Ni Hyperaccumulator Research Articles

Effects of Ni on superoxide dismutase and glutathione reductase activities and thiol groups: a comparative study between Alyssum hyperaccumulator and non-accumulator species

In this study we investigated the activities of superoxide dismutase (SOD), glutathione reductase (GR) and total thiol groups in two Ni hyperaccumulator species, Alyssum inflatum Nyárády and Alyssum bracteatum Boiss. & Bushe, and a non-accumulator species (Alyssum saxatile (L.) Desv.). Plants were exposed to different concentrations of Ni (0, 100, 300 and 500 μM for hyperaccumulators and 0, 10 and 20 μM Ni for the non-accumulator) in solution culture. The accumulation of Ni, the influence of Ni on dry biomass, and SOD, GR and total thiol activities were measured. The results showed a growth retardation effect by Ni in all plants. The activity of SOD strongly decreased with increase in Ni concentration in A. inflatum whereas the other Ni hyperaccumulator, A. bracteatum, showed less decrease. In contrast, a positive link between SOD activity and Ni concentrations was observed in shoots and roots of A. saxatile. The GR and total thiol activities were increased in the hyperaccumulator plants by increase in Ni concentration, but no changes were observed in A. saxatile. The results indicate that in hyperaccumulators, the accumulation of Ni could negatively affect some vital physiological processes.

Nickel accumulation by Alyssum serpyllifolium subsp. lusitanicum (Brassicaceae) from serpentine soils of Bragança and Morais (Portugal) ultramafic massifs: plant–soil relationships and prospects for phytomining

The aim of the present study was to evaluate the possibility of using Alyssum serpyllifolium Desf. susbp. lusitanicum T.R.Dudley & P.Silva (Brassicaceae) for phytomining nickel (Ni)-rich bio-ore from serpentine soils. This species is endemic to serpentine soils of the Bragança and Morais massifs and is a Ni hyperaccumulator, containing more than 7000 mg kg–1 (dry weight) of this element. The variability of the Ni concentrations contained in the plant at different locations was verified as was its relationship with the total Ni contained in the soil or with the bioavailable Ni fraction extracted with ammonium acetate. We evaluated the biomass produced under natural conditions, and, on the basis of these values, we estimated the amount of Ni that can be removed per unit area. The results showed that considering only the aerial parts of the plant, the biomass varies between 7.04 and 9.37 t ha–1, containing a range of 0.12–0.70% Ni, allowing a withdrawal of between 12.2 and 44.0 kg Ni ha–1 per crop under natural conditions.

The flora of ultramafic soils in the Australia–Pacific Region: state of knowledge and research priorities

In the Australia–Pacific Region ultramafic outcrops are both widespread and extensive, covering thousands of km2. Soils derived from ultramafic bedrock impose edaphic challenges and are widely known to host highly distinctive floras with high levels of endemism. In the Australia–Pacific Region, the ultramafics of the island of New Caledonia are famed for harbouring 2150 species of vascular plants of which 83% are endemic. Although the ultramafic outcrops in Western Australia are also extensive and harbour 1355 taxa, only 14 species are known to be endemic or have distributions centred on ultramafics. The ultramafic outcrops in New Zealand and Tasmania are small and relatively species-poor. The ultramafic outcrops in Queensland are much larger and host 553 species of which 18 (or possibly 21) species are endemic. Although New Caledonia has a high concentration of Ni hyperaccumulator species (65), only one species from Western Australia and two species from Queensland have so far been found. No Ni hyperaccumulator species are known from Tasmania and New Zealand. Habitat destruction due to forest clearing, uncontrolled fires and nickel mining in New Caledonia impacts on the plant species restricted to ultramafic soils there. In comparison with the nearby floras of New Guinea and South-east Asia, the flora of the Australia–Pacific Region is relatively well studied through the collection of a large number of herbarium specimens. However, there is a need for studies on the evolution of plant lineages on ultramafic soils especially regarding their distinctive morphological characteristics and in relation to hyperaccumulation.

