Heavy Metals in Plants: Phytoremediation

Heavy metals in plants: phytoremediation: Plants used to remediate heavy metal pollution 1*Varsha Mudgal, 1Nidhi Madaan and 2Anurag Mudgal 1Department of Biotechnology, College of Engineering and Technology, IFTM Campus, Moradabad, UP, India, 2Department of Mechanical Engineering, College of Engineering and Technology IFTM Campus, Moradabad, UP, India Email: [email protected] ABSTRACT In current time the environment is heavily polluted by various toxic metals, which create a danger for all living beings. These metals are retarding farming efficiency and destructing the health of the plants and animals. Now a day "green technology" minimized this problem to some extent. In this technology to clean up the contamination, such plants grow which can tolerant metal. The application of genetic manipulation and the use of naturally occurring metal tolerant plants should accelerate the process of transmitting this technology from experimental place to field. Thus careful investigation of the mechanism of tolerance of heavy metal at physiological and genetic level is essential Key words: hyperaccumulator plants, phytoremediation, toxic metals. INTRODUCTION Phytoremediation of toxic metals from the Heavy metals like As, Cd, Co, Cu, Ni, Zn, and Cr are contaminated soil basically involves the extraction or phytotoxic either at all concentrations or above certain inactivation of these metals in soils. Figure-1 shows threshold levels. Toxic metals are biologically phytoextraction and phytomining processes which are magnified through the food chain. They infect the used to recover the accumulated toxic metals in plant environment by affecting soil properties its fertility, tissues for its reuse and Figure 2 shows the role of biomass and crop yields and ultimately human health. genetic engineering in production of It is a big issue of accumulation of heavy metals in hyperaccumulators. soils as a result of industrial effluents and atmospheric Phytoremediation is one new approach that offers emissions like paper mill, fertilizers, glasses and more ecological benefits and a cost efficient Mining wastes. The presence of heavy metals in toxic alternative. Although it is cheaper method but requires concentrations can result in the formation of - technical strategy, expert project designers with field superoxide radicals (O2 ), hydrogen peroxide (H2O2), - experience that choose the proper species and hydroxyl radicals (OH ), etc., can cause severe cultivars for particular metals and regions. During oxidative damage to biomolecules like lipids, proteins various researches main focus is to understand the and nucleic acids. Cr, Cu and Zn can induce the physiological mechanisms of metal absorption, activity of various antioxidant enzymes and also non- transportation and assimilation, but little is know enzymes like ascorbate and glutathione. Certain regarding the genetic basis of hyperaccumulation plants absorb these toxic metals and help to clean up (Pollard et al., 2002). them from soils these plants are termed hyper accumulators. These plants have been shown to be The plant used in the phytoremediation technique resistant to heavy metals and are capable of must have a considerable capacity of metal accumulating them into their roots and leaves and absorption, its accumulation and strength to decrease transporting these soil pollutants to high the treatment time. Many families of vascular plants concentrations. Thus, biologically engineered have been identified as metal hyperaccumulator methods designed to improve the use of (Reeves and Baker, 2000; Prasad and Freitas 2003), phytoremediation to reduce the amount of heavy and many of them belongs to Brassicaceae. These metals in contaminated soils. Such a process has hyperaccumulator are metal selective, having slow been used to clean up heavy metals, toxic aromatic growth rate, produce small amounts of biomass and pollutants, acid mine drainage, pesticides and can be used in their natural habitats only (Kamnev xenobiotics and organic compounds. and van der Lelie, 2000). Phytoremediation is an environmentally friendly, safe and cheap technique to eliminate the pollutants. Agric. Biol. J. N. Am., 2010, 1(1): 40-46

Extraction Composting

Incineration

Recovery and use

Harvested biomass processed for metal recovery and utilized in appropriate industry

Fig 1. Sustainable development via Agrobiotechnology (Source: Gratao et al., 2005 reproduce with some modification)

Fig 2. Use of genetic engineering to accelerate “Phytoremediation” (Source: Gratao et al., 2005 reproduce with some modification) The capacity of metal accumulation and tolerance modified genes encoding antioxidant enzymes or could be enhanced by over expressing natural or those that are involved in the biosynthesis of

