Differential cadmium stress tolerance in five indian mustard (Brassica juncea L.) cultivars: an evaluation of the role of antioxidant machinery

Screening of chili cultivars with low cadmium accumulation and analysis of their physiological properties of tolerance

Chili (Capsicum annuum L.) is a widely eaten condiment that may accumulate the toxic element cadmium (Cd) from environments, thus presenting a human health risk. This experiment aimed to identify cultivars with low Cd-uptake characteristics that could be used to produce safe spicy food in Cd-contaminated soil. Five chili cultivars responses to Cd exposure in soils with 0.18 mg kg-1 (CK), 2.88 mg kg-1 (T1), 7.69 mg kg-1 (T2), 16.72 mg kg-1 (T3), and 33.46 mg kg-1 (T4) in a greenhouse were compared. The results showed that Cd concentration in roots, shoots, and fruits of the cultivar Bolafengxiang was the lowest. Additionally, its biomass was not reduced compared to the CK, and both the enrichment factor (EF) and translocation factor (TF) were all lower than 1. Notably, under soil Cd concentrations of 2.88 mg kg-1, the Cd content in the fruits of Bolafengxiang was 0.07 mg kg-1, which is below the safety standard limit (0.1 mg kg-1) for "Green Food Chili Products." This indicates its potential for low Cd accumulation. The above research indicates that selecting and cultivating low-Cd-accumulating chili cultivars is an effective approach to reduce Cd accumulation in edible parts, thereby ensuring agricultural food safety.

The role of nitric oxide and nitrogen in mediating copper stress in Brassica juncea L

Copper (Cu) holds a significant importance in plant metabolism as it serves as an essential micronutrient but becomes toxic at higher concentrations. Nitric oxide (NO), a key signaling molecule, and nitrogen (N) play essential roles in combating toxicity of some metals. This study explores the potential of interactive effects of NO as 100 µM SNP (sodium nitroprusside, NO source) and N (80 mg N kg-1 soil) in mitigating Cu (100 mg Cu kg-1 soil) stress in mustard (Brassica juncea L.) plants. The impaired physio-biochemical changes, photosynthetic efficiency, and the expression level of genes associated with photosynthesis, and N assimilation under Cu stress were ameliorated with the exogenous application of NO and N. The combined treatment of NO and N conspicuously lowered reactive oxygen species (ROS) and its related impacts. It also enhanced the activity and relative expression of antioxidant enzymes, including ascorbate peroxidase (APX), glutathione reductase (GR), and superoxide dismutase (SOD) as well as N assimilation enzymes, such as nitrate reductase (NR) and nitrite reductase (NiR). The supplementation of NO and N also triggered the expression of rbcL (large subunit of Rubisco), photosystem (photosystem II D1 protein; psbA and photosystem II protein B; psbB) and markedly improved photosynthetic capacity under Cu stress. The study highlights the significance of NO and N as a potential strategy to counteract Cu-induced stress in crops. It suggests a synergistic or interactive effect between the two substances as a phytoremediation strategy for enhancing crop growth and productivity in Cu-contaminated soils. Understanding the mechanisms behind NO and N mediated stress alleviation could facilitate the development of targeted approaches to enhance plant resilience against heavy metal stress.

Unveiling the variability in cadmium accumulation and tolerance characteristics: a comparative study of Basma and Yunyan 87 tobacco varieties

Tobacco (Nicotiana tabacum L.) shows promise for remediating Cd-contaminated soil due to its significant Cd accumulation capabilities. Although various tobacco varieties exhibit distinct Cd bioaccumulation capacities, a comprehensive understanding of the underlying mechanisms is lacking. This study, conducted using hydroponics, explores differences in Cd accumulation and tolerance mechanisms between two tobacco varieties, Basma and Yunyan 87. The results showed that Cd stress reduced the dry weight, tolerance index, and root morphology for both varieties. Basma exhibited a relatively smaller decline in these indices compared to Yunyan 87. Moreover, Basma demonstrated a higher Cd bioconcentration factor (BCF), concentration, and accumulated content, signifying its superior tolerance and bioaccumulation capacity to Cd compared to Yunyan 87. The Carbonyl Cyanide3-ChloroPhenylhydrazone (CCCP) addition resulted in reduced Cd accumulation and BCFs in both tobacco species. This effect was more pronounced in Basma, suggesting that Basma relies more on an active transport process than Yunyan 87. This could potentially explain its enhanced bioaccumulation ability. Subcellular Cd distribution analysis revealed Basma's preference for distributing Cd in soluble fractions, while Yunyan 87 favoured the cell wall fractions. Transmission electron microscope showed that Basma's organelles were less damaged than Yunyan 87's under Cd stress, possibly contributing to the superior tolerance of Basma. Therefore, these results provided a theoretical foundation for development of Cd-contaminated soil tobacco remediation technology.

Exogenously applied nano-zinc oxide mitigates cadmium stress in Zea mays L. through modulation of physiochemical activities and nutrients homeostasis

The increasing levels of cadmium (Cd) pollution in agricultural soil reduces plant growth and yield. This study aims to determine the impact of green synthesized zinc oxide nanoparticles (ZnO-NPs) on the physiochemical activities, nutrition, growth, and yield of Zea mays L. under Cd stress conditions. For this purpose, ZnO-NPs (450 ppm and 600 ppm) synthesized from Syzygium aromaticum were applied through foliar spray to Z. mays and also used as seed priming agents. A significant decline in plant height (35.24%), biomass production (43.86%), mineral content, gas exchange attributes, and yield (37.62%) was observed in Cd-spiked plants compared to the control. While, 450 ppm ZnO-NPs primed seed increased plant height (18.46%), total chlorophyll (80.07%), improved ascorbic acid (25.10%), DPPH activity (26.66%), and soil mineral uptake (Mg+2 (38.86%), K+ (27.83%), and Zn+2 (43.68%) as compared to plants only spiked with Cd. On the contrary, the foliar-applied 450 ppm ZnO-NPs increased plant height (8.22%), total chlorophyll content (73.59%), ascorbic acid (21.39%), and DPPH activity (17.61%) and yield parameters; cob diameter (19.45%), and kernels numbers 6.35% enhanced compared to plants that were spiked only with Cd. The findings of the current study pave the way for safer and more cost-effective crop production in Cd-stressed soils by using green synthesized NPs and provide deep insights into the underlying mechanisms of NPs treatment at the molecular level to provide compelling evidence for the use of NPs in improving plant growth and yield.

Enhancing maize growth and resilience to environmental stress with biochar, gibberellic acid and rhizobacteria

Background

Zea mays (maize) is a globally significant cereal crop with diverse applications in food, feed, and industrial products. However, maize cultivation is often challenged by environmental stressors such as heavy metal toxicity and drought stress (DS). Heavy metals like cadmium (Cd) and lead (Pb) can accumulate in soil through industrial activities and improper waste disposal, posing significant threats to plant growth and development. Drought stress further exacerbates these challenges by limiting water availability and affecting physiological processes in plants. This study explores the impact of Cd and Pb toxicity, as well as DS, on maize growth and development, and investigates the potential mitigating effects of various treatments, including gibberellic acid (GA3), biochar (BC), rhizobacteria (RB), and their combinations.

Methods

The experiment involved maize plants subjected to different stress conditions: cadmium (Cd) at concentrations of 0, 6, and 12 ppm, lead (Pb) at 0 and 400 ppm, and drought stress (DS). Treatments included the application of 10 ppm GA3, 0.75% BC, a combined treatment of 10 ppm GA3 and 0.75% BC, rhizobacteria (RB), and a combined treatment of 0.5% BC and RB. The study measured germination rates, shoot and root lengths, and biochemical parameters such as shoot and root protein, phenolics, and chlorophyll contents under these conditions.

Results

In the absence of Cd stress (0 Cd), the application of 10 ppm GA3 and 0.75% BC significantly enhanced germination rates by 72% and 76%, respectively, compared to the control, with the combined treatment exhibiting the highest enhancement of 86%. Under Cd stress (6 ppm Cd), GA3 and BC individually improved germination by 54% and 57%, respectively, with the combined treatment showing the largest increase of 63%. Drought stress influenced germination, with notable improvements observed with the application of 0.5% BC (50% increase) and RB (49% increase). Similar trends were observed in shoot and root lengths, where the combined treatment of GA3 and BC resulted in the most significant improvements. The treatments positively influenced shoot and root protein, phenolics, and chlorophyll contents, particularly under stress conditions.

Conclusion

These findings highlight the potential of combined treatments, such as the application of GA3 and BC or BC with RB, in alleviating the adverse effects of heavy metals (Cd and Pb) and drought stress in maize cultivation. The combined treatments not only improved germination rates but also significantly enhanced shoot and root growth, as well as important biochemical parameters under stress conditions. This suggests that GA3 and BC, alone or in combination with RB, can play a crucial role in enhancing maize resilience to environmental stressors. The study highlights the importance of exploring sustainable agricultural practices to mitigate the impacts of heavy metal toxicity and drought stress. Future research should focus on long-term field trials to validate these findings and further investigate the mechanistic pathways involved in stress mitigation by these amendments, as well as their economic feasibility and environmental impact on a larger scale to ensure their practical applicability in real-world agricultural settings.

