The Role of Metal Transport and Tolerance in Nickel Hyperaccumulation by Thlaspi goesingense Halacsy

Abstract

400 hyperaccumulator species have been identified, according to the analysis of field-collected specimens. Metal hyperaccumulators are interesting model organisms to study for the development of a phytoremediation technology, the use of plants to remove pollutant metals from soils. However, little is known about the molecular, biochemical, and physiological processes that result in the hyperaccumulator phenotype. We investigated the role of Ni tolerance and transport in Ni hyperaccumulation by Thlaspigoesingense, using plant biomass production, evapotranspiration, and protoplast viability assays, and by following short-and long-tem uptake of Ni into roots and shoots. As long as both species (T. goesingense and Thlaspi arvense) were unaffected by Ni toxicity, the rates of Ni translocation from roots to shoots were the same in both the hyper-and nonaccumulator species. Our data suggest that Ni tolerance is sufficient to explain the Ni hyperaccumulator phenotype observed in hydroponically cultured T. goesingense when compared with the Ni-sensitive nonhyperaccumulator T. arvense. ~~~~ ~~~~~~~~~~~~~~~~~~The Ni requirement of plants is generally very low, 1.7 nmol g-' Ni or less in tissue dry biomass (Brown et al., 1988;Dalton et al., 1988). Symptoms of Ni toxicity can be observed between 0.19 and 0.85 pmol g-' Ni in plant dry biomass. These symptoms include the inhibition of root elongation and interveinal chlorosis, the latter possibly a consequence of the interference of Ni with chlorophyll formation (Woolhouse, 1983;Gabbrielli et al., 1990;Brune and Dietz, 1995;Marschner, 1995). The majority of plants that occur on metalliferous soils are known to exclude toxic metals from their shoots (Baker and Walker, 1990). A number of species, however, have developed an unusual adaptation to metal-rich soils. Instead of excluding toxic metals, so-called hyperaccumulators accumulate metals such as Ni, Zn, or Co in their aboveground biomass. This trait is

Comparison between fractionation and bioavailability of trace elements in rhizosphere and bulk soils

“…Principally, the methods to study on the bioavailability of trace elements in soils should be analogous to the uptake of metals by biota and need a detailed knowledge of biological uptake mechanisms, particularly the transport of metals through the biological membrane (Town, 2000). In order to achieve these goals, many efforts should be made in the future to understand what speciation of metals present in a sample (Kaneta et al, 1986;Kot and Namiesnik, 2000), where the different forms of metals are localized in plants (Vazquez et al, 1992;Kramer et al, 1996;Kelley et al, 1999), how the different compounds of metals enter the plants apoplastically or intercellularly (Kramer et al, 1997;Salt and Kramer, 2000). Due to the extremely complicated mechanisms involved between the speciation of metals and their bioavailability, at the present time, various empirical approaches are often used to study the speciations of trace elements in soils and the contents of those elements in plants, including single regression (Randall et al, 1976;LeClaire et al, 1984;Chlopecka and Adriano, 1996) and multiple regression analyses (Davies, 1992;Maiz et al, 2000).…”

Zinc tolerance and hyperaccumulation are genetically independent characters

“…The relationship between tolerance and hyperaccumulation is unclear. In a recent review, Chaney et al (1997) asserted that hypertolerance is the key property which makes hyperaccumulation possible, while KrÌmer et al (1997) suggested that nickel hyperaccumulation is simply a manifestation of nickel tolerance in Thlaspi goesingense. To understand the relationship fully, we need to know whether the two characters are essentially governed by the same genes (as implied by KrÌmer et al (1997)) or are genetically independent.…”

Phytoextraction: a review on enhanced metal availability and plant accumulation

“…Blaylock et al (1997) demonstrated that the ability of soil-applied EDTA to increase metal uptake in a multi-contaminated soil is not limited to Pb, since EDTA was also efficient in increasing Cd, Cu, Ni, and Zn concentrations in shoots of B. juncea. However, synthetic chelators such as EDTA are barely degradable by microorganisms and can pose a threat to the environment by metal leaching to groundwater (Sun et al, 2001;Wenzel et al, 2003;Madrid et al, 2003;Chen et al, 2004), and adverse effects on soil microbiota (Welper & Brummer, 1997;Bouwman et al, 2005). We also observed EDTA toxicity and drastic plant growth reduction in Indian mustard grown on a metal multicontaminated soil (Nascimento et al, 2006).…”

“…Krämer et al (2000) provided evidence that free histidine may be also involved in shuttling Ni across the cytoplasm into the vacuole in T. goesingense, which could be responsible for Ni tolerance and accumulation. Krämer et al (1996) have already reported a 36-fold increase in the concentration of free histidine in the xylem exudates of the Ni hyperaccumulator Alyssum lesbiacum after exposure to Ni, suggesting that histidine could be involved in the transport and storage of Ni in such species. Kerkeb & Krämer (2003) recently provided further evidence that histidine enhances the release of Ni from roots into the xylem, not only in A. lesbiacum but also in the nonhyperaccumulator B. juncea.…”

“…Notwithstanding the fate of exudates in the rhizosphere, and the nature of reactions involved in phytoextraction and transport of metals by plants being not yet fully understood, it is recognized that they contribute significantly to the accumulation of metals in plants. Chemical compounds likely to occur in the rhizosphere are clearly associated with increase of metals uptake from soil and their translocation to shoots (Mench & Martin, 1991;Salt et al, 1995;Krishnamurti et al, 1997;Lin et al, 2003;Wenzel et al, 2003).…”