By Sandra Hines
Scientists 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. Then the 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, a process called phytoremediation, often hasn't made
sense given the timetables required by regulatory agencies at remediation
sites.

Scientists led by the University of Washington's Sharon Doty,
reporting in this week's Proceedings of the National Academy of Sciences,
say that genetically engineered poplar plants being grown in a laboratory
were able to 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 removed 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.

While federal 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 work being published this week 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," Doty, an assistant professor of forest resources, said.

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 and Doty said
scientists hope to eventually sort them to find ways to manipulate the
plant's own genes to ramp up pollution degradation. 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," Doty said. "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."

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.

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

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. "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," Doty said.

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," the co-authors write in their paper.

Other co-authors are from the UW, Oregon State University and Purdue
University.


[www.pnas.org]