Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids(hereinafter referred to as trace metals)from the soil,some of which are essential but become toxic at high concentrations(e.g.,Cu,Zn,Ni,Co),while others are non-essential and toxic even at relatively low concentrations(e.g.,As,Cd,Cr,Pb,and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities.Trace metal contamination can cause toxicity and growth inhibition in plants,as well as accum...     

Interaction of heavy metals with the sulphur metabolism in angiosperms from an ecological point of view

The metabolism of sulphur in angiosperms is reviewed under the aspect of exposure to ecologically relevant concentrations of sulphur, heavy metals and metalloids. Because of the inconsistent use of the term 'metal tolerance', in this review the degree of tolerance to arsenic and heavy metals is divided into three categories: hypotolerance, basal tolerance and hypertolerance. The composition of nutrient solutions applied to physiological experiments let see that the well-known interactions of calcium, sulphate and zinc supply with uptake of heavy metals, especially cadmium are insufficiently considered. Expression of genes involved in reductive sulphate assimilation pathway and enzyme activities are stimulated by cadmium and partially by copper, but nearly not by other heavy metals. The synthesis of the sulphur-rich compounds glucosinolates, metallothioneins and phytochelatins is affected in a metal-specific way. Phytochelatin levels are low in all metal(loid)-hypertolerant plant species growing in the natural environment on metal(loid)-enriched soils. If laboratory experiments mimic the natural environments, especially high Zn/Cd ratios and good sulphur supply, and chemical analyses are extended to more mineral elements than the single metal(loid) under investigation, a better understanding of the impact of metal(loid)s on the sulphur metabolism can be achieved.     

Metalloids in plants: A systematic discussion beyond description

Metalloids represent a wide range of elements with intermediate physiochemical properties between metals and non-metals. Many of the metalloids, like boron, selenium, and silicon are known to be essential or quasi-essential for plant growth. In contrast, metalloids viz. arsenic and germanium are toxic to plant growth. The toxicity of metalloids largely depends on their concentration within the living cells. Some elements, at low concentration, may be beneficial for plant growth and development; however, when present at high concentration, they often exert negative effects. In this regard, understanding the molecular mechanisms involved in the uptake of metalloids by roots, their subsequent transport to different tissues and inter/intra-cellular redistribution has great importance. The mechanisms of metalloids' uptake have been well studied in plants. Also, various transporters, as well as membrane channels involved in these processes, have been identified. In this review, we have discussed in detail the aspects concerning the positive/negative effects of different metalloids on plants. We have also provided a thorough account of the uptake, transport, and accumulation, along with the molecular mechanisms underlying the response of plants to these metalloids. Additionally, we have brought up the previous theories and debates about the role and effects of metalloids in plants with insightful discussions based on the current knowledge.     

Microbial formation and transformation of organometallic and organometalloid compounds

Abstract: Microbial formation and transformation of organometallic and organometalloid compounds comprise significant components of biogeochemical cycles for the metals mercury, lead and tin and the metalloids arsenic, selenium, tellurium and germanium. Methylated derivatives of such elements can arise as a result of chemical and biological mechanisms and this frequently results in altered volatility, solubility, toxicity and mobility. The major microbial methylating agents are methylcobalamin (CH₃CoB₁₂), involved in the methylation of mercury, tin and lead, and S -adenosylmethionine (SAM), involved in the methylation of arsenic and selenium. Evidence for the methylation of other toxic metal(loid)s is sparse. Biomethylation may result in metal(loid) detoxification since methylated derivatives may be excreted readily from cells, are often volatile and may be less toxic, e.g. organoarsenicals. However, for mercury, low yields of methylated derivatives and the existence of more efficient resistance mechanisms, e.g. reduction of Hg²⁺ to Hg⁰, suggest a lower significance in detoxification. Bioalkylation has only been characterised in detail for arsenic. Microorganisms can accumulate organometal(loid)s, a phenomenon relevant to toxicant transfer to higher organisms. As well as bioaccumulation, many microorganisms are capable of the degradation and detoxification of organometal(loid) compounds by, e.g. demethylation and dealkylation. Several organometal(loid) transformations have potential for environmental bioremediation.     

