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

Synergistic interplay between melatonin and hydrogen sulfide enhances cadmium-induced oxidative stress resistance in stock (Matthiola incana L.)

Ornamental crops particularly cut flowers are considered sensitive to heavy metals (HMs) induced oxidative stress condition. Melatonin (MLT) is a versatile phytohormone with the ability to mitigate abiotic stresses induced oxidative stress in plants. Similarly, signaling molecules such as hydrogen sulfide (H2S) have emerged as potential options for resolving HMs related problems in plants. The mechanisms underlying the combined application of MLT and H2S are not yet explored. Therefore, we evaluated the ability of individual and combined applications of MLT (100 μM) and H2S in the form of sodium hydrosulfide (NaHS), a donor of H2S, (1.5 mM) to alleviate cadmium (Cd) stress (50 mg L-1) in stock (Matthiola incana L.) plants by measuring various morpho-physiological and biochemical characteristics. The results depicted that Cd-stress inhibited growth, photosynthesis and induced Cd-associated oxidative stress as depicted by excessive ROS accumulation. Combined application of MLT and H2S efficiently recovered all these attributes. Furthermore, Cd stress-induced oxidative stress markers including electrolyte leakage, malondialdehyde, and hydrogen peroxide are partially reversed in Cd-stressed plants by MLT and H2S application. This might be attributed to MLT or H2S induced antioxidant plant defense activities, which effectively reduce the severity of oxidative stress indicators. Overall, MLT and H2S supplementation, favorably regulated Cd tolerance in stock; yet, the combined use had a greater effect on Cd tolerance than the independent application.

Nitric oxide alleviates programmed cell death induced by cadmium in Solanum lycopersicum seedlings through protein S-nitrosylation

Cadmium (Cd), as a non-essential and toxic heavy metal in plants, has deleterious effects on plant physiological and biochemical processes. Nitric oxide (NO) is one of the most important signaling molecules for plants to response diverse stresses. Here, we found that Cd-induced programmed cell death (PCD) was accompanied by NO bursts, which exacerbated cell death when NO was removed and vice versa. Proteomic analysis of S-nitrosylated proteins showed that the differential proteins in Cd-induced PCD and in NO-alleviated PCD mainly exist together in carbohydrate metabolism and amino acid metabolism, while some of the differential proteins exist alone in metabolism of cofactors and vitamins and lipid metabolism. Meanwhile, S-nitrosylation of proteins in porphyrin and chlorophyll metabolism and starch and sucrose metabolism could explain the leaf chlorosis induced by PCD. Moreover, protein transport protein SEC23, ubiquitinyl hydrolase 1 and pathogenesis-related protein 1 were identified to be S-nitrosylated in vivo, and their expressions were increased in Cd-induced PCD while decreased in NO treatment. Similar results were obtained in tomato seedlings with higher S-nitrosylation. Taken together, our results indicate that NO might be involved in the regulation of Cd-induced PCD through protein S-nitrosylation, especially proteins involved in PCD response.

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

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

Metabolic regulation mechanism of melatonin for reducing cadmium accumulation and improving quality in rice

Melatonin acts as a potential regulator of cadmium (Cd) tolerance in rice. However, its practical value in rice production remains unclear. To validate the hypothesis that melatonin affects Cd accumulation and rice quality, a series of experiments were conducted. The results showed that exogenous melatonin application was associated with reduced Cd accumulation (23-43%) in brown rice. Fourier transform infrared spectroscopy (FTIR) analysis showed that exogenous melatonin affected the rice protein secondary structure and starch short-range structure. Metabolomics based on LC-MS/MS revealed that exogenous melatonin altered the brown rice metabolic profile, decreased fatty acid metabolite content, but increased amino acid metabolite, citric acid, melatonin biosynthetic metabolite, and plant hormone contents. These findings indicate that exogenous melatonin can effectively reduced Cd accumulation and improve rice quality through metabolic network regulation, serving as an effective treatment for rice cultivated in Cd-contaminated soil.

Enhancing soil health to minimize cadmium accumulation in agro-products: the role of microorganisms, organic matter, and nutrients

Agro-products accumulate Cd from the soil and are the main source of Cd in humans. Their use must therefore be minimized using effective strategies. Large soil beds containing low-to-moderate Cd-contamination are used to produce agro-products in many developing countries to keep up with the demand of their large populations. Improving the health of Cd-contaminated soils could be a cost-effective method for minimizing Cd accumulation in crops. In this review, the latest knowledge on the physiological and molecular mechanisms of Cd uptake and translocation in crops is presented, providing a basis for developing advanced technologies for producing Cd-safe agro-products. Inoculation of plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi, application of organic matter, essential nutrients, beneficial elements, regulation of soil pH, and water management are efficient techniques used to decrease soil Cd bioavailability and inhibiting the uptake and accumulation of Cd in crops. In combination, these strategies for improving soil health are environmentally friendly and practical for reducing Cd accumulation in crops grown in lightly to moderately Cd-contaminated soil.