Metal-accumulating plants from serpentine habitats of Kızıldaǧ, Konya Province, Turkey

Serpentine (ultramafic) soils are generally deficient in essential plant nutrients such as phosphorus (P), potassium (K) and calcium (Ca) and often also have elevated concentrations of toxic trace elements such as, for example, nickel (Ni), chromium (Cr) and manganese (Mn). However, some serpentine areas have a species-rich plant cover, often with a few endemics. Thus, serpentine areas host valuable bioresources for understanding plant–metal interactions. In the present study, metal-accumulating plants from serpentine habitats in Kızıldağ, Konya Province, Turkey, viz., Aethionema spicatum, Alyssum murale ssp. murale var. murale, Arenaria acerosa, Bornmuellera kiyakii, Cerastium macranthum, Dianthus crinitus. var. crinitus, Dianthus zonatus var. hypochlorus, Iberis sempervirens, Minuartia anatolica var. anatolica, Noccaea camlikensis, Saponoria kotschyi and Silene ozyurtii, belonging to the families Brassicaceae and Caryophyllaceae, were investigated. All plant specimens collected were deposited in the Herbarium of the Biology Department of Akdeniz University. Samples of different plant parts, namely roots, stems, leaves and flowers, were analysed for Ni, Cr, cobalt (Co), Mn, copper (Cu), zinc (Zn), and iron (Fe) by inductively coupled plasma–optical emission spectroscopy. Noccaea camlikensis, Alyssum murale and Bornmuellera kiyakii accumulated 16 650 μg g–1, 12 570 μg g–1 and 8780 μg g–1 Ni, respectively. Noccaea camlikensis is a new addition to the list of Ni hyperaccumulators. The majority of the floristic elements investigated were found to be tolerant to serpentine soil chemistry. A small proportion of the plant assemblages were endemics. Nickel hyperaccumulators were the least abundant in terms of number of species. The study assumes importance in understanding bioconcentration in different plants and, eventually, for gaining a knowledge of plant–metal interactions and applications in biogeochemistry and bioprospecting for metals, including phytoremediation.

Nickel and zinc isotope fractionation in hyperaccumulating and nonaccumulating plants.

Until now, there has been little data on the isotope fractionation of nickel (Ni) in higher plants and how this can be affected by plant Ni and zinc (Zn) homeostasis. A hydroponic cultivation was conducted to investigate the isotope fractionation of Ni and Zn during plant uptake and translocation processes. The nonaccumulator Thlaspi arvense, the Ni hyperaccumulator Alyssum murale and the Ni and Zn hyperaccumulator Noccaea caerulescens were grown in low (2 μM) and high (50 μM) Ni and Zn solutions. Results showed that plants were inclined to absorb light Ni isotopes, presumably due to the functioning of low-affinity transport systems across root cell membrane. The Ni isotope fractionation between plant and solution was greater in the hyperaccumulators grown in low Zn treatments (Δ(60)Ni(plant-solution) = -0.90 to -0.63‰) than that in the nonaccumulator T. arvense (Δ(60)Ni(plant-solution) = -0.21‰), thus indicating a greater permeability of the low-affinity transport system in hyperaccumulators. Light isotope enrichment of Zn was observed in most of the plants (Δ(66)Zn(plant-solution) = -0.23 to -0.10‰), but to a lesser extent than for Ni. The rapid uptake of Zn on the root surfaces caused concentration gradients, which induced ion diffusion in the rhizosphere and could result in light Zn isotope enrichment in the hyperaccumulator N. caerulescens. In high Zn treatment, Zn could compete with Ni during the uptake process, which reduced Ni concentration in plants and decreased the extent of Ni isotope fractionation (Δ(60)Ni(plant-solution) = -0.11 to -0.07‰), indicating that plants might take up Ni through a low-affinity transport system of Zn. We propose that isotope composition analysis for transition elements could become an empirical tool to study plant physiological processes.

Culturable endophytic bacteria enhance Ni translocation in the hyperaccumulator Noccaea caerulescens

In this work, both culture-dependent and independent approaches were used to identify and isolate endophytic bacteria from roots of the Ni hyperaccumulator Noccaea caerulescens. A total of 17 isolates were cultured from root samples, selected for tolerance to 6mM Ni and grouped by restriction analysis of 16S rDNA. Bacterial species cultivated from roots belonged to seven genera, Microbacterium, Arthrobacter, Agreia, Bacillus, Sthenotrophomonas, Kocuria and Variovorax. The culture-independent approach confirmed the presence of Microbacterium and Arthrobacter while only other five clones corresponding to different amplified ribosomal DNA restriction patterns were detected. Five selected highly Ni-resistant bacteria showing also plant growth promoting activities, were inoculated into seeds of N. caerulescens, and in vivo microscopic analysis showed rapid root colonisation. Inoculated plants showed increased shoot biomass, root length and root-to-shoot Ni translocation. Root colonisation was also evident, but not effective, in the non-hyperaccumulating Thlaspi perfoliatum. Seed inoculation with selected Ni-resistant endophytic bacteria may represent a powerful tool in phytotechnologies, although transferring it to biomass species still requires further studies and screening.

