Toxic metal accumulation, responses to exposure and mechanisms of tolerance in plants

Health risk assessment through consumption of vegetables rich in heavy metals: the case study of the surrounding villages from Panasqueira mine, Central Portugal

Panasqueira mine is a tin-tungsten mineralization hosted by metasediments with quartz veins rich in ferberite. The mineralization also comprises wolframite, cassiterite, chalcopyrite, several sulfides, carbonates and silver sulfosalts. The mining and beneficiation processes produce arsenic-rich mine wastes laid up in huge tailings (Barroca Grande and Rio tailings). The contents of As, Cd, Cr, Cu, Pb and Zn were estimated in rhizosphere soils, irrigation waters, road dusts and in potatoes, cabbages, lettuces and beans, collected on local gardens of four neighborhood Panasqueira mine villages: S. Francisco de Assis (SFA) and Barroca suffering the influence of tailings; Unhais-o-Velho and Casegas considered as non-polluted areas. The mean concentrations of metals in rhizosphere soils and vegetables exceed the reference guidelines values and seem to be linked to the sulfides. The rhizosphere ecological risks were ranked in the order of Cd > As > Cu > Pb > Zn > Cr and SFA > Barroca > Casegas > Unhais-o-Velho. Metal concentrations, in vegetables, were found in the order of lettuce > cabbage > potatoes and SFA > Barroca > Casegas > Unhais-o-Velho. For cabbages and lettuces, the tendency of contamination is roots > leaves and for potatoes is roots > leaves > tubers. The risk for residents, due to ingesting of metals/metalloid, by consuming vegetables grown around the sampling area, was calculated and the result indicates that the inhabitants of these villages are probably exposed to some potential health risks through the intake of heavy metals and metalloids via consuming their vegetables.

The Sulfate Supply Maximizing Arabidopsis Shoot Growth Is Higher under Long- than Short-Term Exposure to Cadmium

The processes involved in cadmium detoxification in plants deeply affect sulfate uptake and thiol homeostasis and generate increases in the plant nutritional request for sulfur. Here, we present an analysis of the dependence of Arabidopsis growth on the concentration of sulfate in the growing medium with the aim of providing evidence on how plants optimize growth at a given sulfate availability. Results revealed that short-term (72 h) exposure to a broad range of Cd concentrations (0.1, 1, and 10 μM) inhibited plant growth but did not produce any significant effects on the growth pattern of both shoots and roots in relation to the external sulfate. Conversely, long-term (22 days) exposure to 0.1 μM Cd significantly changed the pattern of fresh weight accumulation of the shoots in relation to the external sulfate, without affecting that of the roots, although their growth was severely inhibited by Cd. Moreover, under long-term exposure to Cd, increasing the sulfate external concentration up to the critical value progressively reduced the inhibitory effects exerted by Cd on shoot growth, indicating the existence of sulfate-dependent adaptive responses protecting the shoot tissues against Cd injury. Transcriptional induction of the high-affinity sulfate transporter genes (SULTR1; 1 and SULTR1; 2) involved in sulfate uptake by roots was a common adaptive response to both short- and long-term exposure to Cd. Such a response was closely related to the total amount of non-protein thiols accumulated by a single plant under short-term exposure to Cd, but did not showed any clear relation with thiols under long-term exposure to Cd. In this last condition, Cd exposure did not change the level of non-protein thiols per plant and thus did not alter the nutritional need for sulfur. In conclusion, our results indicate that long term-exposure to Cd, although it induces sulfate uptake, decreases the capacity of the Arabidopsis roots to efficiently absorb the sulfate ions available in the growing medium making the adaptive response of SULTR1; 1 and SULTR1; 2 "per se" not enough to optimize the growth at sulfate external concentrations lower than the critical value.

Plants prioritize phytochelatin synthesis during cadmium exposure even under reduced sulfate uptake caused by the disruption of SULTR1;2

Glutathione and phytochelatins are sulfur containing compounds playing an important role in cadmium (Cd) detoxification. We examined the Cd-induced changes in the percentage of sulfur containing compounds to total sulfur in wild-type and sulfate transporter 1;2 knockout mutant, sel1-10. Cd treatment increased the proportion of sulfate and thiols in the total sulfur content. Among the thiols analyzed, the proportion of cysteine and glutathione were decreased by the Cd treatment and that of the phytochelatins were increased. Although the total sulfur content in sel1-10 was decreased compared with that in wild-type, the percentages of individual thiol in the total thiol content were similarly maintained between sel1-10 and wild-type, suggesting that plants tightly controlled the balance of each thiol under Cd treatment.

Effects of different sewage sludge applications on heavy metal accumulation, growth and yield of spinach (Spinacia oleracea L.)

In this study, we present the response of spinach to different amendment rates of sewage sludge (0, 10, 20, 30, 40 and 50 g kg^-1) in a greenhouse pot experiment, where plant growth, biomass and heavy metal uptake were measured. The results showed that sewage sludge application increased soil electric conductivity (EC), organic matter, chromium and zinc concentrations and decreased soil pH. All heavy metal concentrations of the sewage sludge were below the permissible limits for land application of sewage sludge recommended by the Council of the European Communities. Biomass and all growth parameters (except the shoot/root ratio) of spinach showed a positive response to sewage sludge applications up to 40 g kg^-1 compared to the control soil. Increasing the sewage sludge amendment rate caused an increase in all heavy metal concentrations (except lead) in spinach root and shoot. However, all heavy metal concentrations (except chromium and iron) were in the normal range and did not reach the phytotoxic levels. The spinach was characterized by a bioaccumulation factor <1.0 for all heavy metals. The translocation factor (TF) varied among the heavy metals as well as among the sewage sludge amendment rates. Spinach translocation mechanisms clearly restricted heavy metal transport to the edible parts (shoot) because the TFs for all heavy metals (except zinc) were <1.0. In conclusion, sewage sludge used in the present study can be considered for use as a fertilizer in spinach production systems in Saudi Arabia, and the results can serve as a management method for sewage sludge.

Association of urinary cadmium with risk of diabetes: a meta-analysis

The association between urinary cadmium and diabetes risk remains controversial. PubMed, Web of Science, China National Knowledge Infrastructure, and Wanfang Data updated on 21 June 2016 were searched for eligible publications. Pooled odds ratio (OR) with 95% confidence interval (CI) of diabetes for highest versus lowest level of urinary cadmium was calculated by using fixed-effect model or random-effect model. Dose-response relationship between urinary cadmium and diabetes was estimated by restricted cubic spline. A total of nine studies with 28,691 participants were included in this meta-analysis. The pooled OR of diabetes for the highest versus lowest level of urinary cadmium was 1.02 (95% CI, 1.00, 1.05; I ² = 42.3%). In subgroup analysis, the ORs were 1.02 (95% CI 1.00, 1.05; I ² = 0.9%) for studies conducted in Asia and 1.11 (95% CI 0.88, 1.41; I ² = 86.3%) in America. For dose-response analysis, a linear relationship was found between urinary cadmium and the risk of diabetes (P _{for nonlinear} = 0.5856). For every l μg/g creatinine increment of urinary cadmium, the risk of diabetes increased by 16% (1.16, 95% CI 1.08, 1.25). This meta-analysis suggests that cadmium exposure might be significantly associated with prevalence of diabetes, but large prospective studies are needed to confirm this finding.

The effects of endophytic bacterium SaMR12 on Sedum alfredii Hance metal ion uptake and the expression of three transporter family genes after cadmium exposure

A hydroponic experiment was conducted to investigate the effects of an endophytic bacterium SaMR12 on Sedum alfredii Hance metal ion accumulation, chlorophyll concentration, and the expression of three metal transporter families, zinc-regulated transporters, iron-regulated transporter-like protein (ZIP); natural resistance-associated macrophage protein; and heavy metal ATPase (HMA) at different Cd treatment levels. The results showed that at relatively low Cd conditions (≤25 μM), SaMR12 demonstrated a 19.5-27.5% increase in Fe, a 46.7-90.7% increase in Zn, and a 7.9-43.7% increase in Cu content in the shoot and elevated expression of SaIRT1, SaZIP3, SaHMA2, and SaNramp3 in the shoot and SaZIP1, SaHMA2, SaNramp1, and SaNramp3 in the root. At high Cd conditions (100 and 400 μM), SaMR12 demonstrated a 16.4-18.5% increase in leaf chlorophyll concentration, a 18.9-23.2% increase in Fe, and a 15.4-17.5% increase in Mg content in the shoot and elevated expression of SaZIP3, SaNramp6, SaHMA2, and SaHMA3 in the shoot and SaZIP3, SaNarmp1, SaNarmp3, and SaNarmp6 in the root. These results indicated that SaMR12 can elevate essential metal ion uptake and regulate the expression of transport genes to promote plant growth and enhance Cd tolerance and uptake to improve Cd accumulation up to 118-130%.