Variety-dependent responses of common tobacco with differential cadmium resistance: Cadmium uptake and distribution, antioxidative activity, and gene expression

Cadmium (Cd), which accumulates in tobacco leaves, enters the human body through inhalation of smoke, causing harmful effects on health. Therefore, identifying the pivotal factors that govern the absorption and resistance of Cd in tobacco is crucial for mitigating the harmful impact of Cd. In the present study, four different Cd-sensitive varieties, namely, ZhongChuan208 (ZC) with resistance, ZhongYan100 (ZY), K326 with moderate resistance, and YunYan87 (YY) with sensitivity, were cultivated in hydroponic with different Cd concentrations (20 µM, 40 µM, 60 µM and 80 µM). The results indicated that plant growth was significantly decreased by Cd. Irrespective of the Cd concentration, ZC exhibited the highest biomass, while YY had the lowest biomass; ZY and K326 showed intermediate levels. Enzymatic (APX, CAT, POD) and nonenzymatic antioxidant (Pro, GSH) systems showed notable variations among varieties. The multifactor analysis suggested that the ZC and ZY varieties, with higher levels of Pro and GSH content, contribute to a decrease in the levels of MDA and ROS. Among all the Cd concentrations, ZC exhibited the lowest Cd accumulation, while YY showed the highest. Additionally, there were significant differences observed in Cd distribution and translocation factors among the four different varieties. In terms of Cd distribution, cell wall Cd accounted for the highest proportion of total Cd, and organelles had the lowest proportion. Among the varieties, ZC showed lower Cd levels in the cell wall, soluble fraction, and organelles. Conversely, YY exhibited the highest Cd accumulation in all tissues; K326 and ZY had intermediate levels. Translocation factors (TF) varied among the varieties under Cd stress, with ZC and ZY showing lower TF compared to YY and K326. This phenomenon mainly attributed to regulation of the NtNramp3 and NtNramp5 genes, which are responsible for the absorption and transport of Cd. This study provides a theoretical foundation for the selection and breeding of tobacco varieties that are resistant to or accumulate less Cd.

Physiological and biochemical characteristics of high and low Cd accumulating Brassica napus genotypes

Phytoremediation is a widely used and cost-effective technique for in situ remediation of heavy metals. Brassica napus L. genotype with high Cd accumulation and strong Cd tolerance is an ideal candidate for phytoremediation. In this study, a hydroponic experiment was conducted to select a Brassica napus genotype with either high or low Cd accumulation from a panel of 55 genotypes. The physiological mechanisms governing Cd accumulation and Cd tolerance were then explored. BN400 and BN147 were identified as the high and low Cd accumulating genotypes, respectively. Additionally, BN400 exhibited greater tolerance to Cd stress compared to BN147. Root morphology analysis revealed that BN400 exhibited longer root length, smaller root surface area and root volume, and less root tips but bigger root diameter than BN147. Subcellular Cd distribution showed that the Cd concentrations in the cell wall and vacuole in shoot were significantly higher in BN400 than in BN147, whereas the opposite trend was observed in the roots.. Pectate/protein-integrated Cd was found to be the predominant form of Cd in both shoots and roots, with significantly higher levels in BN400 compared to BN147 in the shoot, but the opposite trend was observed in the roots. These results suggest that the long fine roots play a role in Cd accumulation. The high Cd accumulating genotype was able to retain Cd in leaf cell walls and vacuoles, and Cd was mainly present in the form of pectate/protein-integrated Cd, which contributes to its strong Cd tolerance. These findings have important implications for the screening and breeding of Brassica napus genotypes with high Cd accumulation for phytoremediation purposes.

Mechanisms of combined bioremediation by phosphate-solubilizing fungus and plants and its effects on cadmium contamination in phosphate-mining wastelands

Owing to uncontrolled mining activities and lack of ecological protection measures, phosphate-mining wastelands are contaminated with the heavy metal Cd. In this study, Penicillium oxalicum strain ZP6, a Cd-resistant phosphate-solubilizing fungus, was used in combination with the fast-growing, high-biomass plant Brassica juncea L. to enhance Cd remediation in phosphate-mining wastelands. Further, the bioremediation mechanisms were explored and elucidated. In pot experiments, strain ZP6 and Brassica juncea L. alone were significantly effective in removing Cd from phosphate-mining wastelands; however, their combination was more effective, exhibiting a high removal rate of 88.75%. The presence of phosphorite powder increases soil-enzyme activity, promotes plant growth, and reduces the bioaccumulation and translocation factors. However, Cd-inhibited plant growth and chlorophyll content increased malondialdehyde accumulation, which was alleviated by inoculation with strain ZP6. The results from the study indicate that bioremediation using a combination of strain ZP6 and plants is a restoration strategy with appreciable potential to resolve Cd contamination in phosphate-mining wastelands.

Nitrate-induced AHb1 expression aggravates Cd toxicity in plants

Cadmium (Cd) causes severe toxicity in plants. However, the molecular mechanisms underlying plant resistance to Cd in relation to nitrogen (N) supply remain unclear. The non-symbiotic hemoglobin gene Hb1 plays an important role in scavenging nitric oxide (NO) in plants. In this study, there was no differential effect of Cd on the biomass of wild-type (WT) and AHb1-overexpressing (H7) plants when NH4+-N was used as a nitrogen source. However, under NO3--N conditions, Cd exerted less biomass stress on AHb1-silenced (L3) plants and more stress on H7 plants than on WT plants. The Cd tolerance index followed the order: L3 > WT > H7. However, there was no difference in Cd concentrations in the roots or shoots of the WT, L3, and H7 plants, indicating that differences in AHb1 expression were unrelated to Cd uptake. Further investigation showed that Cd exposure enhanced H2O2 accumulation and aggravated oxidative damage in H7 plants. The application of an NO donor effectively reversed growth inhibition, H2O2 burst, and oxidative stress induced by Cd in H7 plants. Thus, we suggest that NO3--induced AHb1 expression suppresses Cd-induced NO production in plants, increasing the ROS burst and exacerbating Cd toxicity.

Interactive effect of silicon and zinc on cadmium toxicity alleviation in wheat plants

Silicon (Si) and Zinc (Zn) have been frequently used to alleviate cadmium (Cd) toxicity, which are feasible strategies for crop safety production. However, the mechanisms underlying the interaction of Si and Zn on alleviating Cd toxicity are not well understood. A hydroponic system was adopted to evaluate morphological, physiological-biochemical responses, and related gene expression of wheat seedlings to Si (1 mM) and Zn (50 µM) addition under Cd stress (10 µM). Cd induced obvious inhibition of wheat growth by disturbing photosynthesis and chlorophyll synthesis, provoking generation of reactive oxygen species (ROS) and interfering ion homeostasis. Cd concentration was decreased by 68.3%, 43.1% and 73.3% in shoot, and 78.9%, 44.1% and 85.8% in root by Si, Zn, and combination of Si with Zn, relative to Cd only, respectively. Si and Zn effectively ameliorated Cd toxicity and enhanced wheat growth; but single Si or combination of Si with Zn had more efficient ability on alleviating Cd stress than only Zn, indicating Si and Zn have synergistic effect on Cd toxicity; Interaction of them alleviated oxidative stress by reducing ROS content, improving AsA-GSH cycle and antioxidant enzymes activities, and regulating Cd into vacuole through PC-Cd complexes transported by HMA3 transporter. Our results suggest that fertilizers including Si and Zn should be made to reduce Cd content, which will beneficial for food production and safety.

The Malus domestica metal tolerance protein MdMTP11.1 was involved in the detoxification of excess manganese in Arabidopsis thaliana

Ion homeostasis is maintained in plant cells by specialized transporters. However, functional studies on Mn transporters in apple trees have not been reported. MdMTP11.1, which encodes a putative Mn-MTP transporter in Malus domestica, was expressed highly in leaves and induced by Mn stress. Subcellular localization analysis of the MdMTP11.1-GFP fusion protein indicated that MdMTP11.1 was targeted to the Golgi. Meanwhile, overexpression of MdMTP11.1 in Arabidopsis thaliana conferred increased resistance to plants under toxic Mn levels, as evidenced by increased biomass of whole plant and length of primary root. Analysis of Mn bioaccumulation indicated that overexpression of MdMTP11.1 effectively reduced the content of Mn in every subcellular component and chemical forms when the plants were subjected with Mn stress. The majority of Mn of action were bound to cell wall and combined with un-dissolved phosphate. Besides, contents of malondialdehyde (MDA), proline and hydrogen peroxide (H2O2) were significantly lower, while content of chlorophyll and activities of CAT, SOD, POD and APX were significantly higher in MdMTP11.1-over-expressing plants compared with that in wild type plants under Mn stress. Taken together, these results suggest that MdMTP11.1 is a Mn specific transporter localized to the Golgi can maintain the phenotype, reduce the Mn accumulation and alleviate damage of oxidative stress, conferring the positive role of Mn tolerance.

