A major goal of phytoremediation is to transform fast-growing plants with genes from plant species that hyperaccumulate toxic trace elements. We overexpressed the gene encoding selenocysteine methyltransferase (SMT) from the selenium (Se) hyperaccumulator Astragalus bisulcatus in Arabidopsis and Indian mustard (Brassica juncea). SMT detoxifies selenocysteine by methylating it to methylselenocysteine, a nonprotein amino acid, thereby diminishing the toxic misincorporation of Se into protein. Our Indian mustard transgenic plants accumulated more Se in the form of methylselenocysteine than the wild type. SMT transgenic seedlings tolerated Se, particularly selenite, significantly better than the wild type, producing 3-to 7-fold greater biomass and 3-fold longer root lengths. Moreover, SMT plants had significantly increased Se accumulation and volatilization. This is the first study, to our knowledge, in which a fast-growing plant was genetically engineered to overexpress a gene from a hyperaccumulator in order to increase phytoremediation potential.Although selenium (Se) is a necessary micronutrient for humans and animals at very low doses, it is extremely toxic at higher doses (Wilber, 1983). Excess Se has been implicated in birth defects, sterility, and disease in animals, fish, and wildlife and loss of hair, teeth, and nails, fatigue, and even death in humans (Moxon, 1937;Eisler, 1985;Lemly and Smith, 1987;Sorenson, 1991). Environmental Se pollution is a worldwide problem. Anthropogenic Se pollution arises from many sources, such as aqueous discharges from electric power plants, coal ash leachates, refinery effluents, and industrial wastewater (American Medical Association, 1989). Selenium also occurs naturally in soils formed from Se-bearing shales. This leads to Se-contaminated irrigation drainage water, one of the most serious agricultural problems in the western United States and other areas with similar environments and geological conditions (Presser and Ohlendorf, 1987).Cleaning up Se-contaminated soil and water is a major concern. Phytoremediation, using plants to remove, stabilize, or detoxify pollutants, is a promising technology for remediating Se-contaminated soil and water (Terry et al., 2000). In a process called phytoextraction, plants extract Se from soils and water into their tissues, which can be harvested and removed. Selenium is unusual among trace elements because it can also be removed from the ground ecosystem by phytovolatilization, i.e. plants metabolize inorganic Se to relatively nontoxic, volatile forms (dimethyl selenide [DMSe] and dimethyl diselenide [DMDSe]), which escape to the atmosphere (Lewis et al., 1966;Terry et al., 2000). Selenium phytoremediation has been achieved under field conditions using fast-growing plant species, such as Indian mustard (Brassica juncea; Bañ uelos et al., 1997), which accumulates Se to hundreds of parts per million (Bañ uelos and Schrale, 1989). Hyperaccumulating plant species, such as Astragalus bisulcatus, have adapted to seleniferous soils and accum...

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Three-week-old alfalfa (Medicago sativa), barley (Hordeum vulgare) and maize (Zea mays) were exposed for 7 d to 30 mM of mercury (HgCl2) to characterize the Hg speciation in root, with no symptoms of being poisoned. The largest pool (99%) was associated with the particulate fraction, whereas the soluble fraction (SF) accounted for a minor proportion (<1%). Liquid chromatography coupled with electro-spray/time of flight mass spectrometry showed that Hg was bound to an array of phytochelatins (PCs) in root SF, which was particularly varied in alfalfa (eight ligands and five stoichiometries), a species that also accumulated homophytochelatins. Spatial localization of Hg in alfalfa roots by microprobe synchrotron X-ray fluorescence spectroscopy showed that most of the Hg co-localized with sulphur in the vascular cylinder. Extended X-ray Absorption Fine Structure (EXAFS) fingerprint fitting revealed that Hg was bound in vivo to organic-S compounds, i.e. biomolecules containing cysteine. Albeit a minor proportion of total Hg, Hg-PCs complexes in the SF might be important for tolerance to Hg, as was found with Arabidopsis thaliana mutants cad2-1 (with low glutathione content) and cad1-3 (unable to synthesize PCs) in comparison with wild type plants. Interestingly, high-performance liquid chromatography-electrospray ionization-time of flight analysis showed that none of these mutants accumulated Hg-biothiol complexes.

Three transgenic Indian mustard [Brassica juncea (L.) Czern.] lines were tested under field conditions for their ability to remove selenium (Se) from Se- and boron-contaminated saline sediment. The transgenic lines overexpressed genes encoding the enzymes adenosine triphosphate sulfurylase (APS), gamma-glutamyl-cysteine synthetase (ECS), and glutathione synthetase (GS), respectively. The APS, ECS, and GS transgenic plants accumulated 4.3, 2.8, and 2.3-fold more Se in their leaves than wild type, respectively (P < 0.05). GS plants significantly tolerated the contaminated soil better than wild type, attaining an aboveground biomass/area almost 80% of that of GS plants grown on clean soil, compared to 50% for wild type plants. This is the first report showing that plants genetically engineered for phytoremediation can perform successfully under field conditions.