Melatonin affects the growth and cadmium accumulation of Malachium aquaticum and Galinsoga parviflora

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

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

Phytomelatonin maintained chromium toxicity induced oxidative burst in Brassica juncea L. through improving antioxidant system and gene expression

Chromium (Cr) contamination in soils reduces crop yields and poses a remarkable risk to human and plant system. The main objective of this study was to observe the protective mechanisms of exogenously applied melatonin (Mel- 0.05, 0.1, and 0.15 μM) in seedlings of Brassica juncea L. under Cr (0.2 mM) stress. This was accomplished by analysing the plant's morpho-physiological, biochemical, nuclear, membrane, and cellular characteristics, as well as electrolyte leakage. Superoxide, malondialdehyde, and hydrogen peroxide increased with Cr toxicity. Cr also increased electrolyte leakage. Seedlings under Cr stress had 86.4% more superoxide anion and 27.4% more hydrogen peroxide. Electrolyte leakage increased 35.7% owing to Cr toxicity. B. juncea L. cells with high radical levels had membrane and nuclear damage and decreased viability. Besides this, the activities of the antioxidative enzymes, as POD, APOX, SOD, GST, DHAR, GPOX and GR also elevated in the samples subjected to Cr toxicity. Conversely, the activity of catalase was downregulated due to Cr toxicity. In contrast, Mel reduced oxidative damage and conserved membrane integrity in B. juncea seedlings under Cr stress by suppressing ROS generation. Moreover, the activity of antioxidative enzymes that scavenge reactive oxygen species was substantially upregulated by the exogenous application of Mel. The highest concentration of Mel (Mel c- 0.15 μM) applied showed maximum ameliorative effect on the toxicity caused by Cr. It causes alleviation in the activity of SOD, CAT, POD, GPOX, APOX, DHAR, GST and GR by 51.32%, 114%, 26.44%, 48.91%, 87.51%, 149%, 42.30% and 40.24% respectively. Histochemical investigations showed that Mel increased cell survival and reduced ROS-induced membrane and nuclear damage. The findings showed that Mel treatment upregulated several genes, promoting plant development. Its supplementation decreased RBOH1 gene expression in seedling sunder stress. The results supported the hypothesis that Mel concentrations reduce Cr-induced oxidative burst in B. juncea.

Nitric oxide reduces cadmium uptake in wheat (Triticum aestivum L.) by modulating growth, mineral uptake, yield attributes, and antioxidant profile

Wheat (Triticum aestivum L.) is among the plants that are at risk from cadmium (Cd), a hazardous heavy metal that can be fatal due to its rapid absorption and high mobility. Being taken up from the soil and moving to the shoots and roots of edible plants, it enters the food chain and poses a health concern to people worldwide. A strategically important cereal crop, wheat has a demonstrated role in human health systems, particularly in poor nations. In this study, we describe the effects of nitric oxide (NO) on the growth, nutrition, and physiological functions of commercially cultivated wheat cvs. Galaxy 2013 and Akbar 2019 under Cd stress. Four-week-old plants were subjected to Cd (0.5 mM) stress, and after 2 weeks of Cd toxicity, foliar application of nitric oxide (100 and 150 μM) was carried out. As evident from excessive antioxidant production, Cd toxicity increased reactive oxygen species (ROS) level like H2O2 and significantly (p ≤ 0.001) decreased nutrient acquisition, growth, and yield attributes of plants under experiment. The severity of the effect varied between cultivars under investigation. A minimum accumulation of MDA (44%) and H2O2 (55%) was found in the cv. Akbar 2019 under Cd stress, whilst cv. Galaxy 2013 showed the highest accumulation of the oxidative stress indicators malondialdehyde content (MDA) (48%) and H2O2 (60%). Reduced and oxidized glutathione contents were also increased under Cd-induced toxicity. The application of NO resulted in a significant improvement of 22, 25, 25, and 30% in shoot fresh weight, root fresh weight, shoot dry weight, and root dry weight, respectively. Additionally, there was an increased uptake of Ca+2 (16%), K+1 (5%), chlorophyll a (46%), b (32%), a/b ratio (41%), and carotenoid (28%). When compared with Cd-stressed plants, yield parameters like 100 grain weight, number of tillers plant-1, and grain yield plant-1 improved by 14, 17, and 33%, respectively, under NO application. We concluded from the results of this study that NO treatments increased plant development by lowering oxidative stress and limiting Cd uptake. It is inferred from the results of this study that wheat production with reduced heavy metal uptake may be facilitated using NO due to its cytoprotective properties and its interaction with ROS.

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.

