Changes of chloroplast ultrastructure and total chlorophyll concentration in cabbage leaves caused by excess of organic Ni(II) complexes

Leaf chlorophyll concentration, chloroplast ultrastructure, Ni concentration in leaves and Ni accumulation in chloroplasts were examined in cabbage plants treated with Ni as organic complexes, i.e. Ni(II)–Glu, Ni(II)–citrate and Ni(II)–EDTA, in the concentrations of 40 and 85 μM. The plants were grown on half-strength Hoagland's nutrient solution at pH 5.3 for a period of 21 days. The Ni(II) complexes have different effects on leaf chlorophyll concentration and degree and specificity of ultrastructural changes of chloroplasts. After Ni(II)–Glu and Ni(II)–citrate treatment at both concentrations of Ni, the leaf chlorophyll concentration was reduced. After Ni(II)–EDTA treatment, it was reduced only at the concentration of 85 μM Ni. After Ni(II)–Glu and Ni(II)–citrate treatment, the electron density of chloroplast stroma and the number of grana were reduced. Accumulation of starch was observed only in those chloroplasts that were located close to vascular bundles. After Ni(II)–Glu treatment, the thylakoids were swollen, but after Ni(II)–citrate treatment, the thylakoids were condensed. After Ni(II)–EDTA treatment, an accumulation of starch and an increase of the number of plastoglobuli were observed. Damage of the thylakoids was observed sporadically. The experiment also showed that there is a relationship between the degree of toxicity of the examined Ni complexes and Ni concentration in leaves. As far as toxicity and Ni bioaccumulation is concerned, the Ni complexes can be put in the following order: Ni(II)–Glu≥Ni(II)–citrate≫Ni(II)–EDTA. The tissue localization of Ni by the silver sulphide method showed differences in Ni accumulation in the chloroplasts. After Ni(II)–Glu and Ni(II)–citrate treatment, Ni accumulation in chloroplasts was higher than after Ni(II)–EDTA treatment.     

High-nickel insects and nickel hyperaccumulator plants: A review

Insects can vary greatly in whole‐body elemental concentrations. Recent investigations of insects associated with Ni hyperaccumulator plants have identified insects with relatively elevated whole‐body Ni levels. Evaluation of the limited data available indicates that a whole‐body Ni concentration of 500 μg Ni/g is exceptional: I propose that an insect species with a mean value of 500 μg Ni/g or greater, in either larval/nymphal or adult stages, be considered a “high‐Ni insect”. Using the 500 μg Ni/g criterion, 15 species of high‐Ni insects have been identified to date from studies in Mpumalanga (South Africa), New Caledonia and California (USA). The highest mean Ni concentration reported is 3 500 μg Ni/g for nymphs of a South African Stenoscepa species (Orthoptera: Pyrgomorphidae). The majority of high‐Ni insects (66%) are heteropteran herbivores. Studies of high‐Ni insect host preference indicate they are monophagous (or nearly so) on a particular Ni hyperaccumulator plant species. Much of the Ni in bodies of these insects is in their guts (up to 66%–75%), but elevated levels have also been found in Malpighian tubules, suggesting efficient elimination as one strategy for dealing with a high‐Ni diet. Tissue levels of Ni are generally much lower than gut concentrations, but up to 1200 μg Ni/g has been reported from exuviae, suggesting that molting may be another pathway of Ni elimination. One ecological function of the high Ni concentration of these insects may be to defend them against natural enemies, but to date only one experimental test has supported this “elemental defense” hypothesis. Community‐level studies indicate that high‐Ni insects mobilize Ni into food webs but that bioaccumulation of Ni does not occur at either plant‐herbivore or herbivore‐predator steps. Unsurprisingly, Ni bioaccumulation indices are greater for high‐Ni insects compared to other insect species that feed on Ni hyperaccumulator plants. There is some evidence of Ni mobilization into food webs by insect visitors to flowers of Ni hyperaccumulator plants, but no high‐Ni insect floral visitors have been reported.     

