Cadmium accumulation in chloroplasts and its impact on chloroplastic processes in barley and maize

Cadmium/zinc stresses and plant cultivation influenced soil microflora: a pot experiment conducted in field

It's of great challenge to address for heavy metal-contaminated soil. Once the farmland is contaminated with heavy metals, the microbial ecology of the plant rhizosphere will change, which in turn impacts crop productivity and quality. However, few studies have explored the effects of heavy metals on plant rhizosphere microbes in farmland and the role that plant cultivation plays in such a phytoremediation practice. In this study, the impacts of comfrey (Symphytum officinale L.) cultivation and the stresses of cadmium/zinc (Cd/Zn) on rhizosphere soil microflora were examined. Microbial DNA was collected from soils to evaluate the prevalence of bacteria and fungi communities in rhizosphere soils. High-throughput 16 S rRNA sequencing was used to determine the diversity of the bacterial and fungal communities. The results showed that growing comfrey on polluted soils reduced the levels of Cd and Zn from the vertical profile. Both the comfrey growth and Cd/Zn stresses affected the community of rhizosphere microorganisms (bacteria or fungi). Additionally, the analysis of PCoA and NMDS indicated that the cultivation of comfrey significantly changed the bacterial composition and structure of unpolluted soil. Comfrey cultivation in polluted and unpolluted soils did not result in much variance in the fungi's species composition, but the fungal compositions of the two-type soils were noticeably different. This work provided a better understanding of the impacts of Cd/Zn stresses and comfrey cultivation on rhizosphere microbial community, as well as new insight into phytoremediation of heavy metal-contaminated soils.

Synthesis and properties of nano-cadmium oxide and its size-dependent responses by barley plant

Present study included technological methods that made it possible to synthesize CdO nanoparticles and carry out their qualitative and quantitative diagnostics, confirming the as-prepared CdO nanoparticles (NPs) were spherical and had a size of 25 nm. Then, under the conditions of the model experiment the effect of CdO in macro and nanosized particles on absorption, transformation, and structural and functional changes occurring in cells and tissues of Hordeum vulgare L. (spring barley) during its ontogenesis was analyzed. Different analytical techniques were used to detect the transformation of CdO forms: Fourier-transform infrared spectroscopy (FTIR), Dynamic light scattering (DLS), X-ray fluorescence analysis (XRF), Scanning electron microscopy (SEM-EDXMA and TEM), X-ray diffraction (XRD), and X-ray absorption fine structure, consists of XANES - X-ray absorption near edge structure, and EXAFS - Extended X-ray absorption fine structure. Quantitative differences in the elemental chemical composition of barley root and leaf samples were observed. The predominant root uptake of Cd was revealed. CdO-NPs were found to penetrate deeply into barley plant tissues, where they accumulated and formed new mineral phases such as Cd5(PO4)3Cl and CdSO4 according to XRD analysis. The molecular-structural state of the local Cd environment in plant samples corresponding to Cd-O and Cd-Cd. The toxicity of CdO-NPs was found to significantly affect the morphology of intracellular structures are the main organelles of photosynthesis therefore, destructive changes in them obviously reduce the level of metabolic processes ensuring the growth of plants. This study is an attempt to show results how it is possible to combine some instrumental techniques to characterize and behavior of NPs in complex matrices of living organisms.

Expression of OsHARBI1-1 enhances the tolerance of Arabidopsis thaliana to cadmium

BACKGROUND

As one of the major food crops in the world, rice is vulnerable to cadmium (Cd) pollution. Understanding of the molecular mechanisms of Cd uptake, transport and detoxification in rice is essential for the breeding of low-Cd rice. However, the molecular mechanisms underlying the response of rice to Cd stress remains to be further clarified.

RESULTS

In this study, a novel Cd-responsive gene OsHARBI1-1 was identified in the rice genome and its expression pattern and function were characterized. Bioinformatics analysis showed that the promoter region of OsHARBI1-1 had multiple cis-acting elements in response to phytohormones and stress, and the expression of OsHARBI1-1 was induced by phytohormones. OsHARBI1-1 protein was targeted to the nucleus. qRT-PCR analysis results showed that the expression of OsHARBI1-1 in the roots was repressed while the expression in the shoots was increased under Cd stress. Heterologous expression of OsHARBI1-1 in yeast conferred tolerance to Cd and reduced Cd content in the cells. Meanwhile, the expression of OsHARBI1-1 in Arabidopsis thaliana (A. thaliana) enhanced the tolerance of A. thaliana to Cd stress. In addition, compared with the wild type plants, the POD activity of transgenic plants was increased, while the SOD and CAT activities were decreased. Interestingly, the accumulation of Cd in the roots of A. thaliana expressing OsHARBI1-1 was significantly increased, whereas the Cd accumulation in the shoots was slightly decreased. Compared to the WT plants, the expression of genes related to Cd absorption and chelation was upregulated in transgenic A. thaliana under Cd stress, while the expression of genes responsible for the translocation of Cd from the roots to the shoots was downregulated. Moreover, the expression of phytohormone-related genes was significantly influenced by the expression of OsHARBI1-1 with and without Cd treatment.

CONCLUSIONS

Findings of this study suggest that OsHARBI1-1 might play a role in the response of plants to Cd response by affecting antioxidant enzyme activities, Cd chelation, absorption and transport, and phytohormone homeostasis and signaling.