Exogenous melatonin regulates chromium stress-induced feedback inhibition of photosynthesis and antioxidative protection in Brassica napus cultivars

Melatonin is an early player in chromium stress response in canola plants; it promotes ROS scavenging and chlorophyll stability, modulates PSII stability and regulates feedback inhibition of photosynthesis conferring chromium tolerance. The development of heavy metals, especially chromium (Cr)-tolerant cultivars is mainly constrained due to poor knowledge of the mechanism behind Cr stress tolerance. In the present study, two Brassica napus contrasting cultivars Ac-Excel and DGL were studied for Cr stress tolerance by using chlorophyll a fluorescence technique and biochemical attributes with and without melatonin (MT) treatments. Cr stress significantly reduced the PSII and PSI efficiency, biomass accumulation, proline content and antioxidant enzymes in both the cultivars. The application of MT minimized the oxidative stress, as revealed via a lower level of reactive oxygen species (ROS) synthesis (H₂O₂ and OH^-). Enhanced enzymatic activities of important antioxidants (SOD, APX, CAT, POD), proline and total soluble protein contents under MT application play an effective role in the regulation of multiple transcriptional pathways involved in oxidative stress responses. Higher NPQ and Y(NPQ) observed in Cr stress tolerant cv Ac-Excel, indicating that the MT-treated tolerant cultivar had better ability to protect PSII under Cr stress by increasing heat dissipation as photo-protective component of NPQ. Reduced PSI efficiency along with increased donor end limitation of PSI in both canola cultivars further confirmed the lower PSII activity and electron transport from PSII. The Cr content was higher in cv. DGL as compared to (that in Ac-Excel). The application of MT significantly decreased the Cr content in leaves of both cultivars. Overall, MT-induced Cr stress tolerance in canola cultivars can be related to improved PSII activity, Y(NPQ), and antioxidant potential and these physiological attributes can effectively be used to select cultivars for Cr stress tolerance.

Melatonin Confers Plant Cadmium Tolerance: An Update

Cadmium (Cd) is one of the most injurious heavy metals, affecting plant growth and development. Melatonin (N-acetyl-5-methoxytryptamine) was discovered in plants in 1995, and it is since known to act as a multifunctional molecule to alleviate abiotic and biotic stresses, especially Cd stress. Endogenously triggered or exogenously applied melatonin re-establishes the redox homeostasis by the improvement of the antioxidant defense system. It can also affect the Cd transportation and sequestration by regulating the transcripts of genes related to the major metal transport system, as well as the increase in glutathione (GSH) and phytochelatins (PCs). Melatonin activates several downstream signals, such as nitric oxide (NO), hydrogen peroxide (H₂O₂), and salicylic acid (SA), which are required for plant Cd tolerance. Similar to the physiological functions of NO, hydrogen sulfide (H₂S) is also involved in the abiotic stress-related processes in plants. Moreover, exogenous melatonin induces H₂S generation in plants under salinity or heat stress. However, the involvement of H₂S action in melatonin-induced Cd tolerance is still largely unknown. In this review, we summarize the progresses in various physiological and molecular mechanisms regulated by melatonin in plants under Cd stress. The complex interactions between melatonin and H₂S in acquisition of Cd stress tolerance are also discussed.

The protective role of melatonin under heavy metal-induced stress in Melissa Officinalis L

Heavy metals, due to their inability to degrade, pose a serious environmental and nutritional problem. The accumulation of essential and non-essential heavy metals in living organisms reduces normal growth and development, resulting in acute poisoning, disease and even death of organisms. Melatonin is a very important multifunctional molecule in protecting plants from oxidative stress due to its ability to directly neutralize reactive oxygen species (ROS). Also, melatonin has a chelating property, which may contribute in reducing metal-induced toxicity. In this paper, the protective role of melatonin in counteracting metal-induced free radical generation was highlighted. Using the HPLC-FLD technique melatonin was identified and quantified in the roots and leaves of lemon balm ( Melissa officinalis L.), grown under photoperiod conditions. Furthermore, the response of plants pre-treated with exogenous 0.1 mM melatonin to the increased zinc (Zn) and cadmium (Cd) concentrations was observed, with changes in mineral (Ca, Mg), physiological and antioxidant status of the plant during heavy metals stress. The obtained melatonin concentrations were the highest published for dry plants so far. Elevated Cd and Zn levels in soil caused alternation in biochemical and physiological parameters of lemon balm leaves and roots. However, melatonin pre-treatment increased plant tolerance to heavy metals stress. Increased Cd and Zn uptake and their translocation into the leaves were also improved, indicating the possible use of melatonin in phytoremediation.

