regulation in the protection against heavy metal toxicity.

Potential application of melatonin in reducing boron toxicity in rice seedlings through improved growth, cell wall composition, proline, and defense mechanisms

Melatonin (MT) has been demonstrated to provide defense against both biotic and abiotic stressors. Boron toxicity (BT) can significantly limit the growth and production of plants. However, few studies have been conducted on whether MT is effective in attenuating B toxicity in different plants. In order to evaluate the efficacy of exogenous MT treatment in reducing the negative impact of BT on rice seedlings, this study examined the influence of MT on growth, antioxidant capacity, cell wall composition, and proline metabolism in rice seedlings under hydroponics. Four treatments were established: MT (50 μM), MT + BT (50 μM MT + 800 μM B), BT (800 μM), and CK (control) in a completely randomized design. The results indicate that BT had a significant detrimental effect on the shoot length, root length, and root and shoot fresh weights of rice seedlings by 11.96%, 27.77%, 25.69%, and 18.67%, respectively as compared to the control treatment. However, exogenous MT application increased these parameters and reduced B accumulation in aboveground parts (14.05%) of the plant. Exogenous MT also increased the endogenous melatonin content and antioxidant enzyme activities (64.45%, 71.61%, 237.64%, and 55.42% increase in superoxide dismutase, ascorbate peroxidase, and peroxidase activities, respectively), while decreasing reactive oxygen species levels and oxidized forms of glutathione and ascorbic acid. Additionally, MT enhanced the biosynthesis of proline by decreasing proline dehydrogenase (ProDH) and increasing the GSH (glutathione) and ASA (ascorbic acid) contents. Exogenous MT also increased cell wall components that can increase B adsorption to the cell wall. Overall, these findings suggest that MT application can be a potential solution for strengthening the stress tolerance of rice seedlings, particularly under conditions of B toxicity. In regions where soil contains high levels of boron, the use of MT could enhance rice crop yields and quality.

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

MAIN CONCLUSION

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

Exogenous melatonin enhances tomato heat resistance by regulating photosynthetic electron flux and maintaining ROS homeostasis

Heat stress reduces plant growth and reproduction and increases agricultural risks. As a natural compound, melatonin modulates broad aspects of the responses of plants to various biotic and abiotic stresses. However, regulation of the photosynthetic electron transfer, reactive oxygen species (ROS) homeostasis and the redox state of redox-sensitive proteins in the tolerance to heat stress induced by melatonin remain largely unknown. The oxygen evolution complex activity on the electron-donating side of photosystem II (PSII) is inhibited, and the electron transfer process from QA to QB on the electron-accepting side of PSII is inhibited. In this case, heat stress decreased the chlorophyll content, carbon assimilation rate, PSII activity, and the proportion of light absorbed by tomato seedlings during electron transfer. The ROS burst led to the breakdown of the PSII core protein. However, exogenous melatonin increased the net photosynthetic rate by 11.3% compared with heat stress, substantially reducing the restriction of photosynthetic systems induced by heat stress. Additionally, melatonin reduces the oxidative damage to PSII by balancing electron transfer on the donor, reactive center, and acceptor sides. Melatonin was used under heat stress to increase the activity of the antioxidant enzyme and preserve ROS equilibrium. In addition, redox proteomics also showed that melatonin controls the redox levels of proteins involved in photosynthesis, and stress and defense processes, which enhances the expression of oxidative genes. In conclusion, melatonin via controlling the photosynthetic electron transport and antioxidant, melatonin increased tomato heat stress tolerance and aided plant growth.

The pathway of melatonin biosynthesis in common wheat (Triticum aestivum)

Melatonin (Mel) is a multifunctional biomolecule found in both animals and plants. In plants, the biosynthesis of Mel from tryptophan (Trp) has been delineated to comprise of four consecutive reactions. However, while the genes encoding these enzymes in rice are well characterized no systematic evaluation of the overall pathway has, as yet, been published for wheat. In the current study, the relative contents of six Mel-pathway-intermediates including Trp, tryptamine (Trm), serotonin (Ser), 5-methoxy tryptamine (5M-Trm), N-acetyl serotonin (NAS) and Mel, were determined in 24 independent tissues spanning the lifetime of wheat. These studies indicated that Trp was the most abundant among the six metabolites, followed by Trm and Ser. Next, the candidate genes expressing key enzymes involved in the Mel pathway were explored by means of metabolite-based genome-wide association study (mGWAS), wherein two TDC genes, a T5H gene and one SNAT gene were identified as being important for the accumulation of Mel pathway metabolites. Moreover, a 463-bp insertion within the T5H gene was discovered that may be responsible for variation in Ser content. Finally, a ASMT gene was found via sequence alignment against its rice homolog. Validations of these candidate genes were performed by in vitro enzymatic reactions using proteins purified following recombinant expression in Escherichia coli, transient gene expression in tobacco, and transgenic approaches in wheat. Our results thus provide the first comprehensive investigation into the Mel pathway metabolites, and a swift candidate gene identification via forward-genetics strategies, in common wheat.

