Unravelling cadmium toxicity and nitric oxide induced tolerance in Cucumis sativus : Insight into regulatory mechanisms using proteomics

Community metagenomics reveals the processes of cadmium resistance regulated by microbial functions in soils with Oryza sativa root exudate input

Plants exert a profound influence on their rhizosphere microbiome through the secretion of root exudates, thereby imparting critical effects on their growth and overall health. The results unveil that japonica rice showcases a remarkable augmentation in its antioxidative stress mechanisms under Cd stress. This augmentation is characterized by the sequestration of heavy metal ions within the root system and the prodigious secretion of a spectrum of flavonoids, including Quercetin, Luteolin, Apigenin, Kaempferide, and Sakuranetin. These flavonoids operate as formidable guardians, shielding the plant from oxidative damage instigated by Cd-induced stress. Furthermore, the metagenomic analyses divulge the transformative potential of flavonoids, as they induce profound alterations in the composition and structural dynamics of plant rhizosphere microbial communities. These alterations manifest through the recruitment of plant growth-promoting bacteria, effectively engineering a conducive milieu for japonica rice. In addition, our symbiotic network analysis discerns that flavonoid compounds significantly improved the positive correlations among dominant species within the rhizosphere of japonica rice. This, in turn, bolsters the stability and intricacy of the microenvironmental ecological network. KEGG functional analyses reveal a notable upregulation in the expression of flavonoid functional genes, specifically cadA, cznA, nccC, and czrB, alongside an array of transporters, encompassing RND, ABC, MIT, and P-ATPase. These molecular orchestrations distinctly demarcated the rhizosphere microbiome of japonica rice, markedly enhancing its tolerance to Cd-induced stress. These findings not only shed light on the establishment of Cd-resistant bacterial consortia in rice but also herald a promising avenue for the precise modulation of plant rhizosphere microbiomes, thereby fortifying the safety and efficiency of crop production.

The combined effects of copper and zinc on Arabidopsis involve differential regulation of chlorophyll synthesis and photosystem function

Copper (Cu) and zinc (Zn) are both oxidation-reducing metal elements that are necessary for plant growth, and their effect often depends on their concentration. However, there are few studies that have investigated how plants are stressed and affected when the two ions are present simultaneously, especially when one ion is beneficial due to a low concentration and the other is detrimental due to a high concentration. To address this question, we treated Arabidopsis plants with either high or/and low concentrations of the two ions and investigated their mutual effects and the underlying molecular mechanism, focusing on photosynthetic function. The results showed that the photosynthetic pigment content and the performance of photosynthetic systems were most affected when both metal ions were present at detrimental concentrations (60 μM Cu for Cu60 and 350 μM Zn for Zn350). These include the effective openness of the photoreaction center, the electron transport rate and efficiency of photosystem II (PSII), the NPQ-dependent energy dissipation and the activity of photosystem I (PSI). However, when the harmful concentration of one of the two metals is combined with the beneficial concentration of the other metal (Cu5+Zn350 or Zn50+Cu60), these photosynthetic indicators are compensated to different degrees but the negative effects of copper ions at high dose are more difficult to eliminate than zinc ions. These results were also confirmed by gene expression analysis, which provides a clue to understanding the interaction between heavy metal ions, reducing metal toxicity and improving the tolerance of plants to heavy metals in practice.

Innovative Adsorbents for Pollutant Removal: Exploring the Latest Research and Applications

The growing presence of diverse pollutants, including heavy metals, organic compounds, pharmaceuticals, and emerging contaminants, poses significant environmental and health risks. Traditional methods for pollutant removal often face limitations in efficiency, selectivity, and sustainability. This review provides a comprehensive analysis of recent advancements in innovative adsorbents designed to address these challenges. It explores a wide array of non-conventional adsorbent materials, such as nanocellulose, metal-organic frameworks (MOFs), graphene-based composites, and biochar, emphasizing their sources, structural characteristics, and unique adsorption mechanisms. The review discusses adsorption processes, including the basic principles, kinetics, isotherms, and the factors influencing adsorption efficiency. It highlights the superior performance of these materials in removing specific pollutants across various environmental settings. The practical applications of these adsorbents are further explored through case studies in industrial settings, pilot studies, and field trials, showcasing their real-world effectiveness. Additionally, the review critically examines the economic considerations, technical challenges, and environmental impacts associated with these adsorbents, offering a balanced perspective on their viability and sustainability. The conclusion emphasizes future research directions, focusing on the development of scalable production methods, enhanced material stability, and sustainable regeneration techniques. This comprehensive assessment underscores the transformative potential of innovative adsorbents in pollutant remediation and their critical role in advancing environmental protection.

Impact of Cd and Pb on the photosynthetic and antioxidant systems of Hemerocallis citrina Baroni as revealed by physiological and transcriptomic analyses

KEY MESSAGE

Cd induces photosynthetic inhibition and oxidative stress damage in H. citrina, which mobilizes the antioxidant system and regulates the expression of corresponding genes to adapt to Cd and Pb stress. Cd and Pb are heavy metals that cause severe pollution and are highly hazardous to organisms. Physiological measurements and transcriptomic analysis were combined to investigate the effect of 5 mM Cd or Pb on Hemerocallis citrina Baroni. Cd significantly inhibited H. citrina growth, while Pb had a minimal impact. Both Cd and Pb suppressed the expression levels of key chlorophyll synthesis genes, resulting in decreased chlorophyll content. At the same time, Cd accelerated chlorophyll degradation. It reduced the maximum photochemical efficiency of photosystem (PS) II, damaging the oxygen-evolving complex and leading to thylakoid dissociation. In contrast, no such phenomena were observed under Pb stress. Cd also inhibited the Calvin cycle by down-regulating the expression of Rubisco and SBPase genes, ultimately disrupting the photosynthetic process. Cd impacted the light reaction processes by damaging the antenna proteins, PS II and PS I activities, and electron transfer rate, while the impact of Pb was weaker. Cd significantly increased reactive oxygen species and malondialdehyde accumulation, and inhibited the activities of antioxidant enzymes and the expression levels of the corresponding genes. However, H. citrina adapted to Pb stress by the recruitment of antioxidant enzymes and the up-regulation of their corresponding genes. In summary, Cd and Pb inhibited chlorophyll synthesis and hindered the light capture and electron transfer processes, with Cd exerting great toxicity than Pb. These results elucidate the physiological and molecular mechanisms by which H. citrina responds to Cd and Pb stress and provide a solid basis for the potential utilization of H. citrina in the greening of heavy metal-polluted lands.

Sulfide-carbonate-mineralized functional bacterial consortium for cadmium removal in flue gas

Sulfide-carbonate-mineralized functional bacterial consortium was constructed for flue gas cadmium biomineralization. A membrane biofilm reactor (MBfR) using the bacterial consortium containing sulfate reducing bacteria (SRB) and denitrifying bacteria (DNB) was investigated for flue gas cadmium (Cd) removal. Cadmium removal efficiency achieved 90%. The bacterial consortium containing Citrobacter, Desulfocurvus and Stappia were dominated for cadmium resistance-nitrate-sulfate reduction. Under flue gas cadmium stress, ten cadmium resistance genes (czcA, czcB, czcC, czcD, cadA, cadB, cadC, cueR, copZ, zntA), and seven genes related to sulfate reduction, increased in abundance; whereas others, nine genes related to denitrification, decreased, indicating that cadmium stress was advantageous to sulfate reduction in the competition with denitrification. A bacterial consortium could capable of simultaneously cadmium resistance, sulfate reduction and denitrification. Microbial induced carbonate precipitation (MICP) and biological adsorption process would gradually yield to sulfide-mineralized process. Flue gas cadmium could transform to Cd-EPS, cadmium carbonate (CdCO3) and cadmium sulfide (CdS) bioprecipitate. The functional bacterial consortium was an efficient and eco-friendly bifunctional bacterial consortium for sulfide-carbonate-mineralized of cadmium. This provides a green and low-carbon advanced treatment technology using sulfide-carbonate-mineralized functional bacterial consortium for the removal of cadmium or other hazardous heavy metal contaminants in flue gas.