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 induced changes in photosynthetic efficiency as probed by OJIP associated with improved chromium stress tolerance in canola (Brassica napus L.)

Chromium toxicity is considered as a major problem for agricultural soil that reduced crop productivity by affecting photosynthetic tissues. Exogenous application of melatonin can alleviate the adverse effects of chromium toxicity on plant growth. However, little is known about its effect on thylakoidal protein complexes responsible for conversion of solar energy to biochemical energy. Chlorophyll fluorescence a transients considered one of the best non-invasive and rapid method for the evaluation of photosynthetic (Photosystem II) efficiency of plants and plant health under environmental stress conditions. In the present study, three-week old plants of two canola cultivars AC-Excel and DGL were applied to melatonin (0, 1, 5, 10 μM) when grown under chromium stress (0, 50 and 100 μM) for further two weeks. Chromium stress reduced the growth (fresh and dry weights of shoots and roots) of both canola cultivars and exogenous application of 5 and 10 μM melatonin improved the growth of canola at 50 or 100 μM chromium stress. This improvement was greater in cv DGL than in AC-Excel. Increasing chromium decreased the photosynthetic pigments (chlorophyll a and chlorophyll b). However, 5 and 10 μM melatonin application improved chlorophyll a at 50 μM chromium stress. Structural stability and efficiency of photosystem II (PSII) measured as performance index (PI_ABS) and ratios of fluorescence (Fv/Fm, Fv/Fo) Fv decreased due to chromium stress. JIP-test parameters showed that chromium stress increased the absorption and trapping fluxes with decrease in electron transport fluxes which caused the damage to reaction centers (RC), detachment of oxygen evolving complex (OEC) from RC or inefficiency of electron transfer from OEC to RC. Such adverse effects were greater in cv AC-Excel. However exogenous application of melatonin improved PI_ABS, electron transport per reaction center (ET/RC), reduced variable fluorescence at J step (V_J) reflecting melatonin protected PSII from chromium stress induced damage by protecting OEC. Thus, OJIP fluorescence transients are quite helpful for understanding the intersystem electron transport beyond photosystem II in canola cultivars due to melatonin application under chromium stress.

Findings

Exogenous application of melatonin alleviated toxic effects of chromium on plant growth of canola by modulating photosynthesis, enhanced photosystem II efficiency and regulation of electron transport flux to protect photo-inhibition of PSII from oxidative damage.

Exogenous Melatonin Application Enhances Rhizophagus irregularis Symbiosis and Induces the Antioxidant Response of Medicago truncatula Under Lead Stress

Melatonin is a new kind of plant growth regulator. The aim of this study was to figure out the effect of melatonin on arbuscular mycorrhizal (AM) symbiosis and heavy metal tolerance. A three-factor experiment was conducted to determine the effect of melatonin application on the growth, AM symbiosis, and stress tolerance of Medicago truncatula. A two-factor (AM inoculation and Pb stress) experiment was conducted to determine the effect of AM fungus on melatonin accumulation under Pb stress. AM plants under Pb stress had a higher melatonin accumulation than non-mycorrhizal (NM) plants under Pb stress. Acetylserotonin methyltransferase (ASMT) is the enzymatic reaction of the last step in melatonin synthesis. The accumulation of melatonin may be related to the expression of MtASMT. Melatonin application increased the relative expression of MtPT4 and AM colonization in AM plants. Melatonin application decreased Pb uptake with and without AM inoculation. Both melatonin application and AM inoculation improved M. truncatula growth and increased antioxidant response with Pb stress. These results indicated that melatonin application has positive effects on AM symbiosis and Pb stress tolerance under Pb stress. AM inoculation improve melatonin synthesis capacity under Pb stress. Melatonin application may improve AM plant growth by enhancing AM symbiosis, stimulating antioxidant response, and inhibiting Pb uptake.

Melatonin promotes metabolism of bisphenol A by enhancing glutathione-dependent detoxification in Solanum lycopersicum L

Bisphenol A (BPA), a widely distributed organic compound, is toxic to animals and plants. Here we show the mechanism of BPA detoxification by melatonin (MEL) in tomato, which is otherwise poorly understood in plants. BPA treatment decreased the quantum yield of photosystem II (Fv/Fm) and increased the membrane lipid peroxidation and reactive oxygen species (ROS) accumulation dose-dependently, whereas exogenous MEL alleviated the BPA effects on Fv/Fm, lipid peroxidation, ROS accumulation and BPA uptake. Furthermore, BPA elevated the glutathione (GSH) content, activities of glutathione S-transferase (GST), and glutathione reductase (GR), and the transcript levels of GSH1, GR1, GST1 and MEL biosynthesis genes (COMT, T5H, and SNAT), whereas BPA + MEL showed even a more profound induction. Silencing GSH1, GR1 and GST1 genes compromised the BPA detoxification potential of tomato plants as revealed by an increased level of ROS, lipid peroxidation and BPA uptake, and a decreased Fv/Fm and GST activity; these changes were alleviated by MEL application. Under in vitro conditions, BPA was glutathionylated by GSH, which was further catalyzed by GST to cysteine and N-acetylcysteine conjugates. These findings suggest a crucial role for MEL in BPA detoxification via GSH and GST, and can be useful to reduce BPA residue for food safety.