The Influence of Cadmium on Fountain Grass Performance Correlates Closely with Metabolite Profiles

The relationship between metabolite changes and biological endpoints in response to cadmium (Cd) stress remains unclear. Fountain grass has good Cd enrichment and tolerance abilities and is widely used in agriculture and landscaping. We analyzed the metabolic responses by detecting the metabolites through UPLC-MS and examined the relationships between metabolite changes and the characteristics of morphology and physiology to different Cd stress in fountain grass. Our results showed that under Cd stress, 102 differential metabolites in roots and 48 differential metabolites in leaves were detected, with 20 shared metabolites. Under Cd stress, most of the carbohydrates in leaves and roots decreased, which contributed to the lowered leaf/root length and fresh weight. In comparison, most of the differential amino acids and lipids decreased in the leaves but increased in the roots. Almost all the differential amino acids in the roots were negatively correlated with root length and root fresh weight, while they were positively correlated with malondialdehyde content. However, most of the differential amino acids in the leaves were positively correlated with leaf length and leaf fresh weight but negatively correlated with malondialdehyde content. Metabolic pathway analysis showed that Cd significantly affects seven and eight metabolic pathways in the leaves and roots, respectively, with only purine metabolism co-existing in the roots and leaves. Our study is the first statement on metabolic responses to Cd stress and the relationships between differential metabolites and biological endpoints in fountain grass. The coordination between various metabolic pathways in fountain grass enables plants to adapt to Cd stress. This study provides a comprehensive framework by explaining the metabolic plasticity and Cd tolerance mechanisms of plants.

Inhibition Roles of Calcium in Cadmium Uptake and Translocation in Rice: A Review

Cadmium (Cd) contamination in rice grains is posing a significant threat to global food security. To restrict the transport of Cd in the soil-rice system, an efficient way is to use the ionomics strategy. Since calcium (Ca) and Cd have similar ionic radii, their uptake and translocation may be linked in multiple aspects in rice. However, the underlying antagonistic mechanisms are still not fully understood. Therefore, we first summarized the current knowledge on the physiological and molecular footprints of Cd translocation in plants and then explored the potential antagonistic points between Ca and Cd in rice, including exchange adsorption on roots, plant cell-wall composition, co-transporter gene expression, and transpiration inhibition. This review provides suggestions for Ca/Cd interaction studies on rice and introduces ionomics research as a means of better controlling the accumulation of Cd in plants.

Integrated physio-biochemical and transcriptomic analysis revealed mechanism underlying of Si-mediated alleviation to cadmium toxicity in wheat

Cadmium (Cd) contamination has resulted in serious reduction of crop yields. Silicon (Si), as a beneficial element, regulates plant growth to heavy metal toxicity mainly through reducing metal uptake and protecting plants from oxidative injury. However, the molecular mechanism underlying Si-mediated Cd toxicity in wheat has not been well understood. This study aimed to reveal the beneficial role of Si (1 mM) in alleviating Cd-induced toxicity in wheat (Triticum aestivum) seedlings. The results showed that exogenous supply of Si decreased Cd concentration by 67.45% (root) and 70.34% (shoot), and maintained ionic homeostasis through the function of important transporters, such as Lsi, ZIP, Nramp5 and HIPP. Si ameliorated Cd-induced photosynthetic performance inhibition through up-regulating photosynthesis-related genes and light harvesting-related genes. Si minimized Cd-induced oxidative stress by decreasing MDA contents by 46.62% (leaf) and 75.09% (root), and helped re-establish redox homeostasis by regulating antioxidant enzymes activities, AsA-GSH cycle and expression of relevant genes through signal transduction pathway. The results revealed molecular mechanism of Si-mediated wheat tolerance to Cd toxicity. Si fertilizer is suggested to be applied in Cd contaminated soil for food safety production as a beneficial and eco-friendly element.

Evaluation of phytoremediation potential and resistance of Gladiolus grandiflora L. against cadmium stress

Although irrigation water is a fundamental need for plant growth, it is also a source of pollutants if contaminated with harmful materials like cadmium (Cd). Irrigation water possessing abundant Cd causes damage to soil, plants, animals and ultimately human beings through the food chain. A pot experiment was conducted to evaluate the gladiolus (Gladiolus grandiflora L.) potential of Cd accumulation and the capability of the plant to be an economically beneficial choice in presence of high Cd irrigation water supply. Artificially prepared four levels of Cd irrigation water were applied to the plants viz., 30, 60, 90 and 120 mg L^-1. The results revealed that 30 mg L^-1 Cd had no difference in all growth-related parameters when compared to the control. Photosynthesis rate, stomatal conductance and transpiration rate along with plant height and spike length were reduced with high accumulation levels of Cd in plants. The main plant portion for Cd storage found in Gladiolus grandiflora L was corm where the amount of Cd was 10-12 times higher than the amount found in leaves, and 2-4 times more than the stem. This deportment was further established by the translocation factor (TF). In corm to shoot TF and corm to stem TF, the factor reduced with increasing Cd levels, while, in corm to leaves TF, Cd levels were statistically non-significant. From corm to shoot TF value of 0.68 and 0.43 in case of 30 and 60 mg L^-1, Cd treatments indicates good phytoremediation potential of Gladiolus in low and moderate Cd-polluted environments. Conclusively, the study reveals the good capability of Gladiolus grandiflora L. to harvest Cd from the soil and water in reasonably good amount with sufficient potential to grow under irrigation-based Cd stress. Under revelations of the study, Gladiolus grandiflora L appeared as a Cd accumulator which could potentially be used as a sustainable approach for phytoremediation of Cd.

Genotypic variation in the tolerance to moderate cadmium toxicity among 20 maize genotypes with contrasting root systems

BACKGROUND

Cadmium (Cd) contamination in farmland is a serious environmental and safety issue affecting plant growth, crop productivity, and human health. This study aimed to investigate genotypic variation in root morphology and Cd accumulations under moderate Cd stress among diverse maize genotypes. Twenty maize genotypes with contrasting root systems were assessed for Cd tolerance 39 days after transplanting (V6, six-leaf stage) under 20 μmol L^-1 CdCl₂ using a semi-hydroponic phenotyping platform in a glasshouse.

