A modest plant related to cabbage has revealed an unexpected path to cleaning up millions of acres of land contaminated by toxic residue from high explosives.
The plant, Arabidopsis thaliana, includes a variety that carries a genetic mutation that allows it to thrive in concentrations of TNT that would stunt the growth of its more-normal relatives.
The mutant variety takes up TNT, alters it, then entombs it in the portions of its cell walls that are highly stable. Once locked up there, "we expect that TNT is not biologically available," even after the plant dies and decomposes, says Neil Bruce, a biologist with the University of York's Center for Novel Agricultural Products at the University of York in Britain.
If followup studies confirm that the altered TNT remains sequestered, even after the plants holding it decompose, the results would point to an elegant solution to a vexing problem.
Arabidopsis itself would probably not be the plant to do most of the TNT cleanup. But the discovery suggests to scientists that other plants might have the same mutation, or could be modified to have it.
TNT has been used in large quantities for military and commercial explosives for more than a century. When a bomb or artillery shell explodes, the blast doesn't consume all of the TNT the device contains. What's left gets spread around the ground.
It doesn't readily break down in the environment. And water moving through through soil can pick it up and carry it into streams and rivers.
In animals, TNT is toxic. And in soils, it stymies plant growth directly while disrupting microbial communities that support robust plant growth.
Over the years, various research groups have looked for ways to enlist plants to clean up pollution, known as phytoremediation. To deal with TNT, researchers have experimented with tobacco plants, Arabidopsis, and even seaweed genetically modified to remove it from soils or coastal waters.
The United States military alone owns an estimated 39,000 square miles of land contaminated to varying degrees with TNT.
"These sites are large, so you need low-cost, sustainable technology" that can keep contamination in check, Dr. Bruce says, adding, "plants are the only option to be able to do this."
Generally, Arabidopsis can detoxify TNT but typically only at low concentrations. The research team looked at several lines of Arabidopsis to see if they could help them identify the plant enzymes responsible for detoxifying TNT.
One line in particular showed a remarkable ability to grow in TNT-contaminated soils, pointing to powerful resistance to TNT's toxic effects.
The trait relied on one mutant gene, which carried the genetic code for the enzyme that was conferring resistance to TNT.
When the researchers worked out the biochemistry behind the mutation's action, they were surprised, says Bruce, who led the study along with York colleague Elizabeth Rylott.
The researchers expected that the mutation would lead to an increase in the amount or performance of enzymes already present that the plant used to detoxify small doses of TNT. Instead, the mutation failed to produce an enzyme that renders TNT toxic to the plant. Moreover, this mutation appears to have increased the plants' tolerance for TNT, but not for a range of other compounds known to stress plants.
Genomes analyzed from several other types of plants reveal the presence of the normal version of the mutant gene, Bruce notes. This suggests that the gene probably is present in all plants, whose varieties might also include ones that show a mutation similar to the one found in Arabidopsis, he says.
So instead of using Arabidopsis itself to do the heavy lifting, fast growing, hardy grasses well suited to military training and proving grounds hosting much of the residual TNT would be more appropriate, Bruce says.
If they don't have varieties that carry the mutation, it can be introduced in other ways, including through genetic modification. But genetically modified organisms would raise another set of issues: "the costs, licensing, and trials," he says, not to mention public resistance.
The long-term sequestration of the altered TNT remains to be demonstrated, Bruce acknowledges.
Still, "I'm fairly confident that the TNT's been locked up in this material, and it's incredibly difficult to extract the transformed TNT once it's been taken up into the plant."
The results are set to appear in Friday's issue of the journal Science.