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12Heavy Metal Toxicity in Plants and Phytoremediation

R.C. Setia, Navjyot Kaur, Neelam Setia and Harsh Nayyar1

Department of Botany, Punjab Agricultural University, Ludhiana-141004, India1Department of Botany, Panjab University, Chandigarh-160014

email: [email protected]

ABSTRACT

The increasing heavy metal concentrations in agricultural lands due to various industrialactivities adversely affect crop growth and metabolism consequently lowering yields withconcomitant quality deteriorations. However, a great deal of research in the past ten yearsindicates that certain plants have the genetic potential to remove many heavy metals fromthe soil. Phytoremediation, the use of plants for environmental restoration, consists of fourdifferent technologies for the remediation of metal polluted soils, sediments or waters namelyphytoextraction, phytostabilization, rhizofiltration and phytovolatilization. These differentphytoremediation technologies are reviewed here with their respective advantages andlimitations. An attempt has been made to review plant-based mechanisms which allow metaluptake, accumulation and translocation in plants whose better understanding is needed tofurther enhance the efficiency of phytoremediation. Genetic engineering approaches to improvethe potential of phytoremediation are also reviewed and discussed. The future challenge forphytoremediation is to further reduce the cost and increase the spectrum of metals amenableto this technology.

Keywords: Heavy metals, plants, phytoremediation, metal toxicity

INTRODUCTIONThe heavy metals are important environmental pollutants and also a cause of potential ecologicalrisk. Large areas of agricultural lands, especially near industrialized areas, are contaminated byheavy metals that mainly originate due to burning of fossil fuels, industrial manufacturing andmunicipal wastes, and application of fertilizers, pesticides and sewage sludge to land. Among anarray of heavy metals, Cu, Co, Fe, Mo, Ni and Zn are essential micronutrient mineral elements,whereas Cd, Pb, Hg, As etc. have no known physiological function in plants and are potentialtoxins. However, elevated levels of both essential and non-essential heavy metals in the ploughlayers of crop lands pose serious threat for human health and agriculture. The excessive uptake ofthese metals from the soil can create dual problem: the harvested crops so contaminated serve as asource of heavy metals in our food supply, and yields are reduced due to adverse effect on plantgrowth (Bala and Setia, 1990; Hall, 2002).

Crop Improvement: Strategies and Applications Editors: R.C. Setia, Harsh Nayyar and Neelam Setia

© 2008 I.K. International Publishing House Pvt. Ltd., New Delhi, pp 206-218

Heavy Metal Toxicity in Plants and Phytoremediation 207

A large number of studies, though spread over different crop plants, indicate that the excessivelyabsorbed heavy metals interfere with various biochemical, physiological and structural aspects ofplant processes that not only lead to inhibited growth but sometimes result in plant death. Thetoxic levels of heavy metals affect structural and permeability properties of inner membranes andorganelles, cause inhibition of enzymatic activities, nutrient imbalances, decreases in rates ofphotosynthesis and transpiration (Green et al., 2003; Setia et al., 1993; Prasad and Hagemeyer,1999; Azevado et al., 2005), stimulate formation of free radicals and reactive oxygen speciesresulting in oxidative stress (Sandalio et al., 2005), suppress seed germination and seedling growth(Beri et al., 1990; Beri and Setia, 1996; Setia et al., 1989b), reproductive development (Setia etal., 1988,1989a), seed yield and seed quality (Beri and Setia, 1995) and induce deleterious anatomicaland ultrastructural changes in crop plants (Setia and Beri, 1993; Setia and Bala, 1994; Liu andKottke, 2004; Maruthi Sridhar et al., 2005). Further, consistently increasing levels of differentheavy metals in the soil render the land unsuitable for plant growth and destroy the biodiversity.Remediation of soils contaminated with heavy metals is particularly challenging. The conventionalengineered based remediation technologies (other than bioremediation) used for in situ and ex situremediation of heavy metal contaminated soils include solidification and stabilization, soil flushing,electrokinetics, chemical reduction/oxidation, soil washing, low temperature thermal desorption,incineration, vitrification, pneumatic fracturing, excavation/ retrieval, landfill and disposal (Saxenaet al., 1999; Wenzel et al., 1999). But these are prohibitively expensive and often disturb the landscape.

Phytoremediation, the use of plants for remediation of soils and waters contaminated withheavy metals, has gained acceptance in the past ten years as a cost effective and non-invasivetechnology. This approach is emerging as an innovative tool with greater potential that is mostuseful when contaminants are within the root zone of the plants (top three to six feet). Further,phytoremediation is an energy efficient, cost-effective, aesthetically pleasing method of remediatingsites with low to moderate levels of contamination (Schnoor, 1997; Salt et al., 1998). The techniqueof phytoremediation exploits the use of either naturally occurring metal hyperaccumulator plants orgenetically engineered plants (Cunningham et al., 1997; Flathman and Lanza, 1998).The base ofphytoremediation is pollutant uptake or bounding by plants under the different processes namelyphytoextraction, phytodegradation, phytostabilization and phytovolatilization. This review aims togive a broad overview of processes involved in uptake and transport of heavy metals in plant cells/tissues and mechanism of phytoremediation of the heavy metal contaminated soils.

PLANT RESPONSES TO HEAVY METALSPlants have evolved several effective mechanisms to deal with the excess of heavy metals in thesoil. They can prevent or restrict the uptake of metals through root and/or into protoplast, orminimize the toxic effects of metal ions inside the protoplast, or take up the metals, accumulateand indicate specific symptoms. Accordingly, the plants have been classified as follows:

Metal Excluders: These plants prevent metal uptake into roots and/or avoid translocation andaccumulation into shoots over a wide range of metal concentrations in the soil (De Vos et al.,1991;Memon et al., 2001). Excluders have a low potential for metal extraction, but they can beused to stabilize the soil, and avoid further contamination spread due to erosion. Such a species isAgrostis tenuis, which avoids Cd, Cu, Pb and Zn uptake by precipitating the metal in the rhizosphere(Lasat, 2002).

208 Crop Improvement: Strategies and Applications

Metal Accumulators: This group of plants can accumulate metals in their above ground tissuesin concentrations far exceeding than those present in the soil, and such plant species are termed ashyperaccumulators (Baker and Brooks, 1989). These plants have evolved specific mechanisms fordetoxifying heavy metals accumulated in their cells. Such mechanisms allow bioaccumulation ofextremely high concentrations of metals.

Metal Indicators: These plants show poor control over metal uptake and transport processes,and accumulate metals in their above ground tissues. The extent of metal accumulation in thetissues of these plants reflects metal concentration in the rhizosphere. Indicator species have beenused for mine prospecting to find new ore bodies (Raskin et al., 1994).