Reduction of lead toxicity effects and enhancing the glutathione reservoir in green beans through spraying sulfur and serine and glutamine amino acids

Acid rain is one of the influential environmental factors in transport of heavy metals, including lead from the atmosphere to the surface of the earth and growing plants. Such situation can not only damage the growing plants but can also toxify the food chain, and endanger human life. In order to reduce stress damage due to lead, on green bean plant, the effect of spraying the plants by sulfur, also amino acids including serine and glutamine, was evaluated. A factorial experiment based on randomized complete block design with three replications was carried out using the green bean Sunray cultivar in 2020. Treatments include foliar application of lead at two levels (0.0 and 1 mmol) as lead acetate, foliar application of liquid sulfur at two levels (0.0 and 2 per thousand), and foliar application of amino acids at four levels (0.0, serine at 200 mg/L, glutamine at 200 mg/L, and co-application of serine and glutamine at the same concentrations) at pre-flowering stage. The results showed that leaf foliar uptake of most of the employed treatments resulted in reduction of leaf area index, leaf, stem and pods dry weight, stem diameter and height, pod yield, photosynthetic pigments such as chlorophyll a, chlorophyll b, and carotenoids, and relative leaf water content. However, grain protein content, hydrogen peroxide, and glutathione antioxidant activity significantly increased. Spraying of sulfur solution and serine and glutamine were effective in reducing the negative effects of lead stress, as it reduced the amount of hydrogen peroxide and grain protein and increased the reservoir of glutathione. These treatments also, compared to the pure lead treatment, significantly reduced lead accumulation in the pod, as the edible organ of green beans. This study results showed that foliar application of sulfur along with amino acids serine and glutamine reduced the lead toxicity effects through improving the physiological functions, and thus can increase the final yield and consequently human access to healthier food (Fig. 1). Fig. 1 Graphical abstract.

Melatonin-mediated resistance to copper oxide nanoparticles-induced toxicity by regulating the photosynthetic apparatus, cellular damages and antioxidant defense system in maize seedlings

The pollution of nanoparticles (NPs) has linked with severe negative effects on crop productivity. Thus, effective strategies are needed to mitigate the phytotoxicity of NPs. The aim of present study was to evaluate the efficacy of exogenously applied melatonin (MT) in mitigating the toxic effects of copper oxide nanoparticles (CuO NPs) from maize seedlings. Therefore, we comprehensively investigated the inhibitory effects of MT against CuO NPs-induced toxicity on morpho-physiological, biochemical and ultrastructural levels in maize. Our results show that CuO NPs (300 mg L^-1) exposure displayed significantly reduction in all plant growth traits and induced toxicity in maize. Furthermore, 50 μM MT provided maximum plant tolerance against CuO NPs-induced phytotoxicity. It was noticed that MT improved plant growth, biomass, photochemical efficiency (Fv/Fm), chlorophyll contents (Chl a and Chl b), SPAD values and gas exchange attributes (stomatal conductance, net photosynthetic rate, intercellular CO₂ concentration and transpiration rate) under CuO NPs stress. In addition, MT enhanced the antioxidant defense system and conferred protection to ultrastructural (mainly chloroplast, thylakoids membrane and plastoglobuli) damages and stomatal closure in maize plants subjected to CuO NPs stress. Together, it can be stated that the exogenous supply of MT improves the resilience of maize plants against the CuO NPs-induced phytotoxicity. Our current findings can be useful for the enhancement of plant growth and yield attributes in CuO NPs-contaminated soils. The reported information can provide insight into the MT pathways that can be used to improve crop stress tolerance in a challenging environment.

High boron stress leads to sugar beet (Beta vulgaris L.) toxicity by disrupting photosystem Ⅱ

This sugar beet acts as a soil remediator in areas where there are high levels of boron (B) in the soil, since it has a high requirement of boron (B) for growth, and has strong resistance to high B levels. Although B toxicity in different plants has been widely researched, little is known about the response of photosystem II (PSII) activity in sugar beet leaves to B toxicity at present. To clarify the growth and photosynthetic physiological response of sugar beet to B toxicity, the effects of different concentrations of H₃BO₃ (0.05, 1.5, 2.5,3.5 mM) on the growth, photosynthetic characteristics and antioxidant defense system of sugar beet seedlings were investigated by hydroponic experiments. In the present study, high B stress inhibited the growth of sugar beet and significantly decreased the biomass of the plants. There was a remarkable increase in the accumulation of B in the shoots, which affected photosynthesis and decreased the photosynthetic pigments. As B toxicity increased, leaf PSII activities and maximum photochemical efficiency of PSII (F_v/F_m) showed a tendency to decrease; at the same time, the photosynthetic performance index based on absorbed light energy (PI_ABS) decreased as well. Meanwhile, the energy allocation parameters of the PSII reaction center were changed, the light energy utilization capacity and the energy used for electron transfer were reduced and the thermal dissipation was increased at the same time. Furthermore, B toxicity decreased catalase (CAT) activity, increased peroxidase (POD) and superoxide dismutase (SOD) activities, and increased malondialdehyde (MDA) accumulation. According to the results obtained in this study, high B concentrations reduced the rate of photosynthesis and fluorescence, thus weakened antioxidant defense systems, and therefore inhibited the growth of sugar beet plants. Thus, in high B areas, sugar beet possesses excellent tolerance to high B levels and has a high B translocation capacity, so it can be used as a phytoremediation tool. This study provides a basis for the feasibility of sugar beet resistant to high B environments.

Molecular characterization of the genome-wide BOR transporter family and their responses to boron conditions in common wheat (Triticum aestivum L.)

