Plants hyperaccumulators

Studies have found many natural plant hyperaccumulators tend to have a higher density of metal transporters at the root-cell plasma membrane (Pence et al., 2000). The higher density of metal transporters allows these plants to readily take up metal cations from the soil solution. Once metals are accumulated, hyperaccumulating plant species usually exhibit a rapid translocation of accumulated metals from roots to shoots (Kramer, 2000). Translocated metals are then stored in vacuoles of the epidermal or mesophyllic cells of the stem to decrease toxicity to the plant (Mathys, 1977). 

Since the electric field efficiently drives increased amounts of soluble heavy metals toward plant roots, which results in stress conditions for plants, hyperaccumulator plants with a rapid growth period are candidates for use in combination... 

Keywords excluder plants hyperaccumulator plants indicator plants natural overexpression of transport proteins phytoremediation phytomining vacuolar metal sequestration... 

The mechanisms by which plants hyperaccumulate heavy metals in their shoots and prevent phytotoxicity of these metals have been the subject of many studies. Nonetheless, many of these mechanisms are still under debate. 

Soil. The first reported field trial of the use of hyperaccumulating plants to remove metals from a soil contaminated by sludge appHcations has been reported (103). The results were positive, but the rates of metal uptake suggest a time scale of decades for complete cleanup. Trials with higher biomass plants, such as B.juncea, are underway at several chromium and lead contaminated sites (88), but data are not yet available. 

The availability of precursor IPP may ultimately be most influential over accumulation of carotenoid metabolites. While over-expression of DXS and DXR in color complementation systems leads to hyperaccumulation of carotenoids (discussed in Section 5.3.3.3), over-expression of plant Dxs genes has not always been effective. Over-expression of DXS resulted in increased carotenoid accumulation in transgenic tomato and Arabidopsis, but over-expression of daffodil DXS in rice endosperm did not increase pigment accumulation. ... 

The phytofiltration of Pb(II) and Cd(II) has been also studied using species of Salvinia. S. minima Baker is a small free-floating aquatic fern native to Mexico, Central America and South America. It has been proved to be an excellent aquatic phytoremediator and hyperaccumulator of Cd(II) and Pb(II).72,76 The relevance of using a compartmentalization analysis (CA) complementary to the use of BCFs and metal removal kinetics by plants has been demonstrated using S. minima... 

Phytoextraction is mainly carried out by certain plants called hyperaccumulators, which absorb unusually large amounts of metals compared to other plants. A hyperaccumulator is a plant species capable of accumulating 100 times more metal than a common nonaccumulating plant. Therefore, a hyperaccumulator will concentrate more than 1000 mg/kg or 0.1% (dry weight) of Co, Cu, Cr, or Pb, or 10,000 mg/kg (1%) of Zn and Ni (dry matter).43-44 Similarly, halophytes are plants that can tolerate and, in many cases, accumulate large amounts of salt (typically sodium chloride but also Ca and Mg chlorides). Hyperaccumulators and halophytes may be selected and planted at a site based on the type of metals or salts present, the concentrations of these constituents, and other site conditions. 

Phytoextraction has several advantages. The contaminants are permanently removed from the soil and the quantity of the waste material produced is substantially decreased. In some cases, the contaminant can be recycled from the contaminated biomass. However, the use of hyperaccumul-ating plants is limited by their slow growth, shallow root systems, and small biomass production. In order for this remediation scheme to be feasible, plants must tolerate high metal concentrations, extract large concentrations of heavy metals into their roots, translocate them into the surface biomass, and produce a large quantity of plant biomass. 

Nickel is localized predominantly in the epidermal and subepidermal of the leaves. However, in leaves of some hyperaccumulator plants such as T. caerulescens and T. goesingense, Ni and Zn are found mainly in vacuoles (Salt and Kramer, 2000). Trace elements also are inactivated in the vacuoles as high-affinity low-molecular-weight metal chelators (such as Cd-phytochelatin complex), providing plants with trace element tolerance. Some Ni in leaves is found to be associated with cell wall pectates as well. 