HEAVY METAL UPTAKE, TRANSLOCATION AND ACCUMULATIONThe uptake, translocation and accumulation of heavy metals in plants is mediated by integratednetwork of physiological, biochemical and molecular mechanisms operative at the extracellular(root surface) level as well as inside the cells/tissue of plants growing in contaminated soils. Thetransfer of heavy metals from soils to plants depends primarily on total amount of potentiallyavailable or the bioavailability of the metal (quantity factor), the activity as well as the ionic ratiosof elements in soil solution (intensity factor), and rate of element transfer from solid to liquidphases and to plant roots (reaction kinetics) (Brümmer et al., 1986). Plants distribute metals internallyin many different ways. They may localize selected metals mostly in roots and stems, or they mayaccumulate and store other metals in non-toxic forms for later distribution and use. A mechanismof tolerance or accumulation in some plants apparently involves binding potentially toxic metals atcell walls of roots and leaves, away from sensitive sites within the cell or storing them in avacuolar compartment (Memon et al., 2001).

Metal BioavailabilityThe degree to which higher plants are able to take up metal ions depends on their concentration inthe soil and bioavailability, modulated by the presence of organic matter, pH, redox potential,temperature and concentration of other elements (Benavides et al., 2005). In soils, metal exist as avariety of chemical species in a dynamic equilibrium governed by soil’s physical, chemical andbiological properties (Chaney, 1988). Heavy metals are retained by soils in three ways: by adsorptiononto the surface with mineral particles, by complexation with humic substances in organic particlesand by precipitation reactions (Walton et al., 1994). In general, only a fraction of soil metal isreadily available (bioavailable) for plant uptake. The bulk of soil metals is commonly found asinsoluble compounds unavailable for transport into roots (Lasat, 2002). Plants possess highlyspecialized mechanisms to stimulate metal bioavailability in the rhizosphere, and to enhance uptakeinto roots (Romheld and Marschner, 1986). Root exudates have an important role in the acquisitionof several essential metals. For example, some grass species have been documented to exude fromroots a class of organic acids called siderophores (mugineic and avenic acids), which were shownto significantly enhance the bioavailability of soil-bound iron (Kanazawa et al., 1994) and possiblyzinc (Cakmak, 1996 a, b). Dicotyledonous species facilitate iron uptake by acidifying the rhizospherevia H+ extrusion from roots. Acidic environment stimulates the reduction of ferric to ferrous ironwhich is readily taken up by plants (Chaney et al., 1972; Bienfait et al., 1982). Pollutantbioavailability may also be affected by various plant and/or microbial activities. Some bacteria areknown to release biosurfactants (e.g., rhamnolipids) that make hydrophobic pollutants more water-


souble (Volkering et al., 1998). Plant exudates or lysates may also contain lipophilic compoundsthat increase pollutant solubility or promote biosurfactant producing microbial populations (Sicilianoand Germida, 1998). According to Pilon-Smits (2005), bioavailability of metals may be enhancedby metal chelators that are released by plants and bacteria. Some reported plant chelators such assiderophores, organic acids and phenolics can release metal cations from soil particles and makethe metals more available for plant uptake.

Metal Uptake by RootsThe movement of metals towards the root surface depends on three factors: a) mass flow due towhich the soluble metal ions move from soil solids to root surface (driven by transpiration), b)diffusion of elements along the concentration gradient formed due to uptake and thereby depletionof the element in root vicinity, and c) root interception, where soil volume is displaced by rootvolume due to root growth (Marschner ,1995). The metal uptake by roots may take place at theapical region or from the entire root surface depending upon the type of element under consideration.Further, the uptake depends upon the uptake capacity and growth characteristics of the root system.

There are two pathways for solubilized heavy metals to enter a plant. These are apoplastic(extracellular) and symplastic (intracellular). The apoplast continuum of the root epidermis andcortex is readily permeable to solutes. The metals are first taken into the apoplast of the rootswhere a significant ion fraction is physically adsorbed at the extracellular negatively charged sites(COO-) of the root cell walls (Lasat, 2000). Then, some of the total amount of metal ions associatedwith root cell walls is translocated into the cell. However, the impermeable suberin layers in thecell wall of the root endodermis (casparian strips) prevent solutes from flowing straight from rootapoplast into the root xylem (Taiz and Zeiger, 2002). Therefore, the solutes have to be taken upinto the root symplasm before they can enter the xylem apoplast. Metal ions require membranetransporter proteins for their transportation from root endodermis into root xylem (Pilon-Smits,2005). Some metals are chelated during xylem transport by organic acids (histidine, malate, citrate),nicotianamine, or thiol-rich peptides (Krämer et al., 1996; Pickering et al., 2000). However, formost metal ions it is still unclear via which transporter proteins they are exported to the root xylemand to which chelators they are bound during transport (Pilon-Smits, 2005).

TransportersThe plant plasma membrane may be regarded as the first living structure that encounters the heavymetal toxicity. Because of their charge, metal ions can not move freely across the cellular membranes,which are lipophilic structures. Therefore, ion transport into cells must be mediated by membraneproteins with transport functions, generally known as transporters (Lasat, 2000). Several classes ofmetal transporters are reported in plants that are involved in metal uptake and homeostasis ingeneral, and thus could play some role in tolerance (Hall, 2002). These include heavy metal CPx-ATPases, the Nramps, and CDF (cation diffusion facilitators) family (Williams et al., 2000), andZIP family (Guerinot, 2000). Further, heavy metal ions such as Cd enters the plant cell by transportersfor essential cations such as Fe2+(Thomine et al., 2000). AtNramp genes in Arabidopsis encode themetal transporter, which transports both the metal nutrient iron and the toxic metal cadmium.

Lasat (2000) has summarized the mechanism of transporter function. According to him,“membrane transporters possess an extracellular binding domain to which the ions attach just beforethe transport, and a transmembrane structure which connects extracellular and intracellular media.


The binding domain is receptive only to specific ions and is responsible for transporter specificity.The transmembrane structure facilitates the transfer of bound ions from extracellular space throughhydrophobic environment of the membrane into the cell. The transporters are characterized bycertain kinetic parameters such as transport capacity (Vmax) and affinity for ion (Km)”. For mostelements, multiple transporters occur in plants. For example, Arabidopsis thaliana has been reportedto have 150 different cation transporters and 14 transporters for sulphate alone (Axelsen andPalmgren, 2001).