Boron (B) deficiency is an agricultural problem that causes significant yield losses in many countries. B transporters (BORs) are responsible for B uptake and distribution and play important roles in yield formation. A comprehensive analysis of the BOR family members in common wheat is still lacking. In the present study, to clarify the molecular characterization and response to B status, genome-wide TaBOR genes and expression patterns were investigated. Fourteen TaBOR genes were identified in common wheat by a homology search. The corresponding phylogenetic tree indicated that 14 TaBOR genes were separately classified into subfamilies of TaBOR1, TaBOR3, and TaBOR4. All TaBOR genes had 12-14 extrons and 11-13 introns. Most TaBOR proteins contained 10 conserved motifs, and motifs 1, 2, 3, 4, and 6 constituted the conserved bicarbonate (HCO₃ ^-) domain. Fourteen TaBOR genes were mapped on 13 chromosomes mainly distributed in the first, third, fifth, and seventh homologous groups. The promoters of TaBOR genes consisted of phytohormones, light responses, and stress-related cis-elements. GO analysis indicated that TaBOR genes were enriched in terms of transmembrane transport and ion homeostasis. TaBOR genes showed diverse expression profiles in different tissues. The members of the TaBOR1 subfamily showed high expression in grains, leaves, roots, stems, and spikes, but members of the TaBOR4 subfamily were highly expressed only in spikes and grains. RT-qPCR indicated that TaBOR1-5A, TaBOR1-5B, and TaBOR1-5D were induced by low B concentrations and had much higher expression in roots than in shoots. TaBOR3-3A, TaBOR3-3B, TaBOR3-3D, TaBOR4-1A, TaBOR4-1B, TaBOR4-1D, and TaBOR3-4B were induced by low and high B concentrations and had high expression in roots and shoots. TaBOR3-4D and TaBOR3-7B were upregulated by low and high B concentrations, respectively, but had expression only in roots. Our results provide basic information on the TaBOR family, which is beneficial for elucidating the functions of TaBOR genes to overcome the problem of B deficiency.

Phytomelatonin and plant mineral nutrition

Plant mineral nutrition is critical for agricultural productivity and for human nutrition; however, the availability of mineral elements is spatially and temporally heterogeneous in many ecosystems and agricultural landscapes. Nutrient imbalances trigger intricate signalling networks that modulate plant acclimation responses. One signalling agent of particular importance in such networks is phytomelatonin, a pleiotropic molecule with multiple functions. Evidence indicates that deficiencies or excesses of nutrients generally increase phytomelatonin levels in certain tissues, and it is increasingly thought to participate in the regulation of plant mineral nutrition. Alterations in endogenous phytomelatonin levels can protect plants from oxidative stress, influence root architecture, and influence nutrient uptake and efficiency of use through transcriptional and post-transcriptional regulation; such changes optimize mineral nutrient acquisition and ion homeostasis inside plant cells and thereby help to promote growth. This review summarizes current knowledge on the regulation of plant mineral nutrition by melatonin and highlights how endogenous phytomelatonin alters plant responses to specific mineral elements. In addition, we comprehensively discuss how melatonin influences uptake and transport under conditions of nutrient shortage.

Effect of Exogenous Glycine Betaine on the Germination of Tomato Seeds under Cold Stress

Cold stress is known to influence tomato growth, development, and yield. In this study, we analyzed the germination of tomato seeds treated with exogenous glycine betaine (GB) at a low temperature (14 °C). The results showed that cold stress inhibited tomato seed germination, and pretreatment with exogenous GB reduced this inhibition and enhanced the germination rate (GR), germination index (GI), and viability of tomato seeds at low temperatures. Analysis of gene expression and metabolism revealed that GB positively regulated endogenous hormone gibberellin (GA) content and negatively regulated abscisic acid (ABA) content, while GB reduced the starch content in the seeds by up-regulating the amylase gene expression. Gene expression analysis showed that the key genes (SlSOD, SlPOD, and SlchlAPX) involved in reactive oxygen species (ROS) scavenging systems were up-regulated in GB-pretreated tomato seeds compared with the control. At the same time, levels of malondialdehyde and hydrogen peroxide were significantly lower, while the proline content and peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) levels were elevated compared with those in the control. These results demonstrate that exogenous GB as a positive regulator effectively alleviated the inhibition of tomato seed germination under cold stress by different signal pathways.

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

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

Taurine modulates dynamics of oxidative defense, secondary metabolism, and nutrient relation to mitigate boron and chromium toxicity in Triticum aestivum L. plants

The present study was undertaken to appraise the efficacy of exogenous taurine in alleviating boron (B) and chromium (Cr) toxicity. Taurine protects cell membranes from lipid peroxidation due to its function as a ROS scavenger. However, there exists no report in the literature on the role of taurine in plants under abiotic stresses. The present investigation indicated the involvement of exogenous taurine in mediating plant defense responses under B and Cr toxicity. Wheat plants manifested a significant drop in growth, chlorophyll molecules, SPAD values, relative water content, nitrate reductase activity, and uptake of essential nutrients under B, Cr, and combined B-Cr toxicity. Plants showed significant oxidative damage due to enhanced cellular levels of superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), relative membrane permeability, and activity of lipoxygenase (LOX). Additionally, a significant negative correlation existed in B and Cr levels with the uptake of essential nutrients. Taurine substantially improved growth, photosynthetic pigments, and nutrient uptake by regulating ROS scavenging, secondary metabolism, and ions homeostasis under stress. Taurine protected plants from the detrimental effects of B and Cr by upregulating the production of nitric oxide, hydrogen sulfide, glutathione, and phenolic compounds.