Different responses to joint exposure to cadmium and zinc depends on the sex in Populus cathayana

The alarming increase in soil contamination by heavy metals, such as cadmium and zinc demands immediate attention. The dioecious tree Populus cathayana, a phytoremediation plant, plays an important role in rehabilitating heavy metal contaminated areas. In this study, male and female P. cathayana plants were treated with Cd (20 mg kg^-1) and different levels of Zn (25, 50, or 100 mg kg^-1) to study their physiological responses. The results showed that Cd exposure alone caused stress by inhibiting the growth of both male and female plants. In both males and females, photosynthesis and antioxidant enzymes activities decreased substantially under Cd stress alone. Cd was largely located in the roots, but Zn was present in the shoots of both sexes. Zn supplementation considerably increased the photosynthetic rate from 14.62 % to 60.45 % and also enhanced the antioxidant enzymes activities from 24.11 % to 86.21 %. Zn treatment decreased the translocation ability of Cd compared to the Cd-only treatment, alleviating Cd toxicity. In addition, when sufficient Zn was made available, males showed a high degree of Cd accumulation, low root-to-shoot translocation, elevated antioxidant defense abilities, and an increased photosynthetic rate, while females were less responsive to Cd stress than males. Thus, combined exposure to Cd and Zn caused differential responses in plant growth and physiological processes between males and females P. cathayana. Male plants exhibit better Cd tolerance and accumulation capacity under optimum Zn supplementation. This study increases the fundamental knowledge regarding P. cathayana plants, which can be applied to enhance their remediation capacity in Cd-contaminated soils.

Comparative evaluation of cadmium phytoremediation potential of five varieties of Helianthus annuus L

Helianthus annuus is a potential metal accumulator plant, which can find application in cadmium (Cd) phytoremediation and provide economic gains in terms of oil yield. This study is focused on Cd accumulation analysis, physiological and biochemical responses of five varieties of H. annuus (DRSF-108, DRSF-113, LSFH-171, Phule Bhaskar and KBSH-44). Plantlets of all varieties were treated with various Cd concentrations (10, 50, 100, 300 and 500 mg kg^-1) for 20, 40 and 60 days. DRSF-108 showed the maximum total Cd accumulation (430.52 mg kg^-1) in whole plant while minimum accumulation was observed in KBSH-44 (150.66 mg kg^-1) at 500 mg kg^-1 Cd after 60 days. The highest level of proline and polyphenol in DRSF-108 were 27.206 µmol g^-1 fw and 6.86 mg g^-1 fw, respectively. Antioxidant enzymes (catalase, ascorbate peroxidase and glutathione reductase) also showed increased activity in response to Cd treatment. SEM-EDX analysis of potential accumulator genotype, DRSF-108, showed the distribution of intracellular Cd into plant tissues. Therefore, it is concluded that among five varieties, DRSF-108 was the most potential Cd accumulator and had a higher capacity for Cd tolerance compared to other varieties. Our findings may allow us to extend variety DRSF-108 for sustainable farming and Cd remediation.

An environmental field assessment of soil quality and phytoremediation of toxic metals from saline soil by selected halophytes

The current study has aims to investigate the soil quality and phytoextraction of cadmium (Cd), chromium (Cr), and lead (Pb) from saline soils using Alhagi maurorum (camelthorn), Tamarix aphylla (saltcedar), Salvadora persica (mustard bush), and Suaeda nigra (bush seep weed). The saline bulk soil, rhizospheric soil, and different parts of selected plants were oxidized using the acid mixture and determined Cd, Cr, and Pb by atomic absorption spectrometry. The bio-concentration factor (BCF) and translocation factor (T_F) of also examined. The quality parameters of soil like pH (< 8.5), and electrical conductivity (EC; > 4.00 dS m^-1) indicated the soil is saline. The salinity of soil was lower the organic matters, and total nitrogen contents in studied saline bulk soil due to deterioration condition of soils. However, the rhizospheric soil showed the improved quality of saline soil reflected the good phytoextraction of salts from saline soil. The high contents of Cd in roots and shoots (1.02 and 0.65 µg g^-1) of Alhagi maurorum, Cr in the roots and shoots (6.20, and 6.75 µg g^-1) of Tamarix aphylla and Pb in the roots and shoots (5.63, and 5.75 µg g^-1) of Suaeda nigra. The BCF and T_F showed the Tamarix aphylla and Alhagi maurorum for Pb, Alhagi maurorum, and Salvadora persica for Cr considered as hyperaccumulator plants. Based on BCF and T_F values of Alhagi maurorum, Tamarix aphylla for Cd, and Salvadora persica for Cr and Pb have the efficiency to uptake toxic metals from saline soil. Thus, it can be concluded that selected plant species may have ability for the phytoextraction the Cd, Cr and Pb from saline soil.

Allele segregation analysis of F1 hybrids between independent Brassica allohexaploid lineages

In the Brassica genus, we find both diploid species (one genome) and allotetraploid species (two different genomes) but no naturally occurring hexaploid species (three different genomes, AABBCC). Although hexaploids can be produced via human intervention, these neo-polyploids have quite unstable genomes and usually suffer from severe genome reshuffling. Whether these genome rearrangements continue in later generations and whether genomic arrangements follow similar, reproducible patterns between different lineages is still unknown. We crossed Brassica hexaploids resulting from different species combinations to produce five F₁ hybrids and analyzed the karyotypes of the parents and the F₁ hybrids, as well as allele segregation in a resulting test-cross population via molecular karyotyping using SNP array genotyping. Although some genomic regions were found to be more likely to be duplicated, deleted, or rearranged, a consensus pattern was not shared between genotypes. Brassica hexaploids had a high tolerance for fixed structural rearrangements, but which rearrangements occur and become fixed over many generations does not seem to show either strong reproducibility or to indicate selection for stability. On average, we observed 10 de novo chromosome rearrangements contributed almost equally from both parents to the F₁ hybrids. At the same time, the F₁ hybrid meiosis produced on average 8.6 new rearrangements. Hence, the increased heterozygosity in the F₁ hybrid did not significantly improve genome stability in our hexaploid hybrids and might have had the opposite effect. However, hybridization between lineages was readily achieved and may be exploited for future genetics and breeding purposes.

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.

Transcriptome and methylome changes in two contrasting mungbean genotypes in response to drought stress

Background

Due to drought stress, the growth, distribution, and production of mungbean is severely restricted. Previous study combining physiological and transcriptomic data indicated different genotypes of mungbean exhibited variable responses when exposed to drought stress. Aside from the genetic variation, the modifications of environmentally induced epigenetics alterations on mungbean drought-stress responses were still elusive.

Results

In this study, firstly, we compared the drought tolerance capacity at seedling stage by detecting physiological parameters in two contrasting genotypes wild mungbean 61 and cultivar 70 in response to drought stress. We found that wild mungbean 61 showed lower level of MDA and higher levels of POD and CAT, suggesting wild mungbean 61 exhibited stronger drought resistance. Transcriptomic analysis indicated totally 2859 differentially expressed genes (DEGs) were detected when 70 compared with 61 (C70 vs C61), and the number increased to 3121 in the comparison of drought-treated 70 compared with drought-treated 61 (D70 vs D61). In addition, when drought-treated 61 and 70 were compared with their controls, the DEGs were 1117 and 185 respectively, with more down-regulated DEGs than up-regulated in D61 vs C61, which was opposite in D70 vs C70. Interestingly, corresponding to this, after drought stress, more hypermethylated differentially methylated regions (DMRs) in 61 were detected and more hypomethylated DMRs in 70 were detected. Further analysis suggested that the main variations between 61 and 70 existed in CHH methylation in promoter. Moreover, the preference of methylation status alterations in D61 vs C61 and D70 vs C70 also fell in CHH sequence context. Further analysis of the correlation between DMRs and DEGs indicated in both D61 vs C61 and D70 vs C70, the DMRs in gene body was significantly negatively correlated with DEGs.

Conclusions

The physiological parameters in this research suggested that wild mungbean 61 was more resistant to drought stress, with more hypermethylated DMRs and less hypomethylated DMRs after drought stress, corresponding to more down-regulated DEGs than up-regulated DEGs. Among the three DNA methylation contexts CG, CHG, and CHH, asymmetric CHH contexts were more dynamic and prone to be altered by drought stress and genotypic variations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08315-z.