Melatonin and nitric oxide enhance cadmium tolerance and phytoremediation efficiency in Catharanthus roseus (L.) G. Don

In this study, a pot experiment was performed to evaluate the effects of foliar spray with sodium nitroprusside (200 μM SNP) and melatonin (100 μM) singly and in combination on tolerance and accumulation of cadmium (Cd) in Catharanthus roseus (L.) G. Don plants exposed to different levels of cadmium (0, 50, 100, and 200 mg Cd kg^-1 soil). The results showed that 50 mg kg^-1 Cd had no significant effect on the fresh and dry weight of roots and shoots and content of chlorophyll (Chl) a and b, but the higher levels of Cd (100 and 200 mg kg^-1) significantly reduced these attributes and induced an increase in the level of leaf electrolyte leakage and disrupted nutrient homeostasis. The activities of catalase (CAT) and peroxidase (POD) in leaves were increased under lower Cd concentrations (50 and 100 mg kg^-1) but decreased under 200 mg kg^-1 Cd. However, foliar spray with melatonin and/or SNP increased shoot biomass and the content of Chl a and b, augmented activities of POD and CAT, lowered electrolyte leakage (EL), and improved essential cations homeostasis in leaves. Cadmium content in shoots of C. roseus was less than roots and TF (transfer factor) was < 1. Interestingly, foliar spray with SNP and/or melatonin increased Cd accumulation and bioconcentration factor (BCF) in both roots and shoots and elevated the Cd transport from roots to shoot, as TF values increased in these treatments. The co-application of melatonin and SNP further than their separate usage augmented Cd tolerance through increasing activities of antioxidant enzymes and regulating mineral homeostasis in C. roseus. Furthermore, co-treatment of SNP and melatonin increased Cd phytoremediation efficiency in C. roseus through increasing biomass and elevating uptake and translocation of Cd from root to shoot.

Role of Melatonin to Enhance Phytoremediation Capacity

Phytoremediation is a green technology that aims to take up pollutants from soil or water. Metals are one of the targets of these techniques due to their high toxicity in biological systems, including plants and animals. Their elimination or, at least, decrease will help keep them from being incorporated in the trophic chain and thus reaching animal and human food. The metal removal efficiency of plants is closely related to their growth rate, tolerance, and their adaptability to different environments. Melatonin (N-acetyl-5-methoxytryptamine) is a ubiquitous molecule present in animals, plants, fungi, and bacteria. In plants, it plays an important role related to antioxidant activity, but also as an important redox network regulator. Thus, melatonin has been defined as a biostimulator of plant growth, especially under environmental stress conditions, whether abiotic (water deficit and waterlogging, extreme temperature, UV radiation, salinity, alkalinity, specific mineral deficit/excess, metals and other toxic compounds, etc.) or biotic (bacteria, fungi, and viruses). Exogenous melatonin treated plants have been seen to have a high tolerance to stressors, minimizing possible harmful effects through the control of reactive oxygen species (ROS) levels and activating antioxidative responses. Furthermore, important gene expression changes in stress specific transcription factors have been demonstrated. Melatonin is capable of mobilizing toxic metals, through phytochelatins, transporting this, while sequestration adds to the biostimulator effect of melatonin on plants, improving plant tolerance against toxic pollutants. Furthermore, melatonin improves the uptake of nitrogen (N), phosphorus (P), and sulfur (S) in stress situations, enhancing cell metabolism. In light of the above, the application of melatonin seems to be a useful option for clearing toxic pollutants from the environment by improving phytoremediation. Interestingly, a variety of stressors induce melatonin biosynthesis in plants, and the study of this endogenous response in hyperaccumulator plants may be even more interesting as a natural response of the phytoremediation of diverse plants.