Enhanced arsenic tolerance of transgenic eastern cottonwood plants expressing gamma-glutamylcysteine synthetase

Arsenic is a metalloid that occurs naturally at parts per million (ppm) levels in the earth's crust. Natural and human activities have contributed to arsenic mobilization and increased concentration in the environment, such that World Health Organization guidelines for arsenic levels in drinking water are exceeded at many locations, worldwide. This translates into an increased risk of arsenic-related illnesses for millions of people. Recent studies demonstrate that increasing thiol-sinks in transgenic plants by overexpressing the bacterial gamma-glutamylcysteine synthetase (ECS) gene results in a higher tolerance and accumulation of metals and metalloids such as cadmium, mercury, and arsenic. We used Agrobacterium-mediated transformation to genetically engineer eastern cottonwood with a bacterial ECS gene. Eastern cottonwood plants expressing ECS had elevated thiol group levels, consistent with increased ECS activity. In addition, these ECS-expressing plants had enhanced growth on levels of arsenate toxic to control plants in vitro. Furthermore, roots of ECS-expressing plants accumulated significantly more arsenic than control roots (approximately twice as much), while shoots accumulated significantly less arsenic than control shoots (approximately two-thirds as much). We discuss potential mechanisms for shifting the balance of plant arsenic distribution from root accumulation to shoot accumulation, as it pertains to arsenic phytoremediation.     

Transgenic Indian mustard (<Emphasis Type="Italic">Brassica juncea</Emphasis>) plants expressing an <Emphasis Type="Italic">Arabidopsis</Emphasis> phytochelatin synthase (<Emphasis Type="Italic">AtPCS1</Emphasis>) exhibit enhanced As and Cd tolerance

Phytochelatins (PCs) are post-translationally synthesized thiol reactive peptides that play important roles in detoxification of heavy metal and metalloids in plants and other living organisms. The overall goal of this study is to develop transgenic plants with increased tolerance for and accumulation of heavy metals and metalloids from soil by expressing an Arabidopsis thaliana AtPCS1 gene, encoding phytochelatin synthase (PCS), in Indian mustard (Brassica juncea L.). A FLAG-tagged AtPCS1 gDNA, under its native promoter, is expressed in Indian mustard, and transgenic pcs lines have been compared with wild-type plants for tolerance to and accumulation of cadmium (Cd) and arsenic (As). Compared to wild type plants, transgenic plants exhibit significantly higher tolerance to Cd and As. Shoots of Cd-treated pcs plants have significantly higher concentrations of PCs and thiols than those of wild-type plants. Shoots of wild-type plants accumulated significantly more Cd than those of transgenic plants, while accumulation of As in transgenic plants was similar to that in wild type plants. Although phytochelatin synthase improves the ability of Indian mustard to tolerate higher levels of the heavy metal Cd and the metalloid As, it does not increase the accumulation potential of these metals in the above ground tissues of Indian mustard plants.     

Phytochelatin–metal(loid) transport into vacuoles shows different substrate preferences in barley and Arabidopsis

Cadmium (Cd) and arsenic (As) are toxic to all living organisms, including plants and humans. In plants, Cd and As are detoxified by phytochelatins (PCs) and metal(loid)‐chelating peptides and by sequestering PC–metal(loid) complexes in vacuoles. Consistent differences have been observed between As and Cd detoxification. Whereas chelation of Cd by PCs is largely sufficient to detoxify Cd, As–PC complexes must be sequestered into vacuoles to be fully detoxified. It is not clear whether this difference in detoxification pathways is ubiquitous among plants or varies across species. Here, we have conducted a PC transport study using vacuoles isolated from Arabidopsis and barley. Arabidopsis vacuoles accumulated low levels of PC₂–Cd, and vesicles from yeast cells expressing either AtABCC1 or AtABCC2 exhibited negligible PC₂–Cd transport activity compared with PC₂–As. In contrast, barley vacuoles readily accumulated comparable levels of PC₂–Cd and PC₂–As. PC transport in barley vacuoles was inhibited by vanadate, but not by ammonium, suggesting the involvement of ABC‐type transporters. Interestingly, barley vacuoles exhibited enhanced PC₂ transport activity when essential metal ions, such as Zn(II), Cu(II) and Mn(II), were added to the transport assay, suggesting that PCs might contribute to the homeostasis of essential metals and detoxification of non‐essential toxic metal(loid)s.     