Reactions of H2, CO, and O2 with active [NiFe]-hydrogenase from Allochromatium vinosum. A stopped-flow infrared study

The Ni-Fe site in the active membrane-bound [NiFe]-hydrogenase from Allochromatium vinosum can exist in three different redox states. In the most oxidized state (Ni(a)-S) the nickel is divalent. The most reduced state (Ni(a)-SR) likewise has Ni(2+), while the intermediate state (Ni(a)-C) has Ni(3+). The transitions between these states have been studied by stopped-flow Fourier transform infrared spectroscopy. It is inferred from the data that the Ni(a)-S --> Ni(a)-C* and Ni(a)-C* --> Ni(a)-SR transitions induced by dihydrogen require one of the [4Fe-4S] clusters to be oxidized. Enzyme in the Ni(a)-S* state with all of the iron-sulfur clusters reduced reacts with dihydrogen to form the Ni(a)-SR state in milliseconds. By contrast, when one of the cubane clusters is oxidized, the Ni(a)-S state reacts with dihydrogen to form the Ni(a)-C state with all of the iron-sulfur clusters reduced. The competition between dihydrogen and carbon monoxide for binding to the active site was dependent on the redox state of the nickel ion. Formation of the Ni(a)-S.CO state (Ni(2+)) by reacting CO with enzyme in the Ni(a)-SR and Ni(a)-S states (Ni(2+)) is considerably faster than its formation from enzyme in the Ni(a)-C* (Ni(3+)) state. Excess oxygen converted hydrogen-reduced enzyme to the inactive Ni(r)* state within 158 ms, suggesting a direct reaction at the Ni-Fe site. With lower O(2) concentrations the formation of intermediate states was observed. The results are discussed in the light of the present knowledge of the structure and mechanism of action of the A. vinosum enzyme.     

Effects of three nickel salts on germinating seeds of Grevillea exul var. rubiginosa, an endemic serpentine Proteaceae

BACKGROUND AND AIMS: Serpentine soils are usually quite infertile, arid and toxic, mainly because they contain high levels of heavy metals such as Ni. The aim of the present work was to assess the effects of Ni on the germinating seeds of Grevillea exul var. rubiginosa, an endemic serpentine Proteaceae of New Caledonia. In addition, the distribution of macronutrients and the Ni levels in germinating seeds were examined. METHODS: Seeds were sown in glass Petri dishes and exposed to increasing concentrations of Ni (5 to 500 mg Ni L(-1)) using Ni chloride, Ni sulphate and Ni acetate. The germination percentage and root length were measured after 40 d. Longitudinal frozen sections of germinating seeds growing in the presence of Ni (500 mg L(-1) for all three salts) were used for X-ray microanalysis and X-ray elemental mapping using scanning electron microscopy (SEM). KEY RESULTS: Ni chloride resulted in the greatest reductions in germination and root growth, particularly at 500 mg L(-1), followed by Ni sulphate and Ni acetate. SEM images revealed Ca crystalline structures in the seed coat for all the samples. S/Ca and Mg/P/K/Mn were found to be distributed differently in Ni-treated samples, whereas they all followed the same pattern in the controls. For all three salts, the Ni added to the medium had accumulated in the seed coat, whereas the endosperm seemed to be devoid of Ni. CONCLUSIONS: It is assumed that the seed coat is able to reduce the amount of Ni entering the seed, and that a high level of Ni induced the mobilization of macronutrients.     

Nickel allergy in mice: enhanced sensitization capacity of nickel at higher oxidation states.

Attempts to induce contact hypersensitivity to nickel in mice using, e.g., Ni(II)Cl2 often failed. Here, we report that sensitization was achieved by injecting Ni(II)Cl2 in combination with either CFA or an irritant, such as SDS and PMA, or IL-12, or by administering nickel at higher oxidation states, i.e., Ni(III) and Ni(IV). Although Ni(II), given alone, was ineffective in T cell priming, it sufficed for eliciting recall responses in vivo and in vitro, suggesting that Ni(II) is able to provide an effective signal 1 for T cell activation, but is unable to provide an adequate signal 2 for priming. Immunization of mice with nickel-binding proteins pretreated with Ni(IV), but not with Ni(II), allowed them to generate nickel-specific CD4+ T cell hybridomas. Ni(II) sufficed for restimulation of T cell hybridomas; in this and other aspects as well, the hybridomas resembled the nickel-specific human T cell clones reported in the literature. Interestingly, restimulation of hybridomas did not require the original Ni(IV)-protein complex used for priming, suggesting either that the nickel ions underwent ligand exchange toward unknown self proteins or peptides or that nickel recognition by the TCR is carrier-independent. In conclusion, we found that Ni(III) and Ni(IV), but not Ni(II) alone, were able to sensitize naive T cells. Since both Ni(III) and Ni(IV) can be generated from Ni(II) by reactive oxygen species, released during inflammation, our findings might explain why in humans nickel contact dermatitis develops much more readily in irritated than in normal skin.     

