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Guillaume Echevarria - One of the best experts on this subject based on the ideXlab platform.
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Phytoextraction of high value elements and contaminants from mining and mineral wastes: opportunities and limitations
Plant and Soil, 2020Co-Authors: Amelia Corzo Remigio, Peter D. Erskine, Alan J. M. Baker, Rufus L. Chaney, Mansour Edraki, Guillaume EchevarriaAbstract:Background Phytoextraction is an in situ technique that can be applied to minerals and mining wastes using hyperaccumulator plants to purposely bio-concentrate high levels of metals or metalloids into their shoots in order to remove them from the substrate, while achieving monetary gain. Phytoextraction can be applied to a limited number of elements depending on the existence of hyperaccumulator plants with suitable characteristics. Although phytoextraction has been trialled in experimental settings, it requires testing at field scale to assess commercial broad-scale potential. Scope The novelty and purported environmental benefits of phytoextraction have attracted substantial scientific inquiry. The main limitation of phytoextraction with Hyperaccumulators is the number of suitable plants with a high accumulation capacity for a target element. We outline the main considerations for applying phytoextraction using selected elemental case studies in which key characteristics of the element, hyperaccumulation and economic considerations are evaluated. Conclusions The metals cobalt, cadmium, thallium and rhenium and the metalloids arsenic and selenium are present in many types of minerals wastes, especially base metal mining tailings, at concentrations amenable for economic phytoextraction. Phytoextraction should focus on the most toxic elements (arsenic, cadmium, and thallium) or especially valuable elements (selenium, cobalt, and rhenium). The value proposition is in the clean-up of contaminated land in the case of toxic elements, whereas it is in the ‘bio-ore’ generated by the process in the case of valuable elements.
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Bacterial community diversity in the rhizosphere of nickel hyperaccumulator species of Halmahera Island (Indonesia)
Applied Soil Ecology, 2019Co-Authors: Séverine Lopez, Guillaume Echevarria, Peter D. Erskine, Jeanlouis Morel, Xavier Goux, Magdalena Calusinska, Émile BenizriAbstract:Ultramafic soils cover large areas in Indonesia, with over 8000 km on Halmahera Island. Nickel hyperaccumulator plants have evolved on these soils which are suspected to host highly nickel tolerant bacteria in their rhizosphere. To date, research has been limited on the characterization of the genetic diversity of such bacterial communities. The objective of this study was to investigate the genetic diversity of bacterial communities in the rhizosphere of nickel hyperaccumulator plants from Halmahera Island to improve knowledge on the factors that drive bacterial community diversity. We collected 45 rhizosphere soils from 10 woody nickel hyperaccumulator species at 16 sites to highlight the influence of plants, soil chemical parameters and site effects on bacterial diversity, richness and composition. A total of 2,508,874 bacterial 16S rRNA gene sequences were obtained and clustered into 6645 OTUs. In total, 40 phyla were identified with the most dominant phyla being Proteobacteria, Acidobacteria, Actinobacteria and Chloroflexi. Redundancy analysis between soil chemical characteristics and bacterial phyla relative abundances for the three main species (Rinorea aff. bengalensis, Ficus trachypison and Trichospermum morotaiense) showed that the main factor driving the bacterial diversity in the rhizosphere of Rinorea aff. bengalensis appears to be the plant itself (regardless of the characteristics of the soil), while the impact of soil conditions was important for the two other hyperaccumulator species. This may be explained by the fact that Rinorea aff. bengalensis (at least in Halmahera Island) is an obligate nickel hyperaccumulator.