Coumarin-Mediated Growth Regulations, Antioxidant Enzyme Activities, and Photosynthetic Efficiency of Sorghum bicolor Under Saline Conditions

Secondary metabolites, such as phenolic compounds, play an important role in alleviating salinity-induced negative effects in plants. The present study focused on seed priming and foliar application of a potent phenolic compound, coumarin, to induce salinity tolerance in Sorghum bicolor var. SS-77. Based on pilot experiment, 100 mg L^-1 concentration of coumarin was applied to mitigate the negative effects of salinity on Sorghum, grown at 0, 100, and 200 mM NaCl under netted greenhouse conditions. Coumarin was applied to each salinity treatment in four different ways (i) non-primed control (NP), (ii) seed priming (COP), (iii) foliar application (COF), and (iv) a combination of seed priming and foliar application (COPF). Salinity stress significantly reduced the plant growth, biochemical attributes, and photosynthetic efficiency of Sorghum, whereas coumarin treatments (COP, COF, and COPF) showed a significant increase (P< 0.01) in above-mentioned parameters at all salinities. Among all, the combined treatment (COPF) showed maximum increase in growth, biochemicals, photosynthetic pigments, antioxidant enzymes, and photosynthetic efficiency parameters. Therefore, it is suggested that a combination of seed priming and foliar spray of 10 mg L^-1 coumarin is more suitable than their individual applications. It is an environment friendly and economically feasible approach that will be used to improve salinity tolerance of Sorghum and helpful to get considerable biomass from saline degraded lands to fulfill food, fodder, and energy demands of the ever-growing population.

Mechanistic insight on boron-mediated toxicity in plant vis-a-vis its mitigation strategies: a review

Boron (B) is an essential micronutrient, crucial for the growth and development of crop plants. However, the essential to a toxic range of B in the plant is exceptionally narrow, and symptoms develop with a slight change in its concentration in soil. The morphological and anatomical response, such as leaf chlorosis, stunted growth, and impairment in the xylem and phloem development occurs under B-toxicity. The transport of B in the plant occurs via transpiration stream with the involvement of B-channels and transporter in the roots. The higher accumulation of B in source and sink tissue tends to have lower photosynthetic, chlorophyll content, infertility, failure of pollen tube formation and germination, impairment of cell wall formation, and disruption of membrane systems. Excess B in the plant hinders the uptake of other micronutrients, hormone transport, and metabolite partitioning. B-mediated reactive oxygen species production leads to the synthesis of antioxidant enzymes which help to scavenge these molecules and prevent the plant from further oxidative damage. This review highlights morpho-anatomical, physiological, biochemical, and molecular responses of the plant under B toxicity and thereby might help the researchers to understand the related mechanism and design strategies to develop B tolerant cultivars.

Sustainable Biochar and/or Melatonin Improve Salinity Tolerance in Borage Plants by Modulating Osmotic Adjustment, Antioxidants, and Ion Homeostasis

Salinity is persistently a decisive feature confining agricultural sustainability and food security in arid and semi-arid regions. Biochar (Bi) has been advocated as a means of lessening climate changes by sequestering carbon, concurrently supplying energy and rising crop productivity under normal or stressful conditions. Melatonin (Mt) has been shown to mediate numerous biochemical pathways and play important roles in mitigating multi-stress factors. However, their integrated roles in mitigating salt toxicity remain largely inexpressible. A completely randomized design was conducted to realize the remediation potential of Bi and/or Mt in attenuation salinity injury on borage plants by evaluating its effects on growth, water status, osmotic adjustment, antioxidant capacity, ions, and finally the yield. Salinity stress significantly decreased the plant growth and attributed yield when compared with non-salinized control plants. The depression effect of salinity on borage productivity was associated with the reduction in photosynthetic pigment and ascorbic acid (AsA) concentrations, potassium (K+) percentage, K+-translocation, and potassium/sodium ratio as well as catalase (CAT) activity. Additionally, borage plants' water status was disrupted by salinity through decreasing water content (WC), relative water content (RWC), and water retention capacity (WTC), as well as water potential (Ψw), osmotic potential (Ψs), and turgor potential (Ψp). Moreover, salinity stress evoked oxidative bursts via hyper-accumulation of hydrogen peroxide (H2O2) and malondialdehyde (MDA), as well as protein carbonyl, which is associated with membrane dysfunction. The oxidative burst was connected with the hyper-accumulation of sodium (Na+) and chloride (Cl-) in plant tissues, coupled with osmolytes' accumulation and accelerating plants' osmotic adjustment (OA) capacity. The addition of Bi and/or Mt had a positive effect in mitigating salinity on borage plants by reducing Cl-, Na+, and Na+-translocation, and oxidative biomarkers as well as Ψw, Ψs, and Ψp. Moreover, Bi and/or Mt addition to salt-affected plants increased plant growth and yield by improving plant water status and OA capacity associated with the activation of antioxidant capacity and osmolytes accumulation as well as increased photosynthetic pigments, K+, and K+/Na+ ratio. Considering these observations, Bi and/or Mt can be used as a promising approach for enhancing the productivity of salt-affected borage plants due to their roles in sustaining water relations, rising solutes synthesis, progressing OA, improving redox homeostasis, and antioxidant aptitude.

