The role of phytochelatins in constitutive and adaptive heavy metal tolerances in hyperaccumulator and non-hyperaccumulator metallophytes

Subcellular localization of cadmium in roots and leaves of Arabidopsis thaliana

We examined the subcellular cadmium (Cd) localization in roots and leaves of wild-type Arabidopsis thaliana (ecotype Columbia) exposed to environmentally relevant Cd concentrations. Energy-dispersive X-ray microanalysis (EDXMA) was performed on high-pressure frozen and freeze-substituted tissues. In the root cortex, Cd was associated with phosphorus (Cd/P) in the apoplast and sulfur (Cd/S) in the symplast, suggesting phosphate and phytochelatin sequestration, respectively. In the endodermis, sequestration of Cd/S was present as fine granular deposits in the vacuole and as large granular deposits in the cytoplasm. In the central cylinder, symplastic accumulation followed a distinct pattern illustrating the importance of passage cells for the uptake of Cd. In the apoplast, a shift of Cd/S granular deposits from the middle lamella towards the plasmalemma was observed. Large amounts of precipitated Cd in the phloem suggest retranslocation from the shoot. In leaves, Cd was detected in tracheids but not in the mesophyll tissue. Extensive symplastic and apoplastic sequestration in the root parenchyma combined with retranslocation via the phloem confirms the excluder strategy of Arabidopsis thaliana.

Copper-induced oxidative stress and responses of antioxidants and phytochelatins in Hydrilla verticillata (L.f.) Royle

Copper, though essential, is potentially toxic heavy metal at supraoptimal level and has widespread contamination. The present investigation was carried out to study the responses induced by lower as well as higher doses of copper (0.1-25 microM) in an aquatic macrophyte, Hydrilla verticillata (L.f.) Royle for a period of 1-7 days. The plants accumulated copper in high amount with a maximum of 770 microg g(-1) dw on day 7 at 25 microM. Biomass and photosynthetic pigments showed less alteration up to 1 microM while at higher concentrations, significant decline occurred. Malondialdehyde (MDA) content and electrical conductivity (EC) also showed sharp increase at higher concentrations indicating oxidative stress. In response to copper exposure, plants showed significant induction of proteins and enzymes like superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), catalase (CAT) and glutathione reductase (GR), however, only up to moderate exposures. Total non-protein thiols (NP-SH) and cysteine levels increased significantly up to 5 microM copper exposure while at 25 microM, their level declined drastically. Reduced glutathione (GSH) showed decrease at all concentrations while oxidized glutathione (GSSG) simultaneously increased. Phytochelatins (PCs) were also induced significantly at studied concentrations of 1 and 5 microM on day 4 in comparison to control. However, copper chelation depicted by PC-SH to copper ratio was found to be low (6.5% at 1 microM and 2.4% at 5 microM) suggesting that PCs play only a part in integrated mechanisms of copper homeostasis and detoxification. Tolerant response of plants to moderate copper exposures and high accumulation potential warrants their suitability for remediation of moderately copper polluted water bodies.

Bioavailability of trace metals to aquatic microorganisms: importance of chemical, biological and physical processes on biouptake

An important challenge in environmental biogeochemistry is the determination of the bioavailability of toxic and essential trace compounds in natural media. For trace metals, it is now clear that chemical speciation must be taken into account when predicting bioavailability. Over the past 20 years, equilibrium models (free ion activity model (FIAM), biotic ligand model (BLM)) have been increasingly developed to describe metal bioavailability in environmental systems, despite the fact that environmental systems are always dynamic and rarely at equilibrium. In these simple (relatively successful) models, any reduction in the available, reactive species of the metal due to competition, complexation or other reactions will reduce metal bioaccumulation and thus biological effects. Recently, it has become clear that biological, physical and chemical reactions occurring in the immediate proximity of the biological surface also play an important role in controlling trace metal bioavailability through shifts in the limiting biouptake fluxes. Indeed, for microorganisms, examples of biological (transport across membrane), chemical (dissociation kinetics of metal complexes) and physical (diffusion) limitation can be demonstrated. Furthermore, the organism can employ a number of biological internalization strategies to get around limitations that are imposed on it by the physicochemistry of the medium. The use of a single transport site by several metals or the use of several transport sites by a single metal further complicates the prediction of uptake or effects using the simple chemical models. Finally, once inside the microorganism the cell is able to employ a large number of strategies including complexation, compartmentalization, efflux or the production of extracellular ligands to minimize or optimize the reactivity of the metal. The prediction of trace metal bioavailability will thus require multidisciplinary advances in our understanding of the reactions occurring at and near the biological interface. By taking into account medium constraints and biological adaptability, future bioavailability modeling will certainly become more robust.

Detection and quantification of unbound phytochelatin 2 in plant extracts of Brassica napus grown with different levels of mercury

The mercury (Hg) accumulation mechanism was studied in rape (Brassica napus) plants grown under a Hg concentration gradient (0 microm-1,000 microm). Hg mainly accumulated in roots. Therefore, the presence of phytochelatins (PCs) was studied in the roots of the plants. The high stability of the PC-Hg multicomplexes (mPC-nHg) seems to be the main reason for the lack of previous Hg-PC characterization studies. We propose a modification of the method to detect and quantify unbound PC of Hg in plant extracts via high-performance liquid chromatography coupled to electrospray tandem mass spectrometry and inductively coupled plasma mass spectrometry in parallel. We separated the PC from the Hg by adding the chelating agent sodium 2,3-dimercaptopropanesulfonate monohydrate. We only detected the presence of PC after the addition of the chelating agent. Some multicomplexes mPC-nHg could be formed but, due to their large sizes, could not be detected. In this study, only PC(2) was observed in plant samples. Hg accumulation was correlated with PC(2) concentration (r(2) = 0.98).