41 Agric. Biol. J. N. Am., 2010, 1(1): 40-46

glutathione and phytochelatins. Now it is clear that to causes a reduction in the intracellular concentrations breed superior phytoremediator plants, it is necessary of SeCys and selenomethionine (SeMet), thus that they should have a high growth rate, produce preventing their incorrect insertion into more biomass, ecologically adaptive to diverse protein(Madan.N.,2008). Brassica juncea habitates, possess the ability to accumulate the target overexpressing the A. bisulcatus SMT gene, exhibited metal in the above-ground parts, can tolerate the high a greatly increased accumulation of MetSeCys and metal concentration and have an adaptive tolerance tolerance to Se compounds, in particular selenite may be essential for several metals simultaneously (LeDuc et al., 2004). (Pilon-Smits and Pilon, 2002; Karenlampi et al., 2000). Phytoremediation of Cadmium Cadmium (Cd) is a Hyperaccumulators Plants: Those plants which can toxic element its concentration greatly increased by accumulate and tolerate greater metal concentrations activities such as zinc mining, iron foundries and the in shoot. Over 400 hyperaccumulator plants have use of sewage sludge as a fertilizer in agriculture. Cd been reported and include members of the may be detoxified in plants by phytochelatins (PCs), a Asteraceae, Brassicaceae, Caryophyllaceae, family of sulphur rich peptides which are able to bind Cyperaceae, Cunouniaceae, Fabaceae, Cd and some other heavy metals (Cobbett and Flacourtiaceae, Lamiaceae, Poaceae, Violaceae, and Goldsbrough, 2002). Phytochelatins (PCs) were first Euphobiaceae. The Brassicaceae is a very important discovered as Cd-binding "Cadystins A and B" in a hyperaccumulator group. The minimum threshold fission yeast and then in many plants as the major tissue concentrations for Co, Cu, Cr, Pb or Ni components of Cd-binding complexes. They are hyperaccumulators should be 0.1% dry weight, while capable of binding to various metals including Cd, Cu, for Zn or Mn the threshold is 1%. (Baker and Brooks, Zn or As via the sulfhydryl and carboxyl residues, but 1989) their biosyntheses are controlled preferentially by the metal Cd or metalloid As. These peptides are related Phytoremediation of Selenium Selenium (Se) is a to glutathione and contain a varying number (normally toxic metal at medium to high concentrations but 2-5) of glutamate and cysteine, linked through the essential as a micronutrient for humans and animals. carboxyl group of glutamate. It occurs naturally in soils as selenate and selenite and often as a pollutant, following the industrial use of Phytochelatin synthase (PCS) an enzyme carries out coal. Se and sulphur (S) have very similar chemical the conversion of glutathione to PCs, and has been properties so it incorporate in proteins as activated by Cd. Cysteine synthase catalyses the last selenomethionine and proceed by the same enzymes step in the assimilation of sulphate into the amino of methionine. Some scientist worked to obtain acid. Through overexpressing genes encoding selenium tolerant plants by overexpression of genes enzymes we can stimulate the synthesis of cysteine encoding key enzymes in sulphur metabolism. Some and glutathione.Transgenic tobacco plants over- plants overexpressing ATP sulphurylase, were shown expressing cysteine synthase in the cytosol or to have higher shoot Se concentrations and enhanced chloroplasts, had elevated concentrations of PCs, Se tolerance than wild type when grown in the were more tolerant to Cd, Se and Ni but did not presence of selenate in either hydroponic systems or accumulate the metal in the leaves (Harada et al., soil like Brassica juncea, Arabidopsis thaliana and 2001). In contrast, Arabidopsis thaliana Astragalus bisulcatus (Pilon-Smits et al., 1999). overexpressed cysteine synthase in the cytosol Transgenic plants that were overexpressing ATP (Dominguez-Solis et al. 2004). Trichomes were shown sulphurylase, were more tolerant than the wild type to to be resistant to Cd and can accumulate high As(III), As(V), Cd, Cu, Hg, and Zn, but less tolerant to concentrations in the leaves. F1 plants exhibited a Mo and V (Wangeline et al., 2004). They also higher tolerance than the other transgenic lines and overexpressing cystathionine--synthase (CGS) accumulated Cd in the shoots with expression in both showed a higher Se volatilization rate, lower shoot Se the cytosol and chloroplast (Kawashima et al., 2004). levels, and higher Se tolerance than wild type. A. Transgenic Indian mustard containing the E. coli gshll bisulcatus has the capacity to accumulate Se to high gene encoding glutathione synthetase (GS) concentrations but it has a slow growth rate. It has accumulated significantly more Cd than the wild type been proposed that in selenocysteine in the shoot and the plants showed enhanced methyltransferase (SMT) specifically methylates tolerance to Cd at both the seedling and mature-plant selenocysteine (SeCys) to produce the nonprotein stages The -ECS(E. coli gshI gene encoding - amino acid methylselenocysteine MetSeCys, which glutamylcysteine synthetase) transgenics