Metal Transport from Root to ShootThe transport of heavy metals from root to shoot primarily takes place through the xylem viaspecialized membrane transport processes (Salt et al., 1995). For example, the xylem loading of Nimay be facilitated by binding of Ni to free histidine (Krämer et al., 1996). The movement of metalions in xylem vessels appears to be mainly dependent on transpiration-driven mass flow (Salt etal., 1995). Since xylem cell walls have high cation exchange capacity (CEC), therefore, non-cationicmetal-chelate complexers, e.g., Cd-citrate, should be transported more efficiently in the transpirationstream (Senden et al., 1990). Involvement of organic acids for Cd translocation has been observed,and phytochelatins and other thiol-containing ligands play no direct role in Cd transport in thexylem (Salt et al., 1995). Bulk flow in the xylem from root to shoot is driven by transpiration fromthe shoot, which creates a negative pressure in the xylem that pulls up water and solutes (Taiz andZeiger, 2002).

Import into leaf cells from leaf xylem involves another membrane transport step. Metals aretaken up by specific membrane transporter proteins. Once inside the leaf symplast, the pollutantmay be compartmentized in certain tissues or cellular locations. In general, toxic pollutants aresequestered in places where they can do the least harm to essential cellular processes. At thecellular level, pollutants are generally accumulated in vacuole or cell wall (Burken, 2003; Cobbettand Goldsbrough, 2000). At the tissue level they may be accumulated in the epidermis or trichomes.

When pollutants are sequestered in leaf tissues, they are often bound by chelators or formconjugates. Chelators that are involved in metal sequestration include the tripeptide GSH (³-glu-cys-gly) and its oligomer, the phytochelatins (see further in text). After chelation by GSH or PCs,an ABC-type transporter activity transports the metal-chelate complex to the vacuole where it isfurther complexed by the sulphate. Additional metal chelating proteins exist (e.g. MTs) that play arole in sequestration, tolerance, and / or in homeostasis of essential metals (Goldsbrough, 2000).

PHYTOREMEDIATIONPhytoremediation, which essentially involves the use of plants for environmental clean up consistsof four different technologies for the remediation of metal polluted soils, sediments or waters.These include phytoextraction, phytostabilization, rhizofiltration and phytovolatilization.

PhytoextractionPhytoextraction is the most well known of all phytoremediative technologies involving uptake ofmetal contaminants from soil through plant roots and thereafter storage of the same in plant stemor leaves. This technology is, however, most suitable for the large areas which are contaminated atshallow depths, and have low to moderate levels of metal contaminants. There are two basic strategiesof phytoextraction—induced and continuous phytoextraction (Salt et al., 1998).


Induced Phytoextraction: Some heavy metals, such as Pb occur as insoluble precipitates ofphosphates, carbonates and hydroxy-oxides which are largely unavailable for plant uptake, resultingin binding and immobilization within the soil matrix, and consequently significantly restrict thepotential for metal phytoextraction. To overcome this limitation, several chelating agents have beentested to increase phytoextraction of toxic metals including Cd and Pb and the process is known asinduced phytoextraction. Induced phytoextraction involves chelate-mediated release of bound metalsinto soil solution vis-à -vis transport of metals to the harvestable shoot (Salt et al., 1998). Blaylocket al. (1997) reported that addition of EDTA (ethylene-diamine-tetra acetic acid) at a rate of 10mmolkg-1 soil stimulated Pb accumulation in maize to levels as high as 1.6% of shoot dry weight comparedto levels only 0.01-0.06% Pb of shoot dry biomass in vegetation growing on heavily leadcontaminated soil. The concept of chelate-assisted phytoextraction is applicable to a broad range ofheavy metals, e.g., an application of EDTA to heavy metal contaminated soil resulted in thesimultaneous accumulation of Pb, Cr, Cu, Ni and Zn in Indian mustard plants. In this way, syntheticchelates having a high affinity for the metal of interest can play a significant role for the reclamationof metal contaminated sites, e.g., EDTA for Pb and Cd, EGTA (Ethylene glycol-[amino ethylether] –N, N, N,’ N’, tetra acetic acid) for Cd and NTA (Nitrilotriacetic acid) for Cu and Cd(Hong and Pintauro, 1996 ; Wu et al., 1999). Chelate-mediated transport of metals to shoots appearsto occur in the xylem sap via the transpiration stream. The metal is transported within the plantfrom roots to shoots as a metal-chelate complex where water evaporates and the metal-chlelatecomplex remains ( Salt et al., 1998).

On the negative side, the enhanced solubilization of soil metal contaminants with chemicaladditives/soil amendments pose a high risk of groundwater contamination because highly solublePb-EDTA complex easily percolates through soil profile (Wu et al., 1999). In order to overcomethis limitation, an amendment formulation combining lower EDTA doses and surfactants has beensuggested as an alternative to higher rates of soil EDTA application for removal of Pb (Elless andBlaylock, 2000). Recent research in this field aims at eliminating the risk of spreading metalcontamination due to high solubility of metal-chelate complexes like Pb-EDTA complex byimplementing alternative chelate formulations and innovative agronomic practices.

Continuous Phytoextraction (The Concept of Hyperaccumulators): An alternative approach tochelate-assisted induced phytoextraction is the continuous phytoextraction that utilizes the uniquegenetic and physiological capacity of specialized hyperaccumulating plants, which can grow onsoils rich in heavy metals, and are able to accumulate, translocate and tolerate high amounts ofmetals in their tissues. Hyperaccumulators are those plant species which can accumulate one ormore inorganic elements to levels 100 fold higher than other species grown under the sameconditions, and will concentrate more than 10 mg kg-1 Hg, 100 mg kg-1 Cd, 1000 mg kg-1 Co, Cr,Cu and Pb and 10000 mg kg-1 Zn and Ni (Baker et al., 2000). Thalaspi caerulescens, a member offamily brassicaceae, is the best known metal hyperaccumulator that has been reported to accumulateup to 26000 mg kg-1Zn without showing any symptoms of toxicity (Brown et al., 1995). This plantis also able to extract up to 20% of soil exchangeable Cd from a contaminated site (Gerrard et al.,2000). The recently discovered As hyperaccumulating fern, Pteris vittata, has been shown toaccumulate as much as 14500 mg kg-1 As in fronds without showing symptoms of toxicity (Ma etal., 2001). Another plant, Alyssum bertolonii, has been found to phytoremediate Ni (Li et al.,2003) and can accumulate Ni at levels as high as 1%, which is over 100-1000 times higher thanother plants (Minguzzi and Vergnano, 1948). The main function of metal accumulation in


hyperaccumulators has been suggested to be protection against fungal and insect attack (Boyd andMartens, 1992). However, most of the hyperaccumulators have limited potential for phytoextractionbecause most of them are slow growing and attain low biomass. Thus, plants having the ability toproduce higher dry biomass even when grown on heavy metal contaminated soils are preferredover well-known hyperaccumulator species that can have considerably higher heavy metalconcentrations in their tissues. For example, Brassica juncea, while having one-third the concentrationof Zn in its tissue, is more effective at Zn removal from soil than T. Caerulescens—a knownhyperaccumulator of Zn—as the former produces ten times more biomass than the latter. It isexpected that further research with classical or molecular genetic methods will produce a range ofcrops that can be used for the phytoextraction of several heavy metals (Schmidt, 2003).