RESULTS

Cadmium stress significantly inhibited plant growth across all genotypes. Genotypic variation in response to Cd toxicity was apparent: shoot dry weight varied from 0.13 (genotype NS2020) to 0.35 g plant^-1 (Dongke301) with deductions up to 63% compared with non-Cd treatment (CK). Root dry weight of 20 genotypes ranged from 0.06 (NS2020) to 0.18 g plant^-1 (Dongke301) with a deduction up to 56%. Root length ranged from 2.21 (NS590b) to 9.22 m (Dongke301) with a maximal decline of 76%. Cadmium-treated genotypes generally had thicker roots and average diameter increased by 34% compared with CK. Genotypes had up to 3.25 and 3.50 times differences in shoot and root Cd concentrations, respectively. Principal component and cluster analyses assigned the 20 genotypes into Cd-tolerant (five genotypes) and Cd-sensitive (15 genotypes) groups.

CONCLUSIONS

Maize genotypes varied significantly in response to moderate Cd stress. Cadmium-tolerant genotypes optimized root morphology and Cd accumulation and distribution. This study could assist in the selection and breeding of new cultivars with improved adaptation to Cd-contaminated soil for food and feed or land remediation purposes. © 2022 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Proanthocyanidins Alleviate Cadmium Stress in Industrial Hemp (Cannabis sativa L.)

Industrial hemp (Cannabis sativa L.), an annual herbaceous cash crop, is widely used for the remediation of heavy metal-contaminated soils due to its short growth cycle, high tolerance, high biomass, and lack of susceptibility to transfer heavy metals into the human food chain. In this study, a significant increase in proanthocyanidins was found in Yunnan hemp no. 1 after cadmium stress. Proanthocyanidins are presumed to be a key secondary metabolite for cadmium stress mitigation. Therefore, to investigate the effect of proanthocyanidins on industrial hemp under cadmium stress, four experimental treatments were set up: normal environment, cadmium stress, proanthocyanidin treatment, and cadmium stress after pretreatment with proanthocyanidins. The phenotypes from the different treatments were compared. The experimental results showed that pretreatment with proanthocyanidins significantly alleviated cadmium toxicity in industrial hemp. The transcriptome and metabolome of industrial hemp were evaluated in the different treatments. Proanthocyanidin treatment and cadmium stress in industrial hemp mainly affected gene expression in metabolic pathways associated with glutathione metabolism, phenylpropanoids, and photosynthesis, which in turn altered the metabolite content in metabolic pathways of phenylalanine, vitamin metabolism, and carotenoid synthesis. The combined transcriptomic and metabolomic analysis revealed that proanthocyanidins mitigated cadmium toxicity by enhancing photosynthesis, secondary metabolite synthesis, and antioxidant synthesis. In addition, exogenous proanthocyanidins and cadmium ions acted simultaneously on EDS1 to induce the production of large amounts of salicylic acid in the plant. Finally, overexpression of CsANR and CsLAR, key genes for proanthocyanidins synthesis in industrial hemp, was established in Arabidopsis plants. The corresponding plants were subjected to cadmium stress, and the results showed that CsLAR transgenic plants were more tolerant to cadmium than the CsANR transgenic and wild-type Arabidopsis plants. The results showed that salicylic acid and jasmonic acid were increased in Arabidopsis overexpressing CsLAR compared to AT wild-type Arabidopsis, and levels of secondary metabolites were significantly higher in Arabidopsis overexpressing CsLAR than in AT wild-type Arabidopsis. These results revealed how proanthocyanidins alleviated cadmium stress and laid the foundation for breeding industrial hemp varieties with higher levels of proanthocyanidins and greater tolerance.

Cadmium toxicity symptoms and uptake mechanism in plants: a review

Cadmium (Cd) is one of non-essential heavy metals which is released into environment naturally or anthropogenically. It is highly persistent toxic metals that are exceptionally distressing industrial and agriculture activities by contaminating soil, water and food. Its long-duration endurance in soil and water results in accumulation and uptake into plants, leading to the food chain. This becomes a serious global problem threatening humans and animals as food chain components. Living organisms, especially humans, are exposed to Cd through plants as one of the main vegetative food sources. This review paper is concentrated on the symptoms of the plants affected by Cd toxicity. The absorption of Cd triggers several seen and unseen symptoms by polluted plants such as stunted growth, chlorosis, necrosis and wilting. Apart from that, factors that affect the uptake and translocation of Cd in plants are elaborated to understand the mechanism that contributes to its accumulation. By insight of Cd accumulation, this review also discussed the phytoremediation techniques-phytoextraction, phytostimulation, phytostabilization, phytovolatization and rhizofiltration in bioremediating the Cd.

Carcinogenic Risk of Pb, Cd, Ni, and Cr and Critical Ecological Risk of Cd and Cu in Soil and Groundwater around the Municipal Solid Waste Open Dump in Central Thailand

Several consequences of health effects from municipal solid waste caused by carcinogenic and noncarcinogenic metals have been recognized. The water quality index ( $\text{WQI}$ ) in the groundwater around this landfill is 2945.58, which is unacceptable for consumption. The contaminated groundwater mainly appears within a 1 km radius around the landfill. The metal pollution levels in the soil in descending order were Cu > Cd > Zn=Cr > Pb > Ni. The pollution degree (ER) of Cd was 2898.88, and the potential ecological risk index (RI) was 2945.58, indicating that the risk level was very high. Surprisingly, the hazard index (HI) of Pb (2.05) and Fe (1.59) in children was higher than 1. This indicated that the chronic risk and cancer risk caused by Pb and Fe for children were at a medium level. Carcinogenic risk by oral (CR oral) consumption of Ni, Cd, and Cr in children was 1.4E − 04, 2.5E − 04, and 1.8E − 04, respectively, while the lifetime carcinogenic risk (LCR) of Ni, Cd, and Cr in children was 1.5E − 04, 2.8E − 04, and 2.0E − 04, respectively. In adults, CR oral of Ni and Cr were 1.6E − 03 and 3.0E − 04, respectively, while LCR of Ni and Cr were 1.6E − 03 and 3.4E − 04, respectively, which exceeded the carcinogenic risks limits. Our study indicated a lifetime carcinogenic risk to humans. Environmental surveillance should focus on reducing health risks such as continuous monitoring of the groundwater, soil, and leachate treatment process.