Exogenous Melatonin Promotes the Salt Tolerance by Removing Active Oxygen and Maintaining Ion Balance in Wheat (Triticum aestivum L.)

Melatonin (MT) is a small molecule indole hormone that plays an important role in the regulation of biological processes and abiotic stress resistance. Previous studies have confirmed that MT promotes the normal development of plants under stress by mediating physiological regulation mechanisms. However, the physiological mechanism of exogenous MT regulating seed germination and seedling growth of wheat under salt stress is still unclear. In this study, NaCl stress decreased germination rate and inhibited seedling growth of wheat, but shoot length, root length, and plant weight of SM15 did not change significantly. The addition of 300 μM MT in the cultivation solution directly promoted the germination rate of SM15 and ZM18, and lateral root production, but decreased the germination rate of JM22 and inhibited the length of germ and radicle of three varieties under salt stress. For wheat seedling, application of MT could increase proline content, soluble protein, soluble sugar, Ca2+ content, and vital amino acid content in leaves to keep high water content, low level of H2O2 content, and low [K+]/[Na+] ratio. MT increased root vigor and [K+]/[Na+] ratio and decreased H2O2 content in root induced by salt stress. In conclusion, MT enhanced salt tolerance in wheat seeds and seedlings by regulating the synthesis of soluble protein and sugar, ion compartmentation in roots and leaves, enhancement of enzymatic systems, and changes in amino acid levels. Salt resistance varied with different varieties under the same environmental condition. SM15 was a higher salt-resistant variety and JM22 was a salt-sensitive one. In wheat production, the application of exogenous MT should consider the differences among varieties of wheat during the sowing and seedling stages.

Exogenous Melatonin Alleviates Alkaline Stress by Removing Reactive Oxygen Species and Promoting Antioxidant Defence in Rice Seedlings

Saline-alkali stress seriously restricts rice growth, development, and production in northern China. The damage of alkaline stress on rice is much greater than that of salt due to ion toxicity, osmotic stress, and especially high pH. As a signal molecule, melatonin (N-acetyl-5-methoxytryptamine, MT) mediates many physiological processes in rice and participates in protecting rice from abiotic stress. The potential mechanism of exogenous melatonin-mediated alkaline stress tolerance is still largely unknown. In this study, the effects of melatonin on the morphological change, physiological property, and corresponding genes expression in rice seedlings were analyzed under alkaline stress (20 mmol L^-1, pH 9.55). The results showed that the expression levels of MT synthesis genes (TDC2, T5H, SNAT, ASMT1, and ASMT2) were induced by both exogenous MT and alkaline stress treatment. The cell membrane was protected by MT, and the MT furtherly play role in scavenging reactive oxygen species (ROS), reducing lipoxygenase (LOX) activity, and malondialdehyde (MDA) content. The scavenging of ROS by melatonin is attributed to the coupling of the improvement of redox homeostasis and the enhancement of antioxidant enzyme activity and antioxidant content by upregulating the transcriptional levels of antioxidase genes. In the meantime, MT pretreatment promoted the accumulation of free proline, sucrose, and fructose by regulating the OsP5CS, OsSUS7, and OsSPS1 gene expression level and increased chlorophyll content upregulating the expression of chlorophyll synthesis-related genes. Ultimately, the alleviating effect of exogenous melatonin on alkaline stress was reflected in increasing the leaf relative water content (RWC) and root-shoot ratio and reducing the leaf tip wilt index (TWI) through a series of physiological and biochemical changes. Melatonin pretreatment changed the expression level of MT synthesis genes which might contribute to MT synthesis in rice, consequently, activated the ROS scavenging system and alleviating the damage of alkaline stress on rice seedlings. Our study comprehensively understands the alleviating effect of exogenous melatonin on rice under alkaline stress.

Melatonin confers fenugreek tolerance to salinity stress by stimulating the biosynthesis processes of enzymatic, non-enzymatic antioxidants, and diosgenin content

Salinity-induced stress is widely considered a main plant-growth-limiting factor. The positive effects of melatonin in modulating abiotic stresses have led this hormone to be referred to as a growth regulator in plants. This study aims to show how melatonin protects fenugreek against the negative effects of salt stress. Different amounts of melatonin (30, 60, and 90 ppm), salinity stress (150 mM and 300 mM), and the use of both salinity and melatonin were used as treatments. The results showed that applying different melatonin levels to salinity-treated fenugreek plants effectively prevented the degradation of chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid contents compared with salinity treatment without melatonin application. Besides, melatonin increases the biosynthesis of enzymatic and non-enzymatic antioxidants, thereby adjusting the content of reactive oxygen species, free radicals, electrolyte leakage, and malondialdehyde content. It was observed that applying melatonin increased the activity of potassium-carrying channels leading to the maintenance of ionic homeostasis and increased intracellular water content under salinity stress. The results revealed that melatonin activates the defense signaling pathways in fenugreek through the nitric oxide, auxin, and abscisic acid-dependent pathways. Melatonin, in a similar vein, increased the expression of genes involved in the biosynthesis pathway of diosgenin, a highly important steroidal sapogenin in medical and food industries, and hence the diosgenin content. When 150 mM salinity stress and 60 ppm melatonin were coupled, the diosgenin concentration rose by more than 5.5 times compared to the control condition. In conclusion, our findings demonstrate the potential of melatonin to enhance the plant tolerance to salinity stress by stimulating biochemical and physiological changes.

