At military sites in the United States and across the globe, routine live-fire trainings produce a downpour of an explosive known as RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). A synthetic chemical, RDX can threaten human health, damaging the nervous system if inhaled or ingested. The EPA also classifies it as a possible human carcinogen. But removing the contaminant from military sites where it builds up over decades is no easy task. “It doesn’t break down in the environment very readily,” says plant biotechnologist Liz Rylott of the University of York in the United Kingdom. “It’s in the groundwater and it is starting to threaten drinking water supplies.”
With funding from the US Department of Defense, Rylott and colleagues developed a transgenic plant that, when aided by genes sourced from a particular bacteria, can soak up and degrade RDX. The multi-step process, made longer by permitting requirements related to genetically modified (GM) plants, took two decades. Now, in Nature Biotechnology, they’ve reported their first test of this phytoremediation tool on an actual military site. “They’ve shown in a field trial that it works, and I think it’s quite compelling evidence,” says Jerald Schnoor, a professor of environmental engineering at the University of Iowa, who was not involved in the study.
Phytoremediation, using plants to decontaminate land or water, can be especially helpful when polluted acreage is too expansive to dig out the polluted soil. Ideally, plants not only draw up a pollutant but also break it down. Unfortunately, plants often retain RDX. “They’ll suck it up like a hoover,” says Rylott. “But it just stays in the leaves and the aerial tissues, and then when the plant senesces naturally, it just goes back down into the ground where it’s released.”
So the team decided that the plants needed an assist. In the early 2000s, Neil Bruce of the University of York, the current study’s senior author, and colleagues found a strain of bacteria in RDX-contaminated soil that could degrade the explosive. They identified the pair of enzymes responsible and, several years later, inserted genes for these enzymes into Arabidopsis, which promptly began degrading RDX in the lab.
But Arabidopsis was never right for a military range. The researchers needed a robust plant that could stand up to tank traffic and fire. The military was already planting Alamo switchgrass, a cultivar of the native Panicum virgatum, at training sites. In 2017, the team reported that this grass did a nice job degrading RDX in the lab after the researchers inserted the bacterial genes. In previous work in bacteria, the team identified the products of RDX degradation, which include formaldehyde. But in transgenic plants, they can’t find any trace of these products, suggesting that the plants’ own enzymes break them down further.
To find out if their transgenic switchgrass worked on a real military range, the team conducted a 3-year field trial, starting in 2016, at the Fort Drum military range in New York. They first created a series of 3×3-meter plots, each lined with polypropylene to a depth of half a meter. They fitted each plot with a pump that drew any excess water into a tank outside of the plot. This closed system allowed the team to better track the movement of RDX in the plants and water.
In each plot, the researchers planted either the unmodified switchgrass or the transgenic switchgrass, or they left the plot empty as a control. The researchers then treated some of the plots with RDX, while leaving others uncontaminated.
The transgenic switchgrass indeed removed RDX from contaminated plots. The RDX level in excess water pumped from the unmodified switchgrass plots in the final growing season, when plants were most mature, was over three times higher than in water pumped from the transgenic plots. Over the course of this same growing season, the researchers estimated that the transgenic plants removed RDX at a rate equivalent to 27 kilograms per hectare.
Despite regular sampling across three growing seasons, the team did not find a single trace of RDX in the leaf tissue of the transgenic switchgrass, suggesting that the plants were breaking down the explosive. Meanwhile, RDX accumulated in the leaves of the unmodified grasses.
“This is a nice solid step in the direction of showing that something like this could be implemented for actual remediation,” says Beth Ahner, a professor of environmental engineering at Cornell University, who was not involved in the study. But Ahner questions whether this transgenic switchgrass could be put to widespread use anytime soon. “My concern,” she says, “would be that we’ve been slow to change public opinions about use of transgenic plants in such applications.” In other words, some worry the GM plants could proliferate.
Indeed, even getting the permit to conduct this field trial with a GM grass took time, says Rylott. The team had to be careful to contain the plant through measures like removing flower heads to prevent GM pollen and seed dispersal. But Rylott remains hopeful. “We’ve done all of the things that need to be done to show that this technology is not only effective but safe,” she says.
She also believes that, given more time to grow, the transgenic switchgrass could perform even better. The plants in this experiment grew slowly, in part due to poor weather conditions, including major storms in year two. By the end of the study, the switchgrass was only at half its potential biomass. “And it was still not saturated with the ability to take up the RDX and break it down,” says Rylott. “We haven’t found their upper limit yet.”