42 Agric. Biol. J. N. Am., 2010, 1(1): 40-46

accumulated 2.4 to 3-fold more Cr, Cu, and Pb, phytoextraction capacities almost equal to Thlaspi relative to the wild type. caerulescens (Kubota and Takenaka, 2003). The Sedum alfredii plants of mined ecotype have a Metallothioneins (MTs) are cysteine-rich, low greater ability to tolerate, transport, and accumulate molecular mass proteins synthesized on ribosomes Cd (Xiong et al., 2004). according to the mRNA information. Four categories of these proteins, class-I MTs from mammalian Phytoremediation of Mercury cells,class II fromYeast MTs, occur in plants, which Pistia stratiotes exhibited different patterns of are encoded by at least seven genes in A. thaliana response to Ag, Cd, Cr, Cu, Hg, Ni, Pb and Zn, and (Cobbett and Goldsbrough, 2002). When MT gene of although concentrations as high as 5 mM, all the Pisum sativum (PsMTA) was expressed in A. thaliana, elements accumulated at high concentrations mainly more Cu (several-fold) accumulated in the roots of in the root system. This plant species exhibited the transformed than of control plants (Evans et al., highest tolerance index to Zn and the lowest to Hg. 1992). The A. thaliana metallothionein proteins Spartina plants have been shown to be 3-fold more AtMT2a and AtMT3 were introduced as fluorescent tolerant to Hg than tobacco plants, due to an ability to protein-fused forms into the guard cells of Vicia faba. absorb organic Hg and transform it into an inorganic The MTs protected guard cell chloroplasts from form (Hg+, Hg2+). The inorganic Hg then accumulates degradation upon exposure to Cd, by reducing the in the underground parts of the plants and is presence of reactive oxygen species. It was transferred back to the soil by diffusion and concluded that the Cd stays bound to the MT in the permeation, indicating that this species may be used cytoplasm and is not sequestered into the vacuole, as in the phytoremediation of an Hg polluted environment occurs when Cd is detoxified by PCs. (Tian et al., 2004). The water Fern Azolla caroliniana Transporters are required for exclusion of a toxic Willd (Azollaceae) could be serve as metal ion, transporting the metal into the apoplastic hyperaccumulator thus can purify waters polluted by space and transporting the metal into the vacuole Hg and Cr (Bennicelli et al., 2004). where it would be less likely to exert a toxic effect Phytoremediation of Lead: (Tong et al., 2004). When overexpressing lines, Helianthus annuus accumulate Pb in the leaf and exposed to lethal concentrations of Zn or Cd, stem so it could be used in the restoration of translocated these metals at a greater extent to the abandoned mines and factories sites contaminated shoot, in contrast, the metal level was found to be with elevated Pb levels in the soil (Boonyapookana et rather similar in roots, indicating that the metal uptake al., 2005). Hemidesmus indicus has also been by the roots compensated for the increased metal shown to be a Pb hyperaccumulating plant species, translocation to the shoot (Verret et al., 2004). The but the heavy metal was mainly accumulated in roots vacuole is considered to be the main storage site for and shoots (Chandra et al., 2005). The Sesbania metals in yeast and plant cells thus phytochelatin– drummondii transform lead nitrate in the nutrient metal complexes are pumped into the vacuole. YCF1 solution to lead acetate and sulfate in its tissues and from Saccharomyces cerevisiae is the best known accumulate in roots and leaves (Sharma et al., 2004). vacuolar transporters. It is a Mg ATP-energized glutathione S-conjugate transporter (Song et al., Phytoremediation of Arsenic 2003). Other transporter proteins that could be of Lemna gibba can be used for phytoremediation of value include:- the A. thaliana antiporter CAX2 As in mine tailing waters because of its high (Hirschi et al., 2000), LCT1, a nonspecific transporter accumulation capacity for AsThus it is used as for Ca2+, Cd2+, Na+ and K+ (Antosiewicz and Hennig, bioindicator of As.(Mkandawire and Dudel, 2005). 2004), the Thlaspi caerulescens heavy metal ATPase, Pteris vittata can moderately phytoremediate As TcHMA4 (Papoyan and Kochian, 2004), a novel family (Caille et al., 2004). In addition to P. vittata, P. of cysteine rich membrane proteins that mediate Cd cretica, P. longifolia and P. umbrosa are also able resistance in A. thaliana (Song et al., 2004) and to hyperaccumulate As to a similar extent (Zhao et al., AtMRP3, an ABC transporter (Bovet et al., 2005). 2002). Solanum nigrum and Conyza canadensis have not Phytoremediation of Nickel only been shown to accumulate high concentration of Alyssum and Thlaspi of Brassicaceae and Cd, but also to be tolerant to the combined action of Phyllanthus, Leucocroton of Euphorbiaceae are Cd, Pb, Cu and Zn (Wei et al., 2004). Arabis used for phytomining of Ni. Alyssum is the main Ni gemmifera is a hyperaccumulator of Cd and Zn, with hyperaccumulator, it stored Ni either in the leaf