Role of phytomelatonin responsive to metal stresses: An omics perspective and future scenario

A pervasive melatonin (N-acetyl-5-methoxytryptamine) reveals a crucial role in stress tolerance and plant development. Melatonin (MT) is a unique molecule with multiple phenotypic expressions and numerous actions within the plants. It has been extensively studied in crop plants under different abiotic stresses such as drought, salinity, heat, cold, and heavy metals. Mainly, MT role is appraised as an antioxidant molecule that deals with oxidative stress by scavenging reactive oxygen species (ROS) and modulating stress related genes. It improves the contents of different antioxidant enzyme activities and thus, regulates the redox hemostasis in crop plants. In this comprehensive review, regulatory effects of melatonin in plants as melatonin biosynthesis, signaling pathway, modulation of stress related genes and physiological role of melatonin under different heavy metal stress have been reviewed in detail. Further, this review has discussed how MT regulates different genes/enzymes to mediate defense responses and overviewed the context of transcriptomics and phenomics followed by the metabolomics pathways in crop plants.

Melatonin Modulates Plant Tolerance to Heavy Metal Stress: Morphological Responses to Molecular Mechanisms

Heavy metal toxicity is one of the most devastating abiotic stresses. Heavy metals cause serious damage to plant growth and productivity, which is a major problem for sustainable agriculture. It adversely affects plant molecular physiology and biochemistry by generating osmotic stress, ionic imbalance, oxidative stress, membrane disorganization, cellular toxicity, and metabolic homeostasis. To improve and stimulate plant tolerance to heavy metal stress, the application of biostimulants can be an effective approach without threatening the ecosystem. Melatonin (N-acetyl-5-methoxytryptamine), a biostimulator, plant growth regulator, and antioxidant, promotes plant tolerance to heavy metal stress by improving redox and nutrient homeostasis, osmotic balance, and primary and secondary metabolism. It is important to perceive the complete and detailed regulatory mechanisms of exogenous and endogenous melatonin-mediated heavy metal-toxicity mitigation in plants to identify potential research gaps that should be addressed in the future. This review provides a novel insight to understand the multifunctional role of melatonin in reducing heavy metal stress and the underlying molecular mechanisms.

Salicylic acid-mediated alleviation of soil boron toxicity in Mentha arvensis and Cymbopogon flexuosus: Growth, antioxidant responses, essential oil contents and components

Boron (B) toxicity is a notable abiotic hindrance that restricts crop productivity by disturbing several physiological and biochemical processes in plants. This study was aimed to elucidate the role of salicylic acid (SA) in conferring tolerance to B stress in Mentha arvensis and Cymbopogon flexuosus. Boron toxicity led to a considerable decrease in shoot height and root length, fresh and dry mass of shoot and root, and physiological and biochemical parameters. However, exogenously applied SA relieved the adverse effects caused by B toxicity and led to an increase in growth parameters under B stress and non-stress conditions. The treatment of B resulted in its increased accumulation in roots and shoots of both the plants which, in turn, caused oxidative damage as evident by increased content of malondialdehyde and catalase, peroxidase, superoxide dismutase and glutathione reductase enzyme activities. However, exogenous SA supply significantly affected antioxidant enzyme activities and protected the plants from excess B. Moreover, the essential oil content of two selected plants declined under B toxicity and significantly enhanced in SA-treated stressed plants. The contents of menthol and menthyl acetate in M. arvensis were lowered in B stressed plants which significantly improved in SA treated B-stressed and in their respective SA alone treatment. Similarly, citral-A and citral-B content of C. flexuosus declined under B toxicity, however, SA reversed the negative effects of B toxicity on essential oil components. This assessment stipulated the promising role of exogenously applied SA in alleviating B toxicity in M. arvensis and C. flexuosus by improving antioxidant machinery and limiting B uptake which protects the structural integrity of leaves and also helps in increasing essential oil content.

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

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

Melatonin Relieves Ozone Stress in Grape Leaves by Inhibiting Ethylene Biosynthesis

Ozone (O₃) stress severely affects the normal growth of grape (Vitis vinifera L.) leaves. Melatonin (MT) plays a significant role in plant response to various abiotic stresses, but its role in O₃ stress and related mechanisms are poorly understood. In order to understand the mechanism of MT in alleviate O₃ stress in grape leaves, we perform a transcriptome analyses of grapes leaves under O₃ stress with or without MT treatment. Transcriptome analysis showed that the processes of ethylene biosynthesis and signaling were clearly changed in "Cabernet Sauvignon" grapes under O₃ and MT treatment. O₃ stress induced the expression of genes related to ethylene biosynthesis and signal transduction, while MT treatment significantly inhibited the ethylene response mediated by O₃ stress. Further experiments showed that both MT and aminoethoxyvinylglycine (AVG, an inhibitor of ethylene biosynthesis) enhanced the photosynthetic and antioxidant capacities of grape leaves under O₃ stress, while ethephon inhibited those capacities. The combined treatment effect of MT and ethylene inhibitor was similar to that of MT alone. Exogenous MT reduced ethylene production in grape leaves under O₃ stress, while ethephon and ethylene inhibitors had little effect on the MT content of grape leaves after O₃ stress. However, overexpression of VvACO2 (1-aminocyclopropane-1-carboxylate oxidase2) in grape leaves endogenously induced ethylene accumulation and aggravated O₃ stress. Overexpression of the MT synthesis gene VvASMT1 (acetylserotonin methyltransferase1) in tobacco (Nicotiana tabacum L.) alleviated O₃ stress and reduced ethylene biosynthesis after O₃ stress. In summary, MT can alleviate O₃ stress in grape leaves by inhibiting ethylene biosynthesis.

