| In a 6-year field study, transgenic poplar (Populas tremula x Populus alba) genetically modified with mammalian cytochrome P450 2E1 (CYP2E1) was evaluated for its ability to enhance degradation of trichloroethylene (TCE) in the subsurface. In previous laboratory studies, the transgenic poplar demonstrated greatly increased metabolism towards a variety of organic contaminants. Degradation of TCE in the field was studied in three test beds: a test bed containing twelve CYP2E1 hybrid poplar trees, a second test bed containing twelve wild-type hybrid poplar trees, and a third unplanted control bed. A mass-balance was performed to determine the fate of TCE and quantify primary loss pathways. Quantitative, real-time PCR (qPCR) assays targeting microbial genes involved in TCE degradation and field soil microcosm studies characterized microbial activity in bed soil. The transgenic CYP2E1 poplars demonstrated enhanced degradation of TCE in the field though not in proportion to enhanced removal observed in laboratory studies. Total chlorinated ethene removal was 87% in the CYP2E1 test bed, 85% in the wild-type, and 34% in the unplanted control bed in the 2012 growing season. Evapotranspiration of TCE from transgenic leaves was reduced by 80% and diffusion of TCE from transgenic stem was reduced by 90%. Chloride ion accumulated in the vadose zone soil of the planted test beds that approximately corresponded to the TCE loss, suggesting that dehalogenation was the primary loss fate. The application of a steady-state plant model indicated that the enhanced rate of metabolism in r2E1 roots was insufficient to substantially increase uptake of TCE in a field setting. This study demonstrates the importance of field tests of transgenic plants for phytoremediation applications. Inherent differences in mass transfer processes between laboratory and field experiments can limit the effectiveness of enhanced in planta metabolism. |