Histidine promotes the loading of nickel and zinc, but not of cadmium, into the xylem in Noccaea caerulescens

Histidine is known to be involved in Ni hyperaccumulation. Recently, histidine-dependent xylem loading of Ni and Zn has been demonstrated in the Zn/Ni/Cd hyperaccumulator, Noccaea caerulescens. Here we tested the hypothesis whether Cd xylem loading is histidine-dependent, too. In contrast to that of Ni and Zn, the xylem loading of Cd was not affected by exogenous histidine. Histidine accumulation in root cells appears to facilitate the radial transport of Ni and Zn, but not Cd, across the roots. This may be due to the relatively high preference of Cd for coordination with sulfur over coordination with nitrogen, in comparison with Ni and Zn.

Rinorea niccolifera (Violaceae), a new, nickel-hyperaccumulating species from Luzon Island, Philippines.

A new, nickel-hyperaccumulating species of Rinorea (Violaceae), Rinorea niccolifera Fernando, from Luzon Island, Philippines, is described and illustrated. This species is most similar to the widespread Rinorea bengalensis by its fasciculate inflorescences and smooth subglobose fruits with 3 seeds, but it differs by its glabrous ovary with shorter style (5 mm long), the summit of the staminal tube sinuate to entire and the outer surface smooth, generally smaller leaves (3–8 cm long × 2–3 cm wide), and smaller fruits (0.6–0.8 cm diameter). Rinorea niccolifera accumulates to >18,000 µg g-1 of nickel in its leaf tissues and is thus regarded as a Ni hyperaccumulator.

Noccaea caerulescens populations adapted to grow in metalliferous and non-metalliferous soils: Ni tolerance, accumulation and expression analysis of genes involved in metal homeostasis

Three populations of the Brassicaceae Noccaea caerulescens: (i) the metallicolous Mt. Prinzera population (Italy), which have adapted to grow on serpentinite, a soil naturally rich in Ni, Co, Cr; (ii) the metallicolous La Calamine population (Belgium), which have adapted to growth on soil highly contaminated by Cd, Zn and Pb, and (iii) the non-metallicolous population growing near the town of Rožnov pod Radhoštěm (Czech Republic), were grown in hydroponics and treated with different Ni concentrations (0, 10 and 100μM NiSO4). Ni tolerance and accumulation were analysed along with the expression of genes belonging to different families involved in plant metal homeostasis: ZNT1, ZNT2, NRAMP3, NRAMP4 coding for non-ATP-hydrolysing plasma membrane and vacuolar metal transporters, HMA3, HMA4 coding for ATP-hydrolising metal transporters, NAS1, NAS3, NAS4 and MT1B involved in metal chelation. The three populations showed different levels of expression of some of the tested genes in condition of 0μM Ni. In addition, the Ni hyperaccumulator Mt. Prinzera showed the highest Ni translocation capacity at 10μM Ni and a specific up-regulation of ZNT1, ZNT2, NAS3, NRAMP3 and NRAMP4 genes. With the same Ni treatment, the La Calamine population induced HMA4 and MT1B genes, while the Rožnov pod Radhoštěm population only displayed an over-expression of all the genes at 100μM Ni, the condition in which this population suffers heavily from Ni stress. The expression of ZNT1, ZNT2, NRAMP3, NRAMP4 was also tested on N. caerulescens plants growing in the natural environment on Mt. Prinzera. Higher transcript levels of ZNT1 and ZNT2 were associated with higher levels of plant's total Ni content. Overall these results suggested that some of the genes considered can have a role in determining metal tolerance and accumulation in metallicolous N. caerulescens populations, while they were not involved in metal homeostasis in the non-metallicolous population.

