Scientists dramatically improve poplar's capacity to clean up polluted sites - potential to couple phytoremediation to bioenergy

Scientists led by the University of Washington's Sharon Doty report that they have succeeded in genetically engineering poplar plants with a dramatically improved capacity to clean up contaminated sites. Doty, an assistant professor of forest resources, told Biopact that the ideal end of phytoremediation projects based on the trees would be to use the plants as a bioenergy feedstock. After all, poplar has been identified as a promising, fast-growing energy crop (previous post).

Researchers since the early '90s have seen the potential for cleaning up contaminated sites by growing plants able to take up nasty groundwater pollutants through their roots. Plants break certain kinds of pollutants into harmless byproducts that the plants either incorporate into their roots, stems and leaves or release into the air.

The problem with plants that are capable of doing this is that the process is slow and halts completely when growth stops in winter. Using plants in this way - the process known as phytoremediation (schematic, click to enlarge) - often hasn't made sense given the timetables required by regulatory agencies at remediation sites.

Together with scientists from Oregon State University and Purdue University, Doty engineered trees that were able to do the cleaning much faster. The transgenic poplars take as much as 91 percent of trichloroethylene, the most common groundwater contaminant at U.S. Superfund sites, out of a liquid solution. Unaltered plants in the same laboratory setting removed only 3 percent. The poplar plants - all cuttings just several inches tall growing in vials - also were able to break down, or metabolize, the pollutant into harmless byproducts at rates 100 times that of the control plants. The scientists publish their findings in this week's Proceedings of the National Academy of Sciences.

While U.S. regulations allow the growing of transgenic trees in greenhouses and controlled field trials for research purposes, they do not allow the commercial growing of transgenic trees. A transgenic plant is one in which its genetic material is manipulated. Sometimes only its own genetic material is altered and sometimes genetic material is added from other plants, bacteria or animals.

The scientists' new achievement raises the interesting question of 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. - Sharon Doty, lead author, Assistant Professor of Forest Resources, University of Washington

Trichloroethylene is a heavily used industrial degreaser that's made its way into groundwater because of improper disposal. Both unaltered poplars and the transgenic poplar plants produce the enzymes to break down trichloroethylene, C2HCl3, into chloride ions - harmless salt that the plant sheds - and recombines the carbon and hydrogen with oxygen to produce water and carbon dioxide.

The transgenic poplar plants just do it 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. Doty said scientists hope to eventually sort them to find ways to manipulate the plant's own genes to ramp up pollution degradation:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: groundwater :: pollution :: phytoremediation :: poplar ::

In the meantime they are conducting experiments inserting a gene that produces cytochrome P450 in mammalian livers, in this case the livers of rabbits. Poplar genes producing cytochrome P450 is expressed in all their cells, but not at the rates achieved by the transgenics.

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. - Sharon Doty

Mammalian cytochrome P450 has already been used in transgenic plants that can detoxify herbicides applied to fields to kill weeds. Japanese researchers, for example, published findings in 2005 about using a human gene to make rice plants degrade a suite of herbicides, something they said could help reduce the load of herbicides in paddy fields and streams.

Along with the trichloroethylene tests, the new results also found improved rates of uptake from solutions of chloroform, the byproduct of disinfecting drinking water; carbon tetrachloride, a solvent; and vinyl chloride, a substance used to make plastics. In air pollution experiments using 6-inch plants in closed containers, the transgenic plants had increased absorption of gaseous trichloroethylene and benzene, a pollutant associated with petroleum.

Doty and her colleagues plan 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.

Sites with contaminated groundwater are treated in a variety of chemical, physical and microbial ways, says Stuart Strand, UW professor of forest resources and a co-author of the paper. In some places the groundwater is pumped out of the ground and the contaminants allowed to evaporate into the air. In other places sugars pumped into the ground can clean contaminants but make the water anaerobic - oxygen starved - and can produce other toxic byproducts, he says. "It's destructive, disruptive and expensive," Strand says.

Some people see transgenic trees as risky. The scientists take these concerns seriously:

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. - Sharon Doty

Because there is concern that transgenic trees might get into regular forests, Doty and her colleagues believe poplars may be a good choice, she said. Poplars are fast growing and can grow for several years without flowering, at which time they could be harvested to prevent seeds from generating. And 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.

Even though these things are true, Doty and her co-authors imagine that transgenic trees planted at contaminated sites would involve high levels of containment around where they are being grown.

Commercial use of these trees requires federal regulatory approval and monitoring, and regulations are becoming increasingly strict for transgenic plants intended for biopharmaceutical or industrial purposes, including phytoremediation. - Sharon Doty, et al

Biopact asked Doty whether she saw any potential in the use of the enhanced poplars in a synergy with bioenergy production:

The ideal end of the phytoremediation projects could be to use the plants as biofuel feedstock. But it would have to be verified first that the pollutants are fully metabolized in field-grown trees. This is one of our next areas of research. - Sharon Doty

Work on phytoremediation at the University of Washington has been funded by the National Institute of Environmental Health Sciences, National Science Foundation, Environmental Protection Agency and Department of Energy.

Earlier scientists have looked specifically at using energy crops for phytoremediation. Examples include hybrid poplars that could be grown to soak up polluted water from coal mining sites (earlier post) and miscanthus to clean up brown fields (more here and here).

Poplar has received attention from the bioenergy community as an ideal biomass crop. It is the first tree to have had its entire genome sequenced (previous post).

References:
Sharon L. Doty, et al. "Enhanced phytoremediation of volatile environmental pollutants with transgenic trees", Proc. Natl. Acad. Sci. USA, Published online before print October 16, 2007, DOI: 10.1073/pnas.0703276104

Eurekalert: Scientists ramp up ability of poplar plants to disarm toxic pollutants - October 15, 2007.

Biopact: Energy crops and phytoremediation - new plants may soak up methane water - August 15, 2006

Biopact: Turning brownfields into greenfields with the help of biofuels - August 9, 2006

Biopact: France to use miscanthus to clean up polluted urban sites - August 22, 2006

Biopact: Virginia Tech researchers receive $1.2 million to study poplar tree as model biomass crop - June 26, 2007

Biopact: The first tree genome is published: Poplar holds promise as renewable bioenergy resource - September 14, 2006