Metal tolerance in plants: Molecular and physicochemical interface determines the "not so heavy effect" of heavy metals

An increase in technological interventions and ruthless urbanization in the name of development has deteriorated our environment over time and caused the buildup of heavy metals (HMs) in the soil and water resources. These heavy metals are gaining increased access into our food chain through the plant and/or animal-based products, to adversely impact human health. The issue of how to restrict the entry of HMs or modulate their response in event of their ingress into the plant system is worrisome. The current knowledge on the interactive-regulatory role and contribution of different physical, biophysical, biochemical, physiological, and molecular factors that determine the heavy metal availability-uptake-partitioning dynamics in the soil-plant-environment needs to be updated. The present review critically analyses the interactive overlaps between different adaptation and tolerance strategies that may be causally related to their cellular localization, conjugation and homeostasis, a relative affinity for the transporters, rhizosphere modifications, activation of efflux pumps and vacuolar sequestration that singly or collectively determine a plant's response to HM stress. Recently postulated role of gaseous pollutants such as SO₂ and other secondary metabolites in heavy metal tolerance, which may be regulated at the whole plant and/or tissue/cell is discussed to delineate and work towards a "not so heavy" response of plants to heavy metals present in the contaminated soils.

Foliar application of seed water extract of Nigella sativa improved maize growth in cadmium-contaminated soil

Cadmium (Cd) is a widespread heavy metal, which commonly exert negative impacts on agricultural soils and living organisms. Foliar application of seed water extract of black cumin (Nigella sativa L.) can mitigate the adverse impacts of Cd-toxicity in plants through its rich antioxidants. This study examined the role of seed water extracts of N. sativa (NSE) in mitigating the adverse impacts of Cd-toxicity on maize growth. Two maize genotypes (synthetic ‘Neelum’ and hybrid ‘P1543’) were grown under 0, 4, 8 and 12 mg Cd kg^-1 soil. The NSE was applied at three different concentrations (i.e., 0, 10 and 20%) as foliar spray at 25 and 45 days after sowing. All Cd concentrations had no effect on germination percentage of both genotypes. Increasing Cd concentration linearly decreased root and allometric attributes, gas exchange traits and relative water contents of hybrid genotype. However, gas exchange traits of synthetic genotype remained unaffected by Cd-toxicity. Overall, hybrid genotype showed better tolerance to Cd-toxicity than synthetic genotype with better germination and allometric attributes and less Cd accumulation. Foliar application of NSE lowered negative effects of Cd-toxicity on all studied traits, except relative water contents. In conclusion, foliar application of NSE seemed a viable option to improve maize growth in Cd-contaminated soil.

Effects of electric fields on Cd accumulation and photosynthesis in Zea mays seedlings

Phytoremediation enhanced by electrokinetic has been considered as a potential technology for remediating contaminated soils. However, the effects of electric fields on Cd accumulation and photosynthesis in Zea mays (as a cathode) is still unclear. In the present study, Zea mays seedlings were exposed to various doses of Cd²⁺ (10, 50, 100 μM) to explore the impact of electric fields on Cd accumulation and photosynthesis of Zea mays. Results showed that upon exposure to a concentration of 100 μM Cd, electric fields significantly altered the Cd contents in maize shoots, whereas the concentration of 50 μM Cd increased the Cd contents in maize roots as well as affected the Cd transport from roots to shoots. Uptake index (UI) increased by 1.34%-66.16% with the application of electric fields. The variation of photosynthetic rates attributed to the open or closure of stoma was similar to the change of shoot fresh weight, particularly in maize exposed to high Cd stress. This study proposes a new technology in Cd phytoremediation and provides important information on physiological processes in maize when exposed to Cd stress and electric fields.

Analysis of 2,4-epibrassinolide created an enhancement tolerance on Cd toxicity in Solanum nigrum L

Contamination of soils with cadmium (Cd) is a serious problem worldwide. Solanum nigrum L. is reported as a Cd hyperaccumulator, but its enrichment capacity is limited. 2,4-Epibrassinolide (2,4-EBL) plays important roles in plant response to various stresses. Little is known about its effect on Cd tolerance in S. nigrum. Current study was performed to demonstrate effects of 2,4-EBL on plant growth, photosynthesis activity, activities of antioxidants, and Cd concentration in plants by nutrient solution contaminated with Cd. Results revealed that S. nigrum exhibited toxicity to Cd stress, including reducing plant height, root length, and chlorophyll content and increasing malondialdehyde (MDA) content. Exogenous application of 2,4-EBL significantly enhanced the contents of proline and soluble sugar and decreased the MDA content. Meanwhile, the levels of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) markedly increased compared with the control. Interesting, 2,4-EBL promoted photosynthesis by increasing the chlorophyll content, Fv/Fm. And increase in chlorophyll content is caused by increased expression of synthetic genes and decreased expression of degraded genes. 2,4-EBL also decreased accumulation of Cd in S. nigrum compared with single Cd stress. According to the present results, 2,4-EBL can effectively be used to alleviate the damage of Cd stress in S. nigrum and probably in other solanaceae.