Physiological and proteomic analyses reveal the effects of exogenous nitrogen in diminishing Cd detoxification in Acacia auriculiformis

Cadmium (Cd) has toxic effects on plants. Nitrogen (N), an essential element, is critical for plant growth, development and stress response. However, their combined effects on woody plants, especially in N-fixing tree species is still poorly understood. Our previous study revealed that the fast-growing Acacia auriculiformis showed strong Cd tolerance but the underlying mechanisms was not clear, which constrained its use in mine land reclamation. Herein, we investigated the physiological and proteomic changes in A. auriculiformis leaves to reveal the mechanisms of Cd tolerance and toxicity without N fertilizer (treatment Cd) and with excess N fertilizer (treatment CdN). Results showed that Cd tolerance in A. auriculiformis was closely associated with the coordinated gas exchange and antioxidant defense reactions under Cd treatment alone. Exogenous excessive N, however, inhibited plant growth, increased Cd concentrations, and weaken photosynthetic performance, thus, aggregated the toxicity under Cd stress. Furthermore, the aggregated Cd toxicity was attributed to the depression in the abundance of proteins, as well as their corresponding genes, involved in photosynthesis, energy metabolism (oxidative phosphorylation, carbon metabolism, etc.), defense and stress response (antioxidants, flavonoids, etc.), plant hormone signal transduction (MAPK, STN, etc.), and ABC transporters. Collectively, this study unveils a previously unknown physiological and proteomic network that explains N diminishes Cd detoxification in A. auriculiformis. It may be counterproductive to apply N fertilizer to fast-growing, N-fixing trees planted for phytoremediation of Cd-contaminated soils.

Ethylene Suppresses Abscisic Acid, Modulates Antioxidant System to Counteract Arsenic-Inhibited Photosynthetic Performance in the Presence of Selenium in Mustard

Arsenic (As) stress provokes various toxic effects in plants that disturbs its photosynthetic potential and hampers growth. Ethylene and selenium (Se) have shown regulatory interaction in plants for metal tolerance; however, their synergism in As tolerance through modification of the antioxidant enzymes and hormone biosynthesis needs further elaboration. With this in view, we investigated the impact of ethylene and Se in the protection of photosynthetic performance against As stress in mustard (Brassica juncea L.). Supplementation with ethephon (2-chloroethylphosphonic acid; ethylene source) and/or Se allayed the negative impact of As-induced toxicity by limiting As content in leaves, enhancing the antioxidant defense system, and decreasing the accumulation of abscisic acid (ABA). Ethylene plus Se more prominently regulated stomatal behavior, improved photosynthetic capacity, and mitigated As-induced effects. Ethephon in the presence of Se decreased stress ethylene formation and ABA accumulation under As stress, resulting in improved photosynthesis and growth through enhanced reduced glutathione (GSH) synthesis, which in turn reduced the oxidative stress. In both As-stressed and non-stressed plants treated with ethylene action inhibitor, norbornadiene, resulted in increased ABA and oxidative stress with reduced photosynthetic activity by downregulating expression of ascorbate peroxidase and glutathione reductase, suggesting the involvement of ethylene in the reversal of As-induced toxicity. These findings suggest that ethephon and Se induce regulatory interaction between ethylene, ABA accumulation, and GSH metabolism through regulating the activity and expression of antioxidant enzymes. Thus, in an economically important crop (mustard), the severity of As stress could be reduced through the supplementation of both ethylene and Se that coordinate for maximum stress alleviation.

Targeting Cd coping mechanisms for stress tolerance in Brassica napus under spiked-substrate system: from physiology to remediation perspective

Cadmium (Cd) is a prevalent, non-essential, carcinogenic, and hazardous heavy metal that reduces plant productivity and capacity of arable land area around the globe. In the present substrate-based pot study, seedlings of Brassica napus 180015 were grown equidistantly in the spiked-substrate medium for 60 days under increasing concentrations of Cd (0, 10, 20, 30, 40, 50 mg kg^-1). Following harvest, the morpho-physio-biochemical, antioxidative, and Cd-induced tolerance responses were evaluated in B. napus under an increasing Cd stress regime. Additionally, these parameters were also investigated to select the plant's threshold tolerance limit for Cd under the spiked-substrate system. B. napus showed dynamic behavior regarding morpho-physio-biochemical attributes, including agronomic features, biomass, photosynthetic pigments, relative water content under increased Cd toxicity. Cd stress-induced hydrogen peroxide (H₂O₂) production with high MDA contents and passive EL, followed by the orchestration of both enzymatic (SOD, POD, APX, CAT, and GR) and non-enzymatic antioxidants (flavonoids, TPC, TPA, proline, and total soluble protein) up to a certain limit. In addition, Cd-induced stress upregulated transcriptional levels of antioxidative enzyme SOD, POD, APX, GR, and MT encoded genes in B. napus. The increasing trend of Cd accumulation in different tissues at the highest Cd concentration was as follows: root > leaf > stem. In spiked substrate system, B. napus demonstrated improved metal extractability performance and a high potential for phyto-management of low to moderate Cd contamination, implying that this study could be used for integrative breeding programs and decontaminating heavy metals in real contaminated scenarios.Novelty statementThis study provides an insight into Cd-coping mechanisms of oilseed rape involved in alleviating toxicity and simultaneous phyto-management of increasing Cd concentration under spiked substrate system. The current study is the first scientific evidence of using a Cd-spiked soilless substrate medium. The present study will further strengthen our understanding of Cd-instigated positive responses in B. napus. Furthermore, it will provide a useful basis for integrative breeding programs and decontaminating heavy metals in real contaminated scenarios.

Using wild relatives and related species to build climate resilience in Brassica crops

Climate change will have major impacts on crop production: not just increasing drought and heat stress, but also increasing insect and disease loads and the chance of extreme weather events and further adverse conditions. Often, wild relatives show increased tolerances to biotic and abiotic stresses, due to reduced stringency of selection for yield and yield-related traits under optimum conditions. One possible strategy to improve resilience in our modern-day crop cultivars is to utilize wild relative germplasm in breeding, and attempt to introgress genetic factors contributing to greater environmental tolerances from these wild relatives into elite crop types. However, this approach can be difficult, as it relies on factors such as ease of hybridization and genetic distance between the source and target, crossover frequencies and distributions in the hybrid, and ability to select for desirable introgressions while minimizing linkage drag. In this review, we outline the possible effects that climate change may have on crop production, introduce the Brassica crop species and their wild relatives, and provide an index of useful traits that are known to be present in each of these species that may be exploitable through interspecific hybridization-based approaches. Subsequently, we outline how introgression breeding works, what factors affect the success of this approach, and how this approach can be optimized so as to increase the chance of recovering the desired introgression lines. Our review provides a working guide to the use of wild relatives and related crop germplasm to improve biotic and abiotic resistances in Brassica crop species.

Using wild relatives and related species to build climate resilience in Brassica crops

Climate change will have major impacts on crop production: not just increasing drought and heat stress, but also increasing insect and disease loads and the chance of extreme weather events and further adverse conditions. Often, wild relatives show increased tolerances to biotic and abiotic stresses, due to reduced stringency of selection for yield and yield-related traits under optimum conditions. One possible strategy to improve resilience in our modern-day crop cultivars is to utilize wild relative germplasm in breeding, and attempt to introgress genetic factors contributing to greater environmental tolerances from these wild relatives into elite crop types. However, this approach can be difficult, as it relies on factors such as ease of hybridization and genetic distance between the source and target, crossover frequencies and distributions in the hybrid, and ability to select for desirable introgressions while minimizing linkage drag. In this review, we outline the possible effects that climate change may have on crop production, introduce the Brassica crop species and their wild relatives, and provide an index of useful traits that are known to be present in each of these species that may be exploitable through interspecific hybridization-based approaches. Subsequently, we outline how introgression breeding works, what factors affect the success of this approach, and how this approach can be optimized so as to increase the chance of recovering the desired introgression lines. Our review provides a working guide to the use of wild relatives and related crop germplasm to improve biotic and abiotic resistances in Brassica crop species.

Soil Sulfur Sources Differentially Enhance Cadmium Tolerance in Indian Mustard (Brassica juncea L.)

The effect of four soil-applied sulfur (100 mg S kg−1 soil (100S) and 200 mg S kg−1 soil (200S)) in different sources (elemental S, ammonium sulfate, gypsum or magnesium sulfate) in protecting mustard (Brassica juncea L. (Czern & Coss.)) from cadmium effects was studied. Based on the observed reduction in growth and photosynthesis in plants subjected to 100 and 200 mg Cd kg−1 soil, B. juncea cv. Giriraj was selected as the most Cd-tolerant among five cultivars (namely, Giriraj, RH-0749, Pusa Agrani, RH-406, and Pusa Tarak). Sulfur applied to soil mitigated the negative impact of Cd on sulfur assimilation, cell viability, and photosynthetic functions, with a lower lipid peroxidation, electrolyte leakage, and contents of reactive oxygen species (ROS: hydrogen peroxide, H2O2, and superoxide anion, O2•−). Generally, added S caused higher activity of antioxidant enzymes (ascorbate peroxidase, catalase and superoxide dismutase), contents of ascorbate (AsA) and reduced glutathione (GSH); increases in the activities of their regenerating enzymes (dehydroascorbate reductase and glutathione reductase); as well as rises in S assimilation, biosynthesis of non-protein thiols (NPTs), and phytochelatins (PCs). Compared to the other S-sources tested, elemental S more prominently protected B. juncea cv. Giriraj against Cd-impacts by minimizing Cd-accumulation and its root-to-shoot translocation; decreasing cellular ROS and membrane damage, and improving Cd-chelation (NPTs and PCs), so strengthening the defense machinery against Cd. The results suggest the use of elemental S for favoring the growth and development of cultivated plants also in Cd-contaminated agricultural soils.