PhytostabilizationPhytostabilization is a plant-based remediative technology that exploits the ability of heavy metal-tolerant plants to reduce the mobility of the metal contaminants as the latter are absorbed andaccumulated by roots, adsorbed onto the roots or precipitated in the rhizosphere. The goal ofphytostabilization is thus not to remove metal contaminants from a site, but rather to stabilize themand reduce the risk to human health and environment by preventing migration of metal contaminantsinto the groundwater or air (Prasad and Freitas, 2003). Plants chosen for phytostabilization shouldbe poor translocators of metal contaminants to shoots, such as grasses, thus minimizing exposureof wildlife to toxic elements. To further reduce the risk of groundwater contamination by downwardleaching, grasses should be grown along with fast transpiring trees, such as poplar, which are deeprooted and transpire at very high rates, creating powerful upward flow (Dawson and Ehleringer,1991).

RhizofiltrationRhizofiltration is a phytoremediative technology concerned with the removal of metals from theaquatic environments. Plants used for rhizofiltration are first hydroponically grown in clean wateruntil a large root system has developed. This is followed by acclimatization of the plant to thepollutant by substituting the clean water supply for a polluted water supply. Then these acclimatizedplants are transplanted into metal-polluted waters where plants absorb and concentrate metals intheir roots and shoots (Salt et al., 1995; Zhu et al., 1999 a). In addition to absorption of metalpollutants, latter may also be adsorbed onto the root surfaces triggered by root exudates and changesin rhizosphere pH. Roots or whole plants are harvested for disposal after they become saturatedwith metal pollutants (Flathman and Lanza, 1998). Several aquatic plant species such as waterhyacinth, pennywort and duckweed have the ability to remove heavy metals from water ( Dierberget al., 1987; Mo et al., 1989; Zhu et al., 1999a). However, these plants have limited potential forrhizofiltration because of their small and slow-growing roots.

PhytovolatilizationPhytovolatilization is the process where plants absorb water soluble elemental forms of metalcontaminants, and biologically convert them to gaseous species within the plant followed by theirrelease into the atmosphere. This process is particularly used to remediate soils contaminated withmetals such as As, Hg and Se (Suszcynsky and Shann, 1995). This technology has the addedbenefits of minimal site disturbance, less erosion and no need to dispose off contaminated plant


material. The major limitation of phytovolatilization is that there is no control over the migrationof metal contaminants that have been removed via volatilization to other areas (Prasad and Freitas,2003).

MECHANISMS FOR METAL SEQUESTRATION AND DETOXIFICATIONWhen toxic metals are sequestered in plant tissues, they are often bound by heavy metal-bindingpolypeptides, also known as chelators. The two best-characterized heavy metal binding polypeptidesknown to exist in plants are metallothioneins and phytochelatins. Metallothioneins (MTs) are gene-encoded, low molecular-weight, cysteine-rich polypeptides. MTs were first identified as Cd- bindingproteins in mammalian tissues and are classified based on the arrangement of cysteine residues(Robinson et al., 1993). Several classes of MT genes (MT1, MT2, MT3, MT4) have now beenidentified in several higher plants, including Arabidopsis(Goldsbrough, 2000). The role of plantmetallothioneins in relation to Cu binding and detoxification has been most widely studied. Murphyand Taiz (1995) demonstrated that expression levels of MT2 mRNA in Arabidopsis thaliana stronglycorrelated with Cu resistance. Earlier, Zhou and Goldsbrough (1994) also reported that MT2 mRNAwas strongly induced in Arbabidopsis seedlings by Cu, but only slightly by Cd and Zn, suggestingthat metallothioneins are involved in Cu resistance.

Phytochelatins (PCs), also known as class III metallothioneins, are short metal-binding, metal-induced, cysteine-rich peptides possessing the general structure: (³-Glu Cys)n-Gly with n=2-11. PCsare synthesized from glutathione (GSH) by a specific transpeptidase named ³-glutamyl cysteinedipeptidyl transpeptidase (EC 2.3.2.15), also known as phytochelatin synthase (PCS) (Vatamaniuket al., 2004), which is a constitutive enzyme requiring post-translational activation by heavy metals(Klapheck et al., 1995). The best activator for the enzyme phytochelatin synthase (PCS) is Cd, butis also activated in the presence of other heavy metals, such as Ag, Bi, Pb, Zn, Cu, Hg, Au andAs, both in vivo and in vitro (Cobbett, 2000). X-ray absorption spectroscopy (XAS) has shown thatmetals that were complexed by PCs in vivo include Cd and As (Pickering et al., 2000). In vitrostudies conducted by Loeffler et al. (1989) had shown that PC biosynthesis continued until theactivating metal ions were chelated either by the PCs formed or by the addition of a metal chelatorssuch as EDTA, thus suggesting that activating metal ions autoregulate the biosynthesis of PCs. Thefinal step in heavy metal detoxification involves the sequestration of metal -PC complexes in thevacuole, and transport of the metal-PC complexes through the tonoplast is mediated by an ATP-dependent ABC-transporter (Schat et al., 2000). Metal-PC complex in the vacuole is further stabilizedby the incorporation of acid-labile sulphide (Cobbett and Goldsbrough, 2000). PCs appear to beimportant in the detoxification of Cd and As, but have a relatively insignificant role in thedetoxification of metal ions such as Cu, Zn, Ni and SeO3 (Cobbett, 2000).

Metal hyperaccumulator species possess additional detoxification mechanisms. For example,studies conducted in a Ni hyperaccumulator-T. goesingense have revealed that the high toleranceof the latter was due to Ni complexation by histidine, which rendered the metal inactive (Krämeret al., 1996).