Physiology and transcriptomic analysis revealed the mechanism of silicon promoting cadmium accumulation in Sedum alfredii Hance

Silicon (Si) effectively promote the yield of many crops, mainly due to its ability to enhance plants resistance to stress. However, how Si helps hyperaccumulators to extract Cadmium (Cd) from soil has remained unclear. In this study, Sedum alfredii Hance (S. alfredii) was used as material to study how exogenous Si affected biomass, Cd accumulation, antioxidation, cell ultrastructure, subcellular distribution and changes in gene expression after Cd exposure. The study has shown that as Si concentration increases (1, 2 mM), the shoot biomass of plants increased by 33.1%-63.6%, the Cd accumulation increased by 31.9%-96.6%, and the chlorophyll, carotenoid content, photosynthetic gas exchange parameters significantly increased. Si reduced Pro and MDA, promoted the concentrations of SOD, CAT and POD to reduce antioxidant stress damage. In addition, Si promoted GSH and PC to chelate Cd in vacuoles, repaired damaged cell ultrastructure, improved the fixation of Cd and cell wall (especially in pectin), and reduced the toxic effects of Cd. Transcriptome analysis found that genes encoding Cd detoxification, Cd absorption and transport were up-regulated by Si supplying, including photosynthetic pathways (PSB, LHCB, PSA), antioxidant defense systems (CAT, APX, CSD, RBOH), cell wall biosynthesis such as pectinesterase (PME), chelation (GST, MT, NAS, GR), Cd absorption (Nramp3, Nramp5, ZNT) and Cd transport (HMA, PCR). Our result revealed the tentative mechanism of Si promotes Cd accumulation and enhances Cd tolerance in S. alfredii, and thereby provides a solid theoretical support for the practical use of Si fertilizer in phytoextraction.

Comprehensive physiology and proteomics analysis revealed the resistance mechanism of rice (Oryza sativa L) to cadmium stress

Cadmium contamination can lead to a decrease in crop yield and quality. However, Cd-tolerant rice can improve rice resistance genes, improve crop tolerance to heavy metals, and protect plants from oxidative damage. In this study, Japonica rice: Chunyou 987 and Indica rice: Chuanzhong you 3607 were used to reveal the molecular response mechanism of Cd-tolerant rice under cadmium concentration of 3 mg/kg through comparative experiments combined with physiology and proteomics. The results showed that compared with indica rice, japonica rice showed more robust resistance to Cd stress and effectively retained many Cd ions in roots. Moreover, it enhanced its enzymatic and non-enzymatic anti-oxidative stress mechanism, which increased the activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) by 47.37%, 21.75%, and 55.42%, respectively. The contents of non-enzymatic antioxidant substances ascorbic acid (AsA), glutathione (GSH), cysteine (Cys), proline (PRO), anthocyanins (OPC), and flavonoids were increased by 25.32%, 42.67%, 21.43%, 50.81%, 33.23%, and 72.16%, respectively. Through proteomics analysis, it was found that in response to the damage caused by cadmium stress, Japonica rice makes Photosynthesis functional proteins (psbO and PetH), Photosynthesis antenna proteins (LHCA and ASCAB9), Carbon fixation functional proteins (PEPC and OsAld), Porphyrin metabolism functional proteins (OsRCCR1 and SE5), Glyoxylate and dicarboxylate The expression of metabolism functional proteins (CATC and GLO4.) and Glutathione metabolism functional proteins (APX8 and OsGSTU13) were significantly up-regulated, which stimulated the antioxidant stress mechanism and photosynthetic system, and constructed a robust energy supply system to ensure the normal metabolic activities of life. Strengthening the mechanisms of plant homeostasis. In summary, this study revealed the molecular mechanism of tolerance to Cd stress in japonica rice, and the results of this study will provide a possible way to improve Cd-resistant rice seedlings.

Fulvic acid alleviates cadmium-induced root growth inhibition by regulating antioxidant enzyme activity and carbon-nitrogen metabolism in apple seedlings

Introduction

Substantial previous studies have reported that fulvic acid (FA) application plays an important role in Chinese agricultural production. However, little is known about the mechanisms for using FA to increase apple trees resistance to Cd toxicity. In order to clarify the mechanism underlying FA alleviation in Cd-induced growth inhibition in apple seedlings.

Methods

Herein, we treated M9T337 seedlings to either 0 or 30 µM/L Cd together with 0 or 0.2 g/L FA and analyzed the root growth, antioxidant enzyme activities, carbon (C) assimilation, nitrogen (N) metabolism, and C and N transport.

Results

The results presented that, compared with CK (without Cd addition or FA spraying application), Cd poisoning significantly inhibited the root growth of apple seedlings. However, this Cd-induced root growth inhibition was significantly alleviated by FA spraying relative to the Cd treatment (Cd addition alone). On the one hand, the mitigation of inhibition effects was due to the reduced oxidative damage by enhancing antioxdiant enzyme (SOD, POD, and CAT) activities in leaves and roots. On the other hand, this growth advantage demonstrated compared to the Cd treatment was found to be associated with the strengthen of photosynthetic performance and the elevation of C and N metabolism enzymes activities. Meanwhile, we also found that under Cd stress condition, the distribution of C and N nutrients in apple seedlings was optimised by FA spraying application relative to the Cd treatment, according to the results of 13C and 15N tracing.

Conclusion

Conclusively, our results suggested that the inhibitory effect of Cd on apple seedlings root growth was alleviated by FA through regulating antioxdiant capacities and C and N metabolism.

How plants respond to heavy metal contamination: a narrative review of proteomic studies and phytoremediation applications

MAIN CONCLUSION

Heavy metal pollution caused by human activities is a serious threat to the environment and human health. Plants have evolved sophisticated defence systems to deal with heavy metal stress, with proteins and enzymes serving as critical intercepting agents for heavy metal toxicity reduction. Proteomics continues to be effective in identifying markers associated with stress response and metabolic processes. This review explores the complex interactions between heavy metal pollution and plant physiology, with an emphasis on proteomic and biotechnological perspectives. Over the last century, accelerated industrialization, agriculture activities, energy production, and urbanization have established a constant need for natural resources, resulting in environmental degradation. The widespread buildup of heavy metals in ecosystems as a result of human activity is especially concerning. Although some heavy metals are required by organisms in trace amounts, high concentrations pose serious risks to the ecosystem and human health. As immobile organisms, plants are directly exposed to heavy metal contamination, prompting the development of robust defence mechanisms. Proteomics has been used to understand how plants react to heavy metal stress. The development of proteomic techniques offers promising opportunities to improve plant tolerance to toxicity from heavy metals. Additionally, there is substantial scope for phytoremediation, a sustainable method that uses plants to extract, sequester, or eliminate contaminants in the context of changes in protein expression and total protein behaviour. Changes in proteins and enzymatic activities have been highlighted to illuminate the complex effects of heavy metal pollution on plant metabolism, and how proteomic research has revealed the plant's ability to mitigate heavy metal toxicity by intercepting vital nutrients, organic substances, and/or microorganisms.

Coupling of different antioxidative systems in rice under the simultaneous influence of selenium and cadmium

Selenium (Se) elevates the antioxidant ability of rice against cadmium (Cd) stress, but previous studies only focused on the variation in antioxidant enzymes or nonenzymatic substances induced by Se under Cd stress and ignored the relationships between different antioxidant parameters during the interaction. Here, hydroponic experiments with rice were performed by adding both Cd and Se at doses in the range of 0-50 μM to explore the physiological responses of rice and their relationships in the presence of different levels of Se and Cd. Exogenous Cd markedly promoted the activity of antioxidant enzymes with the exception of catalase (CAT) and the concentration of nonenzymatic substances in aerial parts. Se enhanced the antioxidant capacity by improving the activities of all the enzymes tested in this study and increasing the concentrations of nonenzymatic compounds. The couplings among different antioxidant substances within paddy rice were then determined based on cluster and linear fitting results and their metabolic process and physiological functions. The findings specifically highlight that couplings among the ascorbic acid (AsA)-glutathione (GSH) cycle, glutathione synthase (GS)-phytochelatin synthetase (PCS) coupling system and glutathione peroxidase (GPX)-superoxide dismutase (SOD) coupling system in aerial parts helps protect plants from Cd stress. These coupling systems form likely due to the fact that one enzyme generated a product that could be the substrate for another enzyme. Noticeably, such coupling systems do not emerge in roots because the stronger damage to roots than other organs activates the ascorbate peroxidase (APX)-GPX-CAT and PCS-GS-SOD systems with distinct functions and structures. This study provides new insights into the detoxification mechanisms of rice caused by the combined effect of Se and Cd.

