Elsewhere, the unaltered poplar plants were able to remove just three percent of the toxins from the soil, the University of Washington led research team found. The scientists found that the poplars - all cuttings just several inches tall growing in vials -were able to break down, or metabolize, the pollutant into harmless byproducts at rates 100 times that of the control plants.
Team leader Sharon Doty said their work has raised interesting questions about the potential for using transgenic trees on sites where toxic plumes of pollutants are on the move in groundwater.
"Small, volatile hydrocarbons, including trichloroethylene, vinyl chloride, carbon tetrachloride, benzene, and chloroform, are common environmental pollutants that pose serious health effects. Some of these are known carcinogens," said Doty, an assistant professor of forest resources at the University of Washington.
Since the early 90s, scientists have been growing plants as a means to soak up groundwater pollutants. The plants soak up the pollutants with their roots, break certain kinds of pollutants into harmless byproducts that the plants either incorporate into their roots, stems and leaves or release into the air.
However, the problem with plants that are capable of doing this is that the process is slow and halts completely when growth stops in winter. The transgenic poplar plants, on the other hand, do the same process a lot faster. The enzymes used to metabolize the contaminants are from a group called cytochrome P450 found in both plants and animals.
Poplars have a lot of P450s, but for the time being, the scientists are conducting experiments inserting a gene that produces cytochrome P450 in mammalian livers, in this case the livers of rabbits. Prof. Doty said the team was eventually sorting to find ways to manipulate the plant's own genes to ramp up pollution degradation.
"We overcame the rate-limiting step by causing the poplar plants to overexpress the first enzyme in the degradative pathway. Using the mammalian gene is just a step toward the day when we understand the poplar P450 genes well enough to use promoters to enhance production of their own enzymes that degrade contaminants. With the plant's own genes, the results should be even better," she said.
The team now plans to do additional experiments to determine the detoxification rates when poplars are grown in soils, and to ensure that plant tissues do not harm non-target organisms, such as bugs that might chew on them. She said as there is a concern that transgenic trees might get into regular forests, poplars would be the best choice.
Poplars, being fast growing could grow for several years without flowering, at which time they could be harvested to prevent seeds from generating. Also unlike some other kinds of trees, branches of the hybrid poplar being studied do not take root in soils when branches fall to the ground, she said.
"As researchers we want to make sure such concerns are addressed and risks minimized. In the case of contaminated sites, we're already facing bad situations where the use of transgenic plants may reduce the known risks from carcinogens and other hazardous pollutants in the environment. Our ultimate goal is to provide a more rapid way to reduce the amount of carcinogens, one that is affordable so many sites can be treated," said Prof. Doty.
The study appears in this week's issue of the Proceedings of the National Academy of Sciences (PNAS).