Specificity of Cd, Cu, and Fe effects on barley growth, metal contents in leaves and chloroplasts, and activities of photosystem I and photosystem II

Cd, Cu, and Fe were used to reveal the specificity of their toxic actions. We studied the effects of heavy metals on the growth of barley seedlings, contents of cations in leaves and chloroplasts, induced chlorophyll fluorescence and P₇₀₀ light absorption. Differences were found at each level of research. We measured the contents of Cd, Cu, Fe, Mn, Zn, Ca, Mg, and K. The proportion of cations in leaves targeted to chloroplasts varied from 0.1% (K) to >90% (Fe). Their levels changed in different ways. We found no correlation between changes in cation contents in leaves and chloroplasts. Treatment with Cd, Cu, and Fe increased the contents of some cations. The extra portions were targeted primarily out of chloroplasts, which was most noticeable in the case of Cu and Fe. Cd treatment decreased non-photochemical quenching with concomitant increases in closed photosystem II. We introduced new coefficients qC for closed photosystem II and X(II) to compare the yields of photosystem II and photosystem I. Cd likely decreased both PSI content in leaves and its quantum yield. In control plants, the quantum yield ratio of PSI/PSII increased gradually from 1.25 under low light to 4 under high light. Cd treatment prevented the increase under moderate light; under high light the ratio reached 2. Cu treatment increased the acceptor side limitation of photosystem I under low light; components of the Calvin cycle likely demand more light for activation in Cu-treated plants.

Probing the Role of the Chloroplasts in Heavy Metal Tolerance and Accumulation in Euglena gracilis

The E. gracilis Zm-strain lacking chloroplasts, characterized in this study, was compared with the earlier assessed wild type Z-strain to explore the role of chloroplasts in heavy metal accumulation and tolerance. Comparison of the minimum inhibitory concentration (MIC) values indicated that both strains tolerated similar concentrations of mercury (Hg) and lead (Pb), but cadmium (Cd) tolerance of the Z-strain was twice that of the Zm-strain. The ability of the Zm-strain to accumulate Hg was higher compared to the Z-strain, indicating the existence of a Hg transportation and accumulation mechanism not depending on the presence of chloroplasts. Transmission electron microscopy (TEM) showed maximum accumulation of Hg in the cytosol of the Zm-strain and highest accumulation of Cd in the chloroplasts of the Z-strain indicating a difference in the ability of the two strains to deposit heavy metals in the cell. The highly abundant heavy metal transporter MTP2 in the Z-strain may have a role in Cd transportation to the chloroplasts. A multidrug resistance-associated protein highly increased in abundance in the Zm-strain could be a potential Hg transporter to either cytosol or mitochondria. Overall, the chloroplasts appear to have major role in the tolerance and accumulation of Cd in E. gracilis.

New insights into cadmium stressful-conditions: Role of ethylene on selenium-mediated antioxidant enzymes

Cadmium (Cd) contamination has generated an environmental problem worldwide, leading to harmful effects on human health and damages to plant metabolism. Selenium (Se) is non essential for plants, however it can improve plant growth and reduce the adverse effects of abiotic stress. In addition, ethylene may interplay the positive effects of Se in plants. In order to investigate the role of ethylene in Se-modulation of antioxidant defence system in response to Cd-stress, we tested the hormonal mutant Epinastic (epi) with a subset of constitutive activation of the ethylene response and Micro-Tom (MT) plants. For this purpose, Se mineral uptake, Cd and Se concentrations, pigments, malondialdeyde (MDA) and hydrogen peroxide (H₂O₂) contents, ethylene production, glutathione (GSH) compound, and superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR) and glutathione peroxidase (GSH-Px) activities were analysed in MT and epi plants submitted to 0.5 mM CdCl₂ and 1 μM of selenate or selenite. MT plants treated with both Se forms increased growth in the presence or not of 0.5 mM CdCl₂, but not change epi growth. Both Se forms reduced Cd uptake in MT plants and cause reverse effect in epi plants. P, Mg, S, K and Zn uptake increased in epi plants with Se application, irrespective to Cd exposure. Chlorophylls and carotenoids contents decreased in both genotypes under Cd exposure, in contrast to what was observed in epi leaves in the presence of Se. When antioxidant enzymes activities were concerned, Se application increased Mn-SOD, Fe-SOD and APX activities. In the presence of Cd, MT and epi plants exhibited decreased SOD activity and increased CAT, APX and GR activities. MT and epi plants with Se supply exhibited increased APX and GR activities in the presence of Cd. Overall, these results suggest that ethylene may be involved in Se induced-defence responses, that triggers a positive response of the antioxidant system and improve growth under Cd stress. These results showed integrative roles of ethylene and Se in regulating the cell responses to stressful-conditions and, the cross-tolerance to stress could be used to manipulate ethylene regulated gene expression to induce heavy metal tolerance.

Enterobacter asburiae Reduces Cadmium Toxicity in Maize Plants by Repressing Iron Uptake-Associated Pathways

Soil microbes have recently been utilized to improve cadmium (Cd) tolerance and lower its accumulation in plants. Nevertheless, whether rhizobacteria can prevent Cd uptake by graminaceous plants and the underlying mechanisms remain elusive. In this study, inoculation with Enterobacter asburiae NC16 reduced transpiration rates and the expression of some iron (Fe) uptake-related genes including ZmFer, ZmYS1, ZmZIP, and ZmNAS2 in maize (Zea mays) plants, which contributed to mitigation of Cd toxicity. However, the inoculation with NC16 failed to suppress the transpiration rates and transcription of these Fe uptake-related genes in plants treated with fluridone, an abscisic acid (ABA) biosynthetic inhibitor, indicating that the impacts of NC16-inoculation observed were dependent on the actions of ABA. We found that NC16 increased the host ABA levels by mediating the metabolism of ABA rather than its synthesis. Moreover, the capacity of NC16 to inhibit plant uptake of Cd was greatly weakened in plants overexpressing ZmZIP, encoding a zinc/iron transporter. Collectively, our findings indicated that E. asburiae NC16 reduced Cd toxicity in maize plants at least partially by hampering the Fe uptake-associated pathways.