Biodegradation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by Phanerochaete chrysosporium in the presence of Cd2

Aerobic biodegradation of 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47) by Phanerochaete chrysosporium in the presence of Cd²⁺ was investigated in this study. The results showed that P. chrysosporium could effectively degrade BDE-47, and its extracellular enzyme played an important role in the process of decomposition. BDE-47 biodegradation by fungi was more tolerant than extracellular enzyme in the presence of Cd²⁺. Also, both of the activity of two typical enzymes, MnP and LiP, descended with ascended Cd²⁺ concentration. Based on the four mono-hydroxylated PBDEs (5-OH-BDE-47, 4'-OH-BDE-17, 6-OH-BDE-47, and 2'-OH-BDE-28) and two bromophenols (2,4-DBP, 4-BP) detected, three possible degradation pathways were proposed, inferring that BDE-47 was more easily to transform via hydroxylation. With addition of Cd²⁺, the types of degradation products did not change, merely a variation of the content of these products observed. Meanwhile, the major metabolites of BDE-47, bromophenol compounds, have been found to be transformed or even mineralized by P. chrysosporium quickly, which also helped better explain why the amounts of BDE-47 decomposed did not match with that of the metabolites detected.

NADPH oxidases differentially regulate ROS metabolism and nutrient uptake under cadmium toxicity

The role of NADPH oxidases under cadmium (Cd) toxicity was studied using Arabidopsis thaliana mutants AtrbohC, AtrbohD and AtrbohF, which were grown under hydroponic conditions with 25 and 100 μM Cd for 1 and 5 days. Cadmium reduced the growth of leaves in WT, AtrbohC and D, but not in AtrbohF. A time-dependent increase in H₂ O₂ and lipid peroxidation was observed in all genotypes, with AtrbohC showing the smallest increase. An opposite behaviour was observed with NO accumulation. Cadmium increased catalase activity in WT plants and decreased it in Atrboh mutants, while glutathione reductase and glycolate oxidase activities increased in Atrboh mutants, and superoxide dismutases were down-regulated in AtrbohC. The GSH/GSSG and ASA/DHA couples were also affected by the treatment, principally in AtrbohC and AtrbohF, respectively. Cadmium translocation to the leaves was severely reduced in Atrboh mutants after 1 day of treatment and even after 5 days in AtrbohF. Similar results were observed for S, P, Ca, Zn and Fe accumulation, while an opposite trend was observed for K accumulation, except in AtrbohF. Thus, under Cd stress, RBOHs differentially regulate ROS metabolism, redox homeostasis and nutrient balance and could be of potential interest in biotechnology for the phytoremediation of polluted soils.

Uptake, sequestration and tolerance of cadmium at cellular levels in the hyperaccumulator plant species Sedum alfredii

Sedum alfredii is one of a few plant species known to hyperaccumulate cadmium (Cd). Uptake, localization, and tolerance of Cd at cellular levels in shoots were compared in hyperaccumulating (HE) and non-hyperaccumulating (NHE) ecotypes of Sedum alfredii. X-ray fluorescence images of Cd in stems and leaves showed only a slight Cd signal restricted within vascular bundles in the NHEs, while enhanced localization of Cd, with significant tissue- and age-dependent variations, was detected in HEs. In contrast to the vascular-enriched Cd in young stems, parenchyma cells in leaf mesophyll, stem pith and cortex tissues served as terminal storage sites for Cd sequestration in HEs. Kinetics of Cd transport into individual leaf protoplasts of the two ecotypes showed little difference in Cd accumulation. However, far more efficient storage of Cd in vacuoles was apparent in HEs. Subsequent analysis of cell viability and hydrogen peroxide levels suggested that HE protoplasts exhibited higher resistance to Cd than those of NHE protoplasts. These results suggest that efficient sequestration into vacuoles, as opposed to rapid transport into parenchyma cells, is a pivotal process in Cd accumulation and homeostasis in shoots of HE S. alfredii. This is in addition to its efficient root-to-shoot translocation of Cd.

Cd-induced Cu deficiency responses in Arabidopsis thaliana: are phytochelatins involved?

Cadmium (Cd) exposure can disturb the homeostasis of essential elements. In Arabidopsis thaliana, Cd induces a squamosa promoter binding protein-like 7 (SPL7)-dependent Cu deficiency response. We investigated how Cd induces a Cu deficiency response. The Cu deficiency response consists of the active SPL7 transcription factor binding to GTAC motifs in promoters of among others several Cu transporters, a Cu chaperone, and cupro-miRNAs to regulate Cu homeostasis. We demonstrated that the addition of supplemental Cu to Cd-exposed A. thaliana plants diminished the Cu deficiency response in roots, while it even disappeared in leaves. Exposure of plants to Cd in combination with extra Cu reduced Cd levels in both roots and leaves resulting in an improved cellular oxidative state. Furthermore, we demonstrated a role for phytochelatins (PCs) in the Cd-induced Cu deficiency response, because it was reduced in roots of cad1-3 mutant plants exposed to Cd. In conclusion, a working mechanism is provided in which it is suggested that Cd increases PC levels that can complex both Cd and Cu. This results in cellular Cu deficiency and subsequently the activation of SPL7 and hence the induction of the Cu deficiency response.

Expression of FlHMA3, a P1B2-ATPase from Festulolium loliaceum, correlates with response to cadmium stress

Heavy metal ATPase 3 (HMA3), a P_1B2-ATPase, is a key tonoplast transporter involved in mediating the vacuolar sequestration of cadmium (Cd) to detoxify the intake of this element by plants. HMA3 expression in response to Cd stress has not been previously examined in the grass hybrid species Festulolium loliaceum (Huds.) P. Fourn. In this study, FlHMA3 isolated from F. loliaceum was found to comprise 833 amino acid residues with 77% homology to the rice OsHMA3. Transient expression of FlHMA3 fused to enhanced green fluorescent protein in Arabidopsis protoplasts suggested its localization to vacuolar membranes. Quantitative real-time RT-PCR analysis of F. loliaceum revealed that FlHMA3 is expressed predominantly within roots and up-regulated by excess Cd. Over the 168 h treatment, Cd content of F. loliaceum roots was significantly higher than that of shoots, regardless of external CdCl₂ concentrations. A significant positive correlation was found between FlHMA3 expression and Cd accumulation in roots of F. loliaceum seedlings subjected to 10-100 mg L^-1 CdCl₂ for 168 h or, in a separate experiment, to 25 or 100 mg L^-1 CdCl₂ for the same duration. These findings provide evidence that FlHMA3 encodes a vacuolar P_1B2-ATPase that may play an important role in Cd²⁺ sequestration into root cell vacuoles, thereby limiting the entry of Cd²⁺ into the cytoplasm and reducing Cd²⁺ toxicity.

Lead (Pb) Toxicity; Physio-Biochemical Mechanisms, Grain Yield, Quality, and Pb Distribution Proportions in Scented Rice

Lead (Pb) caused interruptions with normal plant metabolism, crop yield losses and quality issues are of great concern. This study assessed the physio-biochemical responses, yield and grain quality traits and Pb distribution proportions in three different fragrant rice cultivars i.e., Meixiangzhan-2, Xinagyaxiangzhan and Basmati-385. Plants were exposed to 400, 800, and 1,200 ppm of Pb while pots without Pb were taken as control (0 ppm). Our results showed that Pb toxicity significantly (P < 0.05) reduced photosynthetic pigments (chlorophyll contents and carotenoids) and inducted oxidative stress with increased production of hydrogen peroxide (H2O2), malanodialdehyde (MDA) and leaves leachates; while such effects were more apparent in Xinagyaxiangzhan than other two rice cultivars. Pb stress differentially affected the production protein, proline and soluble sugars; however the production rates were higher at heading stage (HS) than maturity stage (MS). Furthermore, Pb stress altered superoxide dismutase (SOD), peroxidases (POD), catalases (CAT) and ascorbate peroxidases (APX) activities and glutathione (GSH) and oxidized glutathione (GSSG) production in all rice cultivars at both HS and MS. All Pb levels reduced the yield and yield components of all rice cultivars; nonetheless such reductions were observed highest in Xinagyaxiangzhan (69.12%) than Meixiangzhan-2 (58.05%) and Basmati-385 (46.27%) and resulted in grain quality deterioration. Significant and positive correlations among rice yields with productive tillers/pot and grains per panicle while negative with sterility percentage were also observed. In addition, all rice cultivars readily taken up the Pb contents from soil to roots and transported upward in different proportions with maximum in roots followed by stemss, leaves, ears and grains. Higher proportions of Pb contents in above ground plant parts in Xinagyaxiangzhan possibly lead to maximum losses in this cultivar than other two cultivars; while less damage in Basmati-385 might be related to strong anti-oxidative defense system and lower proportions of Pb contents in its aerial parts.