Phosphorus deficiency-induced cell wall pectin demethylesterification enhances cadmium accumulation in roots of Salix caprea

Regions affected by heavy metal contamination frequently encounter phosphorus (P) deficiency. Numerous studies highlight crucial role of P in facilitating cadmium (Cd) accumulation in woody plants. However, the regulatory mechanism by which P affects Cd accumulation in roots remains ambiguous. This study aims to investigate the effects of phosphorus (P) deficiency on Cd accumulation, Cd subcellular distribution, and cell wall components in the roots of Salix caprea under Cd stress. The results revealed that under P deficiency conditions, there was a 35.4% elevation in Cd content in roots, coupled with a 60.1% reduction in Cd content in shoots, compared to the P sufficiency conditions. Under deficient P conditions, the predominant response of roots to Cd exposure was the increased sequestration of Cd in root cell walls. The sequestration of Cd in root cell walls increased from 37.1% under sufficient P conditions to 66.7% under P deficiency, with pectin identified as the primary Cd binding site under both P conditions. Among cell wall components, P deficiency led to a significant 31.7% increase in Cd content within pectin compared to P sufficiency conditions, but did not change the pectin content. Notably, P deficiency significantly increased pectin methylesterase (PME) activity by regulating the expression of PME and PMEI genes, leading to a 10.4% reduction in the degree of pectin methylesterification. This may elucidate the absence of significant changes in pectin content under P deficiency conditions and the concurrent increase in Cd accumulation in pectin. Fourier transform infrared spectroscopy (FTIR) results indicated an increase in carboxyl groups in the root cell walls under P deficiency compared to sufficient P treatment. The results provide deep insights into the mechanisms of higher Cd accumulation in root mediated by P deficiency.

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

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

Melatonin alleviates Hg toxicity by modulating redox homeostasis and the urea cycle in moss

Mercury (Hg) is a highly toxic metal and can cause severe damage to many organisms under natural conditions. As an effective free radical scavenger and antioxidant, Melatonin (MT) has played important protective roles in alleviating oxidative damage caused by environmental cues including heavy metal stress in plants. However, the detailed mechanisms of melatonin in alleviating Hg toxicity still remain unclear in plants. Our results showed that the application of melatonin greatly reduced the concentrations of total and intracellular Hg in Taxiphyllum taxirameum. Meanwhile, melatonin significantly improved the antioxidant capacity and thus alleviated oxidative damage to the chloroplasts of T. taxirameum under Hg stress. Metabolic pathway analysis further revealed that melatonin-treated plants exhibited higher levels of 48 metabolites, including sugars, amino acids, and lipids, than non-melatonin-treated plants under Hg stress. Additionally, we further found that melatonin addition greatly improved the concentrations of four organic acids and three amino acids (Orn, Cit and Arg) related to the urea cycle, and thereby changed the levels of putrescine (Put) and spermidine (Spd) in T. taxirameum exposed to Hg stress. Further experiments showed that the high concentration of Put dramatically caused oxidative damage under Hg stress, while Spd effectively alleviated Hg toxicity in T. taxirameum. Taken together, this study provides new insight into the underlying mechanisms of melatonin in alleviating heavy metal toxicity in plants.

Auxin is involved in cadmium accumulation in rice through controlling nitric oxide production and the ability of cell walls to bind cadmium

Although auxin has been linked to plants' responses to cadmium (Cd) stress, the exact mechanism is yet elusive. The objective of the current investigation was to determine the role and the mechanism of auxin in controlling rice's Cd accumulation. Rice roots with Cd stress have higher endogenous auxin levels, and exogenous auxin combined Cd treatment could reduce root cell wall's hemicellulose content when compared with Cd treatment alone, which in turn reduced its fixation of Cd, as well as decreased the expression of OsCd1 (a major facilitator superfamily gene), OsNRAMP1/5 (Natural Resistance-Associated Macrophage Protein 1/5), OsZIP5/9 (Zinc Transporter 5/9), and OsHMA2 (Heavy Metal ATPase 2) that participated in Cd uptake and root to shoot translocation. Furthermore, less Cd accumulated in the shoots as a result of auxin's impact in increasing the expression of OsCAL1 (Cadmium accumulation in Leaf 1), OsABCG36/OsPDR9 (G-type ATP-binding cassette transporter/Pleiotropic drug resistance 9), and OsHMA3, which were in charge of Cd efflux and sequestering into vacuoles, respectively. Additionally, auxin decreased endogenous nitric oxide (NO) levels and antioxidant enzyme activity, while treatment of a NO scavenger-cPTIO-reduced auxin's alleviatory effects. In conclusion, the rice's ability to tolerate Cd toxicity was likely increased by the auxin-accelerated cell wall Cd exclusion mechanism, a pathway that controlled by the buildup of NO.