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Tools for the Discovery of Hyperaccumulator Plant Species and Understanding Their Ecophysiology
Agromining: Farming for Metals, 2017Co-Authors: Peter D. Erskine, Guillaume Echevarria, Hugh H. Harris, Jolanta Mesjasz-przybyłowicz, Alban D. Barnabas, Wojciech J. Przybyłowicz, Peter M. KopittkeAbstract:Globally the discovery of hyperaccumulator plants has been hindered by systematic screening of plant species, and is highly biased towards Ni Hyperaccumulators. This is mainly due to the existence of a reagent paper test that is only specific to nickel (based on dimethylglyoxime) such that more than 400 of the approximately 500 known Hyperaccumulators species are for Ni. New technical advances now permit massive screening of herbarium specimens using non-destructive, portable X-Ray Fluorescence Spectroscopy (XRF), an approach that has already led to the discovery of numerous hyperaccumulator species new to science. The elemental distribution in selected hyperaccumulator plant tissues can then be further studied using techniques such as desktop or synchrotron micro-XRF, nuclear microprobe (PIXE), scanning/transmission electron microscopy with energy-dispersive spectroscopy (SEM/TEM-EDS), secondary ion mass spectrometry (SIMS) or laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The use of histochemical dyes combined with light microscopy further aids in the identification of anatomical and structural features of the studied plant tissues.
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Delimiting soil chemistry thresholds for nickel hyperaccumulator plants in Sabah (Malaysia)
Chemoecology, 2016Co-Authors: Guillaume Echevarria, Mark TibbettAbstract:Nickel hyperaccumulator plants have been the focus of considerable research because of their unique ecophysiological characteristics that can be exploited in phytomining technology. Comparatively little research has focussed on the soil chemistry of tropical nickel hyperaccumulator plants to date. This study aimed to elucidate whether the soil chemistry associated with nickel hyperaccumulator plants has distinctive characteristics that could be indicative of specific edaphic requirements. The soil chemistry associated with 18 different nickel hyperaccumulator plant species occurring in Sabah (Malaysia) was compared with local ultramafic soils where nickel hyperaccumulator plants were absent. The results showed that nickel Hyperaccumulators in the study area were restricted to circum-neutral soils with relatively high phytoavailable calcium, magnesium and nickel concentrations. There appeared to be a ‘threshold response’ for the presence of nickel hyperaccumulator plants at >20 μg g−1 carboxylic-extractable nickel or >630 μg g−1 total nickel, and >pH 6.3 thereby delimiting their edaphic range. Two (not mutually exclusive) hypotheses were proposed to explain nickel hyperaccumulation on these soils: (1) Hyperaccumulators excrete large amounts of root exudates thereby increasing nickel phytoavailability through intense rhizosphere mineral weathering; and (2) Hyperaccumulators have extremely high nickel uptake efficiency thereby severely depleting nickel and stimulating re-supply of Ni from diffusion from labile Ni pools. It was concluded that since there was an association with soils with highly labile nickel pools, the available evidence primarily supports hypothesis (2).
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Effect of hyperaccumulator plants and associated rhizobacteria on the efficiency of nickel extraction
2014Co-Authors: Marie Rue, Guillaume Echevarria, Émile BenizriAbstract:Many works have attempted to relate the association of different plants on the efficiency of inorganic pollutants extraction, with the hypothesis that these multi-species covers promoted the development and the activity of rhizosphere microorganisms, such as PGPR. Up to now, the focus has been on crop associations (maize, tabacco, Brassica species). Only few studies have concerned the effect of the combination of metal hyperaccumulator plants with other species non-Hyperaccumulators. These experiments showed that co-cropping with nonhyperaccumulator plants could enhance the growth of the hyperaccumulator and metal accumulation. The objective of this work was to study the effect of species richness in vegetation cover (mono- vs co-cropping), which only consisted of four hyperaccumulator plant species (Brassicaceae), on the efficiency of Nickel (Ni) extraction from an ultramafic soil containing significant nickel concentrations (Ni = 1480 mg.kg-1). The effects on some soil physicochemical properties and on microbial communities colonizing the rhizosphere were also evaluated. An experiment was set up with four hyperaccumulator species (Leptoplax emarginata, Noccaea tymphaea, Alyssum murale and Bornmuellera tymphaea). Six treatments had been realised (one mixed cover, four monospecies covers and unplanted soil) with 7 replicates for each. After four months of culture in controlled conditions, the estimation of plant biomass and Ni concentrations in shoots and roots were evaluated. In the meanwhile, microbial biomass carbon, size of cultivable rhizosphere bacterial community (UFC), as well as the potential production of auxin compounds (AIA), were evaluated for each treatment. Bacterial communities were also characterized by genetic (SSCP) and metabolic (Biolog Ecoplate™) structures. Moreover, different microbial enzymes (ACCd - 1-Aminocyclopropane-1-carboxylate deaminase, urease, acid