43 Agric. Biol. J. N. Am., 2010, 1(1): 40-46

epidermal cell vacuoles as a red-stained nickel- disadvantages and limitations that must be carefully dimethylglyoxime complex, or in the basal portions of considered. the numerous stellate trichomes. It accumulates up to CONCLUSION 10,000 ppm of Ni in the leaves (Broadhurst et al., Metabolomics can provide biochemical and 2004). One of the most persuasive ecological physiological knowledge about plants subject to toxic explanations for hyperaccumulation of Ni and other metal stress, providing a much more details toxic metals appears to be the defensive role against understanding of the molecular basis of herbivores or pathogens (Dudley, 1986; Boyd and hyperaccumulation. Many signalling pathways and Martens, 1994). proteins can contribute to the cellular stress response. Analysis of "omics" technologies could further reveal Phytoremediation of Aluminium Al the non-targeted identification of all gene products in a hyperaccumulators can uptake the metal in their specific biological sample, which could be followed by aboveground tissues in quantities above 1000 ppm a refined analysis of quantitative dynamics in 0.1 % dry weight.These hyperaccumulators are biological systems. The genomics can accelerate the particularly common in basal branches of fairly discovery of genes that confer key traits, allowing their advanced groups such as rosids (Myrtales, modification. In addition The development of DNA and Malpighiales, Oxalidales) and asterids (Cornales, RNA microarray chip technologies in systematic Ericales, Gentianales, Aquifoliales) and in 27 other genome mapping, sequencing, functioning and families (Jansen et al., 2002). experimentation may allow the identification and Phytoremediation of Manganese genotyping of mutations and polymorphisms, allowing Austromyrtus bidwillii (Myrtaceae) has been better insight into structure-function interaction of identified as hyperaccumulator of Mn (Bidwell et al., genome complexity under toxic metal stress. Other 2002). Phytolacca acinosa Roxb. (Phytolaccaceae) methods including Mitogen-Activated Protein Kinase is a Mn hyperaccumulator species which grows (MAPK) pathways are activated in response to metal rapidly, has substantial biomass, wide distribution and stress, insertion mutagenesis involving populations of a broad ecological amplitude. (Xue et al., 2004). In the T-DNA, can be used to identify genes involved in case of Mn and Se, Stanleya pinnata is a potentially hyper-accumulation,identifying plant genes encoding useful species for phytoremediation due to its broad metal ion transporters with important functions in adaptation to semi-arid environments, and its uptake, cation transport and homeostasis (Papoyan and metabolism and volatilization of Se (Parker et al., Kochian, 2004; Weber et al., 2004). Molecular 2003). techniques, bioinformatics and computational techniques are effective modern tools for detailed Phytoremediation: limitations and safety measure structure-function genome analysis. Possible risks during the use of transgenic plants as Phytoremediation technology is still in its early phytoremediators should be considered, including the development stages and full scale applications are still uncontrolled spread of the transgenic plants due to limited. For widespread future use of this technique, it interbreeding with populations of wild relatives. is important that public awareness about this Exposure of wildlife to metals, could increase as technology is considered and clear and precise accumulated metal in plant shoots that can be information is made available to the general public to ingested by the wildlife. These hazards can be enhance its acceptability as a global sustainable reduced by limiting the growth period of transgenic technology to be widely used. plants. It is also necessary to restrict the toxic metal from the economically harvested plant part, the seeds. Declaration For full utilization of plant their continuous use in "We used Gratão et al. (2005) as a key reference contaminated soils is necessary, like flowers of paper on the subject and also when deciding the Elsholtzia argyi are used as fragrances and general structure of this paper." antiseptics due to the perfume ingredient and antibacterial components existing in their essential oils, and can be use as hyperaccumulator in Pb/Zn REFERENCES mined area, where they normally occur, revealed that Antosiewicz D.M. and Hennig J. (2004). Overexpression of they can be safely exploited for phytoremediation LCT1 in tobacco enhances the protective action of (Peng and Yang, 2005).Despite of being a useful calcium against cadmium toxicity. Environ. Pollut. 129:237-245 process phytoremediation also has some