Subcellular localization and speciation of nickel in hyperaccumulator and non-accumulator Thlaspi species

The ability of Thlaspi goesingense Hálácsy 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.     

Genetic diversity and differential in vitro responses to Ni in <Emphasis Type="Italic">Cenococcum geophilum</Emphasis> isolates from serpentine soils in Portugal

Amplified fragment length polymorphism (AFLP) analysis was used to investigate the genetic diversity in isolates of the ectomycorrhizal fungus Cenococcum geophilum from serpentine and non-serpentine soils in Portugal. A high degree of genetic diversity was found among C. geophilum isolates; AFLP fingerprints showed that all the isolates were genetically distinct. We also assessed the in vitro Ni sensitivity in three serpentine isolates and one non-serpentine isolate. Only the non-serpentine isolate was significantly affected by the addition of Ni to the growth medium. At 30 microg g(-1) Ni, radial growth rate and biomass accumulation decreased to 73.3 and 71.6% of control, respectively, a highly significant inhibitory effect. Nickel at this concentration had no significant inhibitory effect on serpentine isolates, and so the fitness of serpentine isolates, as evaluated by radial growth rate and biomass yield, is likely unaffected by Ni in the field. In all isolates, the Ni concentration in the mycelia increased with increasing Ni concentration in the growth medium, but two profiles of Ni accumulation were identified. One serpentine isolate showed a linear trend of Ni accumulation. At the highest Ni exposure, the concentration of Ni in the mycelium of this isolate was in the hyperaccumulation range for Ni as defined for higher plants. In the remaining isolates, Ni accumulation was less pronounced and seems to approach a plateau at 30 microg g(-1) Ni. Because two profiles of Ni accumulation emerged among our Ni-insensitive serpentine isolates, this result suggests that different Ni detoxification pathways may be operating. The non-serpentine isolate whose growth was significantly affected by Ni was separated from the other isolates in the genetic analysis, suggesting a genetic basis for the Ni-sensitivity trait. This hypothesis is further supported by the fact that all isolates were maintained on medium without added Ni to avoid carry-over effects. However, because AFLP analysis failed to distinguish between serpentine and non-serpentine isolates, we cannot conclude that Ni insensitivity among our serpentine isolates is due to evolutionary adaptation. Screening a larger number of isolates, from different geographical origins and environments, should clarify the relationships between genetic diversity, morphology, and physiology in this important species.     

Specific nickel(II)-transfer process between the native sequence peptide representing the nickel(II)-transport site of human serum albumin and L-histidine.

The kinetics and mechanism for Ni(II)-transfer of the native sequence tripeptide, L-aspartyl-L-alanyl-L-histidine-N-methylamide (AAHNMA), representing the Ni(II)-transport site of human serum albumin (HSA) and L-histidine (L-His) was studied in forward and reverse reactions in the pH range 6.5 to 9.0 at I = 0.2 and 25 degrees C. For the Ni(II)-transfer from Ni(II)-(L-His)2 to native sequence peptide, the rate-determining step is the formation of a mixed-ligand complex of NiH-1AB by deprotonation of peptide nitrogen from NiAB where A and B denote the anionic forms of AAHNMA and L-His, respectively. For the Ni(II)-transfer from Ni(II)-peptide to L-His, the rate-determining step is a bond breaking between Ni(II) and peptide nitrogen to form NiH-1A by protonation to a peptide nitrogen of NiH-2A. The equilibrium constants for the metal-transfer reaction of MH-2A + 2HB in equilibrium MB2 + A (A = Ni(II), Cu(II] were 10(3.29) and 10(0.78) for Ni(II) and Cu(II), respectively. NiB2 is 324 times as stable as CuB2. Furthermore, the ratio of Ni(II)/Cu(II) in the rate constants for the reaction of MB2 with A was found to be 2.8 x 10(-4). Thus, despite the similarities of Cu(II) and Ni(II) in the metal-binding sites of HSA and in reaction mechanism, Ni(II)-(L-His)2 complex is so stable thermodynamically and kinetically, compared to the Cu(II)-(L-His)2 complex, that Ni(II) is hardly transferred from Ni(II)-(L-His)2 to native sequence peptide. These findings may support specificities in the Ni(II)-transfer, its organ distribution, and its excretion through urine in vivo.     