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

Over the past 200 years emissions of toxic heavy metals have risen tremendously and significantly exceed those from natural sources for practically all metals. Uptake and accumulation by crop plants represents the main entry pathway for potentially health-threatening toxic metals into human and animal food. Of major concern are the metalloids arsenic (As) and selenium (Se), and the metals cadmium (Cd), mercury (Hg), and lead (Pb). This review discusses the molecular mechanisms of toxic metal accumulation in plants and algae, the responses to metal exposure, as well as our understanding of metal tolerance and its evolution. The main emphasis will be on cadmium, which is by far the most widely studied of the non-essential toxic metals/metalloids. Entry via Zn2+, Fe2+, and Ca2+ transporters is the molecular basis of Cd2+ uptake into plant cells. Much less is known about the partitioning of non-essential metals and about the genes underlying the enormous diversity among plants with respect to Cd accumulation in different tissues. Numerous studies have described symptoms and responses of plants upon toxic metal exposure. Mysterious are primary targets of toxicity, the degree of specificity of responses, the sensing and the signaling events that lead to transcriptional activation. All plants apparently possess a basal tolerance of toxic non-essential metals. For Cd and As, this is largely dependent on the phytochelatin pathway. Not understood is the molecular biology of Cd hypertolerance in certain plant species such as the metallophytes Arabidopsis halleri or Thlaspi caerulescens.

Synthesis of low molecular weight thiols in response to Cd exposure in Thlaspi caerulescens

In this study, we investigated the accumulation of phytochelatins (PCs) and other low molecular weight (LMW) thiols in response to Cd exposure in two contrasting ecotypes differing in Cd accumulation. Using a root elongation test, we found that the highly accumulating ecotype Ganges was more tolerant to Cd than the low Cd-accumulation ecotype Prayon. L-buthionine-(S,R)-sulphoximine (BSO), a potent inhibitor of the gamma-glutamylcysteine synthetase gamma-ECS) (an enzyme involved in the PC biosynthetic pathway), increased the Cd sensitivity of Prayon, but had no effect on Ganges. Although PC accumulation increased in response to Cd exposure, no significant differences were observed between the two ecotypes. Cd exposure induced a dose-dependent accumulation of both Cys and a still unidentified LMW thiol in roots of both ecotypes. Root accumulation of Cys and this thiol was higher in Ganges than in Prayon; the ecotypic differences were more pronounced when the plants were treated with BSO. These findings suggest that PCs do not contribute to the Cd hypertolerance displayed by the Ganges ecotype of Thlaspi caerulescens, whereas Cys and other LMW thiols might be involved.

Multivariate analysis of protein profiles of metal hyperaccumulatorThlaspi caerulescens accessions

Thlaspi caerulescens is increasingly acknowledged as one of the best models for studying metal hyperaccumulation in plants. In order to study the mechanisms underlying metal hyperaccumulation, we used proteomic profiling to identify differences in protein intensities among three T. caerulescens accessions with pronounced differences in tolerance, uptake and root to shoot translocation of Zn and Cd. Proteins were separated using two-dimensional electrophoresis and stained with SYPRO Orange. Intensity values and quality scores were obtained for each spot by using PDQuest software. Principal component analysis was used to test the separation of the protein profiles of the three plant accessions at various metal exposures, and to detect groups of proteins responsible for the differences. Spot sets representing individual proteins were analysed with the analysis of variance and non-parametric Kruskal-Wallis test. Clearest differences were seen among the Thlaspi accessions, while the effects of metal exposures were less pronounced. The 48 tentatively identified spots represent core metabolic functions (e.g. photosynthesis, nitrogen assimilation, carbohydrate metabolism) as well as putative signalling and regulatory functions. The possible roles of some of the proteins in heavy metal accumulation and tolerance are discussed.

Biosynthesis of phytochelatins and arsenic accumulation in the marine microalga Phaeodactylum tricornutum in response to arsenate exposure

The arsenate-induced synthesis of phytochelatins (PC), intracellular cysteine-rich metal-binding peptides, and its relationship with toxicity and with As accumulation in the cell have been studied in laboratory cultures of the marine microalga Phaeodactylum tricornutum. The time course of cellular PC and As in short-term exposures showed that the involvement of PC in the As detoxification as well as the pathway of cellular As depend on the extent of As accumulation and on the rate of PC synthesis. At arsenate concentrations causing As accumulation at a rate exceeding that of PC synthesis, cells seem to activate a mechanism of release of As mainly in a chemical form not complexed with PC. At arsenate concentrations at which the synthesis of PC occurs at a rate sufficient to allow a significant portion of As accumulated in the cell to be bound, the fate of cellular As seems to be mainly controlled by PC. The occurrence of these different pathways of As detoxification was discussed to explain the pattern of cellular As and PC in cells grown for three days at growth-inhibitory and at no growth-inhibitory concentration of arsenate.