44 Agric. Biol. J. N. Am., 2010, 1(1): 40-46

Baker A.J.M. and Brooks R.R. (1989). Terrestrial higher metallothionein-like gene PsMTA in Escherichia coli plants which hyperaccumulate metallic elements– a and Arabidopsis thaliana and analysis of trace metal ion review of their distribution, ecology and phytochemistry. accumulation: implications for PsMTA function. Plant Biorecovery. 1:81-126 Mol. Biol. 20:1019-1028 Bennicelli R., Stepniewska Z., Banach A., Szajnocha K. and Gratao P. L., Prasad M. N. V., Cardoso P. F., Lea P. J., Ostrowski J. (2004). The ability of Azolla caroliniana to Azevedo R. A. (2005) Phytoremediation: green remove heavy metals (Hg(II), Cr(III), Cr(VI)) from technology for the clean up of toxic metals in the municipal waste water. Chemosphere. 55:141-146 environment. Braz. J. Plant Physiol. 17: 53-64. Bidwell S.D., Woodrow I.E., Batianoff G.N. and Sommer- Harada E., Choi Y.E., Tsuchisaka A., Obata H. and Sano H. Knudsen J. (2002). Hyperaccumulation of manganese (2001). Transgenic tobacco plants expressing a rice in the rainforest tree Austromyrtus bidwillii (Myrtaceae) cysteine synthase gene are tolerant to toxic levels of from Queensland, Australia. Funct. Plant Biol. 29:899- cadmium. J. Plant Physiol. 158:655-661 905 Hirschi E.D., Korenkey, V.D., Wilganewski N.I. and Wagner Boonyapookana B., Parkplan P., Techapinyawat S., G.I. (2000). Expression of Arabidopsis CAX2 in DeLaune R.D. and Jugsujinda A. (2005). tobacco. Altered metal accumulation and increased Phytoaccumulation of lead by sunflower (Helianthus manganese tolerance. Plant Physiol. 124:128-133 annuus), tobacco (Nicotiana tabacum), and vetiver (Vetiveria zizanioides). J. Environ. Sci. Heal. 40:117- Jansen S., Broadley M.R., Robbrecht E. and Smets E. 137 (2002). Aluminum hyperaccumulation in angiosperms: A review of its phylogenetic significance. Bot. Rev. Bovet L., Feller U. and Martinoia E. (2005). Possible 68:235-269 involvement of plant ABC transporters in cadmium detoxification: a cDNA sub-microarray approach. Kamnev A.A. and van der Lelie D. (2000). Chemical and Environ Intl. 31:263-267 biological parameters as tools to evaluate and improve heavy metal phytoremediation. Bioscience Rep. 20:239- Boyd R.S., Shaw J.J. and Martens S.N. (1994). Nickel 258 hyperaccumulation in S.polygaloids (Brassicaceae) as a defense against pathogens. Am. J. Bot. 81:294-300 Karenlampi S., Schat H., Vangronsveld J. and Verkleij J.A.C. (2000). Genetic engineering in the improvement Broadhurst C.L., Chaney R.L., Angle J.S., Maugel T.K., of plants for phytoremediation of metal polluted soils. Erbe E.F. and Murphy C.A. (2004). Simultaneous Environ. Pollut. 