Salylic acid minimize cadmium accumulation in rice through regulating the fixation capacity of the cell wall to cadmium

Although salylic acid (SA) has been linked to how plants react to cadmium (Cd) stress, the exact mechanism is still unknown. The endogenous SA concentration in the rice (Oryza sativa L.) roots was enhanced by Cd stress in the current investigation, and exogenous SA reduced the hemicellulose content in root cell wall, which in turn inhibited its Cd binding capacity. What's more, exogenous SA also decreased the transcription level of genes such as Natural Resistance-Associated Macrophage Protein 5 (OsNRAMP5) and a major facilitator superfamily gene-OsCd1 that responsible for root Cd absorption. Finally, less Cd was accumulated in the rice as a result of the higher expression of Heavy Metal ATPase 3 (OsHMA3), Cation/Ca exchanger 2 (OsCCX2) and Pleiotropic Drug Resistance 9 (OsPDR9/OsABCG36) that were responsible for separating Cd into vacuole and getting Cd out of cells, respectively. In contrast, mutant with low SA level accumulated more Cd. Additionally, SA enhanced endogenous nitric oxide (NO) levels, and its alleviatory effects were mimicked by a NO donor, sodium nitroprusside (SNP). In conclusion, SA enhanced rice's Cd resistance through regulating the binding capacity of the cell wall to Cd, a pathway that might dependent on the NO accumulation.

Seed priming with nano-silica effectively ameliorates chromium toxicity in Brassica napus

Plant yield is severely hampered by chromium (Cr) toxicity, affirming the urgent need to develop strategies to suppress its phyto-accumulation. Silicon dioxide nanoparticles (SiO2 NPs) have emerged as a provider of sustainable crop production and resistance to abiotic stress. But, the mechanisms by which seed-primed SiO2 NPs palliate Cr-accumulation and its toxic impacts in Brassica napus L. tissues remains poorly understood. To address this gap, present study examined the protective efficacy of seed priming with SiO2 NPs (400 mg/L) in relieving the Cr (200 µM) phytotoxicity mainly in B. napus seedlings. Results delineated that SiO2 NPs significantly declined the accumulation of Cr (38.7/35.9%), MDA (25.9/29.1%), H2O2 (27.04/36.9%) and O2• (30.02/34.7%) contents in leaves/roots, enhanced the nutrients acquisition, leading to improved photosynthetic performance and better plant growth. SiO2 NPs boosted the plant immunity by upregulating the transcripts of antioxidant (SOD, CAT, APX, GR) or defense-related genes (PAL, CAD, PPO, PAO and MT-1), GSH (assists Cr-vacuolar sequestration), and modifying the subcellular distribution (enhances Cr-proportion in cell wall), thereby confer tolerance to ultrastructural damages under Cr stress. Our first evidence to establish the Cr-detoxification by seed-primed SiO2 NPs in B. napus, indicated the potential of SiO2 NPs as stress-reducing agent for crops grown in Cr-contaminated areas.

Reduction in the cadmium (Cd) accumulation and toxicity in pearl millet (Pennisetum glaucum L.) by regulating physio-biochemical and antioxidant defense system via soil and foliar application of melatonin

Cadmium (Cd) is among the toxic pollutants that harms the both animals and plants. The natural antioxidant, melatonin can improve Cd-stress tolerance but its potential role in reducing Cd stress and resilience mechanisms in pearl millet (Pennisetum glaucum L.) is remain unclear. The present study suggests that Cd causes severe oxidative damage by decreasing photosynthesis, and increasing reactive oxygen species (ROS), malondialdehyde content (MDA), and Cd content in different parts of pearl millet. However, exogenous melatonin (soil application and foliar treatment) mitigated the Cd toxicity and enhanced the growth, antioxidant defense system, and differentially regulated the expression of antioxidant-responsive genes i. e superoxide dismutase SOD-[Fe] 2, Fe-superoxide dismutase, Peroxiredoxin 2C, and L-ascorbate peroxidase-6. The results showed that foliar melatonin at F-200/50 significantly increased the plant height, chlorophyll a, b, a+b and carotenoids by 128%, 121%, 150%, 122%, and 69% over the Cd treatment, respectively. The soil and foliar melatonin at S-100/50 and F-100/50 reduced the ROS by 36%, and 44%, and MDA by 42% and 51% over the Cd treatment, respectively. Moreover, F200/50 significantly boosted the activities of antioxidant enzymes i. e SOD by 141%, CAT 298%, POD 117%, and APX 155% over the Cd treatment. Similarly, a significant reduction in Cd content in root, stem, and leaf was found on exposure to higher concentrations of exogenous melatonin. These findings suggest that exogenous melatonin may significantly and differentially improve the tolerance to Cd stress in crop plants. However, field applications, type of plant species, concentration of dose, and type of stress may vary with the degree of tolerance in crop plants.