Biotech Plants Help Clean the Environment
Benefits include removal of toxins and other unwanted materials

Most people think of plant biotechnology as it relates to agriculture. Your first thoughts may be of farmers benefiting from increased yields of corn, soybean or cotton. The first environmental benefits that come to mind may include reduced pesticide applications, less soil tillage and reductions in associated fossil-fuel use. But in addition to reducing the environmental footprint of agricultural crops, scientists are working to bolster other plants' natural abilities to rid the environment of unwanted materials.

Phytoremediation: How Plants Help Clean the Soil
Phytoremediation uses plants to remove, transfer, stabilize and destroy environmental contaminants. As plants take in water and other nutrients through their roots, they remove harmful chemicals from the soil. Trees are particularly adept at phytoremediation because their roots grow much more deeply into the soil than other plants.¹

Once toxins are absorbed by a tree, they are either internalized or broken down into less harmful substances. The internalized chemicals may settle above ground within the trunk, branches or leaves, or remain below ground in the roots. The tree can transform contaminants into less toxic substances before storing or releasing them, or microscopic bugs that associate with the tree's roots may transform the toxins into less harmful substances.

Several steps can prevent phytoremediating trees from reintroducing internalized toxins back into the ground or water. Collecting and incinerating discarded leaves guards against soil recontamination. Energy producers may burn harvested trees, and, depending on the nature of the accumulated toxins, pulp or paper manufacturers may use the trees.

The targeted toxins can harm many phytoremediating trees, especially those selected or engineered to be "hyperaccumulators." To combat this problem, researchers are looking for ways to bolster plant resistance to poisons while enhancing uptake and processing capabilities.²

Danbury, Conn., suffered mercury contamination during its heyday as the world's hat-making capital. Today, to clean up the pollution, scientists are splicing genes from the bacterium Escherichia coli (E. coli) into cottonwood trees. The genes enable the common bacterium — and the trees — to live amid mercury. In a field test, scientists planted 45 biotech cottonwood trees in a polluted lot. The researchers hope the trees treat the mercury as a nutrient and draw the toxic element from the soil with their roots. The trees will store most of the mercury, some of which will vaporize into the air. The scientists plan to cut down and incinerate the trees after several years of growth.³

Researchers also are looking for ways to make poplars more effective phytoremediators of heavy metals such as zinc, cadmium and mercury. Fast growth rates and large biomass make poplars especially well-suited for phytoremediation. Yet the trees have limited tolerance of heavy metals and therefore can remediate only a given amount. Scientists hope that genetic modification can significantly increase poplars' remediative capacity.⁴

In a recent article published in EMBO Reports, researchers Andreas D. Peuke and Heinz Rennenberg describe their work of inserting a bacterial gene into poplars. The gene increased the poplars' level of glutathione, an antioxidant that can reduce environmental stress on trees. With the gene, the biotech trees absorbed larger amounts of potentially dangerous heavy metals compared with their nonbiotech counterparts. These new biotech poplars "are indeed all-purpose performers for phytoremediation in controlled greenhouse conditions: They showed a high potential for the uptake and detoxification of…heavy metals," the researchers said.⁵

Mustards Absorb Pollutants, Detect Threats
Trees aren't the only plants scientists are hoping to use for phytoremediation. A significant amount of research is under way on various relatives of wild mustard. For example, researchers from the U.S. Department of Agriculture and the University of California, Berkeley, recently showed that a new biotech variety of Indian mustard converted a toxic form of selenium into a nontoxic one. The scientists engineered one line of Indian mustard plants to produce more of the enzyme adenosine triphosphate sulfurylase (APS). The enzyme is key to the plant's ability to convert selenate into a nontoxic form of selenium, allowing the plant to accumulate more than four times the amount of contaminant found in nonbiotech plants, without incurring harm. Lead researcher Norman Terry hopes to learn if it's possible "to increase [the plant's] ability to absorb selenium and other pollutants ten-, one hundred- or even one thousand-fold."⁶

Scientists recently adapted another mustard, the thale cress Arabidopsis (considered by many to be the mouse of genetic plant research), to detect land mines when sown in potentially mine-laden fields. The Danish company Aresa Biodetection ApS tweaked the plant's genetic makeup so it would turn from red to green within three to six weeks in the presence of nitrogen dioxide, a gas that leaches from mines and other ordinances.

"This is a pioneering example of how we will see genetically engineered plants applied for humanitarian and environmental purposes in the future," said Professor John Mundy, Department of Plant Physiology at the University of Copenhagen.⁷

Biotech varieties of Arabidopsis also can detect acts of terrorism.

"Plants make good sentinels," said Jack Schultz, a chemical ecologist and professor of entomology in the College of Agricultural Sciences at Pennsylvania State University. The U.S. Defense Advanced Research Projects Agency provided a grant to Schultz and other researchers to develop a plant that can detect traces of chemical weapons or biological agents such as anthrax.⁸

Conclusion
Biotechnology fosters hope for more targeted and streamlined phytoremediative processes. Scientists are finding ways to give plants genetic tools to thrive in a toxic environment and to render pollutants harmless. They also are equipping plants to detect the signs of terrorism and tools of warfare. Such efforts could prove less costly and more environmentally friendly than many of the currently available alternatives.

¹ A Citizen's Guide to Phytoremediation, U.S. Environmental Protection Agency, April 2001.

² Ute Krämer. "Phytoremediation: Novel Aproaches to Cleaning Up Polluted Soils," Current Opinion in Biotechnology, 16:133-141, 2005.

³ Paul Elias. "Biotech Fights Pollution With Plants," 2005, Associated Press and ABC News Internet Ventures.

⁴ Katharina Schoebi. "Poplars Long for Zinc," Checkbiotech, Aug. 25, 2005.

⁵ Andreas D. Peuke and Heinz Rennenberg. "Phytoremediation. Molecular Biology, Requirements for Application, Environmental Protection, Public Attention and Feasibility," EMBO Reports, 6(6): 497-501, 2005.

⁶ Sarah Yang. "Transgenic Plants Remove More Selenium From Contaminated Soil Than Wild-type Plants, New Field Tests Show," University of California, Berkeley, news release, Feb. 1, 2005.

⁷ News release on the Technology of Areas Biodection ApS, Jan. 24, 2004.

⁸ "Researchers Developing 'Sentinel Plants' to Warn of Bioterrorism," Pennsylvania State University news release, March 7, 2003.