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.

Distribution of Cd and other cations between the stroma and thylakoids: a quantitative approach to the search for Cd targets in chloroplasts

Plant growth and photosynthetic activity are usually inhibited due to the overall action of Cd on a whole organism, though few cadmium cations can invade chloroplasts in vivo. We found that in vivo, the major portion of Cd in barley chloroplasts is located in the thylakoids (80%), and the minor portion is in the stroma (20%). Therefore, the electron-transport chain in the thylakoids would be the likely target for direct Cd action in vivo. In vitro, we found the distribution of Cd to be shifted to the stroma (40-60%). In barley chloroplasts, the major portions of Mg, Fe, Mn, and Cu were found to be located in the thylakoids, and most Ca, Zn, and K in the stroma. This finding was true for both control and Cu- or Fe-treated plants. Treatment with Cd affected the contents of all cations, and the largest portions of Ca and Zn were in the thylakoids. Alterations of the K and Mn contents were caused by Cd, Cu, or Fe treatment; the levels of other cations in chloroplasts were changed specifically by Cd treatment. The quantity of Cd in chloroplasts was small in comparison to that of Mg, Ca, and Fe. In thylakoids, the amount of Cd was similar to that of Cu and comparable to the levels of Zn and Mn. Accordingly, the possible targets for direct Cd action in thylakoids are the Mn cluster, plastocyanin, carbonic anhydrase, or FtsH protease. The quantity of Cd in thylakoids is sufficient to replace a cation nearly completely at one of these sites or partially (20-30%) at many of these sites.

Differential impact of heat stress on the expression of chloroplast-encoded genes

Heat shock is one of the major abiotic factors that causes severe retardation in plant growth and development. To dissect the principal effects of hyperthermia on chloroplast gene expression, we studied the temporal dynamics of transcript accumulation for chloroplast-encoded genes in Arabidopsis thaliana and genes for the chloroplast transcription machinery against a background of changes in physiological parameters. A marked reduction in the transcript amounts of the majority of the genes at the early phases of heat shock (HS) was followed by a return to the baseline levels of rbcL and the housekeeping genes clpP, accD, rps14 and rrn16. The decline in the mRNA levels of trnE (for tRNA^glu) and the PSI genes psaA and psaB was opposed by the transient increase in the transcript accumulation of ndhF and the PSII genes psbA, psbD, and psbN and their subsequent reduction with the development of stress. However, the up-regulation of PSII genes in response to elevated temperature was absent in the heat stress-sensitive mutants abi1 and abi2 with the impaired degradation of D2 protein. The expression of rpoA and rpoB, which encode subunits of PEP, was strongly down-regulated throughout the duration of the heat treatment. In addition, heat stress-induced PEP deficiency caused the compensatory up-regulation of the genes for the nuclear-encoded RNA polymerases RPOTp and RPOTmp, the PEP-associated proteins PAP6 and PAP8, the Ser/Thr protein kinase cPCK2, and the stress-inducible sigma factor gene SIG5. Thus, heat stress differentially modulates the transcript accumulation of plastid-encoded genes in A. thaliana at least in part via the expression of HS-responsive nuclear genes for the plastid transcription machinery.

Role of Burkholderia cepacia CS8 in Cd-stress alleviation and phytoremediation by Catharanthus roseus

The current study was performed to assess the effect of Burkholderia cepacia CS8 on the phytoremediation of cadmium (Cd) by Catharanthus roseus grown in Cd-contaminated soil. The plants cultivated in Cd amended soil showed reduced growth, dry mass, gas-exchange capacity, and chlorophyll contents. Furthermore, the plants exhibited elevated levels of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) under Cd stress. The bacterized plants showed higher shoot length, root length; fresh and dry weight. The improved stress tolerance in inoculated plants was attributed to the reduced quantity of MDA and H₂O₂, enhanced synthesis of protein, proline, phenols, flavonoids, and improved activity of antioxidant enzymes including peroxidase, superoxide dismutase, ascorbate peroxidase, and catalase. Similarly, the 1-aminocyclopropane-1-carboxylate deaminase activity, phosphate solubilization, auxin, and siderophore production capability of B. cepacia CS8 improved growth and stress alleviation in treated plants. The bacterial inoculation enhanced the amount of water extractable Cd from soil. Furthermore, the inoculated plants showed higher bioconcentration factor and translocation factor. The current study exhibits that B. cepacia CS8 improves stress alleviation and phytoextraction potential of C. roseus plants growing under Cd stress.

The beneficial role of potassium in Cd-induced stress alleviation and growth improvement in Gladiolus grandiflora L

Heavy metal contaminated agricultural soils are one of the most important constraints for successful cultivation of crops. The current research was conducted to evaluate the role of potassium (K) on plant growth and amelioration of cadmium (Cd) stress in Gladiolus grandiflora under greenhouse conditions. G. grandiflora corms were sown in media contaminated with 0 (C), 50 (Cd50) and 100 (Cd100) mg Cd kg^-1 soil. The plants growing in Cd-contaminated media exhibited reduced gas exchange attributes, chlorophyll (Chl) contents, vegetative and reproductive growth as compared to control. The plants raised in Cd contaminated media showed reduced nutrition yet higher Cd contents. However, supplementation of 60 mg Kg^-1 K in treated plants (C+K, Cd50+K and Cd100+K) improved quantity of total soluble protein and proline (Pro) along with activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) under Cd stress. Similarly, K supplementation reduced the level of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) in treated plants. Potassium supplemented plants exhibited better vegetative and reproductive growth. The improved stress tolerance in K supplemented plants was attributed to the reduced quantity of MDA and H₂O₂, enhanced synthesis of protein, proline, phenols, flavonides and improved activity of antioxidant enzymes. The present research supports the application of K for alleviation of Cd stress in G. grandiflora.