Cadmium impairs the survival and proliferation of cultured adult subventricular neural stem cells through activation of the JNK and p38 MAP kinases

Cadmium (Cd) is a heavy metal with a long biological half-life in humans and is recognized as a toxic pollutant. Cd is also a potential neurotoxicant and its exposure is associated with olfactory impairment in humans. However, the molecular and cellular mechanisms of Cd neurotoxicity are not well defined. Adult neurogenesis is a process that generates functional neurons from adult neural stem/progenitor cells (aNPCs). It occurs in specific regions of the adult brain including the subventricular zone (SVZ) along the lateral ventricles in mammals, a process that is critical for olfaction. Various external stimuli can modulate adult neurogenesis and the effect of neurotoxicants on adult neurogenesis is just beginning to be elucidated. Since Cd exposure can impair olfaction in humans, the goal of this study is to investigate the effects of Cd on SVZ adult neurogenesis and underlying mechanisms using primary cultured SVZ-aNPCs. In this study, we report that low-level Cd exposure decreases cell number, induces apoptosis, and inhibits cell proliferation in SVZ-aNPCs. Furthermore, Cd exposure significantly increases phosphorylation of c-Jun NH2-terminal kinase (JNK), and p38 MAP kinase in these cells, indicative of JNK and p38 activation. Pharmacological inhibition of JNK or p38 MAPK kinases attenuated Cd-induced cell loss and apoptosis. Cd treatment did not cause cell loss or apoptosis in SVZ-aNPCs prepared from transgenic mice null for the neural-specific JNK3 isoform. These data suggest a critical role for p38 and JNK3 MAP kinases in Cd neurotoxicity. These results are, to our knowledge, the first demonstration that Cd impairs SVZ adult neurogenesis in vitro, which may contribute to its neurotoxicity in olfaction.

Cadmium absorption and transportation pathways in plants

Controlling the uptake, transport, translocation, and accumulation of excessive amounts of cadmium from polluted environments is critical for plants and, consequently, humans with regard to food safety. Plants adopt various cellular and molecular mechanisms to minimize Cd toxicity. Upon exposure to Cd, plants initially implement avoidance strategies, such as production of organic acids, chelation, and sequestration, to prevent metal access to root cells. Nevertheless, Cd can be transported through the roots, stems, and leaves via apoplastic and symplastic pathways. These processes have been controlled by specific sites at the root surface and root cortex, in cells responsible for loading the root xylem, at the transition between the vascular systems of the root and the shoot, and in connecting tissues and cells at the stem. Although resistance to heavy metal cadmium can be achieved by either avoidance or tolerance, genetic basis to tolerance is therefore implied, in that these mechanisms are heritable attributes of tolerant mutants or genotypes.

Root avoidance of toxic metals requires the GeBP-LIKE 4 transcription factor in Arabidopsis thaliana

Plants reorganize their root architecture to avoid growth into unfavorable regions of the rhizosphere. In a screen based on chimeric repressor gene-silencing technology, we identified the Arabidopsis thaliana GeBP-LIKE 4 (GPL4) transcription factor as an inhibitor of root growth that is induced rapidly in root tips in response to cadmium (Cd). We tested the hypothesis that GPL4 functions in the root avoidance of Cd by analyzing root proliferation in split medium, in which only half of the medium contained toxic concentrations of Cd. The wild-type (WT) plants exhibited root avoidance by inhibiting root growth in the Cd side but increasing root biomass in the control side. By contrast, GPL4-suppression lines exhibited nearly comparable root growth in the Cd and control sides and accumulated more Cd in the shoots than did the WT. GPL4 suppression also altered the root avoidance of toxic concentrations of other essential metals, modulated the expression of many genes related to oxidative stress, and consistently decreased reactive oxygen species concentrations. We suggest that GPL4 inhibits the growth of roots exposed to toxic metals by modulating reactive oxygen species concentrations, thereby allowing roots to colonize noncontaminated regions of the rhizosphere.

Paxillus involutus-Facilitated Cd2+ Influx through Plasma Membrane Ca2+-Permeable Channels Is Stimulated by H2O2 and H+-ATPase in Ectomycorrhizal Populus × canescens under Cadmium Stress

Using a Non-invasive Micro-test Technique, flux profiles of Cd2+, Ca2+, and H+ were investigated in axenically grown cultures of two strains of Paxillus involutus (MAJ and NAU), ectomycorrhizae formed by these fungi with the woody Cd2+-hyperaccumulator, Populus × canescens, and non-mycorrhizal (NM) roots. The influx of Cd2+ increased in fungal mycelia, NM and ectomycorrhizal (EM) roots upon a 40-min shock, after short-term (ST, 24 h), or long-term (LT, 7 days) exposure to a hydroponic environment of 50 μM CdCl2. Cd2+ treatments (shock, ST, and LT) decreased Ca2+ influx in NM and EM roots but led to an enhanced influx of Ca2+ in axenically grown EM cultures of the two P. involutus isolates. The susceptibility of Cd2+ flux to typical Ca2+ channel blockers (LaCl3, GdCl3, verapamil, and TEA) in fungal mycelia and poplar roots indicated that the Cd2+ entry occurred mainly through Ca2+-permeable channels in the plasma membrane (PM). Cd2+ treatment resulted in H2O2 production. H2O2 exposure accelerated the entry of Cd2+ and Ca2+ in NM and EM roots. Cd2+ further stimulated H+ pumping activity benefiting NM and EM roots to maintain an acidic environment, which favored the entry of Cd2+ across the PM. A scavenger of reactive oxygen species, DMTU, and an inhibitor of PM H+-ATPase, orthovanadate, decreased Ca2+ and Cd2+ influx in NM and EM roots, suggesting that the entry of Cd2+ through Ca2+-permeable channels is stimulated by H2O2 and H+ pumps. Compared to NM roots, EM roots exhibited higher Cd2+-fluxes under shock, ST, and LT Cd2+ treatments. We conclude that ectomycorrhizal P. × canescens roots retained a pronounced H2O2 production and a high H+-pumping activity, which activated PM Ca2+ channels and thus facilitated a high influx of Cd2+ under Cd2+ stress.

Germination, Physiological Responses and Gene Expression of Tall Fescue (Festuca arundinacea Schreb.) Growing under Pb and Cd

Cadmium (Cd) and lead (Pb) are recognized as the most toxic metal ions due to their detrimental effects not only to plants, but also to humans. The objective of this study was to investigate the effects of Cd and Pb treatments on seed germination, plant growth, and physiological response in tall fescue (Festuca arundinacea Schreb.). We employed six treatments: CK (nutrient solution as control), T1 (1000 mg L-1 Pb), T2 (50 mg L-1 Cd), T3 (150 mg L-1 Cd), T4 (1000 mg L-1 Pb+50 mg L-1 Cd), T5 (1000 mg L-1 Pb+150 mg L-1 Cd). Antagonistic and synergistic actions were observed in tall fescue under Pb and Cd combined treatments. Under low Cd, plants exhibited higher relative germination rate, germ length, VSGR, catalase (CAT) and peroxidase (POD) activities. Additionally, in the shoots, the gene expression level of Cu/Zn SOD, FeSOD, POD, GPX, translocation factors, MDA, EL, and soluble protein contents were reduced under Pb stress. Conversely, under high Cd level, there was a decline in NRT, Pb content in shoots, Pb translocation factors, CAT activity; and an increase in VSGR, Pb content in roots, gene expression level of Cu/ZnSOD and POD in tall fescue exposed to Pb2+ regimes. On the other hand, tall fescue plants treated with low Cd exhibited lower relative germination rate, germination index, germ length, NRT, Cd content in roots. On the other hand there was higher Cd content, Cd translocation factor, CAT and POD activities, and gene expression level of Cu/Zn SOD, FeSOD, POD, GPX under Pb treatment compared with single Cd2+ treatment in the shoots. However, after high Cd exposure, plants displayed lower NRT, Cd content, CAT activity, and exhibited higher Cd contents, Cd translocation factor, MDA content, gene expression level of Cu/ZnSOD and GPX with the presence of Pb2+ relative to single Cd2+ treatment. These findings lead to a conclusion that the presence of low Cd level impacted positively towards tall fescue growth under Pb stress, while high level of Cd impacted negatively. In summary, antioxidant enzymes responded to Cd and Pb interaction at an early stage of exposure, and their gene expression profiles provided more details of the activation of those systems.

