Chapter 5
Strategies for the Engineered Phytoremediation of Mercury and Arsenic Pollution Downloaded by UNIV OF GUELPH LIBRARY on September 2, 2012 | http://pubs.acs.org Publication Date: April 19, 2005 | doi: 10.1021/bk-2005-0904.ch005
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Om Parkash Dhankher , Andrew C. P. Heaton , Yujing Li , and Richard B. Meagher 2
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Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, MA 01003 Department of Genetics, University of Georgia, Athens, GA 30602
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We have developed genetics-based phytoremediation strategies for mercury and arsenic pollution. Plants engineered to use bacterial, animal, and plant genes take up and tolerate several times the levels of mercury and arsenic that would kill most plant species. Modified plants expressing the bacterial merB gene breakdown the most toxic and biomagnified methylmercury (MeHg) to ionic mercury (Hg(II)) and those expressing the merA gene detoxify ionic mercury to metallic mercury (Hg(0)). Using these and some other genes, mercury is stored below or above ground, or even volatilized as part of the transpiration process, keeping it out of the food chain. These remediation strategies also work in cultivated and wild plant species including canola, rice, yellow poplar, and cottonwood. For arsenic, we have developed a strategy in which the oxyanion arsenate is transported aboveground, electrochemically reduced to arsenite in leaves, and sequestered in thiol-rich peptide complexes. Arabidopsis plants expressing bacterial ArsC and γ-ECS, showed substantially greater resistance and accumulated more arsenic in shoot tissues than wild type plants. These arsenic and mercury phytoremediation strategies should be applicable to a wide variety of plant species.
© 2005 American Chemical Society In Subsurface Contamination Remediation; Berkey, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Goals and Working Hypothesis We are focused on developing environmentally friendly solutions to cleaning heavy metal- and metalloid-contaminated sediments and water. Our long-term goal is to develop and test highly productive, field-adapted plant species that have been engineered for the phytoremediation of mercury. Our current working hypothesis is that transgenic plants controlling the chemical species, electrochemical state, and aboveground binding of mercury and arsenic will (a) prevent elemental toxins from entering the food-chain, (b) remove mercury and arsenic from polluted sites, and (c) hyperaccumulate these toxic metals in aboveground tissues for later harvest. The strategies suggested by this hypothesis are outlined in Figure 1 as follows. (1) Uptake of mercury and arsenic will be enhanced by overexpression of transporters that recognize mercury and arsenate in roots. (2) Translocation of the elemental toxicant from root to shoot will be enhanced. In the case of arsenate, the endogenous reduction of arsenate to arsenite will be suppressed in roots, because arsenate (As0 ) will be more readily transported as a phosphate ( P 0 ) analogue. In the case of mercury, Hg(II) would have to be converted to soluble nonreactive Hg(0), and Hg(0) should be most readily transported to the top of the plant. (3) The levels of toxic element sinks for Hg(II) and arsenite will be increased; for example, this could be accomplished by increasing the levels of γ-EC (γ-glutamylcysteine) through overexpression of γ-ECS (γ-glutamylcysteine synthetase). (4) Hg(0) and arsenate will be electrochemically reduced back to the reactive Hg(II) and arsenite (As0 " ) species, respectively, in leaves for storage. (5) The transport of peptide metal/metalloid complexes into the vacuole for storage will be enhanced by overexpressing multiple drug resistance protein (MRP)-related glutathione conjugate pumps in leaves. Various parts of this hypothesis are being tested by examining different transgenes in model plants and comparing the results to control plants. 3
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Introduction Mercury and arsenic are extremely toxic heavy metal and metalloid pollutants that adversely affect the health of millions of people worldwide (i). These toxic pollutants have reached unacceptably high levels in the environment due to industrial, defense, agricultural, and municipal properties. The U.S. Department of Energy (DOE) and other government and industrial sites in the United States are heavily contaminated with mercury, arsenic, and other toxic metals such as cadmium, copper, lead, and zinc. Each of these elemental pollutants has common environmentally relevant electrochemical species that are thiol-reactive and thus relevant to the phytoremediation strategies outlined in Figure 1. Hundreds of Superfiind sites in the United States are listed on the
In Subsurface Contamination Remediation; Berkey, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Figure 1. Strategy for engineering the phytoremediation of mercury and arsenic. Expanded diagrams ofplant cells with critical activities are designated by large gray boxes. The element being taken up and concentrated is shown as black dots.
National Priority List (http://www.epa.gov/superiund/sites/npl/index.htm; accessed date July 9, 2004) as having unacceptably high levels of mercury and/or arsenic and are recommended for cleanup. In the majority of cases, these sites are not remediated because of the high costs associated with physical methods of cleanup. Physical remediation methods involving soil removal and reburial are expensive, impractical on the scale that is needed, and environmentally destructive. In contrast, phytoremediation, using plants to extract, detoxify, and sequester pollutants (2, J), is an eco-fhendly alternative or amendment to physical methods. Plants have several natural properties ideal for use in remediation (2-4). For example, their vast root systems are in intimate pervasive contact with the soil (5); they have an excess of reducing power from photosystem I; they can use solar energy to catalyze the remediation process; and they control more than 80% of the energy in most ecosystems (2, 3).
In Subsurface Contamination Remediation; Berkey, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Phytoremediation of Elemental vs Organic Pollutants Strategies for phytoremediation need to distinguish between the remediation of organic pollutants and the remediation of elemental pollutants (3). Organic pollutants include toxic chemicals such as benzene, benzo(a)pyrene, polychlorinated biphenyls, trichloroethylene (TCE), trinitrotoluene (TNT), and dichlorodiphenyltrichloroethane (DDT). Phytoremediation of organic pollutants is focused on their complete mineralization to harmless products and hence has few if any drawbacks. Native plants have some exceptional natural abilities to degrade organics, and there have been several reports of plants engineered for phytoremediation of organics dealing principally with the chlorinated solvent TCE (6) and the explosive TNT (7,