However, some plants can accumulate more than 0.1% of Pb, Co, Cr, and more than 1% of Mn, Ni and Zn in the shoots. These accumulator plants are called hyperaccumulators. To date, there are approximately 400 known metal hyperaccumulator plants in the world (Baker and Walker, 1989). Thlaspi caerulescens, Alyssum murale, A. lesbiacum, A. tenium are Zn and Cd hyperaccumulators. Brassica juncea, a high-biomass plant, can accumulate Pb, Cr(III), Cd, Cu, Ni, Zn, Sr, B and Se. Thlaspi caerulescens accumulates Ni. Hybrid poplar trees are reported to phytoremediate Cd and As contaminated soils. A Chinese brake fem, Pteris vittata, is an As hyperaccumulator (Ma et al., 2001). 

Salt D.E., Kramer U. Mechanisms of metal hyperaccumulation in plants. In Phytoremediation of Toxic Metals Using Plants to Clean Up the Environment, I. Raskin, B.D. Ensley, eds. New York, NY John Wiley Sons, Inc., 2000. 

Nickel hyperaccumulator plants Allysum spp. various locations ... 

Lichen, Umbilicaria sp. whole 16 km vs. 90 km from nickel smelter Terrestrial vegetation Hyperaccumulator plants Most species Vegetables... 

Kramer U, Grime GW, Smith JAC, Hawes CR, Baker AJM. Micro-PIXE as a technique for studying nickel localization in leaves of the hyperaccumulator plant Alyssum lesbiacum. Nucl Instr Methods B 1997 130 346-350. 

Kiipper H, Zhao F J, McGrath SP. Cellular compartmentation of zinc in leaves of the hyperaccumulator Thlaspi caerulescens. Plant Physiol 1999 119 305-311. 

It is often assumed that if something is in the soil, it will be in plants. This is incorrect. Plants do not take up all of the elements or molecules present in their immediate environment. However, there are some plants, called hyperaccumulators, that accumulate higher than normal levels of some toxic elements. These plants still do not take up all the elements in their environment and they are often small, so the total amount of toxic elements removed from soil is limited. In addition, not all species of an element are toxic and some are not biologically available and thus do not enter biological systems. For example, chromium as Cr(VI) is more toxic and more biologically available than is Cr(IH) [1]. 

Brooks R. R., 1998, Plants that hyperaccumulate heavy metals, CAB International, University Press, Cambridge. 

Chaney R. L., Angle J. S., Wang A. S., McIntosh M.S., Broadhurst L., and Reeves R. D., 2005, Phytoextraction of soil Cd, Ni and Zn using hyperaccumulator plants to alleviate risks of metal contaminated soils requiring remediation. International Workshop Current developments in remediation of contaminated lands p. 39, 27-29 October 2005. Pulawy, Poland. 

Assun9ao, A. G. L., Da Costa Martins, P., De Folter, S., Vooijs, R., Schat, H., and Aarts, M. G. M., 2001, Elevated expression of metal transporter genes in three accessions of the metal hyperaccumulator Thlaspi caerulescens. Plant Cell Environ. 24 217-226. 

Becher, M., Tatke, I. N., and Kramer, U., 2004, Cross-species microarray transcript profiling reveals high constitutive expression of metal homeostasis genes in shoots of the zinc hyperaccumulator Arabidopsis halleri. Plant J. 37 251 -268. 

Bert, V., Meeits, P., Samitou-Laprade, P., Sails, P., Gruber, W., and Verbruggen, N., 2003, Genetic basis of Cd tolerance and hyperaccumulation in Arabidopsis halleri, Plant Soil 249 9-18. 

Weber, M., Harada, E., Vess, C., v. Roepenack-Lahaye, E., and Clemens, S., 2004, Comparative microarray analysis of Arabidopsis thaliana and Arabidopsis halleri roots identifies nicotianamine synthase, a ZIP transporter and other genes as potential metal hyperaccumulation factors. Plant J. 37 269-281. 

Baker, A. J. M., and Brooks, R. R., 1989, Terrestrial higher plants which hyperaccumulate metallic elements - A review of their distribution, ecology and ph)4ochemistry. Biorecovery 1 81-126. 

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