Oxidative chemistry of nickel porphyrins

The oxidative chemistry of nickel(II) porphyrins is reviewed. Whether electron abstraction occurs from the metal to yield Ni(III) or from the porphyrin to yield Ni(II) pi cation radicals is discussed in terms of the relative energy levels of the metal and porphyrin orbitals. The effects of axial ligands in further modulating this ordering as well as the orbital occupancy of Ni(III) are also reviewed. Structural considerations, based on existing stereochemical data for Ni(I), high spin Ni(II) and related Ni(III) tetraaza complexes, are used to predict the metrics of Ni(III) porphyrins for which no structural data are available.     

The role of free histidine in xylem loading of nickel in Alyssum lesbiacum and Brassica juncea

Exposure of the hyperaccumulator Alyssum lesbiacum to nickel (Ni) is known to result in a dose-dependent increase in xylem sap concentrations of Ni and the chelator free histidine (His). Addition of equimolar concentrations of exogenous L-His to an Ni-amended hydroponic rooting medium enhances Ni flux into the xylem in the nonaccumulator Alyssum montanum, and, as reported here, in Brassica juncea L. cv Vitasso. In B. juncea, reducing the entry of L-His into the root by supplying D-His instead of L-His, or L-His in the presence of a 10-fold excess of L-alanine, did not affect root Ni uptake, but reduced Ni release into the xylem. Compared with B. juncea, root His concentrations were constitutively about 4.4-fold higher in A. lesbiacum, and did not increase within 9 h of exposure to Ni. Cycloheximide did not affect root His or Ni concentrations, but strongly decreased the release of His and Ni from the root into the xylem of A. lesbiacum, whereas xylem sap concentrations of Ca and Mg remained unaffected. Near-quantitative chelation of Ni with nitrilotriacetate in the rooting medium did not enhance Ni flux into the xylem of A. lesbiacum and B. juncea, suggesting the absence of a significant apoplastic pathway for Ni entry into the xylem. The data suggest that in B. juncea roots, Ni(2+) uptake is independent of simultaneous uptake of His. In both species, enhanced release of Ni into the xylem is associated with concurrent release of His from an increased root free His pool.     

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

Aims

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.

Methods

Total Ni concentration, 17 amino acids, 9 organic acids, and nicotianamine were measured in xylem sap of 100-day old plants of Alyssum.

Results

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.

Conclusion

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.     

In Operando Strain Measurement of Bicontinuous Silicon‐Coated Nickel Inverse Opal Anodes for Li‐Ion Batteries

Elastic strains are measured in operando in a nanostructured silicon‐coated nickel inverse opal scaffold anode, using X‐ray diffraction to study the Si (de)lithiation‐induced Ni strains. The volume expansion upon lithiation of the Si in the anode is constrained by the surrounding Ni scaffold, causing mismatch stresses and strains in the Si and Ni phases during cycling. The Ni strains are measured in operando during (dis)charge cycles, using diffraction peak position and peak broadness to describe the distribution of strain in the Ni. During lithiation, compressive strains in the Ni first increase linearly with charge, after which a gradually decreasing strain rate is observed as the maximum lithiation state is approached; upon delithiation a similar process occurs. In‐plane average compressive strains on the order of 990 ± 40 με are measured in the Ni scaffold during lithiation, corresponding to compressive stresses of 215 ± 9 MPa. The decreasing strain rates and decreasing maximum and recovered strains suggest that plasticity in Ni and/or Si, as well as delamination between Ni and Si, may occur during cycling. Rate sensitivity in capacity is correlated with strain and a maximum Ni compressive stress of 230 ± 40 MPa is measured at the maximum state of lithiation.     