Enhanced arsenate reduction by a CDC25-like tyrosine phosphatase explains increased phytochelatin accumulation in arsenate-tolerant Holcus lanatus

Decreased arsenate [As(V)] uptake is the major mechanism of naturally selected As(V) hypertolerance in plants. However, As(V)-hypertolerant ecotypes also show enhanced rates of phytochelatin (PC) accumulation, suggesting that improved sequestration might additionally contribute to the hypertolerance phenotype. Here, we show that enhanced PC-based sequestration in As(V)-hypertolerant Holcus lanatus is not due to an enhanced capacity for PC synthesis as such, but to increased As(V) reductase activity. Vacuolar transport of arsenite-thiol complexes was equal in both ecotypes. Based on homology with the yeast As(V) reductase, Acr2p, we identified a Cdc25-like plant candidate, HlAsr, and confirmed the As(V) reductase activity of both HlAsr and the homologous protein from Arabidopsis thaliana. The gene appeared to be As(V)-inducible and its expression was enhanced in the As(V)-hypertolerant H. lanatus ecotype, compared with the non-tolerant ecotype. Homologous ectopic overexpression of the AtASR cDNA in A. thaliana produced a dual phenotype. It improved tolerance to mildly toxic levels of As(V) exposure, but caused hypersensitivity to more toxic levels. Arabidopsis asr T-DNA mutants showed increased As(V) sensitivity at low exposure levels and enhanced arsenic retention in the root. It is argued that, next to decreased uptake, enhanced expression of HlASR might act as an additional determinant of As(V) hypertolerance and As transport in H. lanatus.

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.

Phytochelatin synthesis and response of antioxidants during cadmium stress in Bacopa monnieri L

The phytotoxicity imposed by cadmium (Cd) and its detoxifying responses of Bacopa monnieri L. have been investigated. Effect on biomass, photosynthetic pigments and protein level were evaluated as gross effect, while lipid peroxidation and electrolyte leakage reflected oxidative stress. Induction of phytochelatins and enzymatic and non-enzymatic antioxidants were monitored as plants primary and secondary metal detoxifying responses, respectively. Plants accumulated substantial amount of Cd in different plant parts (root, stem and leaf), the maximum being in roots (9240.11 microg g(-1) dw after 7 d at 100 microM). Cadmium induced oxidative stress, which was indicated by increase in lipid peroxidation and electrical conductivity with increase in metal concentration and exposure duration. Photosynthetic pigments showed progressive decline while protein showed slight increase at lower concentrations. Enzymes viz., superoxide dismutase (SOD, EC 1.15.1.1), guaiacol peroxidase (GPX, EC 1.11.1.7) ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) showed stimulation except catalase (CAT, EC 1.11.1.6) which showed declining trend. Initially, an enhanced level of cysteine, glutathione and non-protein thiols was observed, which depleted with increase in exposure concentration and duration. Phytochelatins induced significantly at 10 microM Cd in roots and at 50 microM Cd in leaves. The phytochelatins decreased in roots at 50 microM Cd, which may be correlated with reduced level of GSH, probably due to reduced GR activity, which exerted increased oxidative stress as also evident by the phenotypic changes in the plant like browning of roots and slight yellowing of leaves. Thus, besides synthesis of phytochelatins, availability of GSH and concerted activity of GR seem to play a central role for Bacopa plants to combat oxidative stress caused by metal and to detoxify it. Plants ability to accumulate and tolerate high amount of Cd through enhanced level of PCs and various antioxidants suggest it to be a suitable candidate for phytoremediation.

Metal ion ligands in hyperaccumulating plants

Metal-hyperaccumulating plants have the ability to take up extraordinary quantities of certain metal ions without succumbing to toxic effects. Most hyperaccumulators select for particular metals but the mechanisms of selection are not understood at the molecular level. While there are many metal-binding biomolecules, this review focuses only on ligands that have been reported to play a role in sequestering, transporting or storing the accumulated metal. These include citrate, histidine and the phytosiderophores. The metal detoxification role of metallothioneins and phytochelatins in plants is also discussed.

Responses to Nickel in the Proteome of the Hyperaccumulator Plant Alyssum lesbiacum

A proteomic analysis of the Ni hyperaccumulator plant Alyssum lesbiacum was carried out to identify proteins that may play a role in the exceptional degree of Ni tolerance and accumulation characteristic of this metallophyte. Of the 816 polypeptides detected in root tissue by 2D SDS-PAGE, eleven increased and one decreased in abundance relative to total protein after 6-week-old plants were transferred from a standard nutrient solution containing trace concentrations of Ni to a moderately high Ni treatment (0.3 mM NiSO4) for 48 h. These polypeptides were identified by tandem mass spectrometry and the majority were found to be involved in sulphur metabolism (consistent with a re-allocation of sulphur towards cysteine and glutathione), protection against reactive oxygen species, or heat-shock response. In contrast, very few polypeptides were found to change in abundance in root or shoot tissue after plants were exposed for 28 days to 0.03 mM NiSO4, a concentration representing the optimum for growth of this species but sufficient to lead to hyperaccumulation of Ni in the shoot. Under these conditions, constitutively expressed genes in this highly Ni-tolerant species may be sufficient to allow for effective chelation and sequestration of Ni without the need for additional protein synthesis.

Variations in plant metallothioneins: the heavy metal hyperaccumulator Thlaspi caerulescens as a study case

Plant metallothioneins (MTs) are extremely diverse and are thought to be involved in metal homeostasis or detoxification. Thlaspi caerulescens is a model Zn/Cd hyperaccumulator and thus constitutes an ideal system to study the variability of these MTs. Two T. caerulescens cDNAs (accession: 665511; accession: 665515), that are highly homologous to type 1 and type 2 Arabidopsis thaliana MTs, have been isolated using a functional screen for plant cDNAs that confer Cd tolerance to yeast. However, TcMT1 has a much shorter N-terminal domain than that of A. thaliana and so lacks Cys motifs conserved through all the plant MTs classified as type 1. A systematic search in plant databases allowed the detection of MT-related sequences. Sixty-four percent fulfil the criteria for MT classification described in Cobbett and Goldsbrough (2002) and further extend our knowledge about other conserved residues that might play an important role in plant MT structure. In addition, 34% of the total MT-related sequences cannot be classified strictly as they display modifications in the conserved residues according to the current plant MTs' classification. The significance of this variability in plant MT sequences is discussed. Functional complementation in yeast was used to assess whether these variations may alter the MTs' function in T. caerulescens. Regulation of the expression of MTs in T. caerulescens was also investigated. TcMT1 and TcMT2 display higher expression in T. caerulescens than in A. thaliana. Moreover, their differential expression patterns in organs and in response to metal exposure, suggest that the two types of MTs may have diverse roles and functions in T. caerulescens.

