From firing ranges to war zones, humans have pummeled the Earth with an enormous amount of explosive ordinance. In almost every bomb and artillery shell is the ubiquitous and volatile chemical TNT (trinitrotoluene). TNT's danger lingers long after the boom. Labeled a class C carcinogen by the EPA, TNT pollution from partially-detonated explosives wreaks environmental havoc and can cause ailments like cataracts, hepatitis, and bone marrow cancer in animals. The U.S. Department of Defense estimates that around 10 million hectares worldwide have been contaminated.

"It was quite a surprise—we were really quite flummoxed by what we found."

But there's good news. Today a team of biologists led by Emily Johnston and Elizabeth Rylott at the University of York in the U.K. have discovered a genetic mutation in plants that bolsters their immunity to TNT, allowing them to soak up, digest, and safely store the harmful pollutant. First discovered in a species of scientific research plant called Arabidopsis, the mutation could be bred into an almost limitless number of plants, such as the wild grasses already growing on firing ranges.

"Small genetic changes in plants have the potential to go a long way towards revegetating and cleaning up polluted land," says Johnston. The research is published today in the journal Science.

A Confusing Mutation

How can a plant clean up TNT? Within the mutant organism, "TNT is transformed by enzymes and becomes locked up, out of harm's way, in the plant's cell walls," says Neil Bruce, a biologist with the team who studies how plants degrade and transform explosives. "If an animal were to consume that plant, that transformed TNT would simply pass right through it. That's because it's tightly bound up in non-digestible material. It wouldn't be toxic to the animal."

Phil Roberts

To be clear, the scientists did not simply stumble across this fascinating mutation by chance. Bruce says the research team was growing plants in TNT-contaminated soil, looking for hearty survivors to study, when their odd cultivar emerged. When they peered inside the genome of the plucky plant, "it was quite a surprise—we were really quite flummoxed by what we found," Bruce says.

What confused the scientists so much is that their plant had a perplexing mutation in a gene called MDHAR6. That particular gene bolsters the plants' self-defense by producing vitamin C to fight off a nasty group of toxic molecules called reactive oxygen species, or ROS. In other words, the MDHAR6 gene controls a seemingly vital self-defense mechanism against toxic bombardment. Here's the flummoxing part. Within their hearty, TNT-absorbing plant, that MDHAR6 gene had—poof—just vanished.

How is it that the tough mutated plant totally lacks this toxic-defender gene? The explanation became clear as the scientists invesigated exactly happens when TNT gets involved with MDHAR6, Bruce says. In an ordinary, non-mutated plant (what the scientists call "wild-type"), he says "the TNT gets taken up through the plant's roots, where it diffuses across cell membranes into the plant's cells." Once inside the cells, the TNT molecules make their way into the cell's mitochondria, the tiny power plants inside all cells). There, with the unwitting help of the MDHAR6 gene, TNT wreaks havoc. It hijacks the enzymes produced by the the MDHAR6 gene—the ones that should be creating vitamin C—and starts hyper-producing the very same nasty ROS chemicals that the MDHAR6 gene is supposed to fight off in the first place. TNT also saps the plants' mitochondrial fuel supply. With enough TNT around, this two-pronged attack will—as you might have guessed—kill a plant.

"If an animal were to consume that plant, that transformed TNT would simply pass right through it."

But in the mutant plant with no MDHAR6, none of this happens. No enzymes in the plant's mitochondria are hijacked. No plant fuel is stolen. Instead, the plant's natural immune system ("which acts almost like our liver," says Bruce) scoops up the intruding TNT, breaks it down, and safely stores the debris in the plant's hard, woody cell walls.

Weirdly enough, other than being TNT resistant, "in every single way we've tested, plants without the [MDHAR6] gene are indistinguishable than plants with the gene," says Bruce. Knocking out that gene seems to have no downside. This is still baffling the researchers. Given the important role of the gene, the running theory is that plants without the MDHAR6 gene are finding some other mechanism to produce ROS-defending molecules where they need them.

Battlefield Cleanup

Bruce cautions that this discover isn't a miracle fix—enough TNT can still kill their mutant plant (it's not invincible to the stuff). "But you'd need quite a lot. What's important is that we found the plants could survive concentrations of TNT at the upper end of what you'd expect to find at a contaminated site," he says.

Phil Roberts

The implications for cleaning up pollutant TNT are huge. Even better, they are almost immediate. Plants like these "could be used to clean up undesirable compounds such as TNT by extracting them from the soil and concentrating them within their tissues," says Graham Noctor, a plant biologist at the Institute of Plant Sciences in Paris, in accompanying essay in Science. (Noctor was not involved with the research)

Because the MDHAR6 gene is part of a basic genetic structure found in all plants, Bruce says, you could imagine using pretty much any plant for the