Analyzing dose dependency of antioxidant defense system in the cyanobacterium Nostoc muscorum Meg 1 chronically exposed to Cd2

The aim of the present study was to analyze the dose dependency of oxidant-antioxidant homeostasis in Cd²⁺ exposed Nostoc muscorum Meg 1 cells. Quantification of percent DNA loss, protein oxidation and lipid peroxidation was carried out to assess Cd²⁺ induced ROS mediated damages to the organism. The countermeasures adopted by the cyanobacterium were also evaluated by computing various components of both enzymatic and non-enzymatic antioxidants. Exposure to different Cd²⁺ (0.1, 0.2, 0.3, 0.5, 1, 1.5, 2, 2.5, 3 ppm) doses showed substantial increase in ROS content in the ranges of 20-181% and 116-323% at the end of first and seventh day. The DNA damage, protein oxidation and lipid peroxidation were increased by 11-62%, 7-143% and 13-183% with increasing Cd²⁺ concentrations at the end of seven days. TEM images clearly showed damages to the cell wall, cell membrane and thylakoid organization at higher Cd²⁺ (0.5-3 ppm) concentrations. Cd²⁺ exposure up to 0.5 ppm registered increase in contents of antioxidative enzymes (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione reductase (GR)) and in non-enzymatic antioxidants (glutathione, total thiol, phytochelatin and proline) indicating stimulation of ROS mitigating machinery. However, toxicity of Cd²⁺ was evident as at higher concentrations the cellular morphology and ultra-structures were negatively affected and the capacities of the cells to generate various antioxidant measures were highly compromised. The organism registered 96-98% sorption ability from a solution supplemented with 0.3 ppm Cd²⁺ and thus show realistic potential as Cd²⁺ bioremediator in wastewater treatment.

Enzymatic response to cadmium by Impatiens glandulifera: A preliminary investigation

This paper aims to develop our understanding of the effect of cadmium (Cd) on Impatiens glandulifera, a recently identified potential Cd hyperaccumulator. Impatiens glandulifera plants were exposed to three concentrations of Cd (20, 60 and 90 mg/kg) and were sampled at two timepoints (one and seven days) to investigate the stress response of I. glandulifera to Cd. Cd can induce oxidative stress in plants, triggering overproduction of reactive oxygen species (ROS). The level of activity of catalase (CAT) and ascorbate peroxidase (APX), two crucial antioxidant enzymes responsible for detoxifying ROS, were found to increase in a concentration dependent manner. Though there was no change observed in the level of superoxide dismutase (SOD) activity, the activity of glutathione S-transferase (GST), involved in detoxifying and sequestering Cd, increased after exposure to Cd. Cd did not appear to impact the levels of proline and photosynthetic pigments, indicating the plants weren't stressed by the presence of Cd. These results suggest that the rapid response observed in enzyme activity aid the efficacious mitigation of the toxic effects of Cd, preventing significant physiological stress in I. glandulifera.

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Impatiens glandulifera display an enhance tolerance to Cadmium.

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An early response in a Catalase and Peroxidase ascorbate mediates Cadmium tolerance.

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No impact on stress indicators were shown by Impatiens glandulifera even after 7 days.

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SOD was found to be not involved in the early response to Cadmium.

Mechanisms Regulating the Dynamics of Photosynthesis Under Abiotic Stresses

Photosynthesis sustains plant life on earth and is indispensable for plant growth and development. Factors such as unfavorable environmental conditions, stress regulatory networks, and plant biochemical processes limits the photosynthetic efficiency of plants and thereby threaten food security worldwide. Although numerous physiological approaches have been used to assess the performance of key photosynthetic components and their stress responses, though, these approaches are not extensive enough and do not favor strategic improvement of photosynthesis under abiotic stresses. The decline in photosynthetic capacity of plants due to these stresses is directly associated with reduction in yield. Therefore, a detailed information of the plant responses and better understanding of the photosynthetic machinery could help in developing new crop plants with higher yield even under stressed environments. Interestingly, cracking of signaling and metabolic pathways, identification of some key regulatory elements, characterization of potential genes, and phytohormone responses to abiotic factors have advanced our knowledge related to photosynthesis. However, our understanding of dynamic modulation of photosynthesis under dramatically fluctuating natural environments remains limited. Here, we provide a detailed overview of the research conducted on photosynthesis to date, and highlight the abiotic stress factors (heat, salinity, drought, high light, and heavy metal) that limit the performance of the photosynthetic machinery. Further, we reviewed the role of transcription factor genes and various enzymes involved in the process of photosynthesis under abiotic stresses. Finally, we discussed the recent progress in the field of biodegradable compounds, such as chitosan and humic acid, and the effect of melatonin (bio-stimulant) on photosynthetic activity. Based on our gathered researched data set, the logical concept of photosynthetic regulation under abiotic stresses along with improvement strategies will expand and surely accelerate the development of stress tolerance mechanisms, wider adaptability, higher survival rate, and yield potential of plant species.

Effects of dibutyl phthalate contamination on physiology, phytohormone homeostasis, rhizospheric and endophytic bacterial communities of Brassica rapa var. chinensis

Phthalates are plasticizers and are ubiquitously detected in the environment, frequently at mg/kg levels. The present study aimed to evaluate the effects of dibutyl phthalate (DBP) on germination, growth, enzyme activity, phytohormone homeostasis and bacterial communities of two cultivars of Brassica rapa var. chinensis. The germination rate was decreased up to 20% compared to the control, and the growth of the vegetables was severely inhibited at the early stage when exposed to DBP at 20 mg/kg. Antioxidant defense enzyme activities and malondialdehyde (MDA) content increased upon exposure to DBP. A dose-response of auxin (IAA) was observed after a 2 d exposure. Gibberellin (GA3) and abscisic acid (ABA) responded at day 10 under DBP stress. GA3 did not show a clear dose-response effect and ABA increased about 3 times as the DBP concentration increased from 2 to 20 mg/L. Microbial population shifts were observed, especially in rhizosphere soil and roots. No obvious change occurred for the α diversity of rhizospheric bacteria among different treatments. Chao1, Shannon and Simpson indices of the root endophytic bacteria showed a decreasing trend with increasing DBP supplementation, while all the indices increased in shoot endophytic bacteria in comparison to the control. The results indicated that exposure to DBP may compromise the fitness of the leafy vagetables and alter the endophytic and rhizospheric bacteria, which might further affect the nutrients of the vegetables and alter ecosystem functions.

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.

Comparative assessment of Brassica pekinensis L. genotypes for phytoavoidation of nitrate, cadmium and lead in multi-pollutant field

Information is needed for comparative assessment and agronomic practices for phytoavoidation in multi-pollutant field. A field study was conducted to explore 97 Brassica pekinensis L. genotypes with permissible limit of contaminants growing in a severely Cd, moderately nitrate and slightly Pb multi-polluted field. Thirteen genotypes, i.e. KGZY, CXQW, CAIB, JINL, JQIN, JFEN, WMQF, XLSH, TAIK, BJXS, JUKA, XYJQ and GQBW, were identified with permissible limit for nitrate, Cd and Pb based on their resistance to heavy metal and nitrate accumulation in leaves when grown in co-contaminated soils. Furthermore, the correlation between essential and toxic elements concentrations in plant of B. pekinensis were inconsistent. Generally speaking, application of increasing Ca, K and S fertilizers in appropriate forms and dosages tended to increase the yield and quality of B. pekinensis cultivated in multi-pollutant field.

Effects of EDTA, citric acid, and tartaric acid application on growth, phytoremediation potential, and antioxidant response of Calendula officinalis L. in a cadmium-spiked calcareous soil

The improved efficiency of cadmium (Cd) phytoextraction potential of Calendula officinalis L. was evaluated in Cd-spiked calcareous soil, using various chelating agents. In a greenhouse study, three chelating agents, including EDTA, citric acid (CA), and tartaric acid (TA), were applied to Cd-spiked soils (50 and 100 mg kg^-1) under C. officinalis L. cultivation. According to the results, C. officinalis grew normally without any toxicity signs at various Cd levels of the soil; however, with increasing the Cd levels, the plant dry weight biomass decreased, and activities of antioxidant enzymes (AOEs) increased. The application of CA and TA in Cd-spiked soils improved the physiologic traits of plants and mitigated the Cd stress since the activities of AOEs decreased. Oppositely, due to increasing the Cd excessive permeability to the root of the plant, EDTA application diminished the physiologic traits and increased the activities of AOEs. The results also showed that all the chelators, especially EDTA, markedly increased the Cd mobility factor (from 58.80% to 65.20-89.60%) in Cd-spiked soils. The bioconcentration factor (BCF = 1.3-2.90) and translocation factor (TF = 1.28-1.58) of Cd, which were >1 in all treated and untreated plant samples, as well as the accumulated Cd >100 mg kg^-1, demonstrated that C. officinalis is a Cd-hyperaccumulator plant which could remediate Cd by the phytoextraction process. Regarding the biodegradation of CA, as well as the increased TF efficiency of Cd and plant biomass of CA treatments (by decreasing oxidative stress), compared to EDTA and TA treatments, it is recommended that CA be used as a superior chelating agent to enhance the efficiency of Cd phytoremediation in C. officinalis.