Plant-specific responses to zinc contamination in a semi-field lysimeter and on hydroponics

The species Agrostis stolonifera, Brassica napus and Trifolium repens representing different ecological strategies, were selected to study the effect of Zn contamination on Zn tolerance, uptake and accumulation patterns. Parallel tests were carried out with increasing concentrations of Zn in a semi-field lysimeter and hydroponics in the climate chamber. A significant reduction in biomass production or root length and an increase in shoot Zn concentration was observed for all species at increasing external Zn concentrations. However, shoot biomass production, Zn tolerance and Zn accumulation differed significantly among the tested species. The results in both experimental set-ups were quite similar concerning Zn tolerance and accumulation and improved the validity of the findings. The rather specific responses of the different plant species to Zn contamination interfere with the more generic approach used in risk assessment studies. Maximum amounts of Zn in shoot are not likely to cause a risk to herbivores.

Increase of glutathione in mine population of Sedum alfredii: a Zn hyperaccumulator and Pb accumulator

Phytochelatins (PCs) have been induced in a large range of plant species, but their role in heavy metal tolerance is unclear. Sedum alfredii is a new zinc (Zn) hyperaccumulator and lead (Pb) accumulator found in an old Pb/Zn mine in the Zhejiang Province of China. Until now, the mechanisms of its hyperaccumulation/accumulation and tolerance were poorly understood. The aim of this work was to investigate whether PCs were differentially produced in mine populations of S. alfredii compared with a non-mine control of the same species. The results showed that plants from the mine site were more tolerant to increasing Zn and Pb concentrations than those from the control site. No PCs and cysteine (Cys) were detected by pre-column derivatization with HPLC fluorescence in any tissues of two populations at any treatment, which in turn indicated they were not responsible for Zn and Pb tolerance in the mine population. Instead, Zn and Pb treatments resulted in the increase of glutathione (GSH) for both populations in a tissue-dependent manner. Significant increases were observed in leaf, stem and root tissues of plants grown on the mine site. The results suggest that GSH, rather man PCs, may be involved in Zn and Pb transport, hyperaccumulation/accumulation and tolerance in mine population of S. alfredii.

Growth parameters and photosynthetic pigments in leaf segments of Zea mays exposed to cadmium, as related to protection mechanisms

The influence of cadmium on growth parameters and photosynthetic pigment content was studied in maize leaf segments differing in tissue maturity. Experiments were carried out with maize seedlings Zea mays L. cv. Hidosil treated with 25, 50, 100, 150 and 200 microM Cd for 14 days under low light conditions. Tissue age-dependent decrease of fresh mass, dry mass and area of the leaf segments was correlated with Cd concentration in the growth medium. Cd induced changes in chlorophyll and carotenoid contents, and the specific areas and densities of the leaf segments were dependent on age and metal concentration. Results are discussed with respect to the protection mechanisms in the leaf segments.

Cellular damage induced by cadmium and mercury in Medicago sativa

Alfalfa (Medicago sativa) plantlets were exposed to Cd or Hg to study the kinetics of diverse stress indexes. In the so-called beaker-size hydroponic system, plantlets were grown in 30 microM of Cd or Hg for 7 d. Oxidative stress took place and increased over time, a linear response being observed with Cd but not with Hg. To improve the sensitivity of the stress assays used, a micro-assay system, in which seedlings were exposed for 24 h, was developed. Phytotoxicity of metals, quantified as growth inhibition, was observed well before there was any change in the non-protein thiol tissue concentration. When measured with conventional techniques, oxidative stress indexes did not show significant variation. To trace early and small plant responses to Cd and Hg, a microscopic analysis with novel fluorescent dyes, which had not yet been exploited to any significant extent for use in plants, was conducted. These fluorescent probes, which allowed minute cellular responses to 0, 3, 10, and 30 microM of both metals to be visualized in the roots of the alfalfa seedlings, were: (i) 2',7'-dichlorofluorescin diacetate that labels peroxides; (ii) monochlorobimane that stains reduced glutathione/homoglutathione (GSH/hGSH); and (iii) propidium iodide that marks nuclei of dead cells. Oxidative stress and cell death increased after exposure for 6-24 h to Cd and Hg, but labelling of GSH/hGSH decreased acutely. This diminution might be the result of direct interaction of GSH/hGSH with both Cd and Hg, as inferred from an in vitro conjugation assay. Therefore, both Cd and Hg not only compromised severely the cellular redox homeostasis, but also caused cell necrosis. In plants treated with 1 mM L-buthionine sulphoximine, a potent inhibitor of GSH/hGSH synthesis, only the oxidative stress symptoms appeared, indicating that the depletion of the GSH/hGSH pool was not sufficient to promote cell death, and that other phytotoxic mechanisms might be involved.