Genetic diversity and population structure of the serpentine endemic Ni hyperaccumulator Alyssum lesbiacum

Alyssum lesbiacum is a well-established Ni hyperaccumulator, endemic to serpentine soils of Lesbos Island (Greece). A total of 95 individuals were collected from 17 plots encompassing the only four large populations of this species. ISSR (Inter-simple sequence repeat) markers were used to assess genetic diversity and population structure of A. lesbiacum to provide initial guidance for the development of successful management and conservation measures. A total of ten primers produced 82 bands, 96.34 % being polymorphic. The largest part of total diversity was found within populations, rather than among them. AMOVA analysis partitioned the largest part of diversity within plots (66 %), while 15 % contained among plots within populations and 19 % among populations. Principal coordinates analysis along with dendrogram based on genetic distances among populations showed a high degree of genetic differentiation of the isolated population Loutra. At a smaller scale, distance was not the most significant factor influencing the patterns of genetic diversity, but topography, ecosystem types and connectivity through streams. According to our results, conservation efforts should be organized at the level of watersheds and ecosystem types, considering them as management units. For ex situ conservation and restoration, seed samplings should be representative of the different habitats and watersheds as well as the patches of large and small populations while keeping the east population of Loutra separate from the other three central populations, to avoid any loss of genetic diversity and to preserve the character of the local adapted populations.

Ni hyperaccumulators among North Caucasian plant species of the tribe Alysseae, Brassicaceae

Background. Heavy metals are dangerous industrial pollutants. Phytoremediation technology is a promising way to detoxificate polluted territories. Heavy metals hyperaccumulating plants present the base for development of these technologies. Many hyperaccumulators belong to the family Brassicaceae (mostly to the Alysseae tribe). Results. Analysis of molecular phylogeny and distribution of hyperaccumulation ability within the tribe Alysseae has been performed using sequences of the ITS1-5.8S rDNA-ITS2 region. Neighbor-joining tree has been reconstructed to investigate affinity within Alysseae species. For the first time molecular characters (variable region of the ITS2 structures and compensatory nucleotide substitutions availability) have been used to analyze phylogenetic structure of the tribe. Conclusion. Most of the known hyperaccumulators among species of the Alysseae tribe belong to the Odontarrhena section. ITS2 primary and secondary structure analysis results in the partition of the tribe Alysseae into five clades.

Mechanisms of trichome-specific Mn accumulation and toxicity in the Ni hyperaccumulator Alyssum murale

Background and aims Mechanisms of Mn accumulation and toxicity in and around trichomes on the Ni hyperaccumulator Alyssum murale were investigated.

Localization of Nickel in Tissues of Streptanthus polygaloides Gray (Cruciferae) and Endemic Nickel Hyperaccumulators from California

The genus Streptanthus Nutt. is one of the most important indicators of ultramafic floras in western North America. This genus contains taxa that are endemic or tolerant of ultramafic soils. Streptanthus polygaloides is an annual nickel hyperaccumulator strictly confined to ultramafic soils throughout the Californian Sierra Nevada foothills. Nickel concentration in S. polygaloides populations was evaluated by elemental microanalysis using inductively coupled plasma mass spectrometry (ICP-MS). Representative samples of S. polygaloides roots, stems, leaves, flowers, and fruits were studied by scanning electron microscopy (SEM) coupled to an energy-dispersive X-ray probe (SEM-EDX). Results show Ni accumulation values between 0.09 and 1.18%, and a distribution pattern similar to that observed in other Ni hyperaccumulator taxa, with the leaf epidermis accumulating the largest concentrations.

A Ni hyperaccumulator and a congeneric non-accumulator reveal equally effective defenses against herbivory

The defense hypothesis is commonly used to explain the adaptive role of metal hyperaccumulation. We tested this hypothesis using two Brassicaceae congeneric species: Alyssum pintodasilvae, a Ni hyperaccumulator, and the non-accumulator Alyssum simplex both growing on serpentine soils in Portugal. Artificial diet disks amended with powdered leaves from each plant species were used to compare the performance (mortality, biomass change) and feeding behavior of Tribolium castaneum in no-choice and choice tests. The performance of T. castaneum was not affected at several concentrations of A. pintodasilvae or A. simplex in no-choice tests. However, the consumption of plant-amended disks was significantly lower than that of control disks, irrespectively of the species fed. Accordingly, when insects were given an alternative food choice, disks of both plant species were significantly less consumed than control disks. Moreover, insects did not discriminate between disks in the combination “A. pintodasilvae+A. simplex”. Contrary to our expectations, these results suggest that both plant species have equally effective defenses against herbivory. While Ni is believed to be part of the deterrence mechanism in the hyperaccumulator A. pintodasilvae, it seems likely that organic compounds, possibly glucosinolates, play an important role in the defense of A. simplex or in both species.