Bioremediation of cadmium-trichlorfon co-contaminated soil by Indian mustard (Brassica juncea) associated with the trichlorfon-degrading microbe Aspergillus sydowii: Related physiological responses and soil enzyme activities

Soil co-contaminated with heavy metals and organics is often difficult to remediate. In this study, pot experiments were conducted to investigate the concurrent removal of cadmium (Cd, two levels: Cd_L [10 mg kg^-1] and Cd_H [50 mg kg^-1]) and trichlorfon (TCF, 100 mg kg^-1) from co-contaminated soil by comparing the following remediation methods: natural remediation (NR), soil inoculated with Aspergillus sydowii (AS), soil planted with Brassica juncea (BJ), and soil planted with B. juncea and inoculated with A. sydowii (BJ-AS). The physiological responses of B. juncea and soil enzyme activities after remediation were also studied. B. juncea grew well in co-contaminated soil at both Cd levels. The biomass and chlorophyll content of B. juncea in Cd_H soil were lower than those in Cd_L soil, whereas the malondialdehyde content and activities of catalase, peroxidase and superoxide dismutase of B. juncea in Cd_H soil were higher than those in Cd_L soil. Cd accumulation in B. juncea was high in Cd_H soil, whereas high Cd removal efficiency was observed in Cd_L soil. TCF could be thoroughly degraded within 35 days in NR at both Cd-level soils. AS, BJ and BJ-AS promoted TCF degradation and enhanced the activities of catalase, urease, sucrase and alkaline phosphatase in soil compared with the NR. BJ-AS showed the highest phytoextraction ratio (3.32% in Cd_L and 1.34% in Cd_H soil) and TCF degradation rate (half-life of 2.18 and 2.37 days in Cd_L and Cd_H soil, respectively). These results demonstrate that BJ-AS could effectively remove Cd and TCF from soil and is thus a feasible technology for the bioremediation of these co-contaminated soil.

Evaluation of arsenic induced toxicity based on arsenic accumulation, translocation and its implications on physio-chemical changes and genomic instability in indica rice (Oryza sativa L.) cultivars

Arsenic (As) accumulation in rice is a principal route of As exposure for rice based population. We have tested physiochemical and molecular parameters together to identify low As accumulating rice cultivars with normal growth and vigor. The present study examined potential toxicity caused by arsenate (AsV) among four rice cultivars tested that varied with respect to accumulation of total arsenic, arsenite (AsIII) and their differential translocation rate which had deleterious impact on growth and metabolism. Intracellular homeostasis of rice cultivars viz., TN-1, IR-64, IR-20 and Tulaipanji was hampered by 21 days long As(V) treatment due to generation of reactive oxygen species (ROS) and inadequate activity of catalase (CAT; EC 1.11.1.6). Upregulation of oxidative stress markers viz., H₂O₂, proline and MDA along with alteration in enzymatic antioxidants profile were conspicuously pronounced in cv. Tulaipanji while cv. TN-1 was least affected under As(V) challenged environment. In addition to that genomic template stability and band sharing indices were qualitatively measured by DNA profiling of all tested cultivars treated with 25 μM, 50 μM, and 75 μM As(V). In rice cv. Tulaipanji genetic polymorphism was significantly detected with the application of random amplified polymorphic DNA (RAPD) tool and characterized as susceptible cultivar of As compared to cvs. TN-1, IR-64 and IR-20 that is in correlation with data obtained from cluster analysis. Hence, identified As tolerant cultivars viz., TN-1, IR64 and IR-20 especially TN-1 could be used in As contaminated agricultural field after appropriate field trial. This study could help to gather information regarding cultivar-specific tolerance strategy to avoid pollutant induced toxicity.

Plant growth regulators improve growth, photosynthesis, mineral nutrient and antioxidant system under cadmium stress in menthol mint (Mentha arvensis L.)

Menthol mint (Mentha arvensis L.) cultivation is significantly affected by the heavy metals like cadmium (Cd) which also imposes severe health hazards. Two menthol mint cultivars namely Kosi and Kushal were evaluated under Cd stress conditions. Impact of plant growth regulators (PGRs) like salicylic acid (SA), gibberellic acid (GA₃) and triacontanol (Tria) on Cd stress tolerance was assessed. Reduced growth, photosynthetic parameters, mineral nutrient concentration, and increased oxidative stress biomarkers like electrolyte leakage, malondialdehyde, and hydrogen peroxide contents were observed under Cd stress. Differential upregulation of proline content and antioxidant activities under Cd stress was observed in both the cultivars. Interestingly, low electrolyte leakage, lipid peroxidation, hydrogen peroxide and Cd concentration in leaves were observed in Kushal compared to Kosi. Among all the PGRs tested, SA proved to be the best in improving Cd-stress tolerance in both the cultivars but Kushal responded better than Kosi.

Biopriming with Piriformospora indica ameliorates cadmium stress in rice by lowering oxidative stress and cell death in root cells

Piriformospora indica is known for plant growth promotion and abiotic stress alleviation potential in several agricultural crops. However, a systemic analysis is warranted to explore potential application of this important fungus to augment heavy metal tolerance in rice. The present study explores potential of P. indica in ameliorating the effect of cadmium (Cd) stress in rice cultivars N22 and IR64. Seedlings inoculated with P. indica recorded significantly higher root-shoot length and biomass as compared to non-inoculated plants under control and Cd stress, respectively. Moreover, P. indica inoculated stressed roots accumulated more Cd as compared to non-inoculated stressed roots in both the varieties. Interestingly, cell death and reactive oxygen species (ROS) accumulation were significantly lower in the inoculated plant roots as compare with non-inoculated roots under Cd stress. The results emphasized significantly higher accumulation of Cd in fungal spores could reduce ROS accumulation in root cells resulting in lower cell death.

Aluminium stress modulates the osmolytes and enzyme defense system in Fagopyrum species

The present investigation describes aluminum-induced changes in the leaves of two buckwheat species using both physiological and biochemical indices. With increasing levels of Al (viz. 100, 200 and 300 μM), the mean length of root, shoot as well as their biomass accumulation decreased linearly with respect to control. Tolerance test of F. kashmirianum revealed that it was more tolerant to Al-stress than F. tataricum as revealed by higher accumulation of Al in its roots without any significant damage. Translocation factor (TF) values of both species were found to be < 1, indicating more Al is restrained in roots. Total chlorophyll showed a non-significant increase in F. tataricum while as decreased in F. kashmirianum at 300 μM concentration besides, the carotenoid content exhibited inclined trend in F. tataricum and showed a concomitant decrease in F. kashmirianum. The anthocyanin level showed a non-significant decline in F. kashmirianum. Exposure to different Al-treatments enhances malondialdehyde (MDA), H₂O₂ and membrane stability index (MSI) in both species, with increases being greater in F. kashmirianum than F. tataricum as also revealed by DAB-mediated in vivo histo-chemical detection method. The osmolyte level in general were elevated in both buckwheat species however, enhancement was more in F. tataricum than F. kashmirianum. The activities of antioxidant enzymes viz. superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (POD), glutathione reductase (GR), glutathione-S-transferase (GST) were positively correlated with Al-treatment except catalase (CAT) which exhibits a reverse outcome in F. kashmirianum. The present investigation could play an essential role to better understand the detoxification mechanisms of Al in plants.

Modulation of the defence responses against Cd in willow species through a multifaceted analysis

Willow, due to the extensive root system, high transpiration rates and ability to accumulate large amounts of cadmium, is considered particularly useful for green remediation practices. In this study two different willow species, Salix viminalis and Salix alba, were used to assess possible differences in their ability of cadmium accumulation and to analyse in detail the physiology of their response to treatments with this metal using a multidisciplinary approach. Plants were grown in hydroponics and treated with 0, 50 and 100 μM Cd²⁺ (CdCl₂) for 7 and 14 days. Cadmium content, oxidative stress, both evaluated by biochemical and histochemical techniques, antioxidant response, leaf stomatal conductance and photosynthetic efficiency were measured in control and treated roots and/or leaves. The two willow species removed cadmium with a high efficiency from the growth solution; however, the highest contents of Cd recorded in plants grown in the presence of the lower Cd concentrations suggest a limited capacity of metal accumulation. No photochemical limitation characterised treated plants, probably due to the ability to store large amounts of Cd in the root compartment, with reduction of damage to the photosynthetic machinery. S. viminalis, able to uptake cadmium also in the root apical region, seemed to be a more efficient accumulator than S. alba and, thanks to a relatively higher antioxidant response, did not show a higher level of oxidative stress. On the basis of the above, the two plant species, in particular S. viminalis, are confirmed as useful for cadmium phytostabilisation/phytoextraction.