Identification of the form of Cd in the leaves of a superior Cd-accumulating ecotype of Thlaspi caerulescens using 113Cd-NMR

Thlaspi caerulescens (Ganges ecotype) is a known Cd hyperaccumulator, however, the ligands which coordinate to Cd ions in the leaves have not been identified. In the present study, the chemical form of Cd was investigated by using 113Cd-nuclear magnetic resonance (NMR) spectroscopy. Plants were grown hydroponically with a highly enriched 113Cd stable isotope. Measurements of 113Cd-NMR with intact leaves showed a signal at the chemical shift of around -16 ppm. Crude leaf sap also gave a similar chemical shift. Purification by gel filtration (Sephadex G-10), followed by cationic and anionic exchange chromatography, showed that Cd occurred only in the anionic fraction, which gave the same chemical shift as intact leaves. Further purification of the anionic fraction, combined with 113Cd- and 1H-NMR studies, revealed that only the fraction containing malate showed a chemical shift similar to the intact leaves. These results indicate that Cd was coordinated mainly with malate in the leaves of T. caerulescens. The malate concentration in the leaves was not affected by increasing Cd concentration in the solution, suggesting that malate synthesis is not induced by Cd. Because the Cd-malate complex is relatively weak, we suggest that the complex forms inside the vacuoles as a result of an efficient tonoplast transport of Cd and a constitutively high concentration of malate in the vacuoles, and that the formation of the Cd-malate complex may lead to a decrease of subsequent Cd efflux to the cytoplasm.

Molecular cloning and characterization of a phytochelatin synthase gene, PvPCS1, from Pteris vittata L

Pteris vittata L. is a staggeringly efficient arsenic hyperaccumulator that has been shown to be capable of accumulating up to 23,000 microg arsenic g(-1), and thus represents a species that may fully exploit the adaptive potential of plants to toxic metals. However, the molecular mechanisms of adaptation to toxic metal tolerance and hyperaccumulation remain unknown, and P. vittata genes related to metal detoxification have not yet been identified. Here, we report the isolation of a full-length cDNA sequence encoding a phytochelatin synthase (PCS) from P. vittata. The cDNA, designated PvPCS1, predicts a protein of 512 amino acids with a molecular weight of 56.9 kDa. Homology analysis of the PvPCS1 nucleotide sequence revealed that it has low identity with most known plant PCS genes except AyPCS1, and the homology is largely confined to two highly conserved regions near the 5'-end, where the similarity is as high as 85-95%. The amino acid sequence of PvPCS1 contains two Cys-Cys motifs and 12 single Cys, only 4 of which (Cys-56, Cys-90/91, and Cys-109) in the N-terminal half of the protein are conserved in other known PCS polypeptides. When expressed in Saccharomyces cerevisae, PvPCS1 mediated increased Cd tolerance. Cloning of the PCS gene from an arsenic hyperaccumulator may provide information that will help further our understanding of the genetic basis underlying toxic metal tolerance and hyperaccumulation.

Metal stress consequences on frost hardiness of plants at northern high latitudes: a review and hypothesis

This paper reviews the potential of trace/heavy metal-induced stress to reduce plant frost hardiness at northern high latitudes. The scientific questions are first outlined prior to a brief summary of heavy metal tolerance. The concepts of plant capacity and survival adaptation were used to formulate a hypothesis, according to which heavy metal stress may reduce plant frost hardiness for the following reasons: (1) Heavy metals change membrane properties through impaired resource acquisition and subsequent diminution of the cryoprotectant pool. (2) Heavy metals change membrane properties directly through oxidative stress, i.e. an increase of active oxygen species. (3) The involved co-stress may further increase oxidative stress. (4) The risk of frost injury increases due to membrane alterations. An opposite perspective was also discussed: could metal stress result in enhanced plant frost hardiness? This phenomenon could be based on the metabolism (i.e. glutathione, polyamines, proline, heat shock proteins) underlying a possible general adaptation syndrome of stress (GAS). As a result of the review it was suggested that metal-induced stress seems to reduce rather than increase plant frost hardiness.

Detection and quantification of ligands involved in nickel detoxification in a herbaceous Ni hyperaccumulator Stackhousia tryonii Bailey

Field-collected, young plants of Ni hyperaccumulator Stackhousia tryonii, grown in a glasshouse for 20 weeks, were exposed to low- (available Ni concentration in the native serpentine soil, i.e. 60 microg g(-1) dry soil) and high- (external application of 1000 ppm) Ni concentrations in the substrate. Nickel concentration in the freeze-dried leaf tissues increased from 3700 microg g(-1) to 13 700 microg g(-1) with soil Ni supplementation, of which >60% was extracted with dilute acid (0.025 M HCl). Nickel supplementation also elicited a 575%, 211%, and 37% increase in the final concentrations of oxalic, citric, and malic acids, respectively, in leaf tissues. Malic acid was the dominant organic acid, followed by citric and oxalic acids. The molar ratio of Ni to malic acid was 1.0, consistent with a role for malate as a ligand for Ni in hyperaccumulating plants, supporting detoxification/transport and storage of this heavy metal in S. tryonii. The total amino acid concentrations in the xylem sap did not change with Ni supplementation (21.7+/-3.7 mM and 17.9+/-5 mM, respectively, for low- and high-nickel-treated plants). Glutamine was the major amino acid in both the low- and high-Ni-treated plants. The concentration of glutamine decreased by >60%, with a corresponding increase in alanine, aspartic acid, and glutamic acid, on exposure to high Ni. A role of amino acids in Ni complexation and transport in S. tryonii is not immediately apparent.

Phytoremediation: novel approaches to cleaning up polluted soils

Environmental pollution with metals and xenobiotics is a global problem, and the development of phytoremediation technologies for the plant-based clean-up of contaminated soils is therefore of significant interest. Phytoremediation technologies are currently available for only a small subset of pollution problems, such as arsenic. Arsenic removal employs naturally selected hyperaccumulator ferns, which accumulate very high concentrations of arsenic specifically in above-ground tissues. Elegant two-gene transgenic approaches have been designed for the development of mercury or arsenic phytoremediation technologies. In a plant that naturally hyperaccumulates zinc in leaves, approximately ten key metal homeostasis genes are expressed at very high levels. This outlines the extent of change in gene activities needed in the engineering of transgenic plants for soil clean-up. Further analysis and discovery of genes for phytoremediation will benefit from the recent development of segregating populations for a genetic analysis of naturally selected metal hyperaccumulation in plants, and from comprehensive ionomics data--multi-element concentration profiles from a large number of Arabidopsis mutants.