Comparative analysis of the contribution of phytochelatins to cadmium and arsenic tolerance in soybean and white lupin

The biosynthesis of phytochelatins (PCs) plays a crucial role in the detoxification and homeostasis of heavy metals and metalloids in plants. However, in an increasing number of plant species metal(loid) tolerance is not well correlated with the accumulation of PCs: tolerant ecotypes frequently contain lower levels of PCs than non-tolerant ecotypes. In this study we have compared the responses of soybean (Glycine max L. cv. Resnik) and white lupin (Lupinus albus L. cv. Marta) to cadmium and arsenate in order to assess the role of homophytochelatins (hPCs) in the tolerance of soybean to these toxic elements. Soybean plants treated with Cd and As showed a high contribution of homo-glutathione (hGSH) to the pool of thiols in shoots in comparison to white lupin. Higher levels of hPCs in Cd-treated soybeans compared to PCs in lupins did not prevent growth inhibition. In contrast, the role of hPCs in the detoxification mechanism to arsenate in soybean seems to be clearer, showing higher thiol concentrations and lower growth reductions than those present in lupin plants.     

Evolution and function of phytochelatin synthases

Both essential and non-essential transition metal ions can easily be toxic to cells. The physiological range for essential metals between deficiency and toxicity is therefore extremely narrow and a tightly controlled metal homeostasis network to adjust to fluctuations in micronutrient availability is a necessity for all organisms. One protective strategy against metal excess is the expression of high-affinity binding sites to suppress uncontrolled binding of metal ions to physiologically important functional groups. The synthesis of phytochelatins, glutathione-derived metal binding peptides, represents the major detoxification mechanism for cadmium and arsenic in plants and an unknown range of other organisms. A few years ago genes encoding phytochelatin synthases (PCS) were cloned from plants, fungi and nematodes. Since then it has become apparent that PCS genes are far more widespread than ever anticipated. Searches in sequence databases indicate PCS expression in representatives of all eukaryotic kingdoms and the presence of PCS-like proteins in several prokaryotes. The almost ubiquitous presence in the plant kingdom and beyond as well as the constitutive expression of PCS genes and PCS activity in all major plant tissues are still mysterious. It is unclear, how the extremely rare need to cope with an excess of cadmium or arsenic ions could explain the evolution and distribution of PCS genes. Possible answers to this question are discussed. Also, the molecular characterization of phytochelatin synthases and our current knowledge about the enzymology of phytochelatin synthesis are reviewed.     

Volatilisation of metals and metalloids: An inherent feature of methanoarchaea?

As shown by recent studies, anaerobic members of Archaea and Bacteria are involved in processes that transform ionic species of metals and metalloids (arsenic, antimony, bismuth, selenium, tellurium and mercury) into volatile and mostly toxic derivatives (mainly methyl derivatives or hydrides). Since the fact that these transformations proceed in both environmental settings and in parts of the human body, we have to consider that these processes also interfere directly with human health. The diversity of the volatile derivatives produced and their emission rates were significantly higher in methanoarchaeal than in bacterial strains, which supports the pivotal role of methanoarchaea in transforming metals and metalloids (metal(loid)s) into their volatile derivatives. Compared with methanoarchaea, 14 anaerobic bacterial strains showed a significantly restricted spectrum of volatilised derivatives and mostly lower production rates of volatile bismuth and selenium derivatives. Since methanoarchaea isolated from the human gut (Methanosphaera stadtmanae, Methanobrevibacter smithii) showed a higher potential for metal(loid) derivatisation compared to bacterial gut isolates, we assume that methanoarchaea in the human gut are mainly responsible for the production of these volatile derivatives. The observation that trimethylbismuth ((CH(3))(3)Bi), the main volatile derivative of bismuth produced in human feces, inhibited growing cultures of Bacteroides thetaiotaomicron, a representative member of the human physiological gut flora, suggests that these volatiles exert their toxic effects on human health not only by direct interaction with host cells but also by disturbing the physiological gut microflora.     

A Review on Heavy Metals Contamination in Soil: Effects,Sources, and Remediation Techniques

Soil heavy metal pollution has become a worldwide environmental issue that has attracted considerable public attention, largely from the increasing concern for the security of agricultural products. Heavy metals refer to some metals and metalloids possessing biological toxicity, such as cadmium, mercury, arsenic, lead, and chromium. These elements enter the soil agro-ecosystem through natural processes derived from parent materials, and through anthropogenic activities. Heavy metal pollution poses a great threat to the health and well-being of organisms and human beings due to potential accumulation risk through the food chain. Remediation using chemical, physical, and biological methods has been adopted to solve the problem. Phytoremediation has proven to be a promising alternative to conventional approaches as it is cost effective, environmentally friendly, and aesthetically pleasing. To date, based on the natural ability of extraction, approximately 500 taxa have been identified as hyperaccumulators of one or more metals. In addition, further research integrating biotechnological approaches with comprehensive multidisciplinary research is needed to improve plant tolerance and reduce the accumulation of toxic metals in soils. This review discusses harmful effects, sources of heavy metals, and the remediation technologies for soil contaminated by heavy metals.     

