Spatial distribution of cadmium in leaves of metal hyperaccumulating Thlaspi praecox using micro-PIXE

* Localization of cadmium (Cd) and other elements was studied in the leaves of the field-collected cadmium/zinc (Cd/Zn) hyperaccumulator Thlaspi praecox from an area polluted with heavy metals near a lead mine and smelter in Slovenia, using micro-PIXE (proton-induced X-ray emission). * The samples were prepared using cryofixation. Quantitative elemental maps and average concentrations in whole-leaf cross-sections and selected tissues were obtained. * Cd was preferentially localized in the lower epidermis (820 microg g(-1) DW), vascular bundles and upper epidermis, whereas about twice the lower concentrations were found in the mesophyll. * Taking into account the large volume of the mesophyll compared with the epidermis, the mesophyll is indicated as a relatively large pool of Cd, possibly involved in Cd detoxification/dilution at the tissue and cellular level.     

Nickel and cobalt phytoextraction by the hyperaccumulator Berkheya coddii: implications for polymetallic phytomining and phytoremediation

We investigated the potential of the South African high-biomass Ni hyperaccumulator Berkheya coddii to phytoextract Co and/or Ni from artificial metalliferous media. Plant accumulation of both metals from single-element substrates indicate that the plant/media metal concentration quotient (bioaccumulation coefficient) increases as total metal concentrations increase. Cobalt was readily taken up by B. coddii with and without the presence of Ni. Nickel uptake was, however, inhibited by the presence of an equal concentration of Co. Bioaccumulation coefficients of Ni and Co for the single element substrates (total metal concentration of 1000 micrograms g-1) were 100 and 50, respectively. Cobalt phytotoxicity was observed above a total Co concentration in plant growth media of 20 micrograms g-1. Elevated Co concentrations significantly decreased the biomass production of B. coddii without affecting the bioaccumulation coefficients. The mixed Ni-Co substrate produced bioaccumulation coefficients of 22 for both Ni and Co. Cobalt phytotoxicity in mixed Ni-Co substrate occurred above a total Co concentration of 15 micrograms g-1. When grown in the presence of both Ni and Co, the bioaccumulation coefficients of each metal were reduced, as compared to single-element substrate. This may indicate competition for binding sites in the root zone. The interference relationship between Ni and Co uptake demonstrated by B. coddii suggests a significant limitation to phytoextraction where both metals are present.     

Variation in Nickel Content in the Nickel-Hyperaccumulating Shrub Psychotria douarrei (Rubiaceae) from New Caledonia1

Plants that hyperaccumulate Ni contain > 1000 ppm (dry wt.) in their tissues. Variation of Ni content within hyperaccumulating plant species is poorly explored. Using the Ni-hyperaccumulating shrub Psychotria douarrei, we documented variation of leaf Ni levels within individual shrubs, and variation with respect to plant size and leaf age. Plant size did not correlate significantly with leaf Ni content, and leaf Ni content did not correlate significantly with soil Ni content. Older leaves contained twice as much Ni as younger leaves. Older leaves also contained greater concentrations of Ca, Fe, and Cr but less K, P, and Cu. Five elements (Zn, Pb, Co, Mn, Mg) showed no significant variation due to leaf age. We also examined the effect of leaf age on epiphyll cover, finding increased epiphyll cover on the upper surface of older leaves. The dominant leafy liverwort epiphyll had a relatively high Ni content (400 ppm), suggesting that epiphylls of Ni hyperaccumulators obtain some Ni from host leaves. Individual shrubs differed in mean leaf Ni content almost two-fold (14,900-27,700 ppm). Variation among branches within individuals also ranged widely; however, this intraplant variability was not strongly correlated with the mean leaf Ni content of an individual shrub. We concluded that Ni contents in leaves of P. douarrei vary considerably due to leaf age, among individual shrubs, and among branches within a shrub.     

Variations in plant metallothioneins: the heavy metal hyperaccumulator <Emphasis Type="Italic">Thlaspi caerulescens</Emphasis> 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.     

Cadmium hyperaccumulation protects Thlaspi caerulescens from leaf feeding damage by thrips (Frankliniella occidentalis)

Metal hyperaccumulation has been proposed as a plant defensive strategy. Here, we investigated whether cadmium (Cd) hyperaccumulation protected Thlaspi caerulescens from leaf feeding damage by thrips (Frankliniella occidentalis). Two ecotypes differing in Cd accumulation, Ganges (high) and Prayon (low), were grown in compost amended with 0-1000 mg Cd kg(-1) in two experiments under glasshouse conditions. F2 and F3 plants from the Prayon x Ganges crosses were grown with 5 mg Cd kg(-1). Plants were naturally colonized by thrips and the leaf feeding damage index (LFDI) was assessed. The LFDI decreased significantly with increasing Cd in both ecotypes, and correlated with shoot Cd concentration in a log-linear fashion. Prayon was more attractive to thrips than Ganges, but the ecotypic difference in the LFDI was largely accounted for by the shoot Cd concentration. In the F2 and F3 plants, the LFDI correlated significantly and negatively with shoot Cd, but not with shoot zinc (Zn) or sulphur (S) concentrations. We conclude that Cd hyperaccumulation deters thrips from feeding on T. caerulescens leaves, which may offer an adaptive benefit to the plant.     