Seasonal changes in antioxidative/oxidative profile of mining and non-mining populations of Syrian beancaper as determined by soil conditions

Soil pollution by heavy metals/metalloids (HMMs) is a problem worldwide. To prevent dispersion of contaminated particles by erosion, the maintenance of a vegetative cover is needed. Successful plant establishment in multi-polluted soils can be hampered not only by HMM toxicities, but also by soil nutrient deficiencies and the co-occurrence of abiotic stresses. Some plant species are able to thrive under these multi-stress scenarios often linked to marked fluctuations in environmental factors. This study aimed to investigate the metabolic adjustments involved in Zygophyllum fabago acclimative responses to conditions prevailing in HMM-enriched mine-tailings piles, during Mediterranean spring and summer. To this end, fully expanded leaves, and rhizosphere soil, of three contrasting mining and non-mining populations of Z. fabago grown spontaneously in south-eastern Spain were sampled in two consecutive years. Approximately 50 biochemical, physiological and edaphic parameters were examined, including leaf redox components, primary and secondary metabolites, endogenous levels of salicylic acid, and physicochemical properties of soil (fertility parameters and total concentration of HMMs). Multivariate data analysis showed a clear distinction in antioxidative/oxidative profiles between and within the populations studied. Levels of chlorophylls, proteins and proline characterized control plants whereas antioxidant capacity and C- and S-based antioxidant compounds were biomarkers of mining plants. Seasonal variations were characterized by higher levels of alkaloids and PAL and soluble peroxidase activities in summer, and by soluble sugars and hydroxycinnamic acids in spring irrespective of the population considered. Although the antioxidant systems are subjected to seasonal variations, the way and the intensity with which every population changes its antioxidative/oxidative profile seem to be determined by soil conditions. In short, Z. fabago displays a high physiological plasticity that allow it to successfully shift its metabolism to withstand the multiple stresses that plants must cope with in mine tailings piles under Mediterranean climatic conditions.

The respective effects of soil heavy metal fractions by sequential extraction procedure and soil properties on the accumulation of heavy metals in rice grains and brassicas

This study was carried out to examine heavy metal accumulation in rice grains and brassicas and to identify the different controls, such as soil properties and soil heavy metal fractions obtained by the Community Bureau of Reference (BCR) sequential extraction, in their accumulation. In Guangdong Province, South China, rice grain and brassica samples, along with their rhizospheric soil, were collected from fields on the basis of distance downstream from electroplating factories, whose wastewater was used for irrigation. The results showed that long-term irrigation using the electroplating effluent has not only enriched the rhizospheric soil with Cd, Cr, Cu, and Zn but has also increased their mobility and bioavailability. The average concentrations of Cd and Cr in rice grains and brassicas from closest to the electroplating factories were significantly higher than those from the control areas. Results from hybrid redundancy analysis (hRDA) and redundancy analysis (RDA) showed that the BCR fractions of soil heavy metals could explain 29.0 and 46.5 % of total eigenvalue for heavy metal concentrations in rice grains and brassicas, respectively, while soil properties could only explain 11.1 and 33.4 %, respectively. This indicated that heavy metal fractions exerted more control upon their concentrations in rice grains and brassicas than soil properties. In terms of metal interaction, an increase of residual Zn in paddy soil or a decrease of acid soluble Cd in the brassica soil could enhance the accumulation of Cd, Cu, Cr, and Pb in both rice grains and brassicas, respectively, while the reducible or oxidizable Cd in soil could enhance the plants' accumulation of Cr and Pb. The RDA showed an inhibition effect of sand content and CFO on the accumulation of heavy metals in rice grains and brassicas. Moreover, multiple stepwise linear regression could offer prediction for Cd, Cu, Cr, and Zn concentrations in the two crops by soil heavy metal fractions and soil properties.

Cadmium stress tolerance in wheat seedlings induced by ascorbic acid was mediated by NO signaling pathways

Ascorbic acid (AsA) and nitric oxide (NO) are well known and widespread antioxidants and gaseous molecules that regulate plant tolerance to several stresses. However, the relationship between them in plant response to stress, especially heavy stress, is largely unclear. This study demonstrated that both AsA and NO could enhance the tolerance of wheat seedlings to cadmium stress evidenced by root length change, which resulted from their roles in maintaining the balance in reactive oxygen species (ROS) and reducing the absorption of Cd. Furthermore, exogenous AsA led to a significant increase of NO content and endogenous AsA content in wheat roots, which could be weakened by the NO scavenger c-PTIO. In addition, c-PTIO also inhibits the NO-induced production of endogenous AsA. Although the AsA synthesis inhibitor lycorine significantly inhibited the inductive effect of exogenous AsA on endogenous AsA production, it has little effect on NO content. In addition, we found that the protective effects of NO and AsA on Cd stress were removed by c-PTIO and lycorine. These results indicated that NO accumulation could be necessary for exogenous AsA-induced cadmium tolerance and endogenous AsA production, and the exogenous AsA-induced endogenous AsA production was likely mediated by NO signaling pathways and together they induced the tolerance of wheat to cadmium stress.

Multiple mechanisms of heavy metal tolerance are differentially expressed in ecotypes of Artemisia fragrans

Artemisia fragrans is a plant species with ability of growing on heavy metal-polluted soils. Ecotypes of this species naturally growing in polluted areas can accumulate and tolerate different amounts of heavy metals (HM), depending on soil contamination level at their origin. Heavy metal tolerance of various ecotypes collected from contaminated (AP, SP) and non-contaminated (BG) sites was compared by cultivation on a highly HM-contaminated river sediment and a non-contaminated agricultural control soil. Tissue-specific HM distribution was analyzed by laser ablation-inductively-coupled plasma-mass spectroscopy (LA-ICP-MS) and photosynthetic activity by non-invasive monitoring of chlorophyll fluorescence. Plant-mineral analysis did not reveal ecotype-differences in concentrations of Cd, Zn, Cu in shoots of Artemisia plants, suggesting no differential expression of root uptake or root to shoot translocation of HM. There was also no detectable rhizosphere effect on HM concentrations on the contaminated soil. However, despite high soil contaminations, all ecotypes accumulated Zn only in the concentration range of generally reported for normal growth of plants, while Cu and Cd concentrations were close to or even higher than the toxicity level for most plants. As a visible symptom of differences in HM tolerance, only the AP ecotype was able to enter the generative phase to complete its life cycle. Analysis of tissue-specific metal distribution revealed significantly lower concentrations of Cd in the leaf mesophyll of this ecotype, accumulating Cd mainly in the leaf petioles. A similar mesophyll exclusion was detectable also for Cu, although not associated with preferential accumulation in the leaf petioles. However, high mesophyll concentrations of Cd and Cu in the SP and BG ecotypes were associated with disturbances of the photosynthetic activity. The findings demonstrate differential expression of HM exclusion strategies in Artemisia ecotypes and suggest Cd and Cu exclusion from the photosynthetically active tissues as a major tolerance mechanism of the AP ecotype.

Cadmium-zinc accumulation and photosystem II responses of Noccaea caerulescens to Cd and Zn exposure

A population of the metallophyte Noccaea (Thlaspi) caerulescens originating from a Zn-enriched area at Røros Copper Mine (Norway) was studied. N. caerulescens tolerance to accumulate Cd and Zn was evaluated in hydroponic experiments by chlorophyll fluorescence imaging analysis. In the field-collected N. caerulescens mother plants, Zn shoot concentrations were above Zn hyperaccumulation threshold while, in hydroponic experiments under 40-μM Cd exposure, shoot Cd concentrations were clearly above Cd hyperaccumulation threshold. Cadmium ions and, to a less extent, Zn were mainly retained in the roots. Exposure to Cd enhanced Zn translocation to the shoot, while decreased significant total Ca²⁺ uptake, suggesting that Cd uptake occurs through Ca²⁺ transporters. Nevertheless, it increased Ca²⁺ translocation to the leaf, possibly for photoprotection of photosystem II (PSII). Exposure to 800 μM Zn or 40 μM Cd resulted in increased Fe³⁺ uptake suggesting that in N. caerulescens, Cd uptake does not take place through the pathway of Fe³⁺ uptake and that conditions that lead to Cd and Zn accumulation in plants may also favor Fe accumulation. Despite the significant high toxicity levels of Zn and Cd in leaves, under Zn and Cd exposure, respectively, the allocation of absorbed light energy at PSII did not differ compared to controls. The results showed that N. caerulescens keep Cd and Zn concentrations in the mesophyll cells in non-toxic forms for PSII and that the increased Ca and Fe accumulation in leaves alleviates the toxicity effects. Chlorophyll fluorescence imaging revealed that PSII of N. caerulescens resisted better the phytotoxic effects of 20 times higher Zn than Cd exposure concentration. Overall, it is concluded that the use of chlorophyll fluorescence imaging constitutes a promising basis for investigating heavy metal tolerance of plants.