Analysis of bean (Phaseolus vulgaris L.) proteins affected by copper stress

The effect of excess copper on the expression of soluble proteins in 10-day old Phaseolus vulgaris seedlings was studied with two-dimensional electrophoresis and mass spectrometry, to find sensitive biochemical markers of exposure. Despite major differences in root Cu contents, both 15 and 50 microM Cu treatments resulted in equal enhancements of Cu in the primary leaves. Three proteins, apparently reacting in a dose-dependent manner to Cu exposure, were identified from roots. The levels of an intracellular pathogenesis-related protein and a newly identified protein homologous to PvPR1, PvPR2, were increased with increasing Cu concentration. The level of a newly identified PR-10 protein decreased in a dose-dependent manner. No significant difference was observed in the leaf protein pattern between controls and 15 microM Cu-treated plants. However, at 50 microM Cu exposure, the appearance of PvPR1 and a homologue of Arabidopsis thaliana thylakoid lumenal 17.4kDa protein was observed. Another protein slightly enhanced by Cu treatment had sequence homology to a mitochondrial precursor of glycine cleavage system H protein of Flaveria pringlei.

Comparison of root absorption, translocation and tolerance of arsenic in the hyperaccumulator Pteris vittata and the nonhyperaccumulator Pteris tremula

* Several fern species can hyperaccumulate arsenic, although the mechanisms are not fully understood. Here we investigate the roles of root absorption, translocation and tolerance in As hyperaccumulation by comparing the hyperaccumulator Pteris vittata and the nonhyperaccumulator Pteris tremula. * The two species were grown in a pot experiment with 0-500 mg As kg-1 added as arsenate, and in a short-term (8 h) uptake experiment with 5 microM arsenate under phosphorus-sufficient conditions. * In the pot experiment, P. vittata accumulated up to 2500 mg As kg-1 frond d. wt and suffered no phytotoxicity. P. tremula accumulated<100 mg As kg-1 frond d. wt and suffered severe phytotoxicity with additions of >or=25 mg As kg-1. In the short-term uptake experiment, P. vittata had a 2.2-fold higher rate of arsenate uptake than P. tremula, and distributed more As taken up to the fronds (76%) than did P. tremula (9%). * Our results show that enhanced root uptake, efficient root-to-shoot translocation, and a much elevated tolerance through internal detoxification all contribute to As hyperaccumulation in P. vittata.

Phytochelatins

Phytochelatins (PCs) were first discovered as Cd-binding "Cadystins A and B" in a fission yeast and then in many plants as the major components of Cd-binding complexes. PCs have the general structure of (gamma-glutamyl-cysteinyl)n-glycine (n=2-11) and the variants with the repeated gamma-glutamyl-cysteinyl units are formed in some plants and yeast. They are capable of binding to various metals including Cd, Cu, Zn or As via the sulfhydryl and carboxyl residues, but their biosyntheses are controlled preferentially by the metal Cd or metalloid As. PCs are synthesized from glutathione (gamma-glutamyl-cysteinyl-glycine) in steps mediated by PC synthase. Genes (CAD1, PCS1) of the enzyme have been isolated from plants, fission yeast and some animals. Inhibition studies of PC biosynthesis via glutathione have demonstrated their fundamental roles in the metal detoxification in yeast and fungi, green algae and some aquatic plants, and also in the suspension-cultured cells and intact tissues in higher plants. Over-expression of PC synthase genes increases the Cd-tolerance in yeast and bacteria efficiently but not always in higher plant tissues especially in metal-accumulating species. "Hyperaccumulators" of Cd, Zn, Ni or As in terrestrial plants have a common feature where massive metal transport to shoots prevails, besides the ability of their roots to form PCs. This suggests that PC-based metal detoxification might be an ancient type of defense mechanism established in micro-algae or micro-fungi, and the additional PC-independent mechanism via vascular transport system became established later in higher plants. Readjustment of the PC-dependent and independent mechanisms at the metal-binding sites in the symplast and apoplast of shoots can be effective for further improvement of the metal detoxification activities and the tolerance characteristics of higher plants under various conditions.

Enhancing the first enzymatic step in the histidine biosynthesis pathway increases the free histidine pool and nickel tolerance inArabidopsis thaliana

Naturally selected nickel (Ni) tolerance in Alyssum lesbiacum has been proposed to involve constitutively high levels of endogenous free histidine. Transgenic Arabidopsis thaliana expressing a Salmonella typhimurium ATP phosphoribosyl transferase enzyme (StHisG) resistant to feedback inhibition by histidine contained approximately 2-fold higher histidine concentrations than wild type plants. Under exposure to a toxic Ni concentration, biomass production in StHisG expressing lines was between 14- and 40-fold higher than in wild-type plants. This suggested that enhancing the first step in the histidine biosynthesis pathway is sufficient to increase the endogenous free histidine pool and Ni tolerance in A. thaliana.

