American Society of Agronomy Crop Science Society of America Soil Science Society of America

zation by plant roots (Berti and Cunningham, 2000). In the technology called rhizofiltration, accumulation of Indian mustard [Brassica juncea (L.) Czern.] transgenics overexmetals by roots in a hydroponic setup is followed by pressing ATP sulfurylase (APS plants) were shown previously to have higher levels of total thiols, S, and Se. The present study explores the harvesting of the plant biomass (Dushenkov and Kapuleffect of ATP sulfurylase overexpression on tolerance and accumulanik, 2000). Phytoextraction, another promising technoltion of other metals, both oxyanions and cations, reasoning that some ogy in metal phytoremediation, involves the accumulaanions may react directly with ATP sulfurylase, while other ions tion of metals in shoot tissue followed by harvesting of may be bound by its thiol end products. The APS transgenics were the shoot biomass (Blaylock and Huang, 2000). The compared with wild-type plants with respect to tolerance and accumumetal-laden plant material may be used for nonfood lation of As, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, V, W, and Zn, supplied purposes or ashed, followed by recycling of the metals individually in agar medium (seedlings) or in hydroponics (mature or disposal in a landfill (Chaney et al., 2000). These plants). At the seedling stage, APS transgenics were more tolerant metal phytoremediation technologies are already being than wild type to As(III), As(V), Cd, Cu, Hg, and Zn, but less tolerant used effectively (Salt et al., 1998; Blaylock, 2000), and to Mo and V. The APS seedlings had up to 2.5-fold higher shoot concentrations of As(III), As(V), Hg, Mo, Pb, and V, and somewhat are gaining acceptance, since phytoremediation is relalower Cr levels. Mature APS plants contained up to 2.5-fold higher tively cost-effective and aesthetically pleasing, and may shoot concentrations of Cd, Cr, Cu, Mo, V, and W than wild type. They be used in conjunction with more conventional remediaalso contained 1.5to 2-fold higher levels of the essential elements Fe, tion methods. Mo, and S in most of the treatments. Mature APS plants showed no To further increase the effectiveness of metal phytordifferences in metal tolerance compared with the wild type. Overexemediation, several approaches may be employed, inpression of ATP sulfurylase may be a promising approach to create cluding identification of new suitable plant species via plants with enhanced phytoextraction capacity for mixtures of metals. screening studies, optimization of agronomic practices for maximal element uptake, and improvement of selected plant species by classical breeding or genetic engiT metals and metalloids are increasingly reneering. leased into the environment by human activities Genetic engineering is starting to emerge as a relasuch as industry, mining operations, use of ammunition, tively rapid and effective way to improve the capacity traffic, and agriculture, resulting in contamination that of plants to tolerate and accumulate metals. Transgenic threatens natural ecosystems and human well-being plants with enhanced metal tolerance and accumulation (Lantzy and Mackenzie, 1979; Nriagu, 1979; Ross, 1994). have been created through several approaches, includCurrently, the USA is spending around $3 billion a year ing overexpression of metal transporter proteins (Samon remediation of toxic inorganic elements, comprising uelsen et al., 1998; Arazi et al., 1999; Van der Zaal et 35% of the total U.S. funds spent for environmental al., 1999; Curie et al., 2000; Hirschi et al., 2000), overprocleanup (Glass, 1999, 2000). Some examples of convenduction of metal-chelating molecules (Evans et al., 1992; tional remediation methods for metals include soil washde la Fuente et al., 1997; Hasegawa et al., 1997; Goto ing, excavation and reburial of the soil, and soil stabilizaet al., 1999; Zhu et al., 1999a, 1999b), or even introduction by, for example, concrete capping. tion of a bacterial pathway (Rugh et al., 1996; Bizily et Alternative phytoremediation methods for metals al., 1999, 2000). For a review of the development of and metalloids make use of the natural ability of plants transgenics for metal phytoremediation, see Krämer and to acquire minerals from their environment and to stabiChardonnens (2001) and Pilon-Smits and Pilon (2002). lize soil and create an upward water flow in the process. Phytochelatins are cysteine-rich metal-chelating pepPhytostabilization may involve the prevention of leachtides involved in heavy metal tolerance and sequestraing through an upward water flow resulting from plant tion (Steffens, 1990). The general structure of phytotranspiration, reduced runoff owing to aboveground chelatins is ( -Glu-Cys)n–Gly, where n 2 to 11 vegetation, and reduced soil erosion through soil stabili(Rauser, 1995). The cysteine needed for the biosynthesis of phytochelatins is produced by the sulfate assimilation A.L. Wangeline, J.L. Burkhead, K.L. Hale, S.D. Lindblom, M. Pilon, pathway, which is located primarily in the chloroplast and E.A.H. Pilon-Smits, Biology Department, Colorado State Univerin plants (Schwenn, 1994; Leustek, 1996; Leustek and sity, A/Z Building, Fort Collins, CO 80523. N. Terry, Department Saito, 1999). After uptake by sulfate permease, sulfate of Plant and Microbial Biology, 111 Koshland Hall, University of is activated by ATP sulfurylase to form adenosine phosCalifornia, Berkeley, CA 94270. Received 14 Oct. 2002. *Corresponding author ([email protected]). phosulfate, which is subsequently reduced to free sulfite Published in J. Environ. Qual. 33: – (2004).  ASA, CSSA, SSSA Abbreviations: APS, plants overexpressing ATP sulfurylase; WT, wild type. 677 S. Segoe Rd., Madison, WI 53711 USA