ABSTRACT
Full scale field trials have shown that poplar trees are capable of taking up and fully degrading chlorinated hydrocarbons such as trichloroethylene (TCE), perchloroethylene (PCE), and carbon tetrachloride (CT). This capability can be applied to clean up contaminated groundwaters at DOE nuclear facilities at reduced cost, with plantations that take up groundwater either passively or through irrigation systems. In order to understand the mechanism of this unusual biological attack on fully chlorinated hydrocarbons, we have studied the biotransformation and mineralization of CT by pure cultures of poplar cells. CT metabolism was inhibited by carbon monoxide and by isonizid, a specific inhibitor of cytochrome P450 2E1 (CYP2E1). Along with CO2, chloroform was produced during the transformation of CT. Increasing amounts of chloroform were produced under anoxic conditions. These results support the hypothesis that an enzyme similar to the mammalian CYP2E1 is responsible for the degradation of chlorinated hydrocarbons in plants. To increase the degradative capability of plants we have introduced the gene for mammalian CYP2E1 into tobacco and poplar. Transformed tobacco exposed to TCE produced up to 640 times the amount of trichloro-ethanol, a TCE metabolite, compared to control plants. The level of activity was proportional to the level of CYP2E1 expression. Transformed tobacco plants were able to reduce the concentration of ethylene dibromide, another substrate of CYP2E1, in hydroponic culture, while producing stoichiometric amounts of bromide ion. The significance of these findings for applications of phytoremediation will be discussed.