Remediation of Cd contaminated paddy fields by intercropping of the high- and low- Cd-accumulating rice cultivars

Intercropping cadmium (Cd) hyperaccumulators with crops have been widely applied in the remediation of contaminated farmland soils. However, most studies were done on drylands since the majority of the hyperaccumulators are susceptible to the aquatic environment, making the remediation of Cd-contaminated paddy fields particularly difficult. Our study attempts to address the issue by intercropping the high-Cd-accumulating (henceforth, "high-Cd") rice cultivars with the low-Cd-accumulating (henceforth, "low-Cd") ones, and to study the Cd removal, uptake and translocation during the remediation process. The results indicated that intercropping mode with 20-cm row spacing (intercropping-20 treatment) performed better than the that with 30-cm row spacing (intercropping-30 treatment), while intercropping had stronger impact on late rice compared to early rice. In general, the physiological condition of rice was stable under the intercropping-20 treatment, suggesting the growth of rice was not impeded. For late rice, as the intercropping-20 treatment can significantly reduce soil pH and increase the diethylenetriaminepentaacetic acid extracted Cd (DTPA-extracted Cd) from the rhizosphere soil, Cd accumulated more in the tissues of the high-Cd rice cultivars (H2), and its dry biomass increased. As a result, a drastic improvement in the total Cd removal rate by 38.55 % was noticed. Therefore, the reduction of total Cd concentration in 0-20 cm profile caused by removal, thus it could provide safer soil environment for the growth of low Cd-rice cultivars (L2), leading to a significant drop in the root Cd concentration and safer production of L2. Interestingly, intercropping had no effect on the yield per plant of low-Cd rice cultivars. For early rice, intercropping-20 treatment exerted trivial effects to all aspects. The intercropping-30 treatment has poor representativeness of all indicators because of the large intercropping distance. Our results demonstrate that intercropping of the high-Cd and the low-Cd rice cultivars is a potential mode for Cd remediation in paddy fields.

Different responses of Chlorella vulgaris to silver nanoparticles and silver ions under modulation of nitric oxide

Silver nanoparticles (Ag-NPs) are widely used in daily life because of their antibacterial properties. A fraction of Ag-NPs are released into the ecosystem during their production and utilization. The toxicity of Ag-NPs has been reported. However, it is still disputed whether the toxicity is mainly due to the released silver ions (Ag⁺). In addition, few studies have reported the response of algae to metal nanoparticles under modulation of nitric oxide (NO). In this study, Chlorella vulgaris (C. vulgaris) was used as a model organism to study the toxic effects of Ag-NPs and Ag⁺ released from Ag-NPs on algae under the modulation of NO. The results showed that the biomass inhibition rate of Ag-NPs (44.84%) to C. vulgaris was higher than that of Ag⁺ (7.84%). Compared with Ag⁺, Ag-NPs induced more severe damage to photosynthetic pigments, photosynthetic system II (PSII) performance, and lipid peroxidation. More serious damage to cell permeability led to higher internalization of Ag under Ag-NPs stress. Application of exogenous NO reduced the inhibition ratio of photosynthetic pigments and chlorophyll autofluorescence. Further, NO reduced the MDA levels by scavenging reactive oxygen species induced by Ag-NPs. NO modulated the secretion of extracellular polymers and hampered the internalization of Ag. All these results showed that NO alleviates the toxicity of Ag-NPs to C. vulgaris. However, NO did not improve the toxic effects of Ag⁺. Our results provide new insights into the toxicity mechanism of Ag-NPs to algae modulated by the signal molecule NO.

Cadmium tolerance and hyperaccumulation in plants - A proteomic perspective of phytoremediation

Cadmium (Cd) is a major environmental pollutant and poses a risk of transfer into the food chain through contaminated plants. Mechanisms underlying Cd tolerance and hyperaccumulation in plants are not fully understood. Proteomics-based approaches facilitate an in-depth understanding of plant responses to Cd stress at the systemic level by identifying Cd-inducible differentially abundant proteins (DAPs). In this review, we summarize studies related to proteomic changes associated with Cd-tolerance mechanisms in Cd-tolerant crops and Cd-hyperaccumulating plants, especially the similarities and differences across plant species. The enhanced DAPs identified through proteomic studies can be potential targets for developing Cd-hyperaccumulators to remediate Cd-contaminated environments and Cd-tolerant crops with low Cd content in the edible organs. This is of great significance for ensuring the food security of an exponentially growing global population. Finally, we discuss the methodological drawbacks in current proteomic studies and propose that better protocols and advanced techniques should be utilized to further strengthen the reliability and applicability of future Cd-stress-related studies in plants. This review provides insights into the improvement of phytoremediation efficiency and an in-depth study of the molecular mechanisms of Cd enrichment in plants.

Meta-Analysis of the Effect of Nitric Oxide Application on Heavy Metal Stress Tolerance in Plants

Substantial single-species studies have reported the facility of nitric oxide (NO) in alleviating heavy metal-induced stress in plants. Understanding the mechanisms of NO-involved stress alleviation is progressing; however, a quantitative description of the alleviative capacity of NO against heavy metal stress is still lacking. We combined the results of 86 studies using meta-analysis to statistically assess the responses of heavy metal-stressed plants to NO supply across several metal stresses and plant families. The results showed that plant biomass was consistently improved following NO supply to metal-stressed plants. NO played an important role in mitigating oxidative damage caused by heavy metal stress by significantly stimulating the activities of antioxidant enzymes. Moreover, NO supply consistently increased the Ca, Fe, and Mg contents in both leaves and roots. Plant tissues accumulated less heavy metals when exposed to heavy metal stress after NO addition. Additionally, the best concentration of SNP (an NO donor) for hydroponic culture is in the range of 75-150 μM. We further confirmed that NO application can generally alleviate plant heavy metal stress and its action pathway. The results presented here can help guide future applications of NO as a plant growth regulator in agriculture and breeding plants for heavy metal stress tolerance.

Fulvic acid mitigates cadmium toxicity-induced damage in cucumber seedlings through the coordinated interaction of antioxidant enzymes, organic acid, and amino acid

Fulvic acid (FA) can significantly alleviate cadmium (Cd) stress, but the specific metabolic response of FA to Cd toxicity is still not clarified. In the present study, we used untargeted metabolomic [gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS)] analysis to profile cucumber metabolism in response to Cd stress after spray application of FA. Our results showed that 331 differentially enriched metabolites (DEMs) were identified in leaf materials. These DEMs were enriched in 21 shared pathways in comparative groups of "Cd treatment vs. the control treatment" and "FA + Cd treatment vs. the Cd treatment." Specifically, treatment with FA significantly enhanced the organic acid content (citric acid, isocitric acid, 2-oxoglutaric acid, fumaric acid, and malic acid), which would contribute to provide sufficient substrates for the tricarboxylic acid (TCA) cycle and amino acid biosynthesis, thereby ensuring the normal production of energy and amino acid. At the same time, FA significantly increased the amino acid content (aspartate, citrulline, histidine, leucine, and phenylalanine). The accumulation of organic acid and amino acid can act as chelating agents for heavy metal ions and as scavengers of reactive oxygen species (ROS), thereby reducing intracellular oxidative damage. Furthermore, the application of FA improves antioxidant enzymes and accelerates ROS clearance. The improved contents of organic acid and amino acid, and the increased activity of antioxidant enzymes both played a central role in the reduction of malondialdehyde (MDA, 14.08%), hydrogen peroxide (H₂O₂, 61.70%) contents, and superoxide anion radical (O₂^-, 30.41%) production rate in plants under Cd stress. Taken together, the present study demonstrates the effects of FA on the antioxidant capacity and carbohydrate and amino acid metabolism of cucumber seedlings exposed to Cd stress, which provides comprehensive insights into the regulation of plants' response to Cd toxicity with FA was applied in cucumber.

Trx CDSP32-overexpressing tobacco plants improves cadmium tolerance by modulating antioxidant mechanism

The effects of overexpression of the thioredoxin-like protein CDSP32 (Trx CDSP32) on reactive oxygen species (ROS) metabolism in tobacco leaves exposed to cadmium (Cd) were studied by combining physiological measures and proteomics technology. Thus, the number of differentially expressed proteins (DEPs) in plants overexpressing the Trx CDSP32 gene in tobacco (OE) was observed to be evidently lower than that in wild-type (WT) tobacco under Cd exposure, especially the number of down-regulated DEPs. Cd exposure induced disordered ROS metabolism in tobacco leaves. Although Cd exposure inhibited the activities of superoxide dismutase (SOD), catalase (CAT), and l-ascorbate peroxidase (APX) and the expression of proteins related to the thioredoxin-peroxiredoxin (Trx-Prx) pathway, the increase in the activities of peroxidase (POD), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), and glutathione S-transferase (GST) and their protein expression levels played an important role in the physiological response to Cd exposure. Notably, Trx CDSP32 was observed to alleviate the decrease in the expression and activities of SOD and CAT caused by Cd exposure and enhance the function of POD. Trx CDSP32 was observed to increase the H₂O₂ scavenging capacity of the ascorbic acid-glutathione (AsA-GSH) cycle and Trx-Prx pathway under Cd exposure, and it can especially regulate 2-Cys peroxiredoxin (2-Cys Prx) protein expression and thioredoxin peroxidase (TPX) activity. Thus, overexpression of the Trx CDSP32 gene can alleviate the oxidative damage that occurs in tobacco leaves under Cd exposure by modulating antioxidant defense systems.