Melatonin alleviates drought impact on growth and essential oil yield of lemon verbena by enhancing antioxidant responses, mineral balance, and abscisic acid content

Melatonin has recently emerged as a multifunctional biomolecule with promising aspects in plant stress tolerance. The present study examined the effects of foliar-sprayed melatonin (0, 100, and 200 μM) on growth and essential oil yield attributes of lemon verbena (Lippia citriodora) under water-shortage (mild, moderate and severe). Results revealed that melatonin minimized drought effects on lemon verbena, resulting in improved growth and essential oils yield. Drought impositions gradually and significantly reduced several growth parameters, such as plant height and biomass, whereas melatonin application revived the growth performance of lemon verbena. Melatonin protected the photosynthetic pigments and helped maintain the mineral balance at all levels of drought. Melatonin stimulated the accumulation of proline, soluble sugars and abscisic acid, which were positively correlated with a better preservation of leaf water status in drought-stressed plants. Melatonin also prevented oxidative damages by enhancing the superoxide dismutase, ascorbate peroxidase and catalase activities. Furthermore, increased levels of total phenolic compounds, chicoric acid, caffeic acid and chlorogenic acid, as well as ascorbate and total antioxidant capacity in melatonin-sprayed drought-stressed plants indicated that melatonin helped verbena plants to sustain antioxidant and medicinal properties during drought. Finally, melatonin treatments upheld the concentrations and yield of essential oils in the leaves of lemon verbena regardless of drought severities. These results provided new insights into melatonin-mediated drought tolerance in lemon verbena, and this strategy could be implemented for the successful cultivation of lemon verbena, and perhaps other medicinal plants, in drought-prone areas worldwide.

ROS and NO Phytomelatonin-Induced Signaling Mechanisms under Metal Toxicity in Plants: A Review

Metal toxicity in soils, along with water runoff, are increasing environmental problems that affect agriculture directly and, in turn, human health. In light of finding a suitable and urgent solution, research on plant treatments with specific compounds that can help mitigate these effects has increased, and thus the exogenous application of melatonin (MET) and its role in alleviating the negative effects of metal toxicity in plants, have become more important in the last few years. MET is an important plant-related response molecule involved in growth, development, and reproduction, and in the induction of different stress-related key factors in plants. It has been shown that MET plays a protective role against the toxic effects induced by different metals (Pb, Cd, Cu, Zn, B, Al, V, Ni, La, As, and Cr) by regulating both the enzymatic and non-enzymatic antioxidant plant defense systems. In addition, MET interacts with many other signaling molecules, such as reactive oxygen species (ROS) and nitric oxide (NO) and participates in a wide variety of physiological reactions. Furthermore, MET treatment enhances osmoregulation and photosynthetic efficiency, and increases the concentration of other important antioxidants such as phenolic compounds, flavonoids, polyamines (PAs), and carotenoid compounds. Some recent studies have shown that MET appeared to be involved in the regulation of metal transport in plants, and lastly, various studies have confirmed that MET significantly upregulated stress tolerance-related genes. Despite all the knowledge acquired over the years, there is still more to know about how MET is involved in the metal toxicity tolerance of plants.

ROS and NO Phytomelatonin-Induced Signaling Mechanisms under Metal Toxicity in Plants: A Review

Metal toxicity in soils, along with water runoff, are increasing environmental problems that affect agriculture directly and, in turn, human health. In light of finding a suitable and urgent solution, research on plant treatments with specific compounds that can help mitigate these effects has increased, and thus the exogenous application of melatonin (MET) and its role in alleviating the negative effects of metal toxicity in plants, have become more important in the last few years. MET is an important plant-related response molecule involved in growth, development, and reproduction, and in the induction of different stress-related key factors in plants. It has been shown that MET plays a protective role against the toxic effects induced by different metals (Pb, Cd, Cu, Zn, B, Al, V, Ni, La, As, and Cr) by regulating both the enzymatic and non-enzymatic antioxidant plant defense systems. In addition, MET interacts with many other signaling molecules, such as reactive oxygen species (ROS) and nitric oxide (NO) and participates in a wide variety of physiological reactions. Furthermore, MET treatment enhances osmoregulation and photosynthetic efficiency, and increases the concentration of other important antioxidants such as phenolic compounds, flavonoids, polyamines (PAs), and carotenoid compounds. Some recent studies have shown that MET appeared to be involved in the regulation of metal transport in plants, and lastly, various studies have confirmed that MET significantly upregulated stress tolerance-related genes. Despite all the knowledge acquired over the years, there is still more to know about how MET is involved in the metal toxicity tolerance of plants.