Uptake and transformation of metals and metalloids by microbial mats and their use in bioremediation

Summary Constructed microbial mats, used for studies on the removal and transformation of metals and metalloids, are made by combining cyanobacteria inoculum with a sediment inoculum from a metal-contaminated site. These mats are a heterotrophic and autotrophic community dominated by cyanobacteria and held together by slimy secretions produced by various microbial groups. When contaminated water containing high concentrations of metals is passed over microbial mats immobilized on glass wool, there is rapid removal of the metals from the water. The mats are tolerant of high concentrations of toxic metals and metalloids, such as cadmium, lead, chromium, selenium and arsenic (up to 350 mg L^–1). This tolerance may be due to a number of mechanisms at the molecular, cellular and community levels. Management of toxic metals by the mats is related to deposition of metal compounds outside the cell surfaces as well as chemical modification of the aqueous environment surrounding the mats. The location of metal deposition is determined by factors such as redox gradients, cell surface micro-environments and secretion of extra-cellular bioflocculents. Metal-binding flocculents (polyanionic polysaccharides) are produced in large quantities by the cyanobacterial component of the mat. Steep gradients of redox and oxygen exist from the surface through the laminated strata of microbes. These are produced by photosynthetic oxygen production at the surface and heterotrophic consumption in the deeper regions. Additionally, sulfur-reducing bacteria colonize the lower strata, removing and utilizing the reducing H₂S, rather than water, for photosynthesis. Thus, depending on the chemical character of the microzone of the mat, the sequestered metals or metalloids can be oxidized, reduced and precipitated as sulfides or oxides. For example precipitates of red amorphous elemental selenium were identified in mats exposed to selenate (Se-VI) and insoluble precipitates of manganese, chromium, cadmium, cobalt, and lead were found in mats exposed to soluble salts of these metals. Constructed microbial mats offer several advantages for use in the bioremediation of metal-contaminated sites. These include low cost, durability, ability to function in both fresh and salt water, tolerance to high concentrations of metals and metalloids and the unique capacity of mats to form associations with new microbial species. Thus one or several desired microbial species might be integrated into mats in order to design the community for specific bioremediation applications.     

Toxicity, accumulation, and removal of heavy metals by three aquatic macrophytes

A comprehensive understanding of the uptake, tolerance, and transport of heavy metals by plants will be essential for the development of phytoremediation technologies. In the present paper, we investigated accumulation, tissue and intracellular localization, and toxic effects of cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) in three aquatic macrophytes (the angiosperms Lemna minor and Elodea canadensis, and the moss Leptodictyum riparium). We also tested and compared their capacity to absorb heavy metal from water under laboratory conditions. Our data showed that all the three species examined could be considered good bioaccumulators for the heavy metals tested. L. riparium was the most resistant species and the most effective in accumulating Cu, Zn, and Pb, whereas L. minor was the most effective in accumulating Cd. Cd was the most toxic metal, followed by Pb, Cu, and Zn. At the ultrastructural level, sublethal concentrations of the heavy metals tested caused induced cell plasmolysis and alterations of the chloroplast arrangement. Heavy metal removal experiments revealed that the three macrophytes showed excellent performance in removing the selected metals from the solutions in which they are maintained, thus suggesting that they could be considered good candidates for wastewaters remediation purpose.     

Vacuolar compartmentalization as indispensable component of heavy metal detoxification in plants

Plant cells orchestrate an array of molecular mechanisms for maintaining plasmatic concentrations of essential heavy metal (HM) ions, for example, iron, zinc and copper, within the optimal functional range. In parallel, concentrations of non‐essential HMs and metalloids, for example, cadmium, mercury and arsenic, should be kept below their toxicity threshold levels. Vacuolar compartmentalization is central to HM homeostasis. It depends on two vacuolar pumps (V‐ATPase and V‐PPase) and a set of tonoplast transporters, which are directly driven by proton motive force, and primary ATP‐dependent pumps. While HM non‐hyperaccumulator plants largely sequester toxic HMs in root vacuoles, HM hyperaccumulators usually sequester them in leaf cell vacuoles following efficient long‐distance translocation. The distinct strategies evolved as a consequence of organ‐specific differences particularly in vacuolar transporters and in addition to distinct features in long‐distance transport. Recent molecular and functional characterization of tonoplast HM transporters has advanced our understanding of their contribution to HM homeostasis, tolerance and hyperaccumulation. Another important part of the dynamic vacuolar sequestration syndrome involves enhanced vacuolation. It involves vesicular trafficking in HM detoxification. The present review provides an updated account of molecular aspects that contribute to the vacuolar compartmentalization of HMs.     