Bioavailability of adsorbed and coprecipitated Cu, Ni, Pb, and Cd on iron and iron/aluminum hydroxide to Phragmites australis

The bioavailability of heavy metals strongly depends on their speciation in the environment. Adsorption (ADS) and coprecipitation (CPT) on amorphous metal hydroxides are important processes, controlling the fates of heavy metals in an aqueous environment. This work studied the bioavailability of Cu, Cd, Ni, and Pb adsorbed on and/or coprecipitated with amorphous iron and iron/aluminum mixed hydroxides to the wetland plant Phragmites australis. After a 13-day treatment, there was an apparent uptake of the heavy metals by the plant, and the amount of metal bioaccumulation was measurably different for different association forms (ADS vs. CPT). The bioaccumulation of Cd associated with Fe_0.5Al_0.5(OH)₃ was greater than that with Fe(OH)₃; the adsorbed metals were found to be more bioavailable than the coprecipitated forms for most of the treatments while the aging treatment significantly reduced the bioaccumulation of ADS metals. In the single metal treatment, root metal concentrations in the Fe(OH)₃ ADS system followed the order Ni (68 mg kg^-1) > Cu (32 mg kg^-1) > Cd (28 mg kg^-1) > Pb (9 mg kg^-1), while the CPT system followed the order of Cu (30 mg kg^-1) > Ni (22 mg kg^-1) > Pb (9 mg kg^-1) > Cd (7 mg kg^-1). The order of metal accumulation in a combined metal treatment was similar to that for single metal treatments, but observed Ni concentration declines by 22 and 71 % and Cu and Cd concentrations increase by 30 and 50 % (for CPT and ADS treatments, respectively), while Pb concentrations increased by 30~50 % in both of them. When treated with low-molecular-weight organic acids (LMWOAs), metal desorption, indicative of metal oxide bonding strength and metal bioavailability, was consistent with metal accumulation in the plant.

The Soybean Basic Helix-Loop-Helix Transcription Factor ORG3-Like Enhances Cadmium Tolerance via Increased Iron and Reduced Cadmium Uptake and Transport from Roots to Shoots

Cadmium (Cd) is one of the most dangerous heavy metal pollutants in the environment and is toxic to animal and plant cells. On the other hand, iron (Fe) is an essential element for plant growth and development. The chlorosis of plant leaves under cadmium stress is similar to the typical symptom of iron deficiency. Recently, several Arabidopsis basic/helix-loop-helix (bHLH) transcription factors have been identified that are involved in the interactions between Cd and Fe. In the present study, over-expression the ORG3-like gene GmORG3, a bHLH transcription factor OBP3-responsive gene (ORG), enhanced Cd tolerance and stabilized Fe homeostasis. The domain analysis of GmORG3 showed that the protein contains a conserved 61-residue bHLH domain belonging to subfamily II. Moreover, subcellular localization experiments showed that GmORG3 is a nucleoprotein. GmORG3 was transcribed only in soybean roots and was significantly induced by external Cd stress in soybean plants. Heterologous expression of GmORG3 enhanced Cd tolerance in yeast. Furthermore, the overexpression of GmORG3 in soybean mosaic seedlings using a hairy root system showed that overexpressing plants increased the translocation ratio of Fe but reduced Cd translocation from the roots to shoots. In addition, the ectopic expression of GmORG3 in tobacco reduced phytotoxic effects induced by Cd stress and Fe deficiency, including the blockage of root elongation and decreased chlorophyll content. By integrating all these results, we found that GmORG3 plays an important role in response to Cd stress. The results provide new insights into the molecular mechanisms of Cd tolerance in soybean.

Slow-release nitrogen fertilizers can improve yield and reduce Cd concentration in pakchoi (Brassica chinensis L.) grown in Cd-contaminated soil

Cadmium (Cd) pollution in vegetable crops has become a serious problem in recent years. Owing to the limited availability of arable land resources, large areas of Cd-contaminated lands are inevitably being used for the production of vegetables, posing great risks to human health via the food chain. However, strategies to improve yield and reduce Cd concentration in crops grown in contaminated soils are being developed. In the present study, using pot experiments, we investigated the effects of two slow-release nitrogen fertilizers (SRNFs), resin-coated ammonium nitrate (Osmocote_313s), and resin-coated urea (urea₆₂₀), on the growth and Cd concentration of the Cd-contaminated pakchoi. The results showed that pakchoi grown in soil containing 5 mg kg^-1 of Cd-induced oxidative stress (indicated by malondialdehyde (MDA), H₂O₂, and O₂^·-) and photosynthesis inhibition, which in turn was restored with the application of SRNFs. However, pakchoi grown in Cd-contaminated soil supplied with Osmocote_313s and urea₆₂₀ showed 103 and 203 % increase in fresh weight and 51-55 % and 44-56 % decrease in Cd concentration, respectively, as compared with their controls (pakchoi treated with instant soluble nitrogen fertilizers). On the basis of an increase in their tolerance index (47-238 %) and a decrease in their translocation factor (7.5-21.6 %), we inferred that the plants treated with SRNFs have a stronger tolerance to Cd and a lower efficiency of Cd translocation to edible parts than those treated with instant soluble nitrogen fertilizers. Therefore, in terms of both crop production and food safety, application of SRNFs could be an effective strategy for improving both biomass production and quality in pakchoi grown under Cd stress.

Variation of DNA methylation patterns associated with gene expression in rice (Oryza sativa) exposed to cadmium

We report genome-wide single-base resolution maps of methylated cytosines and transcriptome change in Cd-exposed rice. Widespread differences were identified in CG and non-CG methylation marks between Cd-exposed and Cd-free rice genomes. There are 2320 non-redundant differentially methylated regions detected in the genome. RNA sequencing revealed 2092 DNA methylation-modified genes differentially expressed under Cd exposure. More genes were found hypermethylated than those hypomethylated in CG, CHH and CHG (where H is A, C or T) contexts in upstream, gene body and downstream regions. Many of the genes were involved in stress response, metal transport and transcription factors. Most of the DNA methylation-modified genes were transcriptionally altered under Cd stress. A subset of loss of function mutants defective in DNA methylation and histone modification activities was used to identify transcript abundance of selected genes. Compared with wide type, mutation of MET1 and DRM2 resulted in general lower transcript levels of the genes under Cd stress. Transcripts of OsIRO2, OsPR1b and Os09g02214 in drm2 were significantly reduced. A commonly used DNA methylation inhibitor 5-azacytidine was employed to investigate whether DNA demethylation affected physiological consequences. 5-azacytidine provision decreased general DNA methylation levels of selected genes, but promoted growth of rice seedlings and Cd accumulation in rice plant.

Kinetic Analysis of Zinc/Cadmium Reciprocal Competitions Suggests a Possible Zn-Insensitive Pathway for Root-to-Shoot Cadmium Translocation in Rice

BACKGROUND

Among cereals, rice has a genetic propensity to accumulate high levels of cadmium (Cd) in grains. Xylem-mediated root-to-shoot translocation rather than root uptake has been suggested as the main physiological factor accounting for the genotypic variation observed in Cd accumulation in shoots and grains. Several evidence indicate OsHMA2 - a putative zinc (Zn) transporter - as the main candidate protein that could be involved in mediating Cd- and Zn-xylem loading in rice. However, the specific interactions between Zn and Cd in rice often appear anomalous if compared to those observed in other staple crops, suggesting that root-to-shoot Cd translocation process could be more complex than previously thought. In this study we performed a complete set of competition experiments with Zn and Cd in order to analyze their possible interactions and reciprocal effects at the root-to-shoot translocation level.

RESULTS

The competition analysis revealed the lack of a full reciprocity when considering the effect of Cd on Zn accumulation, and vice versa, since the accumulation of Zn in the shoots was progressively inhibited by Cd increases, whereas that of Cd was only partially impaired by Zn. Such behaviors were probably dependent on Cd-xylem loading mechanisms, as suggested by: i) the analysis of Zn and Cd content in the xylem sap performed in relation to the concentration of the two metals in the mobile fractions of the roots; ii) the analysis of the systemic movement of (107)Cd in short term experiments performed using a positron-emitting tracer imaging system (PETIS).