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

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

Proteome and physiological analyses reveal tobacco (Nicotiana tabacum) peroxidase 7 (POD 7) functions in responses to copper stress

Copper is a micronutrient essential for plant growth and development. However, Cu is also a heavy metal element that has deleterious impacts on plants when excessively accumulated in the environment. To understand the molecular mechanism underlying tobacco in response to Cu stress, iTRAQ based technology was used to identify differentially expressed proteins (DEPs) and important metabolic pathways in tobacco plants treated with excessive CuSO4. The results showed that 180 DEPs were detected between the treatment and control, among which 78 were upregulated and 102 were downregulated. These DEPs can be functionally divided into 65 categories and are closely related to metabolic pathways, carbon metabolism, secondary metabolite biosynthesis, biosynthesis of antibiotics, glyoxylate and dicarboxylate metabolism, and glycolysis/gluconeogenesis. Peroxidase7 was significantly upregulated and was selected and overexpressed in tobacco. Then, positive transgenic lines and wild type plants were exposed to a Cu stress environment. The results showed that Peroxidase7 transgenic tobacco plants exhibited enhanced Cu stress resistance with decreased malondialdehyde and Cu contents, and increased shoot dry weight, root length, secondary root number, SOD, POD and CAT activity. The present study suggests that the ROS scavenging mechanism is essential for tobacco plants in response to Cu stress and that Peroxidase7 functions in tobacco plant resistance to excessive Cu environment.

Physiology and proteomics reveal Fulvic acid mitigates Cadmium adverse effects on growth and photosynthetic properties of lettuce

Understanding the molecular mechanisms of plants in response to Cd stress is crucial for improving plants adaptation to Cd stress. Fulvic acid (FA) is an active humic substance that is often used as a soil conditioner. However, there are few reports on the role of FA against Cd stress. The aim of this study was to determine the effects of Fulvic acid on alleviation of Cd toxicity in lettuce (Lactuca sativa L) under hydroponic conditions. Our results showed that 20 μmol/L Cd stress significantly reduced photosynthetic pigment metabolism and the expression of photosynthetic apparatus-related proteins, thereby inhibiting photosynthetic electron transport, net photosynthetic rate and negatively affecting photosynthetic carbon assimilation and growth of lettuce. However, proteomic findings suggest that the application of FA can reduce the adverse effects of Cd contamination. Compared to Cd stress alone, FA significantly increased the expression of Light-harvesting proteins, reaction center and electron transport-related proteins. Further results showed that FA at 0.5 g/L reduced the uptake of Cd by the roots, resulting in a 23.5% reduction in total Cd content in lettuce. Moreover, FA enhanced S metabolism and rebuilt redox homeostasis in cells. Overall, these findings provide new insights into the mechanism of cadmium toxicity mitigation in lettuce by FA. Which is recommended as an eco-friendly tool for improving the photosynthesis performance and biomass of lettuce under Cd stress.

WRKY74 regulates cadmium tolerance through glutathione-dependent pathway in wheat

Cadmium (Cd) is a toxic heavy metal to plants and human health. Ascorbate (ASA)-glutathione (GSH) synthesis pathway plays key roles in Cd detoxification, while its molecular regulatory mechanism remains largely unknown, especially in wheat. Here, we found a WRKY transcription factor-TaWRKY74, and its function in wheat Cd stress is not clear in previous studies. The expression levels of TaWRKY74 were significantly induced by Cd stress. Compared to control, the activities of GST, GR, or APX were significantly increased by 1.55-, 1.43-, or 1.75-fold and 1.63-, 2.65-, or 2.30-fold in shoots and roots of transiently TaWRKY74-silenced wheat plants under Cd stress. Similarly, the contents of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), GSH, or Cd were also significantly increased by 2.39- or 1.25-fold, 1.54- or 1.20-fold, and 1.34- or 5.94-fold in shoots or roots in transiently TaWRKY74-silenced wheat plants, while ASA content was decreased by 47.4 or 43.3% in shoots, 10.7 or 6.5% in roots in these silenced wheat plants, respectively. Moreover, the expression levels of GSH, GPX, GR, DHAR, MDHAR, and APX genes, which are involved in ASA-GSH synthesis, were separately induced by 2.42-, 2.16-, 3.28-, 2.08-, 1.92-, and 2.23-fold in shoots, or by 10.69-, 3.33-, 3.26-, 1.81-, 16.53-, and 3.57-fold in roots of the BSMV-VIGS-TaWRKY74-inoculated wheat plants, respectively. However, the expression levels of TaNramp1, TaNramp5, TaHMA2, TaHMA3, TaLCT1, and TaIRT1 metal transporters genes were decreased by 21.2-76.3% (56.6%, 59.2%, 76.3%, 53.6%, 35.8%, and 21.2%) in roots of the BSMV-VIGS-TaWRKY74-inoculated wheat plants. Taken together, our results suggested that TaWRKY74 alleviated Cd toxicity in wheat by affecting the expression of ASA-GSH synthesis genes and suppressing the expression of Cd transporter genes, and further affecting Cd uptake and translocation in wheat plants.

The involvement of nitric oxide and ethylene on the formation of endodermal barriers in response to Cd in hyperaccumulator Sedum alfredii

Nitric oxide (NO) and ethylene are both important signaling molecules which participate in numerous plant development processes and environmental stress resistance. Here, we investigate whether and how NO interacts with ethylene during the development of endodermal barriers that have major consequences for the apoplastic uptake of cadmium (Cd) in the hyperaccumulator Sedum alfredii. In response to Cd, an increased NO accumulation, while a decrease in ethylene production was observed in the roots of S. alfredii. Exogenous supplementation of NO donor SNP (sodium nitroprusside) decreased the ethylene production in roots, while NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) had the opposite effect. The exogenous addition of NO affected the ethylene production through regulating the expression of genes related to ethylene synthesis. However, upon exogenous ethylene addition, roots retained their NO accumulation. The abovementioned results suggest that ethylene is downstream of the NO signaling pathway in S. alfredii. Regardless of Cd, addition of SNP promoted the deposition of endodermal barriers via regulating the genes related to Casparian strips deposition and suberization. Correlation analyses indicate that NO positively modifies the formation of endodermal barriers via the NO-ethylene signaling pathway, Cd-induced NO accumulation interferes with the synthesis of ethylene, leading to a deposition of endodermal barriers in S. alfredii.

Proteomic and physiological analyses to elucidate nitric oxide-mediated adaptive responses of barley under cadmium stress

Nitric oxide (NO) is known to induce plant resistance for several environmental stresses. The protective roles of NO in cadmium (Cd) toxicity have been well documented for various plant species; nevertheless, little information is available about its molecular regulation in improving Cd tolerance of barley plants. Therefore, we combined a comparative proteomics with physiological analyses to evaluate the potential roles of NO in alleviating Cd stress (50 μM) in barley (Hordeum vulgare L.) seedlings. Exogenous application of NO donor sodium nitroprusside (SNP, 100 μM) decreased the Cd-mediated seedling growth inhibition. This observation was supported by the reduction of lipid peroxidation as well as the improvement of chlorophyll content and inhibition of hydrogen peroxide accumulation. Activities of the superoxide dismutase and guaiacol peroxidase were reduced following the application of SNP, while ascorbate peroxidase activity was enhanced. In this study, a total of 34 proteins were significantly regulated by NO in the leaves under Cd stress using a gel-based proteomic approach. The proteomic analysis showed that several pathways were noticeably influenced by NO including photosynthesis and carbohydrate metabolism, protein metabolism, energy metabolism, stress defense, and signal transduction. These results provide new evidence that NO induce photosynthesis and energy metabolism which may enhance Cd tolerance in barley seedlings.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01214-3.

Red and blue light function antagonistically to regulate cadmium tolerance by modulating the photosynthesis,antioxidant defense system and Cd uptake in cucumber(Cucumis sativus L.)