Heavy metal specificity of cellular tolerance in two hyperaccumulating plants, Arabidopsis halleri and Thlaspi caerulescens

•  The cellular tolerance to nickel (Ni), zinc (Zn) and cadmium (Cd) of two poly-hyperaccumulators, Arabidopsis halleri and Thlaspi caerulescens, was investigated in order to compare their cellular phenotypes toward various metal ion exposures. •  Protoplasts were kept for 24 h on solutions containing increasing concentrations of the metal ions, and a viability test was performed. Zinc loading of the protoplasts was investigated with Arabidopsis lyrata and A. halleri protoplasts using the Zn fluorescent indicator Newport green diacetate. •  Only T. caerulescens protoplasts showed a clear tolerance to Ni. On the other hand, protoplasts from both hyperaccumulators displayed a very high and constitutive Zn tolerance and an inducible Cd tolerance. The vacuolar storage of Zn was confirmed, but no Zn accumulation at all was observed in A. halleri protoplasts after Zn exposure. •  Specific metal tolerances were found at the cellular level in the hyperaccumulating plants, highlighting that specific adaptations to metal ions exist in the cells as well as in the whole plants.

Thiol synthesis and arsenic hyperaccumulation in Pteris vittata (Chinese brake fern)

Pteris vittata (Chinese brake fern) has potential for phytoremediation of As-contaminated sites. In this study, the synthesis of total thiols and acid-soluble thiols in P. vittata was investigated under arsenic exposure. The strong and positive correlation between As concentration and acid-soluble thiols in plant leaflets suggests that acid-soluble thiols may play a role in As detoxification. A major As-induced thiol was purified and characterized. A molecular ion (M + 1) of 540 m/z suggests that the thiol was a phytochelatin (PC) with two base units (PC(2)). However, the ratios of acid-soluble thiols to As in leaflets exposed to As ranged from 0.012 to 0.026, suggesting that only a very small part of As is complexed by PC(2). PCs could play a minor detoxification role in this hyperaccumulator. A PC-independent mechanism appears to be mainly involved in As tolerance, while PC-dependent detoxification seems to be a supplement.

The role of thiol species in the hypertolerance of Aspergillus sp. P37 to arsenic

Aspergillus sp. P37 is an arsenate-hypertolerant fungus isolated from a river in Spain with a long history of contamination with metals. This strain is able to grow in the presence of 0.2 M arsenate, i.e. 20-fold higher than the reference strain, Aspergillus nidulans TS1. Although Aspergillus sp. P37 reduces As(V) to As(III), which is slowly pumped out of the cell, the measured efflux of oxyanions is insufficient to explain the high tolerance levels of this strain. To gain an insight into this paradox, the accumulation of acid-soluble thiol species in Aspergillus sp. P37 when exposed to arsenic was compared with that of the arsenic-sensitive A. nidulans TS1 strain. Increasing levels of arsenic in the medium did not diminish the intracellular pool of reduced glutathione in Aspergillus sp. P37, in sharp contrast with the decline of glutathione in A. nidulans under the same conditions. Furthermore, concentrations of arsenic that were inhibitory for the sensitive A. nidulans strain (e.g. 50 mM and above) provoked a massive formation of vacuoles filled with thiol species. Because the major fraction of the cellular arsenic was present as the glutathione conjugate As(GS)3, it is plausible that the arsenic-hypertolerant phenotype of Aspergillus sp. P37 is in part due to an enhanced capacity to maintain a large intracellular glutathione pool under conditions of arsenic exposure and to sequester As(GS)3 in vacuoles. High pressure liquid chromatography analysis of cell extracts revealed that the contact of Aspergillus sp. P37 (but not A. nidulans) with high arsenic concentrations (> or =150 mM) induced the production of small quantities of a distinct thiol species indistinguishable from plant phytochelatin-2. Yet, we argue that phytochelatins do not explain arsenic resistance in Aspergillus, and we advocate the role of As(GS)3 complexes in arsenic detoxification.

Increased glutathione biosynthesis plays a role in nickel tolerance in thlaspi nickel hyperaccumulators

Worldwide more than 400 plant species are now known that hyperaccumulate various trace metals (Cd, Co, Cu, Mn, Ni, and Zn), metalloids (As) and nonmetals (Se) in their shoots. Of these, almost one-quarter are Brassicaceae family members, including numerous Thlaspi species that hyperaccumulate Ni up to 3% of there shoot dry weight. We observed that concentrations of glutathione, Cys, and O-acetyl-l-serine (OAS), in shoot tissue, are strongly correlated with the ability to hyperaccumulate Ni in various Thlaspi hyperaccumulators collected from serpentine soils, including Thlaspi goesingense, T. oxyceras, and T. rosulare, and nonaccumulator relatives, including T. perfoliatum, T. arvense, and Arabidopsis thaliana. Further analysis of the Austrian Ni hyperaccumulator T. goesingense revealed that the high concentrations of OAS, Cys, and GSH observed in this hyperaccumulator coincide with constitutively high activity of both serine acetyltransferase (SAT) and glutathione reductase. SAT catalyzes the acetylation of l-Ser to produce OAS, which acts as both a key positive regulator of sulfur assimilation and forms the carbon skeleton for Cys biosynthesis. These changes in Cys and GSH metabolism also coincide with the ability of T. goesingense to both hyperaccumulate Ni and resist its damaging oxidative effects. Overproduction of T. goesingense SAT in the nonaccumulator Brassicaceae family member Arabidopsis was found to cause accumulation of OAS, Cys, and glutathione, mimicking the biochemical changes observed in the Ni hyperaccumulators. In these transgenic Arabidopsis, glutathione concentrations strongly correlate with increased resistance to both the growth inhibitory and oxidative stress induced effects of Ni. Taken together, such evidence supports our conclusion that elevated GSH concentrations, driven by constitutively elevated SAT activity, are involved in conferring tolerance to Ni-induced oxidative stress in Thlaspi Ni hyperaccumulators.