GENETIC ENGINEERING AND PHYTOREMEDIATIONNatural metal hyperaccumulators such as Thalaspi spp. can accumulate and tolerate higher metalconcentrations in their tissues than those usually found in non-accumulators without any visibletoxicity symptoms. However, most of the metal hyperaccumulators have a limited potential for


phytoremediation because of their small size and slow growth (Lasat, 2002). Thus, to overcomethis limitation and to improve the potential for metal phytoextraction, Brown et al. (1995 ) proposedthe transfer of the hyperaccumulator phenotype from small and slow growing hyperaccumulatorspecies to fast growing, high biomass producing nonaccumulator plants. Research data indicatesthat tolerance to toxic metals is regulated by one or few major genes, thus providing a hope thatbiotransformation of selected crop species for tolerance and ultimately superior metal extractionpotential is feasible. However, the use of conventional breeding programmes to improve plants formetal phytoextraction has limited potential because of anatomical constraints that can severelyrestrict sexual compatibility between species (Ow, 1996). The most spectacular application ofbiotechnology for environmental restoration has been the bioengineering of plants capable ofremoving methyl-mercury from the contaminated soil. Methyl-mercury, a strong neurotoxic agent,is biosynthesized in Hg-contaminated soils, and to detoxify this compound, A. thaliana plants weregenetically manipulated to express bacterial genes mer A (Catalyzes proteolysis of the carbon-mercury bond with the release of Hg2+) and mer B (converts Hg2+ taken up by roots to Hgo, a lesstoxic volatile element which is released into the atmosphere). These transgenic A. thaliana plantswere able to grow on media containing 50-fold higher methylmercury concentrations than wildtype plants (Rugh et al., 1996).

The roles of glutathione and phytochelatins in heavy metal tolerance have been well illustratedin Cd-sensitive mutants of Arabidopsis, cad1 and cad2. These mutants are deficient in PC productiondue to mutations in PC synthase and ³-glutamyl-cys synthetase in cad1 and cad 2 mutants,respectively (Howden et al., 1995; Cobbett et al., 1998). To investigate the importance of PC forCd tolerance, Lee et al., (2003) overexpressed the Arabidopsis PC Synthase gene, AtPCS 1. Theresults of this study showed that the normal level of Arabidopsis PC synthase expression wassufficient to synthesize the required PCs in response to supplemented levels of Cd and increasedcapacity of PC synthesis does not lead to Cd tolerance, but paradoxically leads to Cd hypersensitivity.Furthermore, the genes encoding enzymes involved in glutathione synthesis may hold more promiseas overexpression of E. coli gsh 1 gene encoding ³-glutamylcysteine synthetase (³-ECS) or E. coligshII gene encoding glutathione synthetase (GS) in Brassica juncea enhanced PC synthesis and Cdtolerance (Zhu et al., 1999 b,c).

FUTURE STRATEGIES FOR PHYTOREMEDIATIONIn order to achieve practical applications of phytoremediation, hyperaccumulators offer an importanttool for inexpensive soil decontamination for those elements which these plants hyperaccumulate.But these natural hyperaccumulators have limited potential for phytoremediation, as most of theseare slow growing (e.g., mosses, lichens, or the Thalaspi species that take up heavy metals) and/orhave low biomass (Salt et al., 1998). Thus, in order to overcome these limitations, attempts havebeen made to bring heavy metal tolerance from natural hyperaccumulators like Thalaspi speciesinto high biomass crop plant species like Brassica juncea (which has been reported to accumulateCd as well as other toxic metals) by protoplast fusion technique (Dushenkov et al., 2002). Othernew developments in plant genetic engineering are tailored transgenics that overexpress differentenzymes in different plant parts or that express a transgene only under certain environmentalconditions (Dhankher et al., 2002). But, many oppose introducing transgenic or non-transgenicphytoremediating plant species because they pose a risk of spread to adjacent areas, displacingnative or other desirable species or hybridizing with related native species (Gressel and Al-Ahmad,


2005). To overcome the biological risks of plants used for phytoremediation, Ruiz et al. (2003)have shown that genetic engineering of the chloroplast genome can be used as a novel approach toobtain high expression without the risk of spreading the transgene via pollen.

The future challenge for phytoremediation is to further reduce the cost and increase the spectrumof metals amenable to this technology. This goal can be achieved by creating superior plant varietiesfor phytoextraction by using genetic engineering to introduce valuable traits into plants, optimizingagronomic practices for their cultivation, and designing safer and more effective soil amendments(Gleba et al., 1999). Further, manipulating rhizospheric bacteria and introduction of arbuscularmycorrhizal (AM) fungal inoculums into metal contaminated areas could be used as a strategy forenhancing the establishment of mycorrhizal herbaceous species, as AM fungi play a vital role inmetal tolerance and accumulation (Zhu et al., 2001).

REFERENCESAxelsen, K. B., and M. G. Palmgren. 2001. Inventory of the superfamily of P-type ion pumps in Arabidopsis. Plant

Physiol. 126: 696-706.Azevado, H., C.G.G. Pinto, J. Farnandes, S. Loureiro, and C. Santos. 2005. Cadmium effects on sunflower growth and

photosynthesis. J.Plant Nutr. 28: 2211-2220.Baker, A.J.M., and R.R.Brooks. 1989. Terresterial higher plants which hyperaccumulate metallic elements- A review of

their distribution, ecology and phytochemistry. Biorecovery. 1: 81-126.Baker, A.J.M., S.P.McGrath, R. D. Reeves, and J.A.C. Smith. 2000. Metal hyperaccumulator plants: A review of the

ecology and physiology of a biological resource for phytoremediation of metal-polluted soils. p. 85-107. In N.Terry and G. Banuelos (ed.) Phytoremediation of contaminated soil and water. Lewis Publishers, Boca Raton,FL.

Bala, R., and R. C. Setia. 1990. Some aspects of cadmium and lead toxicity in plants. p.167-180. In C. P. Malik, D.S. Bhatia, R C. Setia and P Singh (ed.) Advances in Frontier Areas of Plant Sciences. Narendra PublishingHouse.

Benavides, M. P., S.M. Gallego, and M.L.Tomaro. 2005. Cadmium toxicity in plants. Braz. J. Plant Physiol. 17: 1-18.Beri, A., and R.C. Setia. 1996. Effects of Ni and Cd on seed germination, seedling growth, mobilization of food

reserves and activity of some enzymes in Lens culinaris Medic (Lentil). p.85-193 In T.A.Sarma, S.S. Saina, M.LTrivedi and M.Sharma (ed.) Current Researches in Plant Sciences. BSMP Singh, Dehra Dun, India.