Xylem exudate composition and root-to-shoot nickel translocation in Alyssum species

An improved understanding of the Ni root-to-shoot translocation mechanism in hyperaccumulators is necessary to increase Ni uptake efficiency for phytoextraction technologies. It has been presumed that an important aspect of Ni translocation and storage involves chelation with organic ligands. It has been reported that exposing several Ni hyperaccumulator species of Alyssum to Ni elicited a large increase in the histidine level of the xylem sap. In later studies it was shown that as time progressed the histidine:Ni ratio dropped considerably. Moreover, previous studies analyzed the relationship between Ni and ligands in plants that were exposed to Ni only for a few hours and therefore obtained results that are unlikely to represent field soils where plants are at steady-state Ni uptake. The aim of this study was to understand the quantitative relationship between Ni and organic ligands in the xylem sap of various Alyssum genotypes or species that reached steady-state Ni uptake after being exposed to Ni in either nutrient solution or serpentine soil for up to 6 weeks. Total Ni concentration, 17 amino acids, 9 organic acids, and nicotianamine were measured in xylem sap of 100-day old plants of Alyssum. Results showed that the concentration of Ni in xylem sap of various Alyssum genotypes was 10–100 fold higher than the concentration of histidine, malate, citrate, and nicotianamine, which were the predominant Ni ligands measured in the sap. When the physiology of the whole plant is taken into account, our results indicate that the concentration of organic chelators is too low to account for the complexation of all the Ni present in the xylem sap of Alyssum at steady-state Ni hyperaccumulation, and suggest that most of the Ni in xylem sap of this species is present as the hydrated cation.

Micro-PIXE analyses of frozen-hydrated semi-thick biological sections

Cryo-micro-PIXE system and methodology of microanalysis of frozen-hydrated semi-thick biological sections is described. A commercially available cryotransfer system used in electron microscopy has been adapted for this purpose. The analyzed material was frozen by metal–mirror method and sections of 20–50 micron thickness were prepared. Micro-PIXE and simultaneous proton backscattering was performed using 3MeV proton beam. Monitoring of water vapour composition during the proton bombardment showed good stability of the analyzed material. The results of repetitive analyses of standards prepared from gelatin–glycerol solution with added known concentrations of K, Ni, Cu, Zn were in good agreement with expected, calculated values. Mass losses and changes of elemental composition were monitored. Elemental maps obtained for frozen-hydrated semi-thick section of Ni hyperaccumulator Senecio coronatus showed excellent preservation of leaf morphology and the distribution of elements. Quantitative elemental mapping of frozen-hydrated specimens compared with subsequent analysis of the same areas after freeze-drying revealed similar distribution pattern in both cases. It is clear, however, that freeze-drying induces some distortion of cell morphology and specimen shrinkage.

The metal hyperaccumulators from New Caledonia can broaden our understanding of nickel accumulation in plants

While an excess of metals such as zinc, cadmium or nickel (Ni) is toxic for most plants, about 500 plant species called hyperaccumulators are able to accumulate high amounts of these metals. These plants and the underlying mechanisms are receiving an increasing interest because of their potential use in sustainable biotechnologies such as biofortification, phytoremediation, and phytomining. Among hyperaccumulators, about 400 species scattered in 40 families accumulate Ni. Despite this wide diversity, our current knowledge of the mechanisms involved in Ni accumulation is still limited and mostly restricted to temperate herbaceous Brassicaceae. New Caledonia is an archipelago of the tropical southwest pacific with a third of its surface (5500 km2) covered by Ni-rich soils originating from ultramafic rocks. The rich New Caledonia flora contains 2145 species adapted to these soils, among which 65 are Ni hyperaccumulators, including lianas, shrubs or trees, mostly belonging to the orders Celastrales, Oxalidales, Malpighiales, and Gentianales. We present here our current knowledge on Ni hyperaccumulators from New Caledonia and the latest molecular studies developed to better understand the mechanisms of Ni accumulation in these plants.

Hyperaccumulation of cadmium in Matricaria chamomilla: a never-ending story?