Melatonin application differentially modulates the enzymes associated with antioxidative machinery and ascorbate-glutathione cycle during arsenate exposure in indica rice varieties

Arsenic (As) contamination and accumulation in rice is a serious concern causing severe oxidative damage. Melatonin acts as a protective agent in plant defence against multiple abiotic stresses. The mechanism of antioxidant function of melatonin during As stress in rice genotypes is less studied. In this study, hydroponically-grown As-susceptible (Khitish) and As-tolerant (Muktashri) rice cultivars, subjected to 150 µm arsenate stress, were supplemented with exogenously applied melatonin (20 µm) to examine the plant defence mechanism. Melatonin (Mel) increased root and shoot length, fresh and dry weight, chlorophyll a and b content and activated reducing power and free radical scavenging capacity in both rice cultivars. The role of Mel in the sensitive variety appeared to be more prominent with respect to reduced water saturation deficit by reducing endogenous As and H₂ O₂ accumulation, and enhancing overall antioxidant capacity by imposing reduced requirement of catalase for ROS detoxification, and restoring As-inhibited activity of glutathione-S-transferase, glutathione peroxidase and dehydroascorbate reductase. In contrast, melatonin treatment in the tolerant cultivar required reduced involvement of ascorbate peroxidase to deal with As toxicity, and complemented the stress-mediated inhibition of guaiacol peroxidase activity. Isozyme profiling also established extensive varietal differences with regard to induction of new isoform(s) by Mel during As treatment. This study provides clear insights into mechanistic details of the regulation of antioxidative enzymes by melatonin in contrasting rice genotypes, which may prove helpful in generating As tolerance in susceptible rice varieties grown in marginalized soils, thereby improving crop yield and productivity.

Physiological and proteomic analysis reveals the impact of boron deficiency and surplus on alfalfa (Medicago sativa L.) reproductive organs

Boron (B), an essential element for increasing seed yield and germinability in alfalfa (Medicago sativa L.), plays a vital role in its reproductive processes. However, effects of B stress on physiological and proteomic changes in reproductive organs related to alfalfa seed yield and germinability are poorly understood. In order to gain a better insight into B response or tolerance mechanisms, field trials were designed for B deficiency (0 mg B L^-1), B sufficiency (800 mg B L^-1), and B surplus (1600 mg B L^-1) application during alfalfa flowering to analyze the proteomics and physiological responses of alfalfa 'Aohan' reproductive organs. Results showed that B deficiency weakened the stress-responsive ability in these organs, while B surplus reduced the sugar utilization of 'Aohan' flowers and caused lipid membrane peroxidation in 'Aohan' seeds. In addition, four upregulated stress responsive proteins (ADF-like protein, IMFP, NAD(P)-binding Rossmann-fold protein and NAD-dependent ALDHs) might play pivotal roles in the response of 'Aohan' reproductive organs to conditions of B deficiency and B surplus. All of the above results would be helpful to understand the tolerance mechanisms of alfalfa reproductive organs to both B deficiency and B surplus conditions, and also to give insight into the regulatory role of B in improving seed yield and germinability in alfalfa seed production. In summary, B likely plays a structural and regulatory role in relation to lipid metabolism, carbohydrate metabolism, amino acid metabolism, and signal transduction, thus regulates alfalfa reproductive processes eventually affecting the seed yield and germinability of alfalfa seeds.

Nitric Oxide Functions as a Downstream Signal for Melatonin-Induced Cold Tolerance in Cucumber Seedlings

Melatonin (MT) and nitric oxide (NO) are two multifunctional signaling molecules that are involved in the response of plants to abiotic stresses. However, how MT and NO synergize in response to cold stress affecting plants is still not clear. In this study, we found that endogenous MT accumulation under cold stress was positively correlated with cold tolerance in different varieties of cucumber seedlings. The data presented here also provide evidence that endogenous NO is involved in the response to cold stress. About 100 μM MT significantly increased the nitrate reductase (NR) activity, NR-relative messenger RNA (mRNA) expression, and endogenous NO accumulation in cucumber seedlings. However, 75 μM sodium nitroprusside (SNP, a NO donor) showed no significant effect on the relative mRNA expression of tryptophan decarboxylase (TDC), tryptamine-5-hydroxylase (T5H), serotonin-N-acetyltransferase (SNAT), or acetylserotonin O-methyltransferase (ASMT), the key genes for MT synthesis and endogenous MT levels. Compared with H₂O treatment, both MT and SNP decreased electrolyte leakage (EL), malondialdehyde (MDA), and reactive oxygen species (ROS) accumulation by activating the antioxidant system and consequently mitigated cold damage in cucumber seedlings. MT and SNP also enhanced photosynthetic carbon assimilation, which was mainly attributed to an increase in the activity and mRNA expression of the key enzymes in the Calvin-Benson cycle. Simultaneously, MT- and SNP-induced photoprotection for both photosystem II (PSII) and photosystem I (PSI) in cucumber seedlings, by stimulating the PsbA (D1) protein repair pathway and ferredoxin-mediated NADP⁺ photoreduction, respectively. Moreover, exogenous MT and SNP markedly upregulated the expression of chilling response genes, such as inducer of CBF expression (ICE1), C-repeat-binding factor (CBF1), and cold-responsive (COR47). MT-induced cold tolerance was suppressed by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, a specific scavenger of NO). However, p-chlorophenylalanine (p-CPA, a MT synthesis inhibitor) did not affect NO-induced cold tolerance. Thus, novel results suggest that NO acts as a downstream signal in the MT-induced plant tolerance to cold stress.