Cadmium (Cd) is highly toxic to both plants and humans.Light plays crucial roles in plant growth, development and stress responses, but how light functions in plant Cd response remain unclear.Here,we found that Cd treatment significantly induced the expression of PHYB but not PHYA and CRY1 in leaves and roots of cucumber. Correspondingly,compared with white light (W) during Cd stress,red light(R) increased Cd sensitivity,whereas blue light (B) enhanced Cd tolerance as evidenced by decreased Cd-induced chlorosis, growth inhibition, photosynthesis inhibition and chloroplast ultrastructure damage.Furthermore,B markedly increased the transcripts and activities of the antioxidant enzymes including ascorbate peroxidase (APX),catalase (CAT),superoxide dismutase (SOD) and glutathione reductase (GR),as well as glutathione (GSH) content and GSH1 expression, resulting in hydrogen peroxide (H₂O₂) and superoxide (O₂^.-) reduction,but R treatment showed the opposite trend. Moreover, R and B markedly up-regulated and down-regulated the expression levels of Cd uptake and transport genes including IRT1, NRAMP1 and HMA3, leading to more and less Cd accumulation than the W-treated plants in both shoots and roots, respectively under Cd stress. Collectively, our data clearly demonstrate that R and B function antagonistically to regulate Cd tolerance in cucumber via modulating the photosynthesis, antioxidant defense system and Cd uptake, providing a novel light quality control strategy to enhance crop Cd tolerance and food safety.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

Melatonin alleviates aluminum-induced growth inhibition by modulating carbon and nitrogen metabolism, and reestablishing redox homeostasis in Zea mays L

Melatonin, a regulatory molecule, performs pleiotropic functions in plants, including aluminum (Al) stress mitigation. Here, we conducted transcriptomic and physiological analyses to identify metabolic processes associated with the alleviated Al-induced growth inhibition of the melatonin-treated (MT) maize (Zea mays L.) seedlings. Melatonin decreased Al concentration in maize roots and leaves under Al stress. Al stress reduced the total dry weight (DW) by 41.2% after 7 days of treatment. By contrast, the total DW was decreased by only 19.4% in MT plants. According to RNA-Seq, enzyme activity, and metabolite content data, MT plants exhibited a higher level of relatively stable carbon and nitrogen metabolism than non-treated (NT) plants. Under Al stress, MT plants showed higher photosynthetic rate and sucrose content by 29.9% and 20.5% than NT plants, respectively. Similarly, the nitrate reductase activity and protein content of MT plants were 34.0% and 15.0% higher than those of NT plants, respectively. Furthermore, exogenous supply of melatonin mitigated Al-induced oxidative stress. Overall, our results suggest that melatonin alleviates aluminum-induced growth inhibition through modulating carbon and nitrogen metabolism, and reestablishing redox homeostasis in maize. Graphical Abstarct.

Metalloprotein-Specific or Critical Amino Acid Residues: Perspectives on Plant-Precise Detoxification and Recognition Mechanisms under Cadmium Stress

Cadmium (Cd) pollution in cultivated land is caused by irresistible geological factors and human activities; intense diffusion and migration have seriously affected the safety of food crops. Plants have evolved mechanisms to control excessive influx of Cd in the environment, such as directional transport, chelation and detoxification. This is done by some specific metalloproteins, whose key amino acid motifs have been investigated by scientists one by one. The application of powerful cell biology, crystal structure science, and molecular probe targeted labeling technology has identified a series of protein families involved in the influx, transport and detoxification of the heavy metal Cd. This review summarizes them as influx proteins (NRAMP, ZIP), chelating proteins (MT, PDF), vacuolar proteins (CAX, ABCC, MTP), long-distance transport proteins (OPT, HMA) and efflux proteins (PCR, ABCG). We selected representative proteins from each family, and compared their amino acid sequence, motif structure, subcellular location, tissue specific distribution and other characteristics of differences and common points, so as to summarize the key residues of the Cd binding target. Then, we explain its special mechanism of action from the molecular structure. In conclusion, this review is expected to provide a reference for the exploration of key amino acid targets of Cd, and lay a foundation for the intelligent design and breeding of crops with high/low Cd accumulation.

Fungal Seed Endophyte FZT214 Improves Dysphania ambrosioides Cd Tolerance Throughout Different Developmental Stages

Phytoremediation is a promising remediation method of heavy metal (HM)-contaminated soils. However, lower HM tolerance of metal accumulator inhibits its practical application and effects. The current study was aimed to illustrate the role of fungal seed endophyte (FZT214) in improving Dysphania ambrosioides Cd tolerance during different developmental stages under various Cd stresses (5, 15, 30 mg kg-1) by pot experiments. The results showed that FZT214 significantly (p < 0.05) improved the host plant's growth at the flowering and fruiting stage in most of the treatment, while at the growing stage the increase was less (p > 0.05). The seed yield was also improved (p < 0.05) in the FZT214-inoculated plants (E+) and induced early flowering was observed. Moreover, the inoculation also positively affected total chlorophyll content, antioxidant process, and lipid peroxidation in most of the treatments throughout three developmental stages. Not all but in most cases, IAA and GA were more in E+ plants while JA was more in the E- plants (non-inoculated plants) during three developmental stages. The results suggested that the colonization of FZT214 to the D. ambrosioides might trigger multiple and comprehensive protective strategies against Cd stress, which mainly include activation of the dilution effects, induced biochemical changes to overcome damage from Cd toxicity, and alteration of the endogenous phytohormones. FZT214 can find competent application in the future to improve the growth of other crop plants.

Transcriptome Profiling Reveals the Effects of Nitric Oxide on the Growth and Physiological Characteristics of Watermelon under Aluminum Stress

Excessive aluminum ions (Al³⁺) in acidic soil can have a toxic effect on watermelons, restricting plant growth and reducing yield and quality. In this study, we found that exogenous application of nitric oxide (NO) could increase the photochemical efficiency of watermelon leaves under aluminum stress by promoting closure of leaf stomata, reducing malondialdehyde and superoxide anion in leaves, and increasing POD and CAT activity. These findings showed that the exogenous application of NO improved the ability of watermelon to withstand aluminum stress. To further reveal the mitigation mechanism of NO on watermelons under aluminum stress, the differences following different types of treatments—normal growth, Al, and Al + NO—were shown using de novo sequencing of transcriptomes. In total, 511 differentially expressed genes (DEGs) were identified between the Al + NO and Al treatment groups. Significantly enriched biological processes included nitrogen metabolism, phenylpropane metabolism, and photosynthesis. We selected 23 genes related to antioxidant enzymes and phenylpropane metabolism for qRT-PCR validation. The results showed that after exogenous application of NO, the expression of genes encoding POD and CAT increased, consistent with the results of the physiological indicators. The expression patterns of genes involved in phenylpropanoid metabolism were consistent with the transcriptome expression abundance. These results indicate that aluminum stress was involved in the inhibition of the photosynthetic pathway, and NO could activate the antioxidant enzyme defense system and phenylpropane metabolism to protect cells and scavenge reactive oxygen species. This study improves our current understanding by comprehensively analyzing the molecular mechanisms underlying NO-induced aluminum stress alleviation in watermelons.

A transcriptomic view of cadmium retention in roots of cadmium-safe rice line (Oryza sativa L.)

A better understanding of the mechanisms controlling cadmium (Cd) accumulation in rice will benefit the development of strategies to minimize Cd accumulation in grains. A Cd-safe rice line designated D62B accumulated less than 0.2 mg Cd kg^-1 in brown rice due to its strong capacity for Cd retention in roots. Here transcriptomic was used to clarify the underlying mechanisms of Cd response in roots of D62B compared with a high Cd-accumulating line (Wujin4B). There were 777, 1058 differentially expressed genes (DEGs) in D62B and Wujin4B, respectively, when exposed to Cd. The functions of DEGs were clearly line-specific. Cell wall biosynthesis responded more intensively to Cd stress in D62B, facilitating Cd restriction. Meanwhile, more glutathione (GSH) and phytochelatins synthesized in D62B with the upregulation of sulphur and GSH metabolism. Besides, membrane proteins played critical roles in Cd response in D62B, whereas 18 terms involved in regulation were enriched in Wujin4B. Exogenous GSH further induced the expression of genes related to GSH metabolism and cell wall biosynthesis, leading to the retention of more Cd. Great responsiveness of cell wall biosynthesis and GSH metabolism could be considered the most important specific mechanisms for Cd retention in the roots of Cd-safe rice line.