107:225-231 hyperaccumulation of nickel, manganese, and calcium in Alyssum leaf trichomes. Environ. Sci. Kawashima C.G., Noji M., Nakamura M., Ogra Y., Suzuki Technol.38:5797-5802 K.T. and Saito K. (2004). Heavy metal tolerance of transgenic tobacco plants over-expressing cysteine Caille N., Swanwick S., Zhao F.J. and McGrath S.P. (2004). synthase. Biotechnol. Lett. 26:153-157 Arsenic hyperaccumulation by Pteris vittata from arsenic contaminated soils and the effect of liming and Kubota H. and Takenaka C. (2003). Arabis gemmifera is a phosphate fertilisation. Environ. Pollut. 132:113-120 hyperaccumulator of Cd and Zn. Intl. J. Phytoremediation. 5:197-120 Chandra S.K., Kamala C.T., Chary N.S., Balaram V. and Garcia G. (2005). Potential of Hemidesmus indicus for Lee J., Shim D., Song W.Y., Hwang I., Lee Y. (2004). phytoextraction of lead from industrially contaminated Arabidopsis metallothioneins 2a and 3 enhance soils. Chemosphere. 58:507-514 resistance to cadmium when expressed in Vicia faba guard cells. Plant Mol. Biol. 54:805-815 Cobbett C. and Goldsbrough P. (2002). Phytochelatins and metallothioneins: Roles in heavy metal detoxification LeDuc D.L., Tarun A.S., Montes-Bayon M., Meija J., Malit and homeostasis. Ann. Rev. Plant Biol. 53:159-182 M.F., Wu C.P., AbdelSamie M., Chiang C.Y., Tagmount A., DeSouza M., Neuhierl B., Bock A., Caruso J. and Dominguez-Solis J.R., Lopez-Martin M.C., Ager F.J., Ynsa Terry N (2004). Overexpression of selenocysteine M.D., Romero L.C. and Gotor C. (2004). Increased methyltransferase in Arabidopsis and Indian mustard cysteine availability is essential for cadmium tolerance increases selenium tolerance and accumulation. Plant and accumulation in Arabidopsis thaliana. Plant Physiol. 135:377-383 Biotechnol. J. 2:469-476 Madaan N. (2008). Ph.D. Thesis, “Analysis of genetic Dudley T.R. (1986). A new nickelophilous species of variability and inheritance for some heavy metal Alyssum (Cruciferae) from Portugal, Alyssum tolerance in oil crop Carthamus tinctorius L.” CCS pintodasilvae. Fedd. Repertorium 97:135–138 University, India Evans K.M., Gatehouse J.A., Lindsay W.P., Shi J., Tommey Mkandawire M. and Dudel E.G. (2005). Accumulation of A.M., Robinson N.J. (1992). Expression of the pea arsenic in Lemna gibba L. (duckweed) in tailing waters