Beri, A., and R.C.Setia. 1995. Assessment of growth and yield in Lens culinaris Medic var. Massar 9-12 treated withheavy metals under N-supplied conditions. J. Indian Bot. Soc. 74: 293-297.

Beri, A., R.C. Setia, and N. Setia. 1990. Germination responses of lentil (Lens culinaris) seeds to heavy metal ionsand plant growth regulators. J.Pl. Sci.Res. 6: 24-27.

Bienfait, H.F., J. Duivenvoorden, and W. Verkerke. 1982. Ferric reduction by roots of chlorotic bean plants: Indicationsfor an enzymatic process. J. Plant Nutr. 5: 451-456.

Blaylock, M.J., D.E. Salt, S. Dushenkov, O. Zakharova, and C. Gussman .1997. Enhanced accumulation of Pb inIndian mustard by soil applied chelating agents. Environ. Sci. Technol. 31: 860-865.

Boyd, R.S., and S.N. Martens. 1992. The raison d’être for metal hyperaccumulation by plants. p.279-289. In A.J.M.Baker, J.Proctor and R.D.Reeves (ed.) The vegetation of Ultramafic (Serpentine) Soils. Andover, UK.

Brown, S.L., R.L. Chaney, J.S. Angle, and A.J.M.Baker. 1995. Zinc and cadmium uptake by hyperaccumulator Thalaspicaerulescens and metal tolerant Silene vulgaris grown on sludge- amended soils. Environ. Sci. Technol. 29:1581-1585.

Brümmer, G., J. Gerth, and U. Herms. 1986. Heavy metals species, mobility and availability in soils. Z. Pflanzenernaehr.Bodenkd. 149: 382-398.

Burken, J. G. 2003. Uptake and metabolism of organic compounds: green-liver model. p. 59-84. In S.C. McCutcheonand J. L. Schnoor (ed.) Phytoremediation: Transformation and control of contaminants. Wiley Publishers, NY.

Cakmak, I., L. Ozturk, S. Karanlik, H. Marschner, and H. Ekiz. 1996a. Zinc-efficient wild grasses enhance release ofphytosiderophoeres under Zn deficiency. J. Plant Nutr. 19: 551-563.

Cakmak, I., N. Sari, H. Marschner, H. Ekiz, and M. Kalayci. 1996b. Phytosiderophore release in bread and durumwheat genotypes differing in zinc efficiency. Plant Soil. 180: 183-189.

Chaney, R. L., J.C. Brown, and L.O. Tiffin. 1972. Obligatory reduction of ferric chelates in iron uptake by soybeans.Plant Physiol. 50: 208-213.


Chaney, R.L. 1988. Metal speciation and interactions among elements affecting trace-element transfer in agriculturaland environmental food chains. p.218-260. In J.R. Kramer and H.E. Allen (ed.) Metal speciation: Theory,Analysis and Applications. Lewis Publishers Inc., Chelsea, MI.

Cobbett, C. S., M. J. May, R. Howden, and B. Rolls. 1998. The glutathione-deficient, cadmium-sensitive mutant, cad2-1, of Arabidopsis thaliana is deficient in ³-glutamylcysteine synthetase. Plant J. 16: 73-78.

Cobbett, C.S., and P.B. Goldsbrough. 2000. Mechanism of metal resistance: Phytochelatins and metallothioneins. p.247-269. In I. Raskin and B.D. Ensley (ed.) Phytoremediation of toxic metals: using plants to clean up theenvironment. John Wiley & Sons, NY.

Cobbett, C.S.2000. Phytochelatin biosynthesis and function in heavy metal detoxification. Curr. Opin. Plant Biol. 3:211-216.

Cunningham, S.D., J.R.Shann, D.E. Crowley, and T.A. Anderson. 1997. Phytoremediation of contaminated water andSoil. p.2-19. In E.L. Kruger, T.A. Anderson and J.R. Coats (ed.) Phytoremediation of soil and water contaminants.ACS symposium series 664. Am. Chem. Soc., Washington, DC.

Dawson, T.E., and J.R. Ehleringer. 1991. Streamside trees do not use stream water. Nature. 350: 335-337.De Vos, C.H.R., H. Schat, M.A.N. De Waal, R. Voojs, and W.H.O. Ernst. 1991. Increased resistance to copper-

induced damage of root cell plasmalemma in copper tolerant Silene cucubalus. Physiol. Plantarum. 82: 523-528.

Dhankher, O.P., Y. Li, B.P. Rosen, J. Shi, and D. Salt. 2002. Engineering tolerance and hyperaccumulation of arsenicin plants by combining arsenate reductase and gamma- glutamylcysteine synthetase expression. Nat. Biotechnol.20: 1140-1145.

Dierberg, F.E., T.A. Debusk, and N.A.Goulet. Jr. 1987. Removal of copper and lead using a thin- film technique. p.497-504. In K.R. Reddy and W.H. Smith (ed.) Aquatic plants for water treatment and resource recovery. MagnoliaPulishing.

Dushenkov, S., M. Skarzhinskaya, K.Glimelius, D. Gleba, and I. Raskin. 2002. Bioengineering of a phytoremediationplant by means of somatic hybridization. Int. J. Phytoremed. 4: 117-126.

Elless, M.P., and M.J. Blaylock. 2000. Amendment optimization to enhance lead extractability from contaminated soilsfor phytoremediation. Int. J. Phytol. : 75-89.

Flathman, P.E., and G.R. Lanza. 1998. Phytoremediation: Current views on an emerging green technology. J. SoilContam. 7: 415-432.

Gerrard, E., G. Echevarria, T. Sterckeman, and J.L. Morel. 2000. Cadmium availability to three plant species varyingin cadmium accumulation pattern. J. Environ. Qual. 29: 117-1123.

Gleba, D., N.V. Borisjuk, L.G. Borisjuk, R.Kneer, A. Poulev, M.Skarzhinskaya, S.Dushenkov, S.Logendra, Y.Y.Gleba,and I. Raskin. 1999. Use of Plant roots for phytoremediation and molecular farming. Proc. Natl. Acad. Sci.USA. 96: 5973-5977.

Goldsbrough, P.2000. Metal tolerance in Plants: The role of phytochelatins and metallothioneins. p.221-234. In N.Terry and G. Bañuelos (ed.) Phytoremediation of contaminated soil and water. Lewis Publishers, Boca Raton,FL.

Green, C., R. Vhaney, and J. Bouwkamp. 2003. Interactions between cadmium and phytotoxic levels of zinc in hardred spring wheat. J. Plant Nutr. 26: 417-430.

Gressel, J., and H. Al-Ahmad. 2005. Assessing and managing biological risks of plants used for bioremediation,including risks of transgene flow. Z. Naturforsch. 60:154-165.