Interaction between macrocyclic nickel complexes and the nucleotides GMP,AMP and ApG

Reactions between the nucleotides GMP, AMP and ApG and the complexes Ni(tren), Ni(cyclam) and NiCR in aqueous solution have been monitored by (1)H, (15)N NMR and UV spectroscopy. The three nickel complexes display different properties in reactions with nucleotides. Ni(tren) which has a pseudo-octahedral coordination geometry was shown to bind to all three nucleotides. Ni(cyclam) and NiCR, both with four nitrogen atoms in a square planar arrangement are not able to bind to nucleotides efficiently because of steric hindrance. Oxidation of Ni(cyclam) by KHSO(5) to produce trivalent Ni(III)(cyclam) improves the coordination capacity, while oxidation of NiCR does not produce a similar effect. The nucleotides interact with trivalent nickel complexes to different extent. Ni(III)CR is seen to oxidize GMP gradually but does not affect AMP significantly. Ni(III)(cyclam), on the other hand, does not oxidize either GMP or AMP at the 1:1 concentration of oxidant used. This result is probably due to the lower redox potential of Ni(cyclam). ApG binds less efficiently to the Ni complexes but is easier oxidized than the mononucleotides.     

Extracytoplasmic storage as the nickel resistance mechanism in a natural isolate of Pseudomonas putida S4

Metal resistances in microbes are important to study not only to understand metal homeostasis but also to use such organisms further in environmental bioremediation. Nickel (Ni2+) is an important micronutrient, which at higher concentration becomes toxic. Many Ni2+-resistant organisms are known, which resist metal by active efflux. Pseudomonas putida S4, a natural isolate from India, is reported to show a multi-metal resistance profile. In the present study, the Ni2+-resistance mechanism in strain S4 was examined. Wild-type cells gradually accumulated Ni2+ but kept it preferentially in the periplasmic space in a bound form. In Ni2+-sensitive mutants, periplasmic storage was disturbed and more metal accumulated cytoplasmically, producing toxicity. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis of periplasmic proteins revealed a band of approximately 18 kDa, which appeared only in Ni2+-exposed wild-type cells, and which was absent from cells not exposed to Ni2+ as well as from Ni2+-sensitive mutants. On the basis of these observations, we propose a Ni2+-resistance mechanism in P. putida S4 based on sequestration of metal in the periplasmic space. This is the first study of sequestration-based Ni2+ resistance.     

Lygus hesperus （Heteroptera： Miridae） tolerates high concentrations of dietary nickel

Nickel hyperaccumulator plants contain unusually elevated levels of Ni （〉 1 000 μg Ni/g）. Some insect herbivores, including Lygus hesperus （Western tarnished plant bug）, have been observed feeding on the California Ni hyperaccumulator Streptanthus polygaloides. This bug may be able to utilize S. polygaloides as a host either through its feeding behavior or by physiological tolerance of Ni. This experiment determined the Ni tolerance of L hesperus by offering insects artificial diet amended with 0, 0.4, 1, 2, 4.5, 10, 20 and 40 mmol Ni/L and recording survival. Survival varied due to Ni concentration, with diets containing 10 mmol Ni/L and greater resulting in significantly lower survival compared to the control （0 mmol Ni/L） treatment. Insects tolerated diet containing as much as 4.5 mmol Ni/L, a relatively elevated Ni concentration. I conclude that L hesperus can feed on S. polygaloides because it is Ni-tolerant, probably due to physiological mechanisms that provide it with resistance to plant chemical defenses including elemental defenses such as hyperaccumulated Ni.     