Improved physiological defense responses by application of sodium nitroprusside in Isatis cappadocica Desv. under cadmium stress

Isatis cappadocica is a well-known arsenic-hyperaccumulator, but there are no reports of its responses to cadmium (Cd). Nitric oxide (NO) is a signaling molecule, which induces cross-stress tolerance and mediates several physio-biochemical processes related to heavy metal toxicity. In this study, the effects of Cd and sodium nitroprusside (SNP as NO donor) on the growth, defense responses and Cd accumulation in I. cappadocica were investigated. When I. cappadocica was treated with 100 and 200 μM Cd, there was an insignificant inhibition of shoot growth. However, Cd stress at Cd400 treatment decreased significantly the dry weight of root and shoot by 73 and 38%, respectively, as compared to control. The application of SNP significantly improved the growth parameters and mitigated Cd toxicity. In addition, SNP decreased reactive oxygen species (ROS) production induced by Cd. The increased total thiol and glutathione (GSH) concentrations after SNP application may play a decisive role in maintaining cellular redox homeostasis, thereby protecting plants against oxidative damage under Cd stress. Bovine hemoglobin (Hb as NO scavenger) reduced the protective role of SNP, suggesting a major role of NO in the defensive effect of SNP. Furthermore, the reduction in shoot growth and the increase of oxidative damage were more severe after the addition of Hb, which confirms the protective role of NO against Cd-induced oxidative stress. The protective role of SNP in decreasing Cd-induced oxidative stress may be related to NO production, which can lead to stimulation of the thiols synthesis and improve defense system.

Methyl Jasmonate and Sodium Nitroprusside Jointly Alleviate Cadmium Toxicity in Wheat (Triticum aestivum L.) Plants by Modifying Nitrogen Metabolism, Cadmium Detoxification, and AsA-GSH Cycle

The principal intent of the investigation was to examine the influence of joint application of methyl jasmonate (MeJA, 10 μM) and a nitric oxide-donor sodium nitroprusside (SNP, 100 μM) to wheat plants grown under cadmium (Cd as CdCl2, 100 μM) stress. Cd stress suppressed plant growth, chlorophylls (Chl), and PSII maximum efficiency (F v /F m ), but it elevated leaf and root Cd, and contents of leaf proline, phytochelatins, malondialdehyde, and hydrogen peroxide, as well as the activity of lipoxygenase. MeJA and SNP applied jointly or singly improved the concentrations of key antioxidant biomolecules, e.g., reduced glutathione and ascorbic acid and the activities of the key oxidative defense system enzymes such as catalase, superoxide dismutase, dehydroascorbate reductase, glutathione S-transferase, and glutathione reductase. Exogenously applied MeJA and SNP jointly or singly also improved nitrogen metabolism by activating the activities of glutamine synthetase, glutamate synthase, and nitrate and nitrite reductases. Compared with individual application of MeJA or SNP, the combined application of both showed better effect in terms of improving plant growth and key metabolic processes and reducing tissue Cd content, suggesting a putative interactive role of both compounds in alleviating Cd toxicity in wheat plants.

MAIN FINDINGS

The main findings are that exogenous application of methyl jasmonate and nitric oxide-donor sodium nitroprusside alleviated the cadmium (Cd)-induced adverse effects on growth of wheat plants grown under Cd by modulating key physiological processes and up-regulating enzymatic antioxidants and the ascorbic acid-glutathione cycle-related enzymes.

Comparative methylome reveals regulatory roles of DNA methylation in melon resistance to Podosphaera xanthii

Powdery mildew caused by Podosphaera xanthii (P. xanthii) severely endangers melon (Cucumis melo L.) production, while the mechanistic understanding about its resistance to powdery mildew remains largely limited. In this study, we integrated transcriptomic and methylomic analyses to explore whether DNA methylation was involved in modulating transcriptional acclimation of melon to P. xanthii infection. Net photosynthetic rate (Pn), stomatal conductance (Gs), actual photochemical efficiency (ФPSII) and maximum PSII quantum yield (Fv/Fm) were significantly decreased in P. xanthii-infected plants relative to uninfected ones (Control), revealing apparent physiological disorders. Totally 4808 differentially expressed genes (DEGs) were identified by global analysis of gene expression in Control and P. xanthii-infected plants. Comparative methylome uncovered that 932 DEGs were associated with hypermethylation, while 603 DEGs were associated with hypomethylation in melon upon P. xanthii infection. Among these differential methylation-involved DEGs, a set of resistance-related genes including R genes and candidate genes in metabolic and defense pathways were further identified, demonstrating that DNA methylation might function as a new regulatory layer for melon resistance to P. xanthii infection. Altogether our study sheds new insights into the molecular mechanisms of melon against powdery mildew and provides some potential targets for improving melon disease resistance in future.

The outcomes of the functional interplay of nitric oxide and hydrogen sulfide in metal stress tolerance in plants

Phytotoxicity of metals constraints plants health, metabolism and productivity. The sustainable approaches for minimizing major metals-accrued phytotoxicity have been least explored. The gasotransmitters signaling molecules such as nitric oxide (NO) and hydrogen sulfide (H₂S) play a significant role in the mitigation of major consequences of metals stress. Versatile gaseous signaling molecules, NO and H₂S are involved in the regulation of various physiological processes in plants and their tolerance to abiotic stresses. However, literature available on NO or H₂S stand alone, and the major insights into the roles of NO and/or H₂S in plant tolerance, particularly to metals, remained unclear. Given above, this paper aimed to (a) briefly overview metals and highlight their major phytotoxicity; (b) appraises literature reporting potential mechanisms underlying the roles of NO and H₂S in plant-metal tolerance; (c) crosstalk on NO and H₂S in relation to plant metal tolerance. Additionally, major aspects so far unexplored in the current context have also been mentioned.

Phytoremediation of Cadmium: Physiological, Biochemical, and Molecular Mechanisms

Cadmium (Cd) is one of the most toxic metals in the environment, and has noxious effects on plant growth and production. Cd-accumulating plants showed reduced growth and productivity. Therefore, remediation of this non-essential and toxic pollutant is a prerequisite. Plant-based phytoremediation methodology is considered as one a secure, environmentally friendly, and cost-effective approach for toxic metal remediation. Phytoremediating plants transport and accumulate Cd inside their roots, shoots, leaves, and vacuoles. Phytoremediation of Cd-contaminated sites through hyperaccumulator plants proves a ground-breaking and profitable choice to combat the contaminants. Moreover, the efficiency of Cd phytoremediation and Cd bioavailability can be improved by using plant growth-promoting bacteria (PGPB). Emerging modern molecular technologies have augmented our insight into the metabolic processes involved in Cd tolerance in regular cultivated crops and hyperaccumulator plants. Plants’ development via genetic engineering tools, like enhanced metal uptake, metal transport, Cd accumulation, and the overall Cd tolerance, unlocks new directions for phytoremediation. In this review, we outline the physiological, biochemical, and molecular mechanisms involved in Cd phytoremediation. Further, a focus on the potential of omics and genetic engineering strategies has been documented for the efficient remediation of a Cd-contaminated environment.

The mechanism of chelator improved the tolerance and accumulation of poplar to Cd explored through differential expression protein based on iTRAQ

Appropriate chelator may increase plant tolerance and accumulation for Cd in soil, but its molecular mechanism is unclear. In this experiment, the technology of isobaric tags for relative and absolute quantitation (iTRAQ) was used to compare the differential expression proteins (DEPs) and differential expression genes (DEGs) characteristics of poplar accumulating Cd combined with EDTA and/or EGTA. The results showed that the Cd concentrations, biomasses and activities of antioxidant enzymes of poplar were significantly increased in the treatments of chelator addition compared to the T_Cd. The number of co-intersecting specific proteins of T_Cd/CK, T_Cd+EDTA/T_Cd, T_Cd+EGTA/T_Cd and T_Cd+EDTA+EGTA/T_Cd was 49. Using the GO function and KEGG analysis, it was found that EDTA and EGTA might improve some main metabolic pathways of poplar leaves, which were involved in the enhancement of the expression of carbohydrate and energy metabolism-related proteins, regulation of cell energy metabolism, complementing and cooperating with each other in various ways, and dynamic regulation of energy metabolism. Particularly, chelator might induce the regulation of protein synthesis, folding and transport, and degradation of abnormal proteins in response to Cd toxicity. These results provided a theoretical basis for further elucidation of molecular mechanisms of poplar response to Cd stress.