Clonal integration facilitates spread of Paspalum paspaloides from terrestrial to cadmium-contaminated aquatic habitats

Cadmium (Cd) is a hazardous environmental pollutant with high toxicity to plants, which has been detected in many wetlands. Clonal integration (resource translocation) between connected ramets of clonal plants can increase their tolerance to stress. We hypothesised that clonal integration facilitates spread of amphibious clonal plants from terrestrial to Cd-contaminated aquatic habitats. The spread of an amphibious grass Paspalum paspaloides was simulated by growing basal older ramets in uncontaminated soil connected (allowing integration) or not connected (preventing integration) to apical younger ramets of the same fragments in Cd-contaminated water. Cd contamination of apical ramets of P. paspaloides markedly decreased growth and photosynthetic capacity of the apical ramets without connection to the basal ramets, but did not decrease these properties with connection. Cd contamination did not affect growth of the basal ramets without connection to the apical ramets, but Cd contamination of 4 and 12 mg·l^-1 significantly increased growth with connection. Consequently, clonal integration increased growth of the apical ramets, basal ramets and whole clones when the apical ramets were grown in Cd-contaminated water of 4 and 12 mg·l^-1 . Cd was detected in the basal ramets with connection to the apical ramets, suggesting Cd could be translocated due to clonal integration. Clonal integration, most likely through translocation of photosynthates, can support P. paspaloides to spread from terrestrial to Cd-contaminated aquatic habitats. Amphibious clonal plants with a high ability for clonal integration are particularly useful for re-vegetation of degraded aquatic habitats caused by Cd contamination.

Application of Bacillus megaterium MCR-8 improved phytoextraction and stress alleviation of nickel in Vinca rosea

The current research was performed to evaluate the effect of Bacillus megaterium MCR-8 on mitigation of nickel (Ni) stress in Vinca rosea grown on Ni-contaminated soil (50, 100, and 200 mg Ni kg^-1 soil). The treated plants exhibited reduced growth, biomass, gas exchange capacity, and chlorophyll (Chl) content under Ni stress. The inoculated plants growing in Ni-contaminated media exhibited relatively higher growth, total soluble protein, and proline contents. Similarly, bacterial inoculation improved the activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) under Ni stress. The Ni stress alleviation in inoculated plants was attributed to the reduced level of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), enhanced synthesis of protein, proline, phenols, and flavonides in conjunction with improved activity of antioxidant enzymes. The growth-promoting characteristics of microbe such as 1-aminocyclopropane-1-carboxylate deaminase (ACCD) and phosphate solubilization activity, siderophore, and auxin production capability also improved the growth and stress mitigation in inoculated plants. Furthermore, the inoculated plants exhibited higher value for bioconcentration factor (BCF), translocation factor (TF), and resulted in higher loss of Ni content from soil. The current results exhibited the beneficial role of B. megaterium MCR-8 regarding stress alleviation and Ni phytoextraction by V. rosea.

The wheat mutant DELLA-encoding gene (Rht-B1c) affects plant photosynthetic responses to cadmium stress

Тhe sensitivity to cadmium (Cd) stress of two near-isogenic wheat lines with differences at the Rht-B1 locus, Rht-B1a (tall wild type, encoding DELLA proteins) and Rht-B1c (dwarf mutant, encoding modified DELLA proteins), was investigated. The effects of 100 μM CdCl₂ on plant growth, pigment content and functional activity of the photosynthetic apparatus of wheat seedlings grown on a nutrient solution were evaluated through a combination of PAM chlorophyll fluorescence, oxygen evolution, oxidation-reduction kinetics of P700 and 77 K fluorescence. The results showed that the wheat mutant (Rht-B1c) was more tolerant to Cd stress compared to the wild type (Rht-B1a), as evidenced by the lower reductions in plant growth and pigment content, lower inhibition of photosystem I (PSI) and photosystem II (PSII) photochemistry and of the oxygen evolution measured with Clark-type and Joliot-type electrodes. Furthermore, the enhanced Cd tolerance was accompanied by increased Cd accumulation within mutant plant tissues. The molecular mechanisms through which the Rht-B1c mutation improves plant tolerance to Cd stress involve structural alterations in the mutant photosynthetic membranes leading to better protection of the Mn cluster of oxygen-evolving complex and increased capacity for PSI cyclic electron transport, protecting photochemical activity of the photosynthetic apparatus under stress. This study suggests a role for the Rht-B1c-encoded DELLA proteins in protective mechanisms and tolerance of the photosynthetic apparatus in wheat plants exposed to heavy metals stress.

Long-term hyperthermia impairs activity of both photosystems

This is the first study to perform a simultaneous analysis of the activity of photosystem 1 and photosystem 2 after long-term exposure to elevated temperature. It was found that the quantum yield of photochemical reactions decreases in both photosystems. It is shown that, in photosystem 2, the regulated nonphotochemical quenching decreases whereas the unregulated non-photochemical quenching increases. In photosystem 1, limitation on both acceptor and donor sides increases.

Use of Maize (Zea mays L.) for phytomanagement of Cd-contaminated soils: a critical review

Maize (Zea mays L.) has been widely adopted for phytomanagement of cadmium (Cd)-contaminated soils due to its high biomass production and Cd accumulation capacity. This paper reviewed the toxic effects of Cd and its management by maize plants. Maize could tolerate a certain level of Cd in soil while higher Cd stress can decrease seed germination, mineral nutrition, photosynthesis and growth/yields. Toxicity response of maize to Cd varies with cultivar/varieties, growth medium and stress duration/extent. Exogenous application of organic and inorganic amendments has been used for enhancing Cd tolerance of maize. The selection of Cd-tolerant maize cultivar, crop rotation, soil type, and exogenous application of microbes is a representative agronomic practice to enhance Cd tolerance in maize. Proper selection of cultivar and agronomic practices combined with amendments might be successful for the remediation of Cd-contaminated soils with maize. However, there might be the risk of food chain contamination by maize grains obtained from the Cd-contaminated soils. Thus, maize cultivation could be an option for the management of low- and medium-grade Cd-contaminated soils if grain yield is required. On the other hand, maize can be grown on Cd-polluted soils only if biomass is required for energy production purposes. Long-term field trials are required, including risks and benefit analysis for various management strategies aiming Cd phytomanagement with maize.