45 Agric. Biol. J. N. Am., 2010, 1(1): 40-46

of two abandoned uranium mining sites in Saxony, Tian J.L., Zhu H.T., Yang Y.A. and He Y.K. (2004). Organic Germany. Sci. Total Environ. 336:81-89 mercury tolerance, absorption and transformation in Spartina plants. Zhi Wu Sheng Li Yu Fen Zi Sheng Wu Papoyan A. and Kochian L.V. (2004). Identification of Xue Xue Bao. 30:577-582 Thlaspi caerulescens genes that may be involved in heavy metal hyperaccumulation and tolerance. Tong Y.P., Kneer R. and Zhu Y.G. (2004). Vacuolar Characterization of a novel heavy metal transporting compartmentalization: a second-generation approach to ATPase. Plant Physiol. 136:3814-3823 engineering plants for phytoremediation. Trends Plant Sci. 9:7-9 Parker D.R., Feist L.J. Varvel T.W., Thomason D.N. and Zhang Y.Q. (2003). Selenium phytoremediation Verret F., Gravot A., Auroy P., Leonhardt N., David P., potential of Stanleya pinnata. Plant Soil 249:157-165 Nussaume L., Vavasseur A. and Richaud P. (2004). Overexpression of AtHMA4 enhances root-to-shoot Peng H.Y. and Yang X.E. (2005). Volatile constituents in the translocation of zinc and cadmium and plant metal flowers of Elsholtzia argyi and their variation: a possible tolerance. FEBS Lett. 576:306-312 utilization of plant resources after phytoremediation. J Zhejiang Univ. Sci. 6:91-95 Wangeline A.L., Burkhead J.L., Hale K.L., Lindblom S.D., Terry N., Pilon M. and Pilon-Smits E.A.H. (2004). Pilon-Smits E., Pilon M. (2002). Phytoremediation of metals Overexpression of ATP sulfurylase in Indian mustard: using transgenic plants. Crit. Rev. Plant Sci. 21:439-456 Effects on tolerance and accumulation of twelve metals. Pilon-Smits EAH, Hwang S, Lytle CM, Zhu Y, Tai JC, Bravo J. Env. Qual. 33:54-60 RC, Chen Y, Leustek T, Terry N (1999) Overexpression Weber M., Harada E., Vess C., von Roepenack-Lahaye E. of ATP sulfurylase in indian mustard leads to increased and Clemens S. (2004). Comparative microarray selenate uptake, reduction and tolerance. Plant Physiol. analysis of Arabidopsis thaliana and Arabidopsis halleri 119:123-132. roots identifies nicotianamine synthase, a ZIP Pollard A.J., Powell K.D., Harper F.A. and Smith J.A.C. transporter and other genes as potential metal (2002). The genetic basis of metal hyperaccumulation hyperaccumulation factors. Plant J. 37:269-281 in plants. Crit. Rev. Plant Sci. 21:539-566 Wei S.H., Zhou Q.X., Wang X., Cao W., Ren L.P. and Song Prasad M.N.V. and Freitas H. (2003) Metal Y.F. (2004). Potential of weed species applied to hyperaccumulation in plants - Biodiversity prospecting remediation of soils contaminated with heavy metals. J. for phytoremediation technology. Electronic Environ. Sci. (China) 16:868-873 J.Biotechnol. 6:275-321 Xiong Y.H., Yang X.E., Ye Z.Q. and He Z.L. (2004). Reeves R.D. and Baker A.J.M. (2000). Metal-accumulating Characteristics of cadmium uptake and accumulation by plants. In: Raskin I, Ensley BD (eds), Phytoremediation two contrasting ecotypes of Sedum alfredii Hance. J of Toxic Metals, pp. 193-229. Jonh Wiley, NY, USA Environ. Sci. Health A Tox. Hazard Subst. Environ. Eng. 39:2925-2940 Sharma N.C., Gardea-Torresdey J.L., Parsons J. and Sahi S.V. (2004). Chemical speciation and cellular deposition Xue S.G., Chen Y.X., Reeves R.D., Baker A.J. and Lin Q., of lead in Sesbania drummondii. Environ. Toxicol. Fernando D.R. (2004). Manganese uptake and Chem. 23:2068-2073 accumulation by the hyperaccumulator plant Phytolacca acinosa Roxb. (Phytolaccaceae). Environ. Pollut. Song W.Y., Sohn E.J., Martinoia E., Lee Y.J., Yang Y.Y., 131:393-399 Jasinski M., Forestier C., Hwang I. and Lee Y. (2003). Engineering tolerance and accumulation of lead and Zhao F.J., Dunham S.J. and McGrath S.P. (2002). Arsenic cadmium in transgenic plants. Nature Biotech. 21:914- hyperaccumulation by different fern species. New 919 Phytol. 15:27-31