Nitric Oxide Pre-Treatment Advances Seed Germination and Alleviates Copper-Induced Photosynthetic Inhibition in Indian Mustard

This investigation tested the efficiency of nitric oxide (NO) in alleviation of Cu-induced adverse impacts on seed germination and photosynthesis in Indian mustard (Brassica juncea L.). Pre-treatment of B. juncea seeds with sodium nitroprusside (SNP; NO donor) significantly improved the seed germination rate and also alleviated Cu-accrued oxidative stress. However, in the absence of NO, Cu caused a higher reduction in seed germination rate. The presence of NO strengthened the antioxidant defense system (glutathione reductase, ascorbate peroxidase, and superoxide dismutase) and thereby sustained the lower lipid peroxidation, reduced H2O2 content, and thiobarbituric acid reactive substances in Cu-exposed seeds. NO pre-treated seeds also retained a higher amylase activity and exhibited an improved seed germination rate. This effect of NO under Cu stress was also seen in plants originated from the NO pre-treated seeds, where the role of NO pre-treatment was reflected in the improved photosynthetic potential of B. juncea. Overall, NO pre-treatment not only improved the germination rate in seeds but also carried its effects in the grown seedlings evidenced as improved photosynthesis and growth. Potential mechanisms involved in the action of NO pre-treatment included NO-mediated significant strengthening of the antioxidant defense system and decreases in Cu-caused oxidative stress parameters.

The role of nitrate reductase in brassinosteroid-induced endogenous nitric oxide generation to improve cadmium stress tolerance of pepper plants by upregulating the ascorbate-glutathione cycle

A study was performed to assess if nitrate reductase (NR) participated in brassinosteroid (BR)-induced cadmium (Cd) stress tolerance primarily by accelerating the ascorbate-glutathione (AsA-GSH) cycle. Prior to initiating Cd stress (CdS), the pepper plants were sprayed with 0.5 μM 24-epibrassinolide (EBR) every other day for 10 days. Thereafter the seedlings were subjected to control or CdS (0.1 mM CdCl₂) for four weeks. Cadmium stress decreased the plant growth related attributes, water relations as well as the activities of monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), but enhanced proline content, leaf Cd²⁺ content, oxidative stress-related traits, activities of ascorbate peroxidase (APX) and glutathione reductase (GR), and the activities of antioxidant defence system-related enzymes as well as NR activity and endogenous nitric oxide content. EBR reduced leaf Cd²⁺ content and oxidative stress-related parameters, enhanced plant growth, regulated water relations, and led to further increases in proline content, AsA-GSH cycle-related enzymes' activities, antioxidant defence system-related enzymes as well as NR activity and endogenous nitric oxide content. The EBR and the inhibitor of NR (tungstate) reversed the positive effects of EBR by reducing NO content, showing that NR could be a potential contributor of EBR-induced generation of NO which plays an effective role in tolerance to CdS in pepper plants by accelerating the AsA-GSH cycle and antioxidant enzymes.

Transcription Factor GmWRKY142 Confers Cadmium Resistance by Up-Regulating the Cadmium Tolerance 1-Like Genes

Cadmium (Cd) is a widespread pollutant that is toxic to living organisms. Previous studies have identified certain WRKY transcription factors, which confer Cd tolerance in different plant species. In the present study, we have identified 29 Cd-responsive WRKY genes in Soybean [Glycine max (L.) Merr.], and confirmed that 26 of those GmWRKY genes were up-regulated, while 3 were down-regulated. We have also cloned the novel, positively regulated GmWRKY142 gene from soybean and investigated its regulatory mechanism in Cd tolerance. GmWRKY142 was highly expressed in the root, drastically up-regulated by Cd, localized in the nucleus, and displayed transcriptional activity. The overexpression of GmWRKY142 in Arabidopsis thaliana and soybean hairy roots significantly enhanced Cd tolerance and lead to extensive transcriptional reprogramming of stress-responsive genes. ATCDT1, GmCDT1-1, and GmCDT1-2 encoding cadmium tolerance 1 were induced in overexpression lines. Further analysis showed that GmWRKY142 activated the transcription of ATCDT1, GmCDT1-1, and GmCDT1-2 by directly binding to the W-box element in their promoters. In addition, the functions of GmCDT1-1 and GmCDT1-2, responsible for decreasing Cd uptake, were validated by heterologous expression in A. thaliana. Our combined results have determined GmWRKYs to be newly discovered participants in response to Cd stress, and have confirmed that GmWRKY142 directly targets ATCDT1, GmCDT1-1, and GmCDT1-2 to decrease Cd uptake and positively regulate Cd tolerance. The GmWRKY142-GmCDT1-1/2 cascade module provides a potential strategy to lower Cd accumulation in soybean.

Microbial inoculant and garbage enzyme reduced cadmium (Cd) uptake in Salvia miltiorrhiza (Bge.) under Cd stress

The uptake and accumulation of cadmium (Cd) in Salvia miltiorrhiza (Bge.) negatively affects the quality of its harvested roots, and seriously threatens human health. This study investigates the effect of a microbial inoculant (MI) and garbage enzyme (GE) on Cd uptake, the accumulation of bioactive compounds, and the community composition of microbes in the rhizosphere soil of S. miltiorrhiza under Cd stress. S. miltiorrhiza seedlings were transplanted to Cd-contaminated pots and irrigated with an MI, GE, a combination of an MI and GE (MIGE) or water (control). The results indicated that treatments with an MI, GE or MIGE can reduce Cd uptake in S. miltiorrhiza. The MIGE treatment had greater efficiency in reducing Cd uptake than the control (reduction by 37.90%), followed by the GE (25.31%) and MI (5.84%) treatments. Treatments with an MI, GE and MIGE had no significant impact on fresh and dry root biomass. Relative to the control, the MI treatment had the highest efficiency in increasing the accumulation of total tanshinones (an increase of 40.45%), followed by the GE treatment (40.08%), with the MIGE treatment (9.90%) treatment not having a more favorable effect than the separate application of an MI or GE. The salvianolic acid content for all groups was higher than the standard prescribed by Chinese pharmacopoeia, notwithstanding a slightly lower level in the treated groups relative to the control. In addition, metagenomic analysis indicated changes in the relative abundance of soil microbes associated with the bioremediation of heavy metals. The relative abundances of Brevundimonas, Microbacterium, Cupriavidus and Aspergillus were significantly greater in the treated groups than in the Control. These results suggest that using MI and GE, either separately or together, may not only improve the quality of S. miltiorrhiza but may also facilitate the microbial remediation of soil contaminated with Cd.

Genotypic variation for cadmium tolerance in common bean (Phaseolus vulgaris L.)

Given the limitation of crop production in Cd-polluted areas, the identification and selection of plant genotypes tolerant to Cd stress are of great significance. In the present work, we show the existence of genotypic variation for Cd tolerance in common bean. The laboratory screening of 25 bean genotypes indicated a significant positive correlation of the mean productivity (MP) and the geometric mean productivity (GMP) with plant fresh weight both in control and Cd-treated plants. A principal component analysis further confirmed this variation and, together with other analyses, led to the selection of genotypes G-11867, Taylor, Emerson, and D-81083 as tolerant genotypes. A total of six bean genotypes with different degrees of Cd tolerance were selected, and their long-term physiological responses to Cd (0, 45, and 90 mg/kg soil) were evaluated. Increasing Cd concentrations led to higher Cd accumulation both in roots and shoots, and to significant rises in the levels of the oxidative stress biomarkers malondialdehyde (MDA), dityrosine (D-T), and 8-hydroxy-2'-deoxyguanosine (8-OH-2'-dG). Remarkable reductions in plant hormone levels and chlorophyll contents, as well as in dry and fresh weight, were observed in Cd-treated plants. Among the examined genotypes, Emerson, Taylor, and G-11867 were found to be more tolerant to Cd owing to lower Cd accumulation and lower oxidative stress levels, as well as higher chlorophyll and hormone contents. Our results contribute to the understanding of the physiological and biochemical basis of Cd tolerance in bean plants and may therefore, be useful for breeding programs directed towards obtaining bean varieties showing low Cd accumulation.

Nitric oxide-mediated regulation of oxidative stress in plants under metal stress: a review on molecular and biochemical aspects

Given their sessile nature, plants continuously face unfavorable conditions throughout their life cycle, including water scarcity, extreme temperatures and soil pollution. Among all, metal(loid)s are one of the main classes of contaminants worldwide, posing a serious threat to plant growth and development. When in excess, metals which include both essential and non-essential elements, quickly become phytotoxic, inducing the occurrence of oxidative stress. In this way, in order to ensure food production and safety, attempts to enhance plant tolerance to metal(loid)s are urgently needed. Nitric oxide (NO) is recognized as a signaling molecule, highly involved in multiple physiological events, like the response of plants to abiotic stress. Thus, substantial efforts have been made to assess NO potential in alleviating metal-induced oxidative stress in plants. In this review, an updated overview of NO-mediated protection against metal toxicity is provided. After carefully reviewing NO biosynthetic pathways, focus was given to the interaction between NO and the redox homeostasis followed by photosynthetic performance of plants under metal excess.