Chamomile (Matricaria chamomilla L.) is a widely used medicinal plant but may also accumulate metals including cadmium. Because recent paper (Masarovicova et al. 2010 Acta Physiol Plant 32:823–829) contradicts previous results of our research group and suggested Cd “hyper/accumulation” in chamomile, present study examined Cd uptake in a long-term experiment (7 weeks) using low Cd concentration (1.5 μM). Besides, typical Cd hyperaccumulator Thlaspi caerulescens and Ni hyperaccumulator Thlaspi goesingense were also cultured under identical conditions. Data obtained indicate that shoot Cd content and bioaccumulation factor (BAF) in chamomile was ca. 5–10 times lower than that in Thlaspi. In this view, chamomile is not a hyperaccumulator species. Comparison of shoot Cd content, BAF and translocation factor (TF) in selected crops is also provided.

Ecophysiology of nickel phytoaccumulation: a simplified biophysical approach

Solute active transport or exclusion by plants can be identified by the values of the Transpiration Stream Concentration Factor (TSCF=xylem:solution solute concentration ratio). The aim of this study was to estimate this parameter for Ni uptake by the Ni-hyperaccumulator Leptoplax emarginata or the Ni-excluder Triticum aestivum cultivar 'Fidel'. The Intact Plant TSCF for nickel (IPTSCF(Ni)) was calculated as the ratio between the nickel mass accumulation in the leaves and the nickel concentration in solution per volume of water transpired. Predominantly, Ni active transport occurred for L. emarginata, with IPTSCF(Ni) values of 4.7-7.2 and convective component proportions of the root Ni uptake flow of only 15-20% for a range of Ni concentrations in solutions of 2-16 µmol Ni l(-1), regardless of the growth period and the time of Ni uptake. Hyperaccumulator roots were permeable to both water and nickel (mean reflection coefficient for Ni, σ(Ni), of 0.06), which was mainly attributed to an absence of exodermis. Results provide a new view of the mechanisms of Ni hyperaccumulation. By contrast, the wheat excluder was characterized by an extremely low mean IPTSCF(Ni) value of 0.006, characterizing a predominantly Ni sequestration in roots. From a methodological viewpoint, the 'microscopic' TSCF(Ni), measured directly on excised plants was 2.4 times larger than its recommended 'macroscopic' IPTSCF(Ni) counterpart. Overall, IPTSCF and σ determined on intact transpiring plants appeared to be very useful biophysical parameters in the study of the mechanisms involved in metal uptake and accumulation by plants, and in their modelling.

Elemental and metabolite profiling of nickel hyperaccumulators from New Caledonia

Leaf material from nine Ni hyperaccumulating species was collected in New Caledonia: Homalium kanaliense (Vieill.) Briq., Casearia silvana Schltr, Geissois hirsuta Brongn. & Gris, Hybanthus austrocaledonicus Seem, Psychotria douarrei (G. Beauvis.) Däniker, Pycnandra acuminata (Pierre ex Baill.) Swenson & Munzinger (syn Sebertia acuminata Pierre ex Baill.), Geissois pruinosa Brongn. & Gris, Homalium deplanchei (Viell) Warb. and Geissois bradfordii (H.C. Hopkins). The elemental concentration was determined by inductively-coupled plasma optical emission spectrometry (ICP-OES) and from these results it was found that the species contained Ni concentrations from to 250-28,000 mg/kg dry mass. Gas chromatography mass spectrometry (GC-MS)-based metabolite profiling was then used to analyse leaves of each species. The aim of this study was to target Ni-binding ligands through correlation analysis of the metabolite levels and leaf Ni concentration. Approximately 258 compounds were detected in each sample. As has been observed before, a correlation was found between the citric acid and Ni concentrations in the leaves for all species collected. However, the strongest Ni accumulator, P. douarrei, has been found to contain particularly high concentrations of malonic acid, suggesting an additional storage mechanism for Ni. A size exclusion chromatography separation protocol for the separation of Ni-complexes in P. acuminata sap was also applied to aqueous leaf extracts of each species. A number of metabolites were identified in complexes with Ni including Ni-malonate from P. douarrei. Furthermore, the levels for some metabolites were found to correlate with the leaf Ni concentration. These data show that Ni ions can be bound by a range of small molecules in Ni hyperaccumulation in plants.