UW researchers genetically engineered a switchgrass (foreground) to break down the explosive chemical RDX using genes from a soil bacterium. The researchers also grew a few thousand of the plantlets in the lab to prepare them to be transplanted in the field. From left to right: Ryan Routsong, Long Zhang, Stuart Strand. This photo was taken in 2018.Mark Stone/University of Washington
Large swaths of U.S. military land are covered with munitions components, including the explosive chemical RDX. This molecule is toxic to people and can cause cancer. It also doesn't naturally break down and can contaminate groundwater.
Now researchers have genetically engineered a grass commonly used to fight soil erosion so that it can remove RDX from the soil, according to a new paper published May 3 in Nature Biotechnology.
The team, which includes researchers from the University of Washington, demonstrated that over the course of three years, genetically engineered switchgrass broke down RDX in plots of soil at a military range. This is the first time researchers have used a genetically engineered plant in the field to remove pollutants that are resistant to degradation, the team said.
"Our lab's expertise is in genetic engineering useful plants - such as grasses and houseplants - so that they can degrade pollutants," said co-author Stuart Strand, a UW research professor emeritus in the civil and environmental engineering department. "The big goal here is to prevent cancer-causing chemicals from contaminating groundwater beneath active, live-fire training ranges by planting these grasses in and around target areas."
RDX is designated as a priority pollutant by the U.S. Environmental Protection Agency and is of significant and increasing public concern.
"The recalcitrance of RDX to degradation in the environment, combined with its high mobility through soil and groundwater, mean that plumes of toxic RDX continue to spread below these military sites, threatening drinking water supplies," said senior author Neil Bruce, a biology professor at the University of York in the United Kingdom.
Because of the scale of explosives pollution, there is considerable interest in developing sustainable remediation strategies, such as using plants to help break down toxic compounds, the researchers said.
To create these grasses, the researchers inserted two genes from a soil bacterium that has evolved to break down RDX into switchgrass (Panicum virgatum).
A researcher sampling the switchgrass at a site in New York state.Neil Bruce
Then the team tested the plants at a military range in New York state. The researchers compared three different conditions - no plants, non-genetically engineered grass and the engineered grass - on 27 plots containing RDX at a typical concentration found in contaminated sites.
After three years, the excess water coming off the plots with the engineered grass contained lower levels of RDX compared to the other two types of plots. In addition, the engineered plants had little or no RDX in their tissues compared to the wild-type plants, suggesting that these grasses were taking up and metabolizing this chemical.
"We estimate that grass would need to be grown for several years on most sites with contaminated soil, but less contaminated sites could be remediated more quickly," Strand said. "This technology would be very cheap compared to alternative methods."
Before this system is ready to be used at contaminated sites, researchers will need to perform biosafety experiments to check how these engineered grasses would affect native plants, the team said.
"We demonstrated that by inserting these genes into switchgrass, the plant then had the ability to degrade RDX to non-detectable levels in the plant tissue," said co-author Liz Rylott, a biology lecturer at the University of York.
Additional co-authors are Long Zhang and Ryan Routsong in the UW civil and environmental engineering department, and Timothy Cary and Antonio Palazzo with the U.S. Army Cold Regions Research and Engineering Laboratory. This research was funded by the Strategic Environmental Research and Development Program and the Environmental Security Technology Certification Program of the U.S. Department of Defense.