Protective mechanisms of melatonin against selenium toxicity in Brassica napus: insights into physiological traits, thiol biosynthesis and antioxidant machinery

BACKGROUND

The ubiquitous signaling molecule melatonin (N-acetyl-5-methoxytryptamine) (MT) plays vital roles in plant development and stress tolerance. Selenium (Se) may be phytotoxic at high concentrations. Interactions between MT and Se (IV) stress in higher plants are poorly understood. The aim of this study was to evaluate the defensive roles of exogenous MT (0 μM, 50 μM, and 100 μM) against Se (IV) (0 μM, 50 μM, 100 μM, and 200 μM) stress based on the physiological and biochemical properties, thiol biosynthesis, and antioxidant system of Brassica napus plants subjected to these treatments.

RESULTS

Se (IV) stress inhibited B. napus growth and biomass accumulation, reduced pigment content, and lowered net photosynthetic rate (P_n) and PSII photochemical efficiency (Fv/Fm) in a dose-dependent manner. All of the aforementioned responses were effectively alleviated by exogenous MT treatment. Exogenous MT mitigated oxidative damage and lipid peroxidation and protected the plasma membranes from Se toxicity by reducing Se-induced reactive oxygen species (ROS) accumulation. MT also alleviated osmotic stress by restoring foliar water and sugar levels. Relative to standalone Se treatment, the combination of MT and Se upregulated the ROS-detoxifying enzymes SOD, APX, GR, and CAT, increased proline, free amino acids, and the thiol components GSH, GSSG, GSH/GSSG, NPTs, PCs, and cys and upregulated the metabolic enzymes γ-ECS, GST, and PCS. Therefore, MT application attenuates Se-induce oxidative damage in plants. MT promotes the accumulation of chelating agents in the roots, detoxifies Se there, and impedes its further translocation to the leaves.

CONCLUSIONS

Exogenous MT improves the physiological traits, antioxidant system, and thiol ligand biosynthesis in B. napus subjected to Se stress primarily by enhancing Se detoxification and sequestration especially at the root level. Our results reveal better understanding of Se-phytotoxicity and Se-stress alleviation by the adequate supply of MT. The mechanisms of MT-induced plant tolerance to Se stress have potential implications in developing novel strategies for safe crop production in Se-rich soils.

Exogenous melatonin improves lead tolerance of bermudagrass through modulation of the antioxidant defense system

Lead (Pb) is a major anthropogenic contaminant that can be devastating on both animals and plants. It is essential to develop methods to decrease the Pb contaminant in soil by phytoremediation using plants that are tolerance to Pb. In this study, we investigated the tolerance of bermudagrass (Cynodon dactylon (L.) Pers.) and the role of exogenous application of melatonin for improving its tolerance to Pb. Bermudagrass growing in soil treated with Pb at 1,000 or 2,000 mg kg^-1 were assessed with or without melatonin pretreatment at various concentrations. Under Pb stresses, bermudagrass plants showed stunted growth and increased cellular oxidative stress. Pre-treating bermudagrass plants with melatonin at 20 or 100 μM significantly increased the activities of antioxidant enzymes (superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and glutathione reductase) and the contents of non-enzymatic antioxidants (ascorbic acid and glutathione), and decreased reactive oxygen species (hydrogen peroxide, superoxide), and ultimately reduced membrane lipid peroxidation and permeability. These changes contributed to improvements in the water status, photosynthetic pigment synthesis, and biomass production of bermudagrass under Pb stresses. Our study provides the first evidence that melatonin may be a promising tool for enhancing Pb tolerance and phytoremediation potential of bermudagrass.

Phytomelatonin: Recent advances and future prospects

Melatonin (MEL) has been revealed as a phylogenetically conserved molecule with a ubiquitous distribution from primitive photosynthetic bacteria to higher plants, including algae and fungi. Since MEL is implicated in numerous plant developmental processes and stress responses, the exploration of its functions in plant has become a rapidly progressing field with the new paradigm of involvement in plants growth and development. The pleiotropic involvement of MEL in regulating the transcripts of numerous genes confirms its vital involvement as a multi-regulatory molecule that architects many aspects of plant development. However, the cumulative research in plants is still preliminary and fragmentary in terms of its established functions compared to what is known about MEL physiology in animals. This supports the need for a comprehensive review that summarizes the new aspects pertaining to its functional role in photosynthesis, phytohormonal interactions under stress, cellular redox signaling, along with other regulatory roles in plant immunity, phytoremediation, and plant microbial interactions. The present review covers the latest advances on the mechanistic roles of phytomelatonin. While phytomelatonin is a sovereign plant growth regulator that can interact with the functions of other plant growth regulators or hormones, its qualifications as a complete phytohormone are still to be established. This review also showcases the yet to be identified potentials of phytomelatonin that will surely encourage the plant scientists to uncover new functional aspects of phytomelatonin in plant growth and development, subsequently improving its status as a potential new phytohormone.