Responses of nitric oxide and hydrogen sulfide in regulating oxidative defence system in wheat plants grown under cadmium stress

We conducted a study to evaluate the interactive effect of NO and H₂ S on the cadmium (Cd) tolerance of wheat. Cadmium stress considerably reduced total dry weight, chlorophyll a and b content and ratio of Fv/Fm by 36.7, 48.6, 26.7 and 19.5%, respectively, but significantly enhanced the levels of hydrogen peroxide (H₂ O₂ ) and malondialdehyde (MDA), endogenous H₂ S and NO, and the activities of antioxidant enzymes. Exogenously applied sodium nitroprusside (SNP) and sodium hydrosulfide (NaHS), donors of NO and H₂ S, respectively, enhanced total plant dry matter by 47.8 and 39.1%, chlorophyll a by 92.3 and 61.5%, chlorophyll b content by 29.1 and 27.2%, Fv/Fm ratio by 19.7 and 15.2%, respectively, and the activities of antioxidant enzymes, but lowered oxidative stress and proline content in Cd-stressed wheat plants. NaHS and SNP also considerably limited both the uptake and translocation of Cd, thereby improving the levels of some key mineral nutrients in the plants. Enhanced levels of NO and H₂ S induced by NaHS were reversed by hypotuarine application, but they were substantially reduced almost to 50% by cPTIO (a NO scavenger) application. Hypotuarine was not effective, but cPTIO was highly effective in reducing the levels of NO and H₂ S produced by SNP in the roots of Cd-stressed plants. The results showed that interactive effect of NO and H₂ S can considerably improve plant resistance against Cd toxicity by reducing oxidative stress and uptake of Cd in plants as well as by enhancing antioxidative defence system and uptake of some essential mineral nutrients.

Salicylic Acid Signals Plant Defence against Cadmium Toxicity

Salicylic acid (SA), as an enigmatic signalling molecule in plants, has been intensively studied to elucidate its role in defence against biotic and abiotic stresses. This review focuses on recent research on the role of the SA signalling pathway in regulating cadmium (Cd) tolerance in plants under various SA exposure methods, including pre-soaking, hydroponic exposure, and spraying. Pretreatment with appropriate levels of SA showed a mitigating effect on Cd damage, whereas an excessive dose of exogenous SA aggravated the toxic effects of Cd. SA signalling mechanisms are mainly associated with modification of reactive oxygen species (ROS) levels in plant tissues. Then, ROS, as second messengers, regulate a series of physiological and genetic adaptive responses, including remodelling cell wall construction, balancing the uptake of Cd and other ions, refining the antioxidant defence system, and regulating photosynthesis, glutathione synthesis and senescence. These findings together elucidate the expanding role of SA in phytotoxicology.

Foliar application of salicylic acid alleviate the cadmium toxicity by modulation the reactive oxygen species in potato

Heavy metal toxicity is one of the main factors that limit crop growth and yield in the world. Salicylic acid (SA) is thought to be a plant hormone that plays an important role in plant growth, development, and resistance to abiotic stresses. To uncover the toxic alleviation effects of SA on potato plants to cadmium (Cd) stress, the morphological, physiological, and biochemical indexes including antioxidant defense system were assayed in potato plants under 200 μM Cd stress in 1/2 Hoagland solution with foliar application of 600 μM SA concentration (10 ml/plant). Interestingly, exogenous SA treatment mitigated Cd toxicity by increasing the relative water content (RWC), chlorophyll, proline, and endogenous SA contents along with decline in malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and superoxide anion radicals (O₂^-). Correspondingly, our study also proved that SA may stimulate the antioxidant enzymatic mechanism pathway including superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and glutathione reductase (GR, EC 1.6.4.2) in potato plants subjected to Cd stress. Moreover, the expression level of selected genes relate to SA and reactive oxygen species (ROS) metabolism (StSABP2, StSOD and StAPX) were enhanced in SA-treated potato plants under Cd stress, indicating that SA treatment regulated the expression of these genes, which in turn enhanced potato tolerance to Cd stress. Taken together, our results indicated that exogenous SA can play a positive regulatory role in alleviating Cd toxicity in potato plants.

Responses of duckweed (Lemna minor L.) to aluminum stress: Physiological and proteomics analyses

Aluminum (Al) is commonly considered an abiotic stress factor under acidic conditions. Duckweed (Lemna minor L.) has wide application in ecotoxicological research as a model organism and, in this study, its response to Al bioaccumulation was evaluated at morphological, physiological and proteomic levels. The Al accumulation in L. minor was accompanied by chlorosis and growth inhibition. Overproduction of superoxide and hydrogen peroxide, and decreased chlorophyll and protein contents, suggested that Al exposure induced oxidative stress. Inhibition of photosynthesis was evident in a significant decrease in maximum photosystem II quantum yield. There were 261 proteins, with significant changes in expression, successfully identified and quantified through isobaric tags for relative and absolute quantification (iTRAQ) analysis. Among the KEGG pathway enrichment proteins, those related to the citrate cycle and amino acid metabolism were predominantly up-regulated, whereas those associated with energy metabolism and glyoxylate and dicarboxylate metabolism were predominantly down-regulated. In addition, antioxidant enzyme related proteins played an important role in the response of L. minor to Al. The western blot analysis further validated the changes in photosynthetic related proteins. These results provide comprehensive insights into the physiological and molecular mechanisms of Al toxicity and tolerance in L. minor.

Transcriptome Analysis Reveals Cotton (Gossypium hirsutum) Genes That Are Differentially Expressed in Cadmium Stress Tolerance

High concentrations of heavy metals in the soil should be removed for environmental safety. Cadmium (Cd) is a heavy metal that pollutes the soil when its concentration exceeds 3.4 mg/kg. Although the potential use of cotton to remediate heavy Cd-polluted soils is known, little is understood about the molecular mechanisms of Cd tolerance. In this study, transcriptome analysis was used to identify Cd tolerance genes and their potential mechanisms in cotton. We exposed cotton plants to excess Cd and identified 4627 differentially expressed genes (DEGs) in the root, 3022 DEGs in the stem and 3854 DEGs in the leaves through RNA-Seq analysis. Among these genes were heavy metal transporter coding genes (ABC, CDF, HMA, etc.), annexin genes and heat shock genes (HSP), amongst others. Gene ontology (GO) analysis showed that the DEGs were mainly involved in the oxidation–reduction process and metal ion binding. The DEGs were mainly enriched in two pathways, the influenza A and pyruvate pathway. GhHMAD5, a protein containing a heavy-metal binding domain, was identified in the pathway to transport or to detoxify heavy metal ions. We constructed a GhHMAD5 overexpression system in Arabidopsis thaliana that showed longer roots compared to control plants. GhHMAD5-silenced cotton plants showed more sensitivity to Cd stress. The results indicate that GhHMAD5 is involved in Cd tolerance, which gives a preliminary understanding of the Cd tolerance mechanism in upland cotton. Overall, this study provides valuable information for the use of cotton to remediate soils polluted with Cd and potentially other heavy metals.

Quantitative proteomic analysis reveals novel stress-associated active proteins (SAAPs) and pathways involved in modulating tolerance of wheat under terminal heat

Terminal heat stress has detrimental effect on the growth and yield of wheat. Very limited information is available on heat stress-associated active proteins (SAAPs) in wheat. Here, we have identified 159 protein groups with 4271 SAAPs in control (22 ± 3 °C) and HS-treated (38 °C, 2 h) wheat cvs. HD2985 and HD2329 using iTRAQ. We identified 3600 proteins to be upregulated and 5825 proteins to be downregulated in both the wheat cvs. under HS. We observed 60.3% of the common SAAPs showing upregulation in HD2985 (thermotolerant) and downregulation in HD2329 (thermosusceptible) under HS. GO analysis showed proton transport (molecular), photosynthesis (biological), and ATP binding (cellular) to be most altered under HS. Most of the SAAPs identified were observed to be chloroplast localized and involved in photosynthesis. Carboxylase enzyme was observed most abundant active enzymes in wheat under HS. An increase in the degradative isoenzymes (α/β-amylases) was observed, as compared to biosynthesis enzymes (ADP-glucophosphorylase, soluble starch synthase, etc.) under HS. Transcript profiling showed very high relative fold expression of HSP17, CDPK, Cu/Zn SOD, whereas downregulation of AGPase, SSS under HS. The identified SAAPs can be used for targeted protein-based precision wheat-breeding program for the development of 'climate-smart' wheat.