phosphatase, ß-glucosidase, arylsulphatase and FDA-fluorescein diacetate hydrolysis) were measured in rhizosphere soil samples. The presence of a cover, whether single or multi-species, caused a reduction in the concentration of extractable Ni from the soil. In our case, this effect was most pronounced in the presence of mesocosms planted with B. tymphaea and N. tymphaea. Similarly, the Ni bioconcentration factor showed a good correlation with shoot biomass and Ni concentration in shoot, especially for B. tymphaea and N. tymphaea and in a less extend for cocropping species. Moreover, the treatment with N. tymphaea showed the lowest pH value – which favours Ni solubility in soil. A strong correlation between pH, microbial enzyme activities and the size of the bacterial community was also observed. No significant change in enzyme activities was observed with covers, except in the case of arylsulfatase. The characterization of bacterial communities from soil samples by genetic (SSCP) and metabolic (Biolog Ecoplate™) structures revealed differences between mesocosms. The co-cropping of the four hyperaccumulator species did not significantly improve the process of phytoextraction. However, the biomass produced by B. tymphaea was in the same range as that of N. tymphaea and L. emarginata but B. tymphaea bioconcentration factor was the highest among all four species (i.e. more than 1% Ni in its dried shoots). Therefore, B. tymphaea, and to a lesser extent N. tymphaea, were the two species with the greatest potential of phytoextraction of Ni in co-cropping systems. A combination of different hyperaccumulator plants appears promising in phytoremediation practices, but further research is needed to unravel the links between aboveground hyperaccumulating plants, the belowground rhizosphere microbial communities in metaliferous soils and the implication of these microbial communities in the survival of plants and their ability to extract Ni. In particular, associations of plants should be tested in pairs, to define the plant cover providing the best phytoextraction.
Henk Schat - One of the best experts on this subject based on the ideXlab platform.
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Hyperaccumulators of metal and metalloid trace elements: Facts and fiction
Plant and Soil, 2012Co-Authors: Alan J. M. Baker, Roger D. Reeves, A. Joseph Pollard, Henk SchatAbstract:Background: Plants that accumulate metal and metalloid trace elements to extraordinarily high concentrations in their living biomass have inspired much research worldwide during the last decades. Hyperaccumulators have been recorded and experimentally confirmed for elements such as nickel, zinc, cadmium, manganese, arsenic and selenium. However, to date, hyperaccumulation of lead, copper, cobalt, chromium and thallium remain largely unconfirmed. Recent uses of the term in relation to rare-earth elements require critical evaluation. Scope: Since the mid-1970s the term 'hyperaccumulator' has been used millions of times by thousands of people, with varying degrees of precision, aptness and understanding that have not always corresponded with the views of the originators of the terminology and of the present authors. There is therefore a need to clarify the circumstances in which the term 'hyperaccumulator' is appropriate and to set out the conditions that should be met when the terms are used. We outline here the main considerations for establishing metal or metalloid hyperaccumulation status of plants, (re)define some of the terminology and note potential pitfalls. Conclusions: Unambiguous communication will require the international scientific community to adopt standard terminology and methods for confirming the reliability of analytical data in relation to metal and metalloid Hyperaccumulators. © 2012 Springer Science+Business Media B.V
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Identification and functional analysis of two ZIP metal transporters of the hyperaccumulator Thlaspi caerulescens
Plant and Soil, 2009Co-Authors: Jian Wu, Henk Schat, Fang-jie Zhao, Artak Ghandilyan, Barbara Logoteta, Myriam Olortegui Guzman, Xiaowu Wang, Mark G. M. AartsAbstract:The heavy metal hyperaccumulator Thlaspi caerulescens expresses several ZIP-like genes at higher levels than their orthologues in non-hyperaccumulator species, but it is not clear why. To elucidate the function of the T. caerulescens orthologues of the Arabidopsis thaliana ZIP5 and ZIP6 genes, full-length cDNAs of TcZNT5-LC and TcZNT6-LC were cloned, their expression was examined and genes were expressed in A. thaliana. Transcript level analysis revealed the constitutively high expression of these two genes in T. caerulescens compared to AtZIP5 and AtZIP6 genes and differential expression of both genes when comparing two accessions of T. caerulescens with different metal accumulation properties. Expression of TcZNT5-LC in A. thaliana did not modify Cd or Zn tolerance, but mildly affected the root and shoot Zn concentrations towards a hyperaccumulator shoot to root concentration ratio. A. thaliana zip5 knock-out mutants showed increased tolerance to Cd and decreased seed mineral concentrations. Expression of TcZNT6-LC enhanced the Cd sensitivity of A. thaliana, but no phenotype was observed for the zip6 mutant. In conclusion, the changes in expression of TcZNT5-LC and TcZNT6-LC upon changes in Zn or Cd exposure indicate both genes act in metal homeostasis, but their CaMV 35S-mediated expression in A. thaliana does not create T. caerulescens hyperaccumulator phenotypes.