CONCLUSIONS

Our results suggest that at least two pathways may mediate root-to-shoot Cd translocation in rice. The former could involve OsHMA2 as Zn(2+)/Cd(2+) xylem loader, whereas the latter appears to involve a Zn-insensitive system that still needs to be identified.

Calcium Deficiency Triggers Phloem Remobilization of Cadmium in a Hyperaccumulating Species

Understanding cadmium (Cd) accumulation in plants is critical for the development of plant-based strategies for soil remediation and crop safety. Sedum alfredii is a nonbrassica plant species known to hyperaccumulate Cd. The characteristics of Cd uptake, distribution, and retranslocation affected by the Ca status were investigated at cellular levels in S. alfredii Low Ca supply significantly increased Cd contents in shoots of S. alfredii, particularly in the young leaves. Micro x-ray fluorescence images confirmed that sequestration of Cd was greatly enhanced in the young leaves under Ca deficiency stress, with a significant amount of Cd localized in mesophyll cells, compared to the young leaves supplied with high Ca levels. Cd influx into protoplasts isolated from young leaves was significantly inhibited by the addition of Ca channel inhibitors, but not by pre-exposure to Ca deficiency. In stems, the Cd signal in vascular systems under low Ca levels was 10-fold higher than in those treated with higher Ca levels. A detailed investigation of vascular bundles revealed that an extremely high Cd signal induced by low Ca supply occurred in the phloem tissues, but not in the xylem tissues. Transfer of Cd pretreated plants to nutrient solutions at different Ca levels confirmed that a much higher amount of Cd was reallocated to the new growth tissues under low Ca stress compared to plants supplied with sufficient Ca. These results suggest that Ca deficiency triggered a highly efficient phloem remobilization of Cd in S. alfredii and subsequently enhanced Cd accumulation in its young leaves.

Microbial community dynamics in the rhizosphere of a cadmium hyper-accumulator

Phytoextraction is influenced by the indigenous soil microbial communities during the remediation of heavy metal contaminated soils. Soil microbial communities can affect plant growth, metal availability and the performance of phytoextraction-assisting inocula. Understanding the basic ecology of indigenous soil communities associated with the phytoextraction process, including the interplay between selective pressures upon the communities, is an important step towards phytoextraction optimization. This study investigated the impact of cadmium (Cd), and the presence of a Cd-accumulating plant, Carpobrotus rossii (Haw.) Schwantes, on the structure of soil-bacterial and fungal communities using automated ribosomal intergenic spacer analysis (ARISA) and quantitative PCR (qPCR). Whilst Cd had no detectable influence upon fungal communities, bacterial communities underwent significant structural changes with no reduction in 16S rRNA copy number. The presence of C. rossii influenced the structure of all communities and increased ITS copy number. Suites of operational taxonomic units (OTUs) changed in abundance in response to either Cd or C. rossii, however we found little evidence to suggest that the two selective pressures were acting synergistically. The Cd-induced turnover in bacterial OTUs suggests that Cd alters competition dynamics within the community. Further work to understand how competition is altered could provide a deeper understanding of the microbiome-plant-environment and aid phytoextraction optimization.

Genetic variation in Fe toxicity tolerance is associated with the regulation of translocation and chelation of iron along with antioxidant defence in shoots of rice

Excess iron (Fe) is phytotoxic and causes reduced growth and productivity in rice. In this study we elucidated the mechanisms conferring differential tolerance to Fe-toxicity in rice seedlings. Excess Fe caused retardation in roots of both Pokkali and BRRI 51, but it caused no significant changes on growth parameters, Fe accumulation and OsIRT1 expression in shoots of Pokkali only compared with control plants. These results suggest that the Pokkali genotype does have mechanisms in shoots to withstand Fe toxicity. Pokkali maintained membrane stability and total soluble protein in shoots due to Fe toxicity, further confirming its ability to tolerate excess Fe. Furthermore, a significant decrease of Fe-chelate reductase activity and OsFRO1 expression in shoots of Pokkali suggests that limiting Fe accumulation is possibly regulated by Fe-reductase activity. Our extensive expression analysis on the expression pattern of three chelators (OsDMAS1, OsYSL15, OsYSL2 and OsFRDL1) showed no significant changes in expression in shoots of Pokkali due to Fe toxicity, whereas these genes were significantly upregulated under Fe-toxicity in sensitive BRRI 51. These results imply that regulation of Fe chelation in shoots of Pokkali contributes to its tolerance to Fe toxicity. Finally, increased catalase (CAT), peroxidase (POD), glutathione reductase (GR) and superoxide dismutase (SOD), along with elevated ascorbic acid, glutathione, cysteine, methionine and proline in shoots of Pokkali caused by Fe toxicity suggests that strong antioxidant defence protects rice plants from oxidative injury under Fe toxicity. Taking these results together, we propose that genetic variation in Fe-toxicity tolerance in rice is shoot based, and is mainly associated with the regulation of translocation and chelation of Fe together with elevated antioxidant metabolites in shoots.

Increased lead and cadmium tolerance of Typha angustifolia from Huaihe River is associated with enhanced phytochelatin synthesis and improved antioxidative capacity

Heavy metal contamination of water is an increasing environmental problem worldwide, and the use of aquatic plants for phytoremediation of heavy metal pollution has become an important subject of research. One key to successful phytoremediation is the identification of plants that are efficient at sequestering heavy metals. In this study, we examined the growth and heavy metal accumulation of Typha angustifolia and compared growth characteristics and tolerance mechanisms in plants from the Huaihe and Chaohu Rivers irrigated with different concentrations of lead (Pb) and cadmium (Cd). T. angustifolia from Huaihe River showed enhanced tolerance and accumulation of Pb and Cd and had greater biomass and more vigorous growth than the ecotype from Chaohu River. In addition, higher phytochelatin (PC) content and significantly higher superoxide dismutase and catalase activities were detected in T. angustifolia from Huaihe River than in T. angustifolia from Chaohu River. These findings suggest that high Pb and Cd accumulation and tolerance in T. angustifolia from Chaohu River is associated with its higher PC synthesis and better antioxidative capacity, and that the Huaihe ecotype of T. angustifolia might also be an efficient species for phytoremediation of Pb and Cd in water contaminated by heavy metals.

Effects of Cadmium Treatment on the Uptake and Translocation of Sulfate in Arabidopsis thaliana

Cadmium (Cd) is a highly toxic and non-essential element for plants, whereas phytochelatins and glutathione are low-molecular-weight sulfur compounds that function as chelators and play important roles in detoxification. Cadmium exposure is known to induce the expression of sulfur-assimilating enzymes and sulfate uptake by roots. However, the molecular mechanism underlying Cd-induced changes remains largely unknown. Accordingly, we analyzed the effects of Cd treatment on the uptake and translocation of sulfate and accumulation of thiols in Arabidopsis thaliana Both wild type (WT) and null mutant (sel1-10 and sel1-18) plants of the sulfate transporter SULTR1;2 exhibited growth inhibition when treated with CdCl₂ However, the mutant plants exhibited a lower growth rate and lower Cd accumulation. Cadmium treatment also upregulated the transcription of SULTR1;2 and sulfate uptake activity in WT plants, but not in mutant plants. In addition, the sulfate, phytochelatin and total sulfur contents were preferentially accumulated in the shoots of both WT and mutant plants treated with CdCl₂, and sulfur K-edge XANES spectra suggested that sulfate was the main compound responsible for the increased sulfur content in the shoots of CdCl₂-treated plants. Our results demonstrate that Cd-induced sulfate uptake depends on SULTR1;2 activity, and that CdCl₂ treatment greatly shifts the distribution of sulfate to shoots, increases the sulfate concentration of xylem sap and upregulates the expression of SULTRs involved in root-to-shoot sulfate transport. Therefore, we conclude that root-to-shoot sulfate transport is stimulated by Cd and suggest that the uptake and translocation of sulfate in CdCl₂-treated plants are enhanced by demand-driven regulatory networks.