Hyperaccumulation of cadmium and zinc in Thlaspi caerulescens and Arabidopsis halleri at the leaf cellular level

Vacuolar compartmentalization or cell wall binding in leaves could play a major role in hyperaccumulation of heavy metals. However, little is known about the physiology of intracellular cadmium (Cd) sequestration in plants. We investigated the role of the leaf cells in allocating metal in hyperaccumulating plants by measuring short-term (109)Cd and (65)Zn uptake in mesophyll protoplasts of Thlaspi caerulescens "Ganges" and Arabidopsis halleri, both hyperaccumulators of zinc (Zn) and Cd, and T. caerulescens "Prayon," accumulating Cd at a lower degree. The effects of low temperature, several divalent cations, and pre-exposure of the plants to metals were investigated. There was no significant difference between the Michaelis-Menten kinetic constants of the three plants. It indicates that differences in metal uptake cannot be explained by different constitutive transport capacities at the leaf protoplast level and that plasma and vacuole membranes of mesophyll cells are not responsible for the differences observed in heavy metal allocation. This suggests the existence of regulation mechanisms before the plasma membrane of leaf mesophyll protoplasts. However, pre-exposure of the plants to Cd induced an increase in Cd accumulation in protoplasts of "Ganges," whereas it decreased Cd accumulation in A. halleri protoplasts, indicating that Cd-permeable transport proteins are differentially regulated. The experiment with competitors has shown that probably more than one single transport system is carrying Cd in parallel into the cell and that in T. caerulescens "Prayon," Cd could be transported by a Zn and Ca pathway, whereas in "Ganges," Cd could be transported mainly by other pathways.

The role of phytochelatins in arsenic tolerance in the hyperaccumulator Pteris vittata

•  Pteris vittata was the first identified arsenic (As) hyperaccumulator. Here we investigated whether phytochelatins (PCs) are involved in the hypertolerance of arsenic by P. vittata. •  P. vittata was exposed to 0-500 µm arsenate for 5 d, or to 50 µm arsenate for 0-7 d. In addition, l-buthionine-sulphoximine (BSO), an inhibitor of γ-glutamylcysteine synthetase, was used in combination with different arsenate exposures. The relationships between As accumulation and the concentrations of PCs and glutathione (GSH) were examined. •  PC synthesis was induced upon exposure to arsenate in P. vittata, with only PC₂ detected in the plant. The As concentration correlated significantly with PC₂ concentration in both roots and shoots, but not with GSH. The molar ratio of PC-SH to As was c. 0.09 and 0.03 for shoots and roots, respectively, suggesting that only a small proportion (1-3%) of the As in P. vittata can be complexed with PCs. In the presence of arsenate, addition of BSO decreased PC₂ concentrations in roots and shoots by 89-96% and 30-33%, respectively. BSO alone was found to inhibit root growth of P. vittata markedly. •  The results suggest that PCs play a limited role in the hypertolerance of As in P. vittata.

Thlaspi caerulescens, an attractive model species to study heavy metal hyperaccumulation in plants

Studying heavy metal hyperaccumulation is becoming more and more interesting for ecological, evolutionary, nutritional, and environmental reasons. One model species, especially in the era of high throughput genomics, transcriptomics, proteomics and metabolomics technologies, would be very advantageous. Although there are several hyperaccumulator species known, there is no single model species yet. The Zn, Cd and Ni hyperaccumulator species Thlaspi caerulescens has been studied to a great extent, especially for Zn and Cd hyperaccumulation and tolerance. Its physiological, morphological and genetic characteristics, and its close relationship to Arabidopsis thaliana, the general plant reference species, make it an excellent candidate to be the plant heavy metal hyperaccumulation model species.

Phytoextraction of metals and metalloids from contaminated soils

The removal of inorganic contaminants by plants is termed phytoextraction. Recent studies have looked at the feasibility of phytoextraction, and demonstrate that both good biomass yields and metal hyperaccumulation are required to make the process efficient. Adding chelating agents to soil to increase the bioavailability of contaminants can sometimes induce hyperaccumulation in normal plants, but may produce undesirable environmental risks. Thus, it is necessary to investigate the mechanisms responsible for hyperaccumulation, using natural hyperaccumulators as model plant species. Recent advances have been made in understanding the mechanisms responsible for hyperaccumulation of Zn, Cd, Ni and As by plants. Attempts to engineer metal tolerance and accumulation have so far been limited to Hg, As and Cd, and although promising results have been obtained they may be some way from practical application. More fundamental understanding of the traits and mechanisms involved in hyperaccumulation are needed so that phytoextraction can be optimised.

Metalloid tolerance based on phytochelatins is not functionally equivalent to the arsenite transporter Acr3p

Active transport of metalloids by Acr3p and Ycf1p in Saccharomyces cerevisiae and chelation by phytochelatins in Schizosaccharomyces pombe, nematodes, and plants represent distinct strategies of metalloid detoxification. In this report, we present results of functional comparison of both resistance mechanisms. The S. pombe and wheat phytochelatin synthase (PCS) genes, when expressed in S. cerevisiae, mediate only modest resistance to arsenite and thus cannot functionally compensate for Acr3p. On the other hand, we show for the first time that phytochelatins also contribute to antimony tolerance as PCS fully complement antimonite sensitivity of ycf1Delta mutant. Remarkably, heterologous expression of PCS sensitizes S. cerevisiae to arsenate, while ACR3 confers much higher arsenic resistance in pcsDelta than in wild-type S. pombe. The analysis of PCS and ACR3 homologues distribution in various organisms and our experimental data suggest that separation of ACR3 and PCS genes may lead to the optimal tolerance status of the cell.