Environmental Factor, November 2007, National Institute of Environmental Health Sciences
Superfund Grantees Engineer Plants to Clean Environmental Pollutants
By Maureen Avakian
In an NIEHS-funded study published in the October 23, 2007 Proceedings of the National Academy of Sciences , University of Washington researchers Stuart Strand, Ph.D., and Sharon Doty, Ph.D., report on their ground-breaking work to transform hybrid poplars with a vector containing rabbit cytochrome P450 2E1. The resulting transgenic plants overexpressed cytochrome P450 2E1, a key enzyme in the metabolism of a variety of halogenated compounds. This modification significantly enhanced the plants' ability to capture and break down volatile environmental pollutants through a process known as phytoremediation.
The study grew out of burgeoning interest among environmental scientists in the use of trees to clean up contaminants found at hazardous waste sites. As a result of natural processes fueled by solar energy, trees offer great potential for environmentally sound and cost-effective clean-up of contaminated soil, groundwater and air.
Some tree species, particularly poplars, grow at a remarkable rate - up to 15 feet per year - and their root systems penetrate large volumes of soil. In addition to drawing water and nutrients out of the ground, these trees also take up organic contaminants and metabolize them to harmless products. It is well documented that poplar trees metabolize trichloroethylene (TCE) to innocuous products. There is no release or sequestration of toxic intermediate metabolites such as vinyl chloride, a known carcinogen.
This promising technology of phytoremediation has many advantages over conventional engineering methods such as "pump and treat," including being significantly less expensive, less intrusive and more aesthetically pleasing. The disadvantages of phytoremediation are that the process is often too slow and may be only seasonally effective or remove only small amounts of pollutant from the environment. Regulatory agencies often require significant progress in remediation to be demonstrated in only a few years, making most phytoremediation applications unsuitable.
Since the 1980's, the NIEHS Superfund Basic Research Program (SBRP) has funded cutting-edge phytoremediation research at the University of Washington. Initiated by the late biochemist Milton Gordon, Ph.D., this work has evolved from identification of plant species best suited to remediation and development of more effective hybrids, to on-going studies to develop a transgenic poplar (Populus tremula x Populus alba) with greatly increased rates of metabolism and removal of volatile hydrocarbons including TCE, vinyl chloride, carbon tetrachloride, benzene and chloroform.
In laboratory studies that exposed apical stem cuttings to TCE, the scientists found that CYP2E1-containing transgenic cuttings had average rates of TCE metabolism nearly 45-fold greater than in the control cuttings. Two of the CYP2E1 transgenic lines had TCE metabolism rates that were more than 100-fold higher than in controls. The transgenic cuttings grew normally and did not display any adverse reaction to the TCE or its metabolites.
Strand and Doty then conducted studies to determine if the increased metabolism of TCE in the poplar cuttings led to increased uptake from solution and/or from air. They found that while control cuttings removed < 3 percent of the TCE from hydroponic solution, the CYP2E1 transgenics removed 51 to 91 percent. When exposed to contaminants in the air, the transgenic plants also demonstrated increased uptake. The CYP2E1 plants removed 79 percent of the TCE from the air, while control plants did not show any statistically significant uptake of TCE. In two weeks, the control plants removed an average of only 13 percent of benzene from air, while two CYP2E1 plant lines removed 36 and 46 percent.
The SBRP researchers believe that this work represents the first development of transgenic trees for increased removal of a broad range of serious environmental pollutants from water and air. Additional studies are needed to verify efficacy under field conditions and to ensure that plant tissues do not cause unacceptable impacts on non-target organisms.