Oxidative stress and antioxidant responses to increasing concentrations of trivalent chromium in the Andean crop species Chenopodium quinoa Willd

Quinoa (Chenopodium quinoa Willd), an ancient Andean seed crop, exhibits exceptional nutritional properties and resistance to abiotic stress. The species' tolerance to heavy metals has, however, not yet been investigated nor its ability to take up and translocate chromium (Cr). This study aimed to investigate the metabolic adjustments occurring upon exposure of quinoa to several concentrations (0.01-5mM) of CrCl3. Young hydroponically grown plants were used to evaluate Cr uptake, growth, oxidative stress, and other biochemical parameters three and/or seven days after treatment. Leaves accumulated the lowest amounts of Cr, while roots and stems accumulated the most at low and at high metal concentrations, respectively. Fresh weight and photosynthetic pigments were reduced only by the higher Cr(III) doses. Substantially increased lipid peroxidation, hydrogen peroxide, and proline levels were observed only with 5mM Cr(III). Except for a significant decrease at day 7 with 5mM Cr(III), total polyphenols and flavonoids maintained control levels in Cr(III)-treated plants, whereas antioxidant activity increased in a dose-dependent manner. Maximum polyamine accumulation was observed in 1mM CrCl3-treated plants. Even though α- and γ-tocopherols also showed enhanced levels only with the 1mM concentration, tyrosine aminotransferase (TAT, EC 2.6.1.5) activity increased under Cr(III) treatment in a dose- and time-dependent manner. Taken together, results suggest that polyamines, tocopherols, and TAT activity could contribute to tolerance to 1mM Cr(III), but not to the highest concentration that, instead, generated oxidative stress.

Identification of a regulation network in response to cadmium toxicity using blood clam Tegillarca granosa as model

Clam, a filter-feeding lamellibranch mollusk, is capable to accumulate high levels of trace metals and has therefore become a model for investigation the mechanism of heavy metal toxification. In this study, the effects of cadmium were characterized in the gills of Tegillarca granosa during a 96-hour exposure course using integrated metabolomic and proteomic approaches. Neurotoxicity and disturbances in energy metabolism were implicated according to the metabolic responses after Cd exposure, and eventually affected the osmotic function of gill tissue. Proteomic analysis showed that oxidative stress, calcium-binding and sulfur-compound metabolism proteins were key factors responding to Cd challenge. A knowledge-based network regulation model was constructed with both metabolic and proteomic data. The model suggests that Cd stimulation mainly inhibits a core regulation network that is associated with histone function, ribosome processing and tight junctions, with the hub proteins actin, gamma 1 and Calmodulin 1. Moreover, myosin complex inhibition causes abnormal tight junctions and is linked to the irregular synthesis of amino acids. For the first time, this study provides insight into the proteomic and metabolomic changes caused by Cd in the blood clam T. granosa and suggests a potential toxicological pathway for Cd.

Molybdenum (Mo) increases endogenous phenolics, proline and photosynthetic pigments and the phytoremediation potential of the industrially important plant Ricinus communis L. for removal of cadmium from contaminated soil

Cadmium (Cd) in agricultural soil negatively affects crops yield and compromises food safety. Remediation of polluted soil is necessary for the re-establishment of sustainable agriculture and to prevent hazards to human health and environmental pollution. Phytoremediation is a promising technology for decontamination of polluted soil. The present study investigated the effect of molybdenum (Mo) (0.5, 1.0 and 2.0 ppm) on endogenous production of total phenolics and free proline, plant biomass and photosynthetic pigments in Ricinus communis plants grown in Cd (25, 50 and 100 ppm) contaminated soils and the potential for Cd phytoextraction. Mo was applied via seed soaking, soil addition and foliar spray. Foliar sprays significantly increased plant biomass, Cd accumulation and bioconcentration. Phenolic concentrations showed significantly positive correlations with Cd accumulation in roots (R ² = 0.793, 0.807 and 0.739) and leaves (R ² = 0.707, 721 and 0.866). Similarly, proline was significantly positively correlated with Cd accumulation in roots (R ² = 0.668, 0.694 and 0.673) and leaves (R ² = 0.831, 0.964 and 0.930). Foliar application was found to be the most effective way to deliver Mo in terms of increase in plant growth, Cd accumulation and production of phenolics and proline.

Potential ecological and human health risks of heavy metals in surface soils associated with iron ore mining in Pahang, Malaysia

The composition of heavy metals (and metalloid) in surface soils of iron ore mine-impacted areas has been evaluated of their potential ecological and human health risks. The mining areas included seven selected locations in the vicinity of active and abandoned iron ore-mining sites in Pahang, Malaysia. Heavy metals such as Fe, Mn, Cu, Zn, Co, Pb, Cr, Ni, and Cd and metalloid As were present in the mining soils of the studied area, while Cu was found exceeding the soil guideline value at all sampling locations. However, the assessment of the potential ecological risk index (RI) indicated low ecological risk (RI between 44 and 128) with respect to Cd, Pb, Cu, As, Zn, Co, and Ni in the surface soils. Contributions of potential ecological risk [Formula: see text]by metal elements to the total potential ecological RI were evident for Cd, As, Pb, and Cu. Contribution of Cu appears to be consistently greater in the abandoned mining area compared to active iron ore-mining site. For non-carcinogenic risk, no significant potential health risk was found to both children and adults as the hazard indices (HIs) were all below than 1. The lifetime cancer risk (LCR) indicated that As has greater potential carcinogenic risk compared to other metals that may induce carcinogenic effects such as Pb, Cr, and Cd, while the LCR of As for children fell within tolerable range for regulatory purposes. Irrespective of carcinogenic or non-carcinogenic risk, greater potential health risk was found among children (by an order of magnitude higher for most metals) compared to adults. The hazard quotient (HQ) and cancer risk indicated that the pathways for the risk to occur were found to be in the order of ingestion > dermal > inhalation. Overall, findings showed that some metals and metalloid were still present at comparable concentrations even long after cessation of the iron ore-mining activities.

Genetic Diversity, Rather than Cultivar Type, Determines Relative Grain Cd Accumulation in Hybrid Rice

Cadmium (Cd) is a toxic element, and rice is known to be a leading source of dietary Cd for people who consume rice as their main caloric resource. Hybrid rice has dominated rice production in southern China and has been adopted worldwide. The characteristics of high yield heterosis of rice hybrids makes the public think intuitively that the hybrid rice accumulates more Cd in grain than do inbred cultivars. A detailed understanding of the genetic basis of grain Cd accumulation in hybrids and developing Cd-safe rice are one of the top priorities for hybrid rice breeders at present. In this study, we investigated genetic diversity and grain Cd levels in 617 elite rice hybrids collected from the middle and lower Yangtze River Valley in China and 68 inbred cultivars from around the world. We found that there are large variations in grain Cd accumulation in both the hybrids and their inbred counterparts. However, we found grain Cd levels in the rice hybrids to be similar to the levels in indica rice inbreds, suggesting that the hybrids do not accumulate more Cd than do the inbred rice cultivars. Further analysis revealed that the high heritability of Cd accumulation in the grain and the single indica population structure increases the risk of Cd over-accumulation in hybrid rice. The genetic effects of Cd-related QTLs, which have been identified in related Cd-QTL mapping studies, were also determined in the hybrid rice population. Four QTLs were identified as being associated with the variation in grain Cd levels; three of these loci exhibited obvious indica-japonica differentiations. Our study will provide a better understanding of grain Cd accumulations in hybrid rice, and pave the way toward effective breeding for high-yielding, low grain-Cd hybrids in the future.

Combined effects of elevated temperature and CO2 concentration on Cd and Zn accumulation dynamics in Triticum aestivum L

A simulated climate warming experiment was conducted to evaluate the combined effects of elevated temperature and CO2 concentration on the bioaccumulation, translocation and subcellular distributions of Cd and Zn in wheat seedlings (Triticum aestivum L. cv. Xihan 1.) at Dingxi, Gansu Province, China. The objective was to find evidence that global climate change is affecting the bioaccumulation of Cd and Zn in T. aestivum L. cv. Xihan 1. The results showed that compared to control A, elevated temperature and CO2 increased Cd bioaccumulation in the shoots by 1.4-2.5 times, and increased that in the roots by 1.2-1.5 times, but decreased Zn levels in wheat shoots by 1.4-2.0 times, while decreased that in the roots by 1.6-1.9 times. Moreover, temperature and CO2 concentration increase also led to increased Cd concentration, and decreased Zn concentration in subcellular compartments of wheat seedlings. The largest Cd concentration increase (174.4%) was observed in the cell wall and debris fractions of shoots after they were subjected to the highest CO2 and temperature treatment (TC3). The largest Zn concentration decrease (53.1%) was observed in the soluble (F3) fractions of shoots after they were subjected to the medium CO2 and temperature treatment (TC2). The temperature and CO2 increase had no significant effect on the proportional distribution of Cd and Zn in the subcellular fractions. The root-to-shoot translocation of Cd increased with the increasing temperature and CO2 concentration. However, the Zn distributions only fluctuated within a small range.

