Arabidopsis and the genetic potential for the phytoremediation of toxic elemental and organic pollutants

Abstract

In a process called phytoremediation, plants can be used to extract, detoxify, and/or sequester toxic pollutants from soil, water, and air. Phytoremediation may become an essential tool in cleaning the environment and reducing human and animal exposure to potential carcinogens and other toxins. Arabidopsis has provided useful information about the genetic, physiological, and biochemical mechanisms behind phytoremediation, and it is an excellent model genetic organism to test foreign gene expression. This review focuses on Arabidopsis studies concerning: 1) the remediation of elemental pollutants; 2) the remediation of organic pollutants; and 3) the phytoremediation genome. Elemental pollutants include heavy metals and metalloids (e.g., mercury, lead, cadmium, arsenic) that are immutable. The general goal of phytoremediation is to extract, detoxify, and hyperaccumulate elemental pollutants in above-ground plant tissues for later harvest. A few dozen Arabidopsis genes and proteins that play direct roles in the remediation of elemental pollutants are discussed. Organic pollutants include toxic chemicals such as benzene, benzo(a)pyrene, polychlorinated biphenyls, trichloroethylene, trinitrotoluene, and dichlorodiphenyltrichloroethane. Phytoremediation of organic pollutants is focused on their complete mineralization to harmless products, however, less is known about the potential of plants to act on complex organic chemicals. A preliminary survey of the Arabidopsis genome suggests that as many as 700 genes encode proteins that have the capacity to act directly on environmental pollutants or could be modified to do so. The potential of the phytoremediation proteome to be used to reduce human exposure to toxic pollutants appears to be enormous and untapped.

Keywords: carcinogenic; genetics; heavy metals; hyperaccumulation; metalloids; mineralization; organics; plants; remediation; teratogenic; toxins; transformation; transport.

Figure 1.

Mechanisms and possible ligand complexes that aid in transport and sequestration of toxin pollutants.Secreted citrate can form tetrahedral metal ion complexes that block Al(III) (Rajan et al., 1981; de la Fuente et al., 1997) and possibly Ni(II) transport into roots.Histidine (His) can participate in forming tetrahedral metal ion complexes with Ni(II) that aid in uptake, transport, hyperaccumulation, and tolerance (Salt, 1999). Water forms the fourth ligand in this model.Metallothioneins are small (45–90 a.a.) peptides with at least two cysteine (S) clusters that chelate thiol-reactive metals like cadmium (Cd) and zinc (Arseniev et al., 1988; Goldsbrough, 1998). The N- and C-terminal ends of the peptide are indicated. Transport of MT-metal complexes is poorly characterized, although recent data demonstrates that MT transport is required for Zn uptake by mitochondria (Ye et al., 2001).Phytochelatins, in this case a trimeric PC3 is shown, form complexes with thiol-reactive metals like cadmium (Cd(II)) enhancing tolerance. These structures aid in the transport into and sequestration of metals in vacuoles via the glutathione S-conjugate pump (GCP). With other metals such as Cu(II) and Zn(II), the a-carboxyl groups of PCs may participate in forming very different metal-ligand structures than the one shown.

Figure 2.

Enzymatic reactions essential to chelation of or the chemical and electrochemical transformation of toxic elemental and organic pollutants.Phytochelatins (PCs) are peptides that are synthesized in three enzymatic steps from amino acids. Gamma glutamyl cysteine synthetase (GCS) catalyzes the condensation of glutamate and cysteine (Cobbett, 2000). The resulting gGlu-Cys (GC) contains an unconventional peptide bond between the g-carboxyl group of Glu and the a-amino group of Cys. Glutathione synthetase (GS) catalyzes the synthesis of glutathione GSH) from GC and Gly. Phytochelatin synthetase (PCS) catalyzes the synthesis of PCs through the transpeptidation of GC from one molecule of GSH to a second or to a previously formed PC oligomer. Transgenic over-expression of the latter two enzymes increased heavy metal resistance in plants and yeast, respectively.Methylmercury (CH3-Hg+) is not only the most toxic natural form of mercury, but in addition methylmercury is biomagnified by orders of magnitude in long aquatic food chains. Mercury is less toxic as ionic mercury (Hg(II)) and least toxic as reduced and volatile metallic mercury (Hg(0)). Hg(0) becomes toxic after reoxidation to Hg(II) (gray arrow). The bacterial enzymes MerB and MerA catalyze the detoxification of methyl and ionic mercury, respectively. This pathway has been engineered to work efficiently in plants (Rugh et al., 1998a; Meagher et al., 2000).Selenium is most toxic when incorporated into amino acid analogues and least toxic as volatile dimethylselenide. Endogenous enzymes carry out these various reactions to different extents in distinct plant species. The reduced intermediate selenite (SeO3=) is more efficiently metabolized to organic forms than selenate (SeO4=) an hence the most toxic.Toxic ferric iron is reduced to ferrous iron via the ferric chelate reductase (FRO2). Fe(II) is readily taken up by plants (Robinson et al., 1999).Chlorinated solvents like trichloroethylene (TCE) can be mineralized to harmless products by endogenous plant enzymes found at high levels in a small percentage of plant species. One important step in the catabolism of TCE is the P-450 catalyzed hydroxylation to chloral (Doty et al., 2000).Dehalogenases (hydrolases, hydratases) catalyze the removal of chloride from TCE to make dichloroacetate, and these activities like many other P-450s appear to be peroxisomal (Everhart et al., 1998; Zhou and Waxman, 1998).Nitroaromatics like trinitrotoluene (TNT) are efficiently broken down and can be mineralized to harmless products in a few plant species. These reactions have been enhanced by transgenic bacterial gene expression. Multiple rounds of reduction leads to an intermediate in the catabolic breakdown of TNT, triaminotoluene (TAT) (Spain, 1995).The first step in benzo(a)pyrene breakdown is often a P-450 catalyzed hyroxylation such as that shown (Shimada and Guengerich, 1990).Chlorinated pesticides like DDT (dichlorodiphenyltrichloroethane) are resistant to degradation, and although only moderately toxic they are particularly harmful, because they are biomagnified in the food chain. Dehalogenases like those that degrade DTT to DDDE (dichlorodiphenyldichloroethane) participate in the mineralization of many chlorinated pesticides (Thomas et al., 1996).Dioxins like TCDD (2,3,7,8-tetrachlorodibenzop-dioxin) are highly toxic and are long lived in the environment. Dioxygenases are thought to play an essential role in dioxin degradation