Bioremediation of Trichloroethylene: Analysis of the Plant Gene Response to TCE and Characterization of a Novel TCE-Degrading Endophyte

Abstract

Trichloroethylene is a common environmental contaminant. High levels of TCE have the potential to cause liver damage and malfunctions in the central nervous system, and it is considered a likely human carcinogen. More than 54% of Superfund sites in the United States are contaminated with TCE. This research explored two approaches for enhancing the phytoremediation of TCE, which includes analysis of transgenic plants and isolating and characterizing natural TCE-degrading bacteria. The hypothesis of the first study was that certain detoxification genes such as cytochrome P450s (CYP), glutathione-S-transferases (GST), glycosyltransferases (UGT), and ATP-binding cassette (ABC) transporters are involved in TCE metabolism in poplar. The study found that poplar trees do have a genetic response to TCE and that many putative detoxification genes are differentially regulated. Previous studies demonstrated that transgenic hybrid poplar plants expressing mammalian cytochrome P450 2E1 had greatly increased metabolism of TCE. In this research, the expression of poplar genes that may be involved in TCE metabolism between wild-type poplar and transgenic poplar were compared by microarray analysis. Through this analysis, many putative detoxification genes have been found and proposed to be involved in TCE metabolism. An important finding of the study was that transgenic poplar automatically upregulated many of these putative genes, reducing the need for further genetic manipulation. The second study hypothesized that endophytes in poplar can degrade TCE more effectively than those currently used in bioremediation. Plants associate with various microbes which live around the plant's root or intercellular spaces of the aerial part of the plant. The research was aimed at isolating and investigating a novel endophyte having the ability to degrade TCE. A unique endophyte, identified as Enterobacter sp. PDN3, was isolated from hybrid poplar and shown to confer high tolerance to TCE. Without the addition of inducers such as toluene or phenol, PDN3 rapidly reduced TCE levels in media. Nearly 80% of TCE (55.3 µM) was dechlorinated by PDN3 in 5 days with 166 µM chloride ion production, suggesting TCE degradation. Overall, the results of this dissertation research supported the hypotheses that specific plant genes and endophytes have strong potential for degrading pollutants.