Can heavy metal pollution defend seed germination against heat stress? Effect of heavy metals (Cu(2+), Cd(2+) and Hg(2+)) on maize seed germination under high temperature

Heavy metal pollution, as well as greenhouse effect, has become a serious threat today. Both heavy metal and heat stresses can arrest seed germination. What response can be expected for seed germination under both stress conditions? Here, the effects of heavy metals (Cu(2+), Cd(2+) and Hg(2+)) on maize seed germination were investigated at 20 °C and 40 °C. Compared with 20 °C, heat stress induced thermodormancy. However, this thermodormancy could be significantly alleviated by the addition of a low concentration of heavy metals. Heavy metals, as well as heat stress induced H2O2 accumulation in germinating seeds. Interestingly, this low concentration of heavy metal that promoted seed germination could be partly blocked by DMTU (a specific ROS scavenger), irrespective of temperature. Accordingly, H2O2 addition reinforced this promoting effect on seed germination, which was induced by a low concentration of heavy metal. Furthermore, we found that the NADPH oxidase derived ROS was required for seed germination promoted by the heavy metals. Subsequently, treatment of seeds with fluridone (a specific inhibitor of ABA) or ABA significantly alleviated or aggravated thermodormancy, respectively. However, this alleviation or aggravation could be partly attenuated by a low concentration of heavy metals. In addition, germination that was inhibited by high concentrations of heavy metals was also partly reversed by fluridone. The obtained results support the idea that heavy metal-mediated ROS and hormone interaction can finally affect the thermodormancy release or not.

Spatially resolved analysis of variation in barley (Hordeum vulgare) grain micronutrient accumulation

Genetic biofortification requires knowledge on natural variation and the underlying mechanisms of micronutrient accumulation. We therefore studied diversity in grain micronutrient concentrations and spatial distribution in barley (Hordeum vulgare), a genetically tractable model cereal and an important crop with widespread cultivation. We assembled a diverse collection of barley cultivars and landraces and analysed grain micronutrient profiles in genebank material and after three independent cultivations. Lines with contrasting grain zinc (Zn) accumulation were selected for in-depth analysis of micronutrient distribution within the grain by micro-proton-induced X-ray emission (μ-PIXE). Also, we addressed association with grain cadmium (Cd) accumulation. The analysis of > 120 lines revealed substantial variation, especially in grain Zn concentrations. A large fraction of this variation is due to genetic differences. Grain dissection and μ-PIXE analysis of contrasting lines showed that differences in grain Zn accumulation apply to all parts of the grain including the endosperm. Cd concentrations exceeded the Codex Alimentarius threshold in most of the representative barley lines after cultivation in a Cd-contaminated agricultural soil. Two important conclusions for biofortification are: first, high-Zn grains contain more Zn also in the consumed parts of the grain; and second, higher micronutrient concentrations are strongly associated with higher Cd accumulation.

Cadmium accumulation is enhanced by ammonium compared to nitrate in two hyperaccumulators, without affecting speciation

Nitrogen fertilization could improve the efficiency of Cd phytoextraction in contaminated soil and thus shorten the remediation time. However, limited information is available on the effect of N form on Cd phytoextraction and associated mechanisms in plants. This study examined the effect of N form on Cd accumulation, translocation, and speciation in Carpobrotus rossii and Solanum nigrum Plants were grown in nutrient solution with 5-15 μM Cd in the presence of 1000 µM NH4 (+) or NO3 (-) Plant growth and Cd uptake were measured, and Cd speciation was analyzed using synchrotron-based X-ray absorption spectroscopy. Shoot Cd accumulation was 30% greater with NH4 (+) than NO3 (-) supply. Carpobrotus rossii accumulated three times more Cd than S. nigrum. However, Cd speciation in the plants was not influenced by N form, but it did vary with species and tissues. In C. rossii, up to 91% of Cd was bound to S-containing ligands in all tissues except the xylem sap where 87-95% were Cd-OH complexes. Furthermore, the proportion of Cd-S in shoots was substantially lower in S. nigrum (44-69%) than in C. rossii (60-91%). It is concluded that the application of NH4 (+) (instead of NO3 (-)) increased shoot Cd accumulation by increasing uptake and translocation, rather than changing Cd speciation, and is potentially an effective approach for increasing Cd phytoextraction.

Leaf-based physiological, metabolic, and ultrastructural changes in cultivated cotton cultivars under cadmium stress mediated by glutathione

Cadmium (Cd) pollution is present in the world over especially in the industrialized parts of the world. To reduce Cd accumulation in various crops especially food crops, alleviating agents such as reduced glutathione (GSH) can be applied, which are capable either to exclude or to sequester Cd contamination. This study investigated the leaf-based spatial distribution of physiological, metabolic, and microstructural changes in two cotton cultivars (Coker 312 and TM-1) under GSH-mediated Cd stress using single levels of Cd (50 μM) and GSH (50 μM) both separately and in mix along with control. Results showed that GSH revived the morphology and physiology of both cotton cultivars alone or in mix with Cd. Cd uptake was enhanced in all segments of leaf and whole leaf upon the addition of GSH. GSH alleviated Cd-induced reduction in the photosynthetic pigment compositions and chlorophyll a fluorescence parameters. Mean data of biomarkers (2,3,5-triphenyltetrazolium (TTC), total soluble protein (TSP), malondialdehyde (MDA), hydrogen peroxide (H2O2)) revealed the adverse effects of Cd stress on leaf segments of both cultivars, which were revived by GSH. The oxidative metabolism induced by Cd stress was profoundly influenced by exogenous GSH application. The microstructural alterations were mainly confined to chloroplastic regions of leaves under Cd-stressed conditions, which were greatly revived upon the GSH addition. As a whole, Cd stress greatly affected TM-1 as compared to Coker 312. These results suggest a positive role of GSH in alleviating Cd-mediated changes in different leaf sections of cotton cultivars.

Enhanced cadmium accumulation and tolerance in transgenic tobacco overexpressing rice metal tolerance protein gene OsMTP1 is promising for phytoremediation

One of the most grievous heavy metal pollutants in the environment is cadmium (Cd), which is not only responsible for the crop yield loss owing to its phytotoxicity, but also for the human health hazards as the toxic elements usually accumulate in the consumable parts of crop plants. In the present study, we aimed to isolate and functionally characterize the OsMTP1 gene from indica rice (Oryza sativa L. cv. IR64) to study its potential application for efficient phytoremediation of Cd. The 1257 bp coding DNA sequence (CDS) of OsMTP1 encodes a ∼46 kDa protein belonging to the cation diffusion facilitator (CDF) or metal tolerance/transport protein (MTP) family. The OsMTP1 transcript in rice plant was found to respond during external Cd stress. Heterologous expression of OsMTP1 in tobacco resulted in the reduction of Cd stress-induced phytotoxic effects, including growth inhibition, lipid peroxidation, and cell death. Compared to untransformed control, the transgenic tobacco plants showed enhanced vacuolar thiol content, indicating vacuolar localization of the sequestered Cd. The transgenic tobacco plants exhibited significantly higher biomass growth (2.2-2.8-folds) and hyperaccumulation of Cd (1.96-2.22-folds) compared to untransformed control under Cd exposure. The transgenic plants also showed moderate tolerance and accumulation of arsenic (As) upon exogenous As stress, signifying broad substrate specificity of OsMTP1. Together, findings of our research suggest that the transgenic tobacco plants overexpressing OsMTP1 with its hyperaccumulating activity and increased growth rate could be useful for future phytoremediation applications to clean up the Cd-contaminated soil.

The PSE1 gene modulates lead tolerance in Arabidopsis

Lead (Pb) is a dangerous heavy metal contaminant with high toxicity to plants. However, the regulatory mechanism of plant Pb tolerance is poorly understood. Here, we showed that the PSE1 gene confers Pb tolerance in Arabidopsis. A novel Pb-sensitive mutant pse1-1 (Pb-sensitive1) was isolated by screening T-DNA insertion mutants. PSE1 encodes an unknown protein with an NC domain and was localized in the cytoplasm. PSE1 was induced by Pb stress, and the pse1-1 loss-of-function mutant showed enhanced Pb sensitivity; overexpression of PSE1 resulted in increased Pb tolerance. PSE1-overexpressing plants showed increased Pb accumulation, which was accompanied by the activation of phytochelatin (PC) synthesis and related gene expression. In contrast, the pse1-1 mutant showed reduced Pb accumulation, which was associated with decreased PC synthesis and related gene expression. In addition, the expression of PDR12 was also increased in PSE1-overexpressing plants subjected to Pb stress. Our results suggest that PSE1 regulates Pb tolerance mainly through glutathione-dependent PC synthesis by activating the expression of the genes involved in PC synthesis and at least partially through activating the expression of the ABC transporter PDR12/ABCG40.