Abscisic acid-generating bacteria can reduce Cd concentration in pakchoi grown in Cd-contaminated soil

Contamination of vegetable plants with cadmium (Cd) has become a serious issue in recent years. In the present study, pakchoi (Brassica chinensis L.) grown in Cd-contaminated soil inoculated with abscisic acid (ABA)-generating bacteria, Azospirillum brasilense and Bacillus subtilis, showed 28%-281% and 26%-255% greater biomass, and 40%-79% and 43%-77% lower Cd concentrations, respectively, than those of the control_{bacteria-free} plants. These treatments also alleviated the Cd-induced photosynthesis inhibition and oxidative damage (indicated by malondialdehyde [MDA], H₂O₂, and O₂^• -). Furthermore, the application of bacteria also remarkably improved the levels of antioxidant-related compounds (total phenolics, total flavonoids, ascorbate, and 2,2-diphenyl-1-picrylhydrazyl [DPPH] activity) and nutritional quality (soluble sugar and soluble protein) in the Cd-supplied plants. Based on these results, we conclude that the application of ABA-generating bacteria might be an alternative strategy for improving the biomass production and quality of vegetable plants grown in Cd-contaminated soil.

ROS management is mediated by ascorbate-glutathione-α-tocopherol triad in co-ordination with secondary metabolic pathway under cadmium stress in Withania somnifera

Being static, plants are frequently exposed to various essential and non-essential heavy metals from the surroundings. This exposure results in considerable ROS generation leading to oxidative stress, the primary response of the plants under heavy metal stress. Withania somnifera is a reputed Indian medicinal plant in Ayurveda, having various pharmacological activities due to the presence of withanolides. The present study deals with the understanding endurance of oxidative stress caused by heavy metal exposure and its management through antioxidant partners in synchronization with secondary metabolites in W. somnifera. The quantitative assessment of enzymatic/non-enzymatic antioxidants revealed significant participation of ascorbate-glutathione-α-tocopherol triad in ROS management. Higher activities of glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) resulted in glutathione and ascorbate accumulation. In addition, superoxide dismutase (SOD), glutathione peroxidase (GPX) and peroxidase (POD) were contributed considerably in ROS homeostasis maintenance. In-situ localization and assays related to ROS generation/scavenging revealed key management of ROS status under Cd stress. Higher antioxidative and reducing power activity attributed to the tolerance capability to the plant. Increased expression of withanolide biosynthetic pathway genes such as WsHMGR, WsDXS, WsDXR and WsCAS correlated with enhanced withanolides. The present study indicated the crucial role of the ascorbate-glutathione-α-tocopherol triad in co-ordination with withanolide biosynthesis in affording the oxidative stress, possibly through a cross-talk between the antioxidant machinery and secondary metabolite biosynthesis. The knowledge may be useful in providing the guidelines for developing abiotic stress resistance in plants using conventional and molecular approaches.

Interactive effect of potassium and flyash: a soil conditioner on metal accumulation, physiological and biochemical traits of mustard (Brassica juncea L.)

At present plants continuously bare to various environmental stresses due to the rapid climate change that adversely affects the growth and nutrient status of the soil and plant. Application of flyash (FA) in combination with potassium (K) fertilizer amendment improves soil physico-chemical characteristics, growth and yield of plants. Mustard grown in combination with FA (0, 20, 40 or 60 t ha^-1) and K (0, 30 or 60 kg ha^-1) treated soil was used to evaluate the effect on heavy metals (Cd, Cr and Pb) concentration and antioxidant system. The experiment was conducted in a net house of the Department of Botany, Aligarh Muslim University, Aligarh. Sampling was done at 70 DAS. The results showed that concentration of metals was found maximum in roots than the leaf and seeds. FA₆₀ accompanied by K₃₀ and K₆₀ cause oxidative stress through lipid peroxidation and showed reduced levels of photosynthesis and enzymatic activity. Proline and ascorbate content increases with increasing flyash doses to combat stress. However, flyash at the rate of 40 t ha^-1 together with K₆₀ followed by K₃₀ significantly boosted crop growth by enhancing antioxidant activity which plays a critical role in ameliorating the oxidative stress. Graphical abstract.

A critical review on the effects of zinc at toxic levels of cadmium in plants

Increasing cadmium (Cd) pollution in agricultural soils has raised serious concerns worldwide. Several exogenous substances can be used to mitigate the toxic effects of Cd in plants. Zinc (Zn) is one of the essential plant micronutrients and is involved in several physiological functions in plants. Zn may alleviate Cd toxicity in plants owing to the chemical similarity of Zn with Cd. Published reports demonstrated that Zn can alleviate toxic effects of Cd in plants by increasing plant growth, regulating Cd uptake, increasing photosynthesis, and reducing oxidative stress. Literature demonstrated that the role of Zn on Cd accumulation by plants is very controversial and depends upon several factors including concentrations of Cd and Zn in the medium, exposure duration, plant species and genotypes, and growth conditions. This review highlights the role of Zn in reducing Cd toxicity in plants and provides new insight that proper level of Zn in plants may enhance plant resistance to excess Cd.

Cadmium phytoremediation potential of Brassica crop species: A review

Cadmium (Cd) is a highly toxic metal released into the environment through anthropogenic activities. Phytoremediation is a green technology used for the stabilization or remediation of Cd-contaminated soils. Brassica crop species can produce high biomass under a range of climatic and growing conditions, allowing for considerable uptake and accumulation of Cd, depending on species. These crop species can tolerate Cd stress via different mechanisms, including the stimulation of the antioxidant defense system, chelation, compartmentation of Cd into metabolically inactive parts, and accumulation of total amino-acids and osmoprotectants. A higher Cd-stress level, however, overcomes the defense system and may cause oxidative stress in Brassica species due to overproduction of reactive oxygen species and lipid peroxidation. Therefore, numerous approaches have been followed to decrease Cd toxicity in Brassica species, including selection of Cd-tolerant cultivars, the use of inorganic and organic amendments, exogenous application of soil organisms, and employment of plant-growth regulators. Furthermore, the coupling of genetic engineering with cropping may also help to alleviate Cd toxicity in Brassica species. However, several field studies demonstrated contrasting results. This review suggests that the combination of Cd-tolerant Brassica cultivars and the application of soil amendments, along with proper agricultural practices, may be the most efficient means of the soil Cd phytoattenuation. Breeding and selection of Cd-tolerant species, as well as species with higher biomass production, might be needed in the future when aiming to use Brassica species for phytoremediation.

Effects of Oxalic Acid on Arsenic Uptake and the Physiological Responses of Hydrilla verticillata Exposed to Different Forms of Arsenic

A hydroponic experiment was conducted to investigate the effects of oxalic acid (OA) on arsenic (As) uptake and the physiological responses of Hydrilla verticillata exposed to 3 mg L^-1 of As in different forms. Plant As(III) uptake was significantly increased by 200-2000 µg L^-1 OA. However, an increase of As(V) uptake was only shown with 1000 µg L^-1 OA, and no significant difference was observed with dimethylarsinate treatment. Peroxidase and catalase activities, and the contents of photosynthetic pigments, soluble sugar and proline, were significantly increased by 1000 µg L^-1 OA during As(III) treatment. Superoxide dismutase and proline were also increased significantly by 1000 µg L^-1 OA when plants were exposed to As(V). In DMA treatment, proline was significantly increased by 500 µg L^-1 OA. Therefore, As-induced oxidative stress is relieved by OA, but it depends on OA concentration and the form of As. Our results may be useful for the phytoremediation of waste water containing As and OA.

Influence of cadmium stress on root exudates of high cadmium accumulating rice line (Oryza sativa L.)

A hydroponic experiment with two different cadmium (Cd) accumulating rice lines of Lu527-8 (the high Cd accumulating rice line) and Lu527-4 (the normal rice line) was carried out to explore the links among Cd stress, root exudates and Cd accumulation. The results showed that (1) Cd stress increased quantities of organic acids, but had no effect on composition in root exudates of the two rice lines. In Cd treatments, the contents of every detected organic acid in root exudates of Lu527-8 were 1.76-2.43 times higher than those of Lu527-4. Significant positive correlations between organic acids contents and Cd contents in plants were observed in both rice lines, except that malic acid was only highly relevant to Lu527-8, but not to Lu527-4. (2) Both composition and quantities of amino acids in root exudates changed a lot under Cd stress and this change differed in two rice lines. In control, four amino acids (glutamic acid, glycine, tyrosine and histidine) were detected in two rice lines. Under Cd stress, eight amino acids in Lu527-8 and seven amino acids in Lu527-4 could be detected, among which phenylalanine was only secreted by Lu527-8 and alanine, methionine and lysine were secreted by both rice lines. The contents of those four newly secreted amino acids from Lu527-8 increased significantly with the increase of Cd dose and each had a high-positive correlation with Cd contents, but the same change did not appear in Lu527-4. The difference between two rice lines in secretion of organic acids and amino acids may be related to their different Cd uptake properties.