Guerinot, M. L. 2000. The ZIP family of metal transporters. BBA. 1465: 190-198.Hall, J. L. 2002. Cellular mechanisms for heavy metal detoxification and tolerance. J. Exp. Bot. 53: 1-11.Hong, J., and P.N. Pintauro. 1996. Desorption-complexation- dissolution characteristics of absorbed cadmium from

kaolin by chelators. Water Air Soil Pollut. 86: 35-50.Howden, R., P.B.Goldsbrough, C.R.Andersen, and C.S.Cobbett. 1995. Cadmium-sensitive cad 1 mutants of Arabidopsis

thaliana are phytochelatin deficient. Plant Physiol. 107: 1059-1066.Kanazawa, K., K. Higuchi, N.K. Nishizawa, S. Fushiya, M. Chino, and S. Mori. 1994. Nicotianamine aminotransferase

activities are correlated to the phytosiderophore secretion under Fe-deficient conditions in Gramineae. J. Exp.Bot. 45: 1903-1906.

Klapheck, S., S. Schlunz, and L. Bergmann. 1995. Synthesis of phytochelatins and homophytochelatins in Pisumsativum L. Plant Physiol. 107: 515-521.

Krämer, U., J.D. Cotter-Howells, J.M.Charnock, A.J.M. Baker, and A.C.Smith. 1996. Free histidine as a metal chelatorin plants that accumulate nickel. Nature. 379: 635-638.

Lasat, M. M. 2002. Phytoextraction of toxic metals: A review of biological mechanisms. J. Environ. Qual. 31: 109-120.Lasat, M.M. 2000. Phytoextraction of metals from contaminated soil: A review of plant/soil/metal interaction and

assessment of pertinent agronomic issues. J. Hazard. Subs. Res. 2: 1-14.


Lee, S., J.S. Moon, T.S.Ko, D. Petros, P.B. Goldsbrough, and S.S. Korban. 2003. Overexpression of Arabidopsisphytochelatin synthase paradoxically leads to hypersensitivity to cadmium stress. Plant Physiol. 131: 656-663.

Li, Y.M., R. Chaney, E. Brewer, R. Roseberg, and S.J. Angle.2003. Development of a technology for commercialphytoextraction of nickel: economic and technical considerations. Plant Soil 249: 107-115.

Liu, D., and I. Kottke. 2004. Subcellular localization of cadmium in root cells of Allium cepa by electron energy lossspectroscopy and cytochemistry. J. Biosci. 29: 329-335.

Loeffler, S., A. Hochberger, E.Grill, E.L. Winnacker, and M.H. Zenk. 1989. Termination of the phytochelatin synthasereaction through sequestration of heavy metals by the reaction product. FEBS Lett. 258: 42-46.

Ma, L. Q., K.M. Komar, C.Tu, W. Zhang, Y. Cai, and E.D. Kenelley. 2001. A fern that hyperaccumulates arsenic.Nature 409: 579.

Marschner, H. 1995. Mineral Nutrition of Higher Plants. Academic Publishers, San Diego.Maruthi Sridhar, B.B., S.V. Diehl, F. X. Han, D.L.Monts, and Y. Su. 2005. Anatomical changes due to uptake and

accumulation of Zn and Cd in Indian mustard (Brassica juncea ). Environ. Expt. Bot. 54: 131-141.Memon, A. R., D. Aktoprakligil, A. Ozdemir, and A. Vertii. 2001. Heavy metal accumulation and detoxification mechanisms

in plants. Turk. J Bot. 25: 111-121.Minguzzi, C., and O. Vergnano. 1948. II Contenuto di nichel nelle ceneri di Alyssum bertolonii Desv. Atti della Societa

Toscana di Scienze Naturali, Mem. Ser. A. 55 :49-77.Mo, S.C., D.S.Choi, and J.W. Robinson. 1989. Uptake of mercury from aqueous solution by duckweed: The effect of

pH, copper, and humic acid. J. Environ.Health. 24: 135-146.Murphy, A.S., and L. Taiz. 1995. Comparison of metallothionein gene expression and nonprotein thiols in ten Arabidopsis

ecotypes. Plant Physiol. 109: 1-10.Ow, D.W. 1996. Heavy metal tolerance genes: Prospective tools for bioremediation. Res. Conserv. Recycling. 18: 135-

149.Pickering, I.J., R.C.Prince, M.J. George, R.D.Smith, G.N.George, and D.E. Salt. 2000. Reduction and coordination of

arsenic in Indian mustard. Plant Physiol. 122: 1171-1177.Pilon-Smits, E.A.H. 2005. Phytoremediation. Annu. Rev. Plant Biol. 56: 15-39.Prasad, M.N.V., and H.Freitas. 2003. Metal hyperaccumulation in Plants-Biodiversity prospecting for phytoremediation

technology. Electronic J. Biotechnol. 6: 275-321.Prasad, M.N.V., and J. Hagemeyer. 1999. Heavy metal stress in plants. From molecules to ecosystem. Springer-

Verlag, Heidelberg, NY.Raskin, I., P.B.A.N.Kumar, S.Dushenkov, and D.E.Salt. 1994. Bioconcentration of heavy metals by Plants. Curr. Opin.

Biotechnol. 5: 285-290.Robinson, N.J., A.M. Tommey, C Kuske, and P.J. Jackson. 1993. Plant metallothioneins. Biochem. J. 295: 1-10.Romheld, V., and H. Marschner. 1986. Mobilization of iron in the rhizosphere of different plant species. Adv. Plant

Nutr. 2: 155-204.Rugh, C.L., H.D. Wlide, N.M. Stack, D.M.Thompson, A.O. Summers, and R.B. Meagher. 1996. Mercuric ion reduction

and resistance in transgenic Arabidopsis thaliana plants expressing a modified bacterial mer A gene. Proc.Natl. Acad. Sci. USA. 93: 3182-3187.

Ruiz, O.N., H.S. Hussein, N. Terry, and H. Daniell. 2003. Phytoremediation of organomercurial compounds via chloroplastgenetic engineering. Plant Physiol.132: 1344-1352.

Salt, D.E., M. Blaylock, P.B.A.N. Kumar, V. Dushenkov, B.D.Ensley, I. Chet, and I. Raskin. 1995. Phytoremediation: Anovel strategy for the removal of toxic metals from the environment using plants. Biotechnol. 13: 468-475.