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Molecular mechanisms of metal hyperaccumulation in plants
New Phytologist, 2009Co-Authors: Nathalie Verbruggen, Cedric Hermans, Henk SchatAbstract:Contents\n\n* Summary 759\n* I. Hyperaccumulation: the phenomenon 759\n* II. Macroevolution of hyperaccumulation 760\n* III. Microevolution of hyperaccumulation: variation within hyperaccumulator species 760\n* IV. Genetic analysis of trace metal accumulation and tolerance 761\n* V. Mechanisms of trace metal accumulation 762\n* VI. General discussion and research perspectives 769\n* Acknowledgements 772\n* References 772\nSummary\nMetal hyperaccumulator plants accumulate and detoxify extraordinarily high concentrations of metal ions in their shoots. Metal hyperaccumulation is a fascinating phenomenon, which has interested scientists for over a century. Hyperaccumulators constitute an exceptional biological material for understanding mechanisms regulating plant metal homeostasis as well as plant adaptation to extreme metallic environments. Our understanding of metal hyperaccumulation physiology has recently increased as a result of the development of molecular tools. This review presents key aspects of our current understanding of plant metal – in particular cadmium (Cd), nickel (Ni) and zinc (Zn) – hyperaccumulation.
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Genetics and Genomics of the Heavy Metal Hyperaccumulator Model Species Thlaspi caerulescens
2007Co-Authors: Antoine X. Deniau, Henk Schat, Mark G. M. AartsAbstract:In the last decade heavy metal hyperaccumulator plants have been increasingly studied, mainly because of their potential use in phytoremediation. Thlaspi caerulescens is an attractive model hyperaccumulator plant, because it accommodates a high of intra-specific variation in the degrees and metal-specificity patterns of tolerance and accumulation. In this review we give an overview of recent progress made in the genetics and genomics of heavy metal hyperaccumulation in this species. QTL analysis for zinc and cadmium accumulation in segregating inter-accession crosses demonstrated that these traits are controlled by multiple genes and that there are accession-specific accumulation mechanisms with distinct metal-affinity patterns. Cross-species transcriptome analyses have revealed a large number of genes with differential expression between Hyperaccumulators and non-Hyperaccumulators. Many of those genes are known to be involved in metal homeostasis, and an even larger number might play a role in this process. However, most of the differentially expressed genes have probably no role in metal homeostasis, owing to the fact that species with different life history and ecology are compared. To confirm the role of candidate genes, mutant research is necessary, but not yet done in Hyperaccumulators. In the absence of physical maps and full genome sequences of Hyperaccumulators, comparative genomics are indispensable. Co-linearity and micro-synteny analysis should enable the identification of the genes responsible for QTL for accumulation traits in intra- and inter-specific crosses.