Cadmium hyperaccumulation and reproductive traits in natural Thlaspi caerulescens populations

During the last decade, the metal hyperaccumulating plants have attracted considerable attention because of their potential use in decontamination of heavy metal contaminated soils. However, in most species, little is known regarding the function, the ecological and the evolutionary significances of hyperaccumulation. In our study, we investigated the parameters influencing the Cd concentration in plants as well as the biological implications of Cd hyperaccumulation in nine natural populations of Thlaspi caerulescens. First, we showed that Cd concentration in the plant was positively correlated with plant Zn, Fe, and Cu concentrations. This suggested that the physiological and/or molecular mechanisms for uptake, transport and/or accumulation of these four heavy metals interact with each other. Second, we specified a measure of Cd hyperaccumulation capacity by populations and showed that T. caerulescens plants originating from populations with high Cd hyperaccumulation capacity had better growth, by developing more and bigger leaves, taller stems, and produced more fruits and heavier seeds. These results suggest a tolerance/disposal role of Cd hyperaccumulation in this species.     

Assessment of plants from the Brassicaceae family as genetic models for the study of nickel and zinc hyperaccumulation

We report on the second phase of a programme to select a relative of Arabidopsis thaliana for use in large-scale molecular genetic studies of nickel (Ni) and zinc (Zn) hyperaccumulation. We also report on the relatedness among Thlaspi caerulescens accessions and the utility of using O-acetyl-L-serine as a marker for Ni and Zn hyperaccumulation potential. Twenty-seven new accessions of metal-accumulating species collected in the Czech Republic, France, Greece, Italy, Slovenia and the USA during Spring-Summer 2002 were evaluated. The criteria established for selection were hyperaccumulation of metals (Ni and Zn); compact growth habit; reasonable time to flowering; production of > or = 1000 seeds per plant; self-fertility; compact diploid genome; high sequence similarity to A. thaliana; > or = 0.1% transformation efficiency with easy selection. We conclude that the best candidate identified in the first phase was the best candidate overall: T. caerulescens accession St Félix de Pallières.     

Ion-exchange aspects of toxic metal uptake by Indian mustard

Uptake of lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) as +2 ions by excised roots of Indian mustard was demonstrated to be an ion-exchange process with existing Ca or protons released to the solution. This initial reaction at the root-aqueous interface is important in the uptake of these toxic metals from contaminated soil. Ethylene diamine tetraacetic acid (EDTA)-amended soil for phytoremediation has Pb in anionic form as [Pb-EDTA]2-, which was not taken up by excised roots. In nonliving B. juncea, Pb2+ was translocated from a solution through a cut stem to petiole and leaves much more quickly than anionic [Pb-EDTA]2-. However, in living plants [Pb-EDTA]2- was more quickly translocated from a solution through roots and petiole to leaves than Pb2+. The final amount of uptake on roots of the living plants was the same for both forms of Pb. The present results are important toward understanding the mechanism of phytoremediation of toxic metal-contaminated soil for two reasons: 1) the initial process, uptake of metal ions by roots, was shown to occur by cation exchange and 2) since [Pb-EDTA]2- was not sorbed by excised roots, other factors such as transpiration and active transport are important in applications using EDTA-amended soils contaminated by Pb.     

Phytoremediation of arsenic-contaminated groundwater by the arsenic hyperaccumulating fern Pteris vittata L

Arsenic concentrations in a much larger fraction of U.S. groundwater sources will exceed the maximum contaminant limit when the new 10 microg L(-1) EPA standard for drinking water takes effect in 2006. Thus, it is important to develop remediation technologies that can meet this new standard. Phytoremediation of arsenic-contaminated groundwater is a relatively new idea. In this research, an arsenic-hyperaccumulating fern, commonly known as Chinese Brake fern (Pteris vittata L.), was grown hydroponically to examine its effectiveness in arsenic removal from what is believed to be herbicide-contaminated groundwater. One plant grown in 600 mL of groundwater effectively reduced the arsenic concentration from 46 to less than 10 microg L(-1) in 3 days. Re-used plants continued to take up arsenic from the groundwater, albeit at a slower rate (from 46 to 20 microg L(-1) during the same time). Young fern plants were more efficient in removing arsenic than were older fern plants of similar size. The addition of a supplement of phosphate-free Hoagland nutrition to the groundwater had little effect on arsenic removal, but the addition of phosphate nutrition significantly reduced its arsenic affinity and, thus, inhibited the arsenic removal. This study suggested that Chinese Brake has some potential to remove arsenic from groundwater.