Photosynthesis and growth response of maize (Zea mays L.) hybrids exposed to cadmium stress

Cadmium (Cd) is a biologically non-essential heavy metal while the cultivation of Cd-tolerant varieties/hybrids (V) seems the most promising strategy for remediation of Cd-contaminated soils. For this, 24-day-old seedlings of seven maize hybrids, DKC 65-25, DKC 61-25, DKC 919, 23-T-16, 32-B-33, 31-P-41, and Syn hybrid, were grown in hydroponic conditions for 21 additional days in various Cd concentrations (0, 5, 10, and 15 μM). Effects of variety, Cd, and their interaction were highly significant (p ≤ 0.05) for studied plant agronomic and physiological traits except the V × Cd interaction for leaf chlorophyll content, root-shoot length, and root dry weight. The Cd accumulation in root and shoot increased gradually with increasing Cd treatments while copper (Cu), zinc (Zn), and manganese (Mn) uptake was decreased in all hybrids. The reduction in root and shoot biomass and Cd uptake was lower in 32-B-33 and 23-T-16 compared to other hybrids. The highest accumulation of Cu, Zn, and Mn was observed in 32-B-33, DK C65-25, and 31-P-41, respectively. The differential uptake and accumulation of Cd by maize hybrids may be useful in selection and breeding for Cd-tolerant genotypes.

Remediation of cadmium toxicity in field peas (Pisum sativum L.) through exogenous silicon

Cadmium (Cd) is an important phytotoxic element causing health hazards. This work investigates whether and how silicon (Si) influences the alleviation of Cd toxicity in field peas at biochemical and molecular level. The addition of Si in Cd-stressed plants noticeably increased growth and development as well as total protein and membrane stability of Cd-stressed plants, suggesting that Si does have critical roles in Cd detoxification in peas. Furthermore, Si supplementation in Cd-stressed plants showed simultaneous significant increase and decrease of Cd and Fe in roots and shoots, respectively, compared with Cd-stressed plants. At molecular level, GSH1 (phytochelatin precursor) and MTA (metallothionein) transcripts predominantly expressed in roots and strongly induced due to Si supplementation in Cd-stressed plants compared with Cd-free conditions, suggesting that these chelating agents may bind to Cd leading to vacuolar sequestration in roots. Furthermore, pea Fe transporter (RIT1) showed downregulation in shoots when plants were treated with Si along with Cd compared with Cd-treated conditions. It is consistent with the physiological observations and supports the conclusion that alleviation of Cd toxicity in pea plants might be associated with Cd sequestration in roots and reduced Cd translocation in shoots through the regulation of Fe transport. Furthermore, increased CAT, POD, SOD and GR activity along with elevated S-metabolites (cysteine, methionine, glutathione) implies the active involvement of ROS scavenging and plays, at least in part, to the Si-mediated alleviation of Cd toxicity in pea. The study provides first mechanistic evidence on the beneficial effect of Si on Cd toxicity in pea plants.

High salinity helps the halophyte Sesuvium portulacastrum in defense against Cd toxicity by maintaining redox balance and photosynthesis

NaCl alleviates Cd toxicity in Sesvium portulacastrum by maintaining plant water status and redox balance, protecting chloroplasts structure and inducing some potential Cd (2+) chelators as GSH and proline. It has been demonstrated that NaCl alleviates Cd-induced growth inhibition in the halophyte Sesuvium portulacastrum. However, the processes that mediate this effect are still unclear. In this work we combined physiological, biochemical and ultrastructural studies to highlight the effects of salt on the redox balance and photosynthesis in Cd-stressed plants. Seedlings were exposed to different Cd concentrations (0, 25 and 50 µM Cd) combined with low (0.09 mM) (LS), or high (200 mM) NaCl (HS) in hydroponic culture. Plant-water relations, photosynthesis rate, leaf gas exchange, chlorophyll fluorescence, chloroplast ultrastructure, and proline and glutathione concentrations were analyzed after 1 month of treatment. In addition, the endogenous levels of stress-related hormones were determined in plants subjected to 25 µM Cd combined with both NaCl concentrations. In plants with low salt supply (LS), Cd reduced growth, induced plant dehydration, disrupted chloroplast structure and functioning, decreased net CO2 assimilation rate (A) and transpiration rate (E), inhibited the maximum potential quantum efficiency (Fv/Fm) and the quantum yield efficiency (Φ PSII) of PSII, and enhanced the non-photochemical quenching (NPQ). The addition of 200 mM NaCl (HS) to the Cd-containing medium culture significantly mitigated Cd phytotoxicity. Hence, even at similar internal Cd concentrations, HS-Cd plants were less affected by Cd than LS-Cd ones. Hence, 200 mM NaCl significantly alleviates Cd-induced toxicity symptoms, growth inhibition, and photosynthesis disturbances. The cell ultrastructure was better preserved in HS-Cd plants but affected in LS-Cd plants. The HS-Cd plants showed also higher concentrations of reduced glutathione (GSH), proline and jasmonic acid (JA) than the LS-Cd plants. However, under LS-Cd conditions, plants maintained higher concentration of salicylic acid (SA) and abscisic acid (ABA) than the HS-Cd ones. We conclude that in S. portulacastrum alleviation of Cd toxicity by NaCl is related to the modification of GSH and proline contents as well as stress hormone levels thus protecting redox balance and photosynthesis.