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Transcription profiling of the metal-hyperaccumulator Thlaspi caerulescens
Zeitschrift Fur Naturforschung Section C-a Journal of Biosciences, 2005Co-Authors: M. Plessi, Henk Schat, Mark G. M. Aarts, Diana Rigola, Viivi H. Hassinen, Dieter ErnstAbstract:Thlaspi caerulescens is a well-studied metal-hyperaccumulator of zinc, cadmium and nickel, belonging to the Brassicaceae family. Moreover it is one of the few Hyperaccumulators that occur on different metalliferous soil types, as well as on nonmetalliferous soils. We are interested in the development of systems to improve phytoremediation of metal contaminated soils through improved metal-accumulation. About 1900 cDNAs isolated from T. caerulescens roots were hybridized with reverse transcribed RNA from zinc-treated T. caerulescens plants of two accessions originating from two different soil types. This comparative transcript profiling of T. caerulescens plants resulted in the identification of genes that are affected by heavy metals. The developed microarray proved to be an appropriate tool for a large scale analysis of gene expression in this metal-accumulator species.
Nathalie Verbruggen - One of the best experts on this subject based on the ideXlab platform.
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tolerance to cadmium in plants the special case of Hyperaccumulators
Biometals, 2013Co-Authors: Nathalie Verbruggen, Michal Juraniec, Cecilia Baliardini, Clairelise MeyerAbstract:On sols highly polluted by trace metallic elements the majority of plant species are excluders, limiting the entry and the root to shoot translocation of trace metals. However a rare class of plants called Hyperaccumulators possess remarkable adaptation because those plants combine extremely high tolerance degrees and foliar accumulation of trace elements. Hyperaccumulators have recently gained considerable interest, because of their potential use in phytoremediation, phytomining and biofortification. On a more fundamental point of view Hyperaccumulators of trace metals are case studies to understand metal homeostasis and detoxification mechanisms. Hyperaccumulation of trace metals usually depends on the enhancement of at least four processes, which are the absorption from the soil, the loading in the xylem in the roots and the unloading from the xylem in the leaves and the detoxification in the shoot. Cadmium is one of the most toxic trace metallic elements for living organisms and its accumulation in the environment is recognized as a worldwide concern. To date, only nine species have been recognized as Cd Hyperaccumulators that is to say able to tolerate and accumulate more than 0.01 % Cd in shoot dry biomass. Among these species, four belong to the Brassicaceae family with Arabidopsis halleri and Noccaea caerulescens being considered as models. An update of our knowledge on the evolution of Hyperaccumulators will be presented here.
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Molecular mechanisms of metal hyperaccumulation in plants
New Phytologist, 2009Co-Authors: Nathalie Verbruggen, Cedric Hermans, Henk SchatAbstract:Contents\n\n* Summary 759\n* I. Hyperaccumulation: the phenomenon 759\n* II. Macroevolution of hyperaccumulation 760\n* III. Microevolution of hyperaccumulation: variation within hyperaccumulator species 760\n* IV. Genetic analysis of trace metal accumulation and tolerance 761\n* V. Mechanisms of trace metal accumulation 762\n* VI. General discussion and research perspectives 769\n* Acknowledgements 772\n* References 772\nSummary\nMetal hyperaccumulator plants accumulate and detoxify extraordinarily high concentrations of metal ions in their shoots. Metal hyperaccumulation is a fascinating phenomenon, which has interested scientists for over a century. Hyperaccumulators constitute an exceptional biological material for understanding mechanisms regulating plant metal homeostasis as well as plant adaptation to extreme metallic environments. Our understanding of metal hyperaccumulation physiology has recently increased as a result of the development of molecular tools. This review presents key aspects of our current understanding of plant metal – in particular cadmium (Cd), nickel (Ni) and zinc (Zn) – hyperaccumulation.
Agnieszka Galuszka - One of the best experts on this subject based on the ideXlab platform.
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prospecting for Hyperaccumulators of trace elements a review
Critical Reviews in Biotechnology, 2015Co-Authors: Karina Krzciuk, Agnieszka GaluszkaAbstract:AbstractSpecific plant species that can take up and accumulate abnormally high concentrations of elements in their aboveground tissues are referred to as “Hyperaccumulators”. The use of this term is justified in the case of enormous element-binding capacity of plants growing in their natural habitats and showing no toxicity symptoms. An increasing interest in the study of Hyperaccumulators results from their potential applications in environmental biotechnology (phytoremediation, phytomining) and their emerging role in nanotechnology. The highest number of plant species with confirmed hyperaccumulative properties has been reported for Hyperaccumulators of nickel, cadmium, zinc, manganese, arsenic and selenium. More limited data exist for plants accumulating other elements, including common pollutants (chromium, lead and boron) or elements of commercial value, such as copper, gold and rare earth elements. Different approaches have been used for the study of Hyperaccumulators – geobotanical, chemical, bioch...