Emerging mechanisms for heavy metal transport in plants

Heavy metal ions such as Cu(2+), Zn(2+), Mn(2+), Fe(2+), Ni(2+) and Co(2+) are essential micronutrients for plant metabolism but when present in excess, these, and non-essential metals such as Cd(2+), Hg(2+) and Pb(2+), can become extremely toxic. Thus mechanisms must exist to satisfy the requirements of cellular metabolism but also to protect cells from toxic effects. The mechanisms deployed in the acquisition of essential heavy metal micronutrients have not been clearly defined although a number of genes have now been identified which encode potential transporters. This review concentrates on three classes of membrane transporters that have been implicated in the transport of heavy metals in a variety of organisms and could serve such a role in plants: the heavy metal (CPx-type) ATPases, the natural resistance-associated macrophage protein (Nramp) family and members of the cation diffusion facilitator (CDF) family. We aim to give an overview of the main features of these transporters in plants in terms of structure, function and regulation drawing on information from studies in a wide variety of organisms.     

Cellular localization and developmental changes of Zip8, Zip14 and transferrin receptor 1 in the inner ear of rats

Prior studies have demonstrated that the inner ear can accumulate a variety of essential and potentially toxic heavy metals including manganese, lead, cobalt and cadmium. Metal accumulation is regulated in part by the functionality and affinity of these metals for the different transport systems responsible for uptake across the blood-cochlea barrier and their subsequent uptake into the different cells within the inner ear. Transport of these metals across cell membranes occurs by many of the same transport systems which include DMT1, Zip8 and Zip14. All three metal transporters have been identified in the cochlea based on quantitative PCR analysis. Prior studies in our laboratory examined the localization and developmental changes of DMT1 in rat cochlea and since the two Zip proteins are also likely to contribute to the transport of essential and non-essential divalent cations, we performed immunolabeling experiments in postnatal day three rat pups and adult rats. For comparison, we also immunolabeled the specimens with antibody against transferrin receptor 1 (TfR1) which is important in DMT1-mediated transport of Fe and Mn. Results presented in this paper demonstrate that the cellular and subcellular distribution of both Zip8 and Zip14 within the different components of the inner ear are distinct from that of DMT1. Nuclear localization for both Zip transporters as well as TfR1 was observed. The findings also reveal that the selective distribution of the three proteins was altered during development presumably to meet the changing needs of the cells to maintain normal and functional levels of iron and other essential metals.     

Comparative strategies of heavy metal accumulation by crustaceans: zinc,copper and cadmium in a decapod,an amphipod and a barnacle

This study investigates the comparative strategies of accumulation under standardised laboratory conditions of the essential metals zinc and copper, and the non-essential metal cadmium by three crustaceans of different taxa; vizPalaemon elegans Rathke (Malacostraca: Eucarida: Decapoda),Echinogammarus pirloti (Sexton & Spooner) (Malacostraca: Peracarida: Amphipoda) and the barnacleElminius modestus Darwin (Cirripedia: Thoracica).The decapodP. elegans regulates body zinc concentrations to a constant level (ca. 79 µg Zn g^–1) over a wide range of dissolved metal availabilities until regulation breaks down at high Zn availabilities and net accumulation begins. The amphipodE. pirloti accumulates zinc at all dissolved zinc concentrations but at a low net rate such that the accumulation strategy approaches that of regulation. The barnacleE. modestus accumulates zinc to high body concentrations with no significant excretion of accumulated zinc. In the case of copper,P. elegans similarly regulates body copper concentrations to a constant level (ca. 129 µg Cu g^–1) over a range of dissolved copper availabilities until regulation breaks down at high copper concentrations. Both the amphipodE. pirloti and the barnacleE. modestus on the other hand accumulate copper at all dissolved copper exposures with no evidence of regulation. All three crustaceans accumulate the non-essential metal cadmium at all dissolved cadmium concentrations without regulation.Heavy metal accumulation strategies therefore vary between crustacean taxa and between metals. Uptake rates for zinc and cadmium have been estimated for the three crustaceans and can be interpreted in terms of cuticle permeability and way of life of each crustacean. Examination of these uptake rates provides an insight into possible reasons behind the adoption of particular metal accumulation strategies.