Association of nickel versus transport of cadmium and calcium in tonoplast vesicles of oat roots

The plant vacuole has long been suspected of being a site for accumulation of Ni in plant roots, but testing this hypothesis directly by vacuole isolation is technically difficult and has not been reported. Here, we have attempted to determine if Ni can be transported into isolated oat (Avena sativa L.) root tonoplast vesicles as an alternative approach toward understanding the importance of the vacuole in Ni accumulation in roots. We found that, in contrast to Ca and Cd, Ni did not affect the proton gradient of vesicles (MgATP energized or artificially created), and further, that Cd/H antiport activity was not affected by the presence of Ni. Nickel was associated with vesicles, but relative rates of accumulation/association of metals with vesicles were Ca > Cd Ni. Protonophores and the potential Ni ligands citrate and histidine, and nucleoside triphosphates or PPi did not stimulate Ni association with vesicles. Comparison of Ni versus Ca and Cd associated with vesicles using various membrane perturbants indicated that while Ca and Cd are rapidly and principally antiported to the vesicle sap, Ni is only slowly associated with the membrane in a not-easily dissociated condition. Our results indicate the absence of an Ni/H antiport or Ni-nucleotide-dependent pump in oat root tonoplasts, and support the contention that the vacuole is not a major compartment for Ni accumulation in oat roots. Received: 2 June 1997 / Accepted: 17 July 1997     

Primary structure of helodermin, a VIP-secretin-like peptide isolated from Gila monster venom

The inhibitory regulatory component of adenylate cyclase (Ni) was highly purified from rat brain synaptic membranes. A low Km GTPase activity was always associated with Ni through the purification, and the recovery of GTPase activity correlated well with that of Ni. Purified Ni was hardly ADP-ribosylated by islet-activating protein (IAP). A heat-labile factor in the fraction of the stimulative regulatory component (Ns) restored ADP-ribosylation and also activated the GTPase about 2-fold. NaF which was reported to interact with Ni markedly reduced GTPase activity. The purified Ni fraction inhibited adenylate cyclase only in the presence of a heat-stable factor found in the partially purified regulatory component. GTPase and inhibitory activities were weak in myelin which contained only a small amount of Ni. These findings support the view that GTPase activity is an intrinsic activity of Ni and some factors are necessary for the function of Ni.     

Natural variation among Arabidopsis accessions reveals malic acid as a key mediator of Nickel (Ni) tolerance

Plants have evolved various mechanisms for detoxification that are specific to the plant species as well as the metal ion chemical properties. Malic acid, which is commonly found in plants, participates in a number of physiological processes including metal chelation. Using natural variation among Arabidopsis accessions, we investigated the function of malic acid in Nickel (Ni) tolerance and detoxification. The Ni-induced production of reactive oxygen species was found to be modulated by intracellular malic acid, indicating its crucial role in Ni detoxification. Ni tolerance in Arabidopsis may actively involve malic acid and/or complexes of Ni and malic acid. Investigation of malic acid content in roots among tolerant ecotypes suggested that a complex of Ni and malic acid may be involved in translocation of Ni from roots to leaves. The exudation of malic acid from roots in response to Ni treatment in either susceptible or tolerant plant species was found to be partially dependent on AtALMT1 expression. A lower concentration of Ni (10?μM) treatment induced AtALMT1 expression in the Ni-tolerant Arabidopsis ecotypes. We found that the ecotype Santa Clara (S.C.) not only tolerated Ni but also accumulated more Ni in leaves compared to other ecotypes. Thus, the ecotype S.C. can be used as a model system to delineate the biochemical and genetic basis of Ni tolerance, accumulation, and detoxification in plants. The evolution of Ni hyperaccumulators, which are found in serpentine soils, is an interesting corollary to the fact that S.C. is also native to serpentine soils.     

Extractability of nickel and its concentration in cultivated plants in Ni rich ultramafic soils of New Caledonia

The presence of higher-than-normal quantities of nickel is one of the most general features of ultramafic soils and is often suspected as the reason for their infertility. This study on the bioavailability of Ni in ultramafic soils derived from peridotites in New Caledonia showed important variations depending on the position of the soil in the landscape. In piedmont and non hydromorphic colluvio-alluvial soils, Ni was poorly absorbed by cultivated plants. In contrast, crops species grown in the plain soils, especially those found in the colluvio-alluvial and plain soils subject to temporary reducing conditions, possessed very high and even toxic Ni concentrations. Extraction of Ni by DTPA 5 mM was an effective method of estimating Ni bioavailability in these soils. The regression equation developed with only DTPA-extractable ni explained 88% of the variability in tomato Ni concentration. Extractable Ni might originate from the association of Ni with primary alterable minerals, organic matter and goethite. ei]Section editor: L V Kochian