Salt, D.E., R.D. Smith, and I. Raskin. 1998. Phytoremediation. Annu. Rev. Plant Physiol. 49: 643-668.Sandalio, L. M., H.C. Dalurzo, M. Gomez, M.C. Romero-Puertas, and L.A.del Rio. 2005. Cadmium induced changes in

growth and oxidative metabolism of pea plants. J. Expt. Bot. 52: 2115-2126.Saxena, P.K., S.Krishnaraj, T. Dan, M.R.Perras, and N.N. Vettakkoruma- Kankav. 1999. Phytoremediation of metal

contaminated and polluted soils. p. 305-329. In M.N.V. Prasad and J. Hagemeyer (ed.) Heavy metal stress inplants. From molecules to ecosystems. Springer-Verlag, Heidelberg, NY.

Schat, H., M. Llugany, and R. Bernhard. 2000. Metal specific patterns of tolerance, uptake and transport of heavymetals in hyperaccumulating and nonhyperaccumulating metallophytes. p. 171-188. In N. Terry and G. Banuelos(ed.) Phytoremediation of contaminated soil and water. Lewis Publishers, Boca Raton, FL.

Schmidt, U. 2003. Enhancing Phytoextraction-The effect of chemical soil manipulation on mobility, plant accumulation,and leaching of heavy metals. J. Environ. Qual. 32: 1939-1954.

Schnoor, J.L. 1997. Phytoremediation: Technical and organisatoric issues, key factors [on line]. Available at http: //www. gwrtac. org / pdf / phyto-e.pdf. Ground-water Remediation Technologies Analysis Center, Pittsburgh.

Senden, M.H.M.N., F. J. M. Van Paassen, A.J.G.M. Van Der Meer, and H.T. Wolterbeek. 1990. Cadmium-citric acid-xylem cell wall interactions in tomato plants. Plant Cell Environ. 15: 71-79.


Setia, N., D. Kaur, and R.C. Setia. 1989a. Influence of heavy metals on growth and reproductive behaviour of pea. J.Pl. Sci. Res. 5: 127-132.

Setia, N., D. Kaur, and R.C.Setia. 1989b. Germination and seedling growth of pea as influenced by Cd and Pb toxicityJ. Pl. Sci. Res. 5: 137-144.

Setia, R.C., and A. Beri. 1993. Cadmium induced anatomical changes in stem and root of lentil (Lens culinaris Medic.).Ibid. 9: 54-58.

Setia, R.C., and R. Bala. 1994. Anatomical changes in root and stem of wheat (Triticum aestivum L.) in response todifferent heavy metals. Phytomorphology. 44: 95-104.

Setia, R.C., J. Kalia, and C.P.Malik. 1988. Effects of NiCl2 toxicity on stem growth and ear development in Triticumaestivum L. Phytomorphology. 38: 21-27.

Setia, R.C., R. Bala, N. Setia, and C.P.Malik. 1993. Photosynthetic characteristics of heavy metal treated wheat(Triticum aestivum L.) plants. J. Pl. Sci. Res. 9: 47-49.

Siciliano, S.D., and J. J. Germida. 1998. Mechanisms of phytoremediation : biochemical and ecological interactionsbetween plants and bacteria. Environ. Rev. 6: 65-79.

Suszcynsky, E.M., and J.R.Shann. 1995. Phytotoxicity and accumulation of mercury subjected to different exposureroutes. Environ. Toxi. Chem. 14: 61-67.

Taiz, L., and E. Zeiger. 2002. Plant Physiology. Sinauer, Sunderland, MA.Thomine, S., R. Wang, J. M. Ward, N. M. Crawford, and J. I. Schroeder. 2000. Cadmium and iron transport by

members of a plant metal transporter family in Arabidopsis with homology to Nramp genes. Proc. Natl. Acad.Sci. USA. 97: 4991-4996.

Vatamaniuk, O.K., S. Mari, A.Lang, S. Chalasani, L.O.Demkiv, and P.A. Rea. 2004. Phytochelatin synthase, adipeptidyltransferase that undergoes multisite acylation with gamma-glutamylcysteine during catalysis-Stoichiometric and site dissected mutagenic analysis of Arabidopsis thaliana PCS 1-catalyzed phytochelatinsynthesis. J. Biol. Chem.. 279: 22449-22460.

Volkering, F., A. M. Breure, and W. H. Rulkens. 1998. Microbial aspects of surfactant use for biological soil remediation.Bioremediation. 8: 401-417.

Walton, B.T., A.M.Hoylman, M.M.Perez, T.A. Anderson, and T.R.Johnson .1994. Rhizosphere microbial community asa plant defense against toxic substances in soils. In T.A. Anderson and J.R. Coats (ed.) Bioremediationthrough rhizosphere technology. Am. Chem. Soc., Washington, DC.

Wenzel, W.W., E. Lombi, and D.C.Adriano. 1999. Biogeochemical processes in the rhizosphere: role in phytoremediationof metal-polluted soils. p. 271-303. In M.N.V. Prasad and J. Hagemeyer (ed.) Heavy metal stress in plants-From molecules to ecosystems. Springer- Verlag, Heidelberg, NY.

Williams, L.E., J.K.Pittman, and J.L. Hall. 2000. Emerging mechanisms for heavy metal transport in plants. Biochimicaet Biophysica Acta. 77803: 1-23.

Wu, J., F. C. Hsu, and S.D. Cunningham. 1999. Chelate-assisted Pb. Phytoexraction: Pb availability, uptake andtranslocation constraints. Environ. Sci. Technol.33:1898-1904.

Zhou, J., and P.B. Goldsbrough. 1994. Functional homologs of fungal metallothionein genes from Arabidopsis. PlantCell. 6:875-884.

Zhu, Y.G., P. Christie, and A.S.Laidlow. 2001. Uptake of Zn by arbuscular mycorrhizal white clover from Zn- contaminatedsoil. Chemosphere. 42: 193-199.

Zhu, Y.L., A.M. Zayed, J.H.Quian, M. De Souza, and N. Terry. 1999a. Phytoaccumulation of trace elements by wetlandplants: II. Water hyacinth. J. Environ. Qual. 28: 339-344.

Zhu, Y.L., E.A.H. Pilon-Smits, A.S.Tarun, S.U.Weber, L.Jouanin, and N. Terry. 1999b. Cadmium tolerance andaccumulation in Indian mustard is enhanced by over expressing ³-glutamylcysteine synthetase. Plant Physiol.121: 1169-1177.

Zhu, Y.L., E.A.H. Pilon-Smits, L. Jouanin, and N. Terry. 1999c. Overexpression of glutathione synthetase in Indianmustard enhances cadmium accumulation and tolerance. 119: 73-79.

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