Researchers from the University of Washington (UW) and the University of York have developed new transgenic grass species that can neutralize and eradicate RDX—a toxic compound widely used in explosives. The development could help provide a cost-effective method for removing the toxic compounds that have contaminated millions of acres of land used for military live-fire training ranges.

UW engineers led by Stuart Strand, professor of civil and environmental engineering, introduced two genes from bacteria that learned to eat RDX and break it down into harmless components in two perennial grass species: switchgrass (Panicum virgatum) and creeping bentgrass (Agrostis stolonifera). The best-performing strains removed all the RDX from a simulated soil in which they were grown in less than two weeks, and they retained none of the toxic chemical in their leaves or stems.

Engineers introduced two genes from bacteria that learned to eat RDX and break it down into harmless components. Image credit: UW/Dennis Wise.“This is a sustainable and affordable way to remove and destroy pollutants on these training ranges," says Strand. “The grasses could be planted on the training ranges, grow on their own and require little to no maintenance. When a toxic particle from the munitions lands in a target area, their roots would take up the RDX and degrade it before it can reach groundwater.”

Unlike other toxic explosive constituents such as TNT—which binds to soils and tends to stay put—RDX dissolves easily in water and is more prone to spread contamination beyond the limits of a military range, manufacturing facility or battleground. Particles are scattered and RDX dissolves in water when it rains, eventually contaminating groundwater.

Neil Bruce and Liz Rylott, respectively biotechnology professor and research scientist at the University of York, and colleagues had previously isolated enzymes found in bacteria that evolved to use the nitrogen found in RDX as a food source. That digestion process has the added benefit of degrading the toxic RDX compound into harmless constituents.

The bacteria themselves are not an ideal cleanup tool because they require other food sources that are not always present on military training ranges. So Bruce and Rylott tried inserting the bacterial genes into plant species commonly used in laboratory settings. Those experiments proved that the new plant strains were able to remove RDX contamination much more successfully than their wild counterparts.

The UW team spent eight years working to express the same genes in plant species that could stand up to real-world use. They needed a hearty perennial species that grows back year after year with strong root systems that can bounce back after fires.

Grasses fit that bill, but they are more difficult to manipulate genetically. In particular, the UW engineers had to build into their gene constructs robust monocot “promoters”—or regions of DNA that cause a particular gene to be expressed—for the process to work in grass species.

Next steps for the UW research team include limited field trials on a military training range to test how the strains perform under different conditions. Wider use would require the U.S. Department of Agriculture's approval to ensure that the genetic modifications pose no threat to wild grass species.