Insight into the expression variation of metal-responsive genes in the seedling of date palm (Phoenix dactylifera)

Phytochelatin synthase and metallothionein gene expressions were monitored via qPCR in order to investigate the molecular mechanisms involved in Cd and Cr detoxification in date palm (Phoenix dactylifera). A specific reference gene validation procedure using BestKeeper, NormFinder and geNorm programs allowed selection of the three most stable reference genes in a context of Cd or Cr contamination among six reference gene candidates, namely elongation factor α1, actin, aldehyde dehydrogenase, SAND family, tubulin 6 and TaTa box binding protein. Phytochelatin synthase (pcs) and metallothionein (mt) encoding gene expression were induced from the first days of exposure. At low Cd stress (0.02 mM), genes were still up-regulated until 60th day of exposure. At the highest metal concentrations, however, pcs and mt gene expressions decreased. pcs encoding gene was significantly up-regulated under Cr exposure, and was more responsive to increasing Cr concentration than mt encoding gene. Moreover, exposure to Cd or Cr influenced clearly seed germination and hypocotyls elongation. Thus, the results have proved that both analyzed genes participate in metal detoxification and their expression is regulated at transcriptional level in date palm subjected to Cr and Cd stress. Consequently, variations of expression of mt and pcs genes may serve as early-warning biomarkers of metal stress in this species.

Cold and Heat Stress Diversely Alter Both Cauliflower Respiration and Distinct Mitochondrial Proteins Including OXPHOS Components and Matrix Enzymes

Complex proteomic and physiological approaches for studying cold and heat stress responses in plant mitochondria are still limited. Variations in the mitochondrial proteome of cauliflower (Brassica oleracea var. botrytis) curds after cold and heat and after stress recovery were assayed by two-dimensional polyacrylamide gel electrophoresis (2D PAGE) in relation to mRNA abundance and respiratory parameters. Quantitative analysis of the mitochondrial proteome revealed numerous stress-affected protein spots. In cold, major downregulations in the level of photorespiratory enzymes, porine isoforms, oxidative phosphorylation (OXPHOS) and some low-abundant proteins were observed. In contrast, carbohydrate metabolism enzymes, heat-shock proteins, translation, protein import, and OXPHOS components were involved in heat response and recovery. Several transcriptomic and metabolic regulation mechanisms are also suggested. Cauliflower plants appeared less susceptible to heat; closed stomata in heat stress resulted in moderate photosynthetic, but only minor respiratory impairments, however, photosystem II performance was unaffected. Decreased photorespiration corresponded with proteomic alterations in cold. Our results show that cold and heat stress not only operate in diverse modes (exemplified by cold-specific accumulation of some heat shock proteins), but exert some associations at molecular and physiological levels. This implies a more complex model of action of investigated stresses on plant mitochondria.

Evaluation of heme oxygenase 1 (HO 1) in Cd and Ni induced cytotoxicity and crosstalk with ROS quenching enzymes in two to four leaf stage seedlings of Vigna radiata

Research on heme oxygenase in plants has received consideration in recent years due to its several roles in development, defense, and metabolism during various environmental stresses. In the current investigation, the role of heme oxygenase (HO) 1 was evaluated in reducing heavy metal (Cd and Ni) uptake and alleviating Cd and Ni toxicity effects in the hydroponically grown seedlings of Vigna radiata var. PDM 54. Seedlings were subjected to Cd- and Ni-induced oxidative stress independently at different concentrations ranging from 10 to 100 μM. After 96 h (fourth day) of treatment, the stressed plants were harvested to study the cellular homeostasis and detoxification mechanism by examining the growth, stress parameters (LPX, H₂O₂ content), and non-enzymatic and enzymatic parameters (ascorbate peroxidase (APX), guaicol peroxidase (GPX), and catalase (CAT)) including HO 1. At 50 μM CdCl₂ and 60 μM NiSO₄, HO 1 activity was found to be highest in leaves which were 1.39 and 1.16-fold, respectively. The greatest HO 1 activity was reflected from the reduction of H₂O₂ content at these metal concentrations (50 μM CdCl₂ and 60 μM NiSO₄) which is correlated with the increasing activity of other antioxidant enzymes (CAT, APX). Thus, HO 1 works within a group that generates the defense machinery for the plant's survival by scavenging ROS which is confirmed by a time-dependent study. Hence, it is concluded that seedlings of V. radiata were more tolerant towards metal-induced oxidative stress in which HO 1 is localized in its residential area (plastids).

Castasterone and citric acid treatment restores photosynthetic attributes in Brassica juncea L. under Cd(II) toxicity

Cadmium(II) toxicity is a serious environmental issue warranting effective measures for its mitigation. In the present study, ameliorative effects of a bioactive brassinosteroid, castasterone (CS) and low molecular weight organic acid, citric acid (CA) against the Cd(II) toxicity to Brassica juncea L. were evaluated. Seeds of B. juncea treated with CS (0, 0.01, 1 and 100nM) were sown in cadmium spiked soils (0 and 0.6mmolkg^-1 soil). CA (0.6mmolkg^-1soil) was added to soil one week after sowing seeds. Plants were harvested 30 days after sowing. Phytotoxicity induced by Cd(II) was evident from stunted growth of the plants, malondialdehyde accumulation, reduction in chlorophyll and carotenoid contents, and leaf gas exchange parameters. Cd(II) toxicity was effectively alleviated by seed soaking with CS (100nM) and/ or soil amendment with CA (0.6mMkg^-1 soil). Relative gene expression of genes encoding for some of the key enzymes of pigment metabolism were also analysed. Expression of chlorophyllase (CHLASE) was reduced, while that of phytoene synthase (PSY), and chalcone synthase (CHS) genes were enhanced with CS and/or CA treatments with respect to plants treated with Cd(II) only. Cd also affected the activities of antioxidative enzymes. Plants responded to Cd(II) by accumulation of total sugars. CS (100nM) and CA treatments further enhanced the activities of these parameters and induced the contents of secondary plant pigments (flavonoids and anthocyanins) and proline. The results imply that seed treatment with CS and soil application with CA can effectively alleviate Cd(II) induced toxicity in B. juncea by strengthening its antioxidative defence system and enhancing compatible solute accumulation.

Comparative study of Cd uptake and tolerance of two Italian ryegrass (Lolium multiflorum) cultivars

Cadmium (Cd) is one of the most toxic heavy metals and is difficult to be removed from contaminated soil and water. Italian ryegrass (Lolium multiflorum), as an energy crop, exhibits a valuable potential to develop Cd polluted sites due to its use as a biofuel rather than as food and forage. Previously, via a screening for Cd-tolerant ryegrass, the two most extreme cultivars (IdyII and Harukaze) with high and low Cd tolerance during seed germination, respectively, were selected. However, the underlying mechanism for Cd tolerance was not well investigated. In this study, we comparatively investigated the growth, physiological responses, and Cd uptake and translocation of IdyII and Harukaze when the seedlings were exposed to a Cd (0–100 μM) solution for 12 days. As expected, excess Cd inhibited seedling growth and was accompanied by an accumulation of malondialdehyde (MDA) and reduced photosynthetic pigments in both cultivars. The effects of Cd on the uptake and translocation of other nutrient elements (Zn, Fe, Mn and Mg) were dependent on Cd concentrations, cultivars, plant tissues and elements. Compared with Harukaze, IdyII exhibited better performance with less MDA and higher pigment content. Furthermore, IdyII was less efficient in Cd uptake and translocation compared to Harukaze, which might be explained by the higher non-protein thiols content in its roots. Taken together, our data indicate that IdyII is more tolerant than Harukaze, which partially resulted from the differences in Cd uptake and translocation.

Effects of triacontanol on ascorbate-glutathione cycle in Brassica napus L. exposed to cadmium-induced oxidative stress

The ability of exogenous triacontanol (TRIA), a plant growth regulator, to reduce Cd toxicity was studied in canola (Brassica napus L.) plants. The following biological parameters were examined in canola seedlings to investigate TRIA-induced tolerance to Cd toxicity: seedling growth, chlorophyll damage and antioxidant response. In particular, TRIA application reduced Cd-induced oxidative damage, as shown by reduction of ROS content, lipoxygenase (LOX) activity and lipid peroxidation level. TRIA pretreatment increased non-enzymatic antioxidant contents (ascorbate, AsA, glutathione and GSH), phytochelatin content (PCs) and activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), monodehydroascorbate reductase (MDHAR), dehydro ascorbate reductase (DHAR), and glutathione reductase (GR), so reducing the oxidative stress. These results clearly indicate the protective ability of TRIA to modulate the redox status through the antioxidant pathway AGC and GSH, so reducing Cd-induced oxidative stress.