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Trees Created to Clean up Pollutants

Researchers have created transgenic poplar trees that can break down a class of common environmental pollutants. With their large size and extensive root systems, these trees may one day help to clean up contaminated sites faster and for less money than current methods.

Many plants naturally process environmental contaminants. They take contaminants up through their roots as they grow, transport them to leaves and other tissues, and either break them down or collect them. However, plants often do this too slowly to be of practical use for environmental cleanup. Researchers have been working to make the process more efficient by adding genes from elsewhere, creating what are called transgenic plants. Scientists have already developed transgenic plants that can remove mercury, lead, cadmium and selenium from polluted soils.

Drs. Sharon Doty and Stuart Strand at the University of Washington in Seattle lead a research team trying to develop a strategy to remove small, volatile hydrocarbons from contaminated soil. This class of common environmental pollutants poses serious health risks to people. It includes trichloroethylene (TCE), the most common pollutant at Superfund sites in the United States, as well as vinyl chloride, carbon tetrachloride, benzene and chloroform. Doty's group had previously created genetically altered tobacco plants with a mammalian gene for cytochrome P450, an enzyme that breaks down a broad range of environmental pollutants. They showed that these plants were more efficient at breaking down TCE.

In the October 23, 2007, issue of Proceedings of the National Academy of Sciences, the team reported that they successfully inserted the gene into hybrid poplars. The transgenic cuttings processed TCE 45 times more efficiently than the control cuttings without the added gene. When the cuttings were grown hydroponically for a week, the transgenic ones were able to take up as much as 91% of the TCE, compared to less than 3% for controls.

The researchers tested several related pollutants and found that the transgenic plants could more efficiently remove them as well. For example, the plants removed 99% of chloroform, compared with only 18-20% removed by controls. Only vinyl chloride seemed to affect the plants, causing their leaves to blacken. However, the environmental levels of vinyl chloride rarely approach the concentrations used in these experiments, so the researchers don't believe it would be a practical problem.

The team also found that the transgenic poplars removed 79% of the TCE from air within a week, compared with none for controls. The plants removed benzene from air more rapidly than control plants as well.

This study, which was supported primarily by NIH's National Institute on Environmental Health Sciences (NIEHS) and the Department of Energy, shows that transgenic trees can remove and degrade several of the most widespread and dangerous pollutants from water and air. Notably, these trees circumvent the widespread concern about genes "escaping" when seeds spread and plants start growing outside areas they've been planted in. While poplars are fast growing, they can grow for several years without flowering and so can be harvested before seeds ever form.

— by Harrison Wein, Ph.D.