David E Salt - One of the best experts on this subject based on the ideXlab platform.
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constitutively elevated salicylic acid signals glutathione mediated nickel tolerance in thlaspi nickel Hyperaccumulators
Plant Physiology, 2005Co-Authors: John L Freeman, Daniel Garcia, Amber Hopf, David E SaltAbstract:Progress is being made in understanding the biochemical and molecular basis of nickel (Ni)/zinc (Zn) hyperaccumulation in Thlaspi; however, the molecular signaling pathways that control these mechanisms are not understood. We observed that elevated concentrations of salicylic acid (SA), a molecule known to be involved in signaling induced pathogen defense responses in plants, is a strong predictor of Ni hyperaccumulation in the six diverse Thlaspi species investigated, including the Hyperaccumulators Thlaspi goesingense, Thlaspi rosulare, Thlaspi oxyceras, and Thlaspi caerulescens and the nonaccumulators Thlaspi arvense and Thlaspi perfoliatum. Furthermore, the SA metabolites phenylalanine, cinnamic acid, salicyloyl-glucose, and catechol are also elevated in the hyperaccumulator T. goesingense when compared to the nonaccumulators Arabidopsis (Arabidopsis thaliana) and T. arvense. Elevation of free SA levels in Arabidopsis, both genetically and by exogenous feeding, enhances the specific activity of serine acetyltransferase, leading to elevated glutathione and increased Ni resistance. Such SA-mediated Ni resistance in Arabidopsis phenocopies the glutathione-based Ni tolerance previously observed in Thlaspi, suggesting a biochemical linkage between SA and Ni tolerance in this genus. Intriguingly, the hyperaccumulator T. goesingense also shows enhanced sensitivity to the pathogen powdery mildew (Erysiphe cruciferarum) and fails to induce SA biosynthesis after infection. Nickel hyperaccumulation reverses this pathogen hypersensitivity, suggesting that the interaction between pathogen resistance and Ni tolerance and hyperaccumulation may have played a critical role in the evolution of metal hyperaccumulation in the Thlaspi genus.
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increased glutathione biosynthesis plays a role in nickel tolerance in thlaspi nickel Hyperaccumulators
The Plant Cell, 2004Co-Authors: John L Freeman, Michael W Persans, Ken Nieman, Carrie Albrecht, Wendy Ann Peer, Ingrid J Pickering, David E SaltAbstract: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.
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Subcellular Localization and Speciation of Nickel in Hyperaccumulator and Non-Accumulator Thlaspi Species
Plant Physiology, 2000Co-Authors: Ute Krämer, Ingrid J Pickering, Roger C. Prince, Ilya Raskin, David E SaltAbstract:The ability of Thlaspi goesingense Halacsy to hyperaccumulate Ni appears to be governed by its extraordinary degree of Ni tolerance. However, the physiological basis of this tolerance mechanism is unknown. We have investigated the role of vacuolar compartmentalization and chelation in this Ni tolerance. A direct comparison of Ni contents of vacuoles from leaves of T. goesingense and from the non-tolerant non-accumulator Thlaspi arvense L. showed that the hyperaccumulator accumulates approximately 2-fold more Ni in the vacuole than the non-accumulator under Ni exposure conditions that were non-toxic to both species. Using x-ray absorption spectroscopy we have been able to determine the likely identity of the compounds involved in chelating Ni within the leaf tissues of the hyperaccumulator and non-accumulator. This revealed that the majority of leaf Ni in the hyperaccumulator was associated with the cell wall, with the remaining Ni being associated with citrate and His, which we interpret as being localized primarily in the vacuolar and cytoplasm, respectively. This distribution of Ni was remarkably similar to that obtained by cell fractionation, supporting the hypothesis that in the hyperaccumulator, intracellular Ni is predominantly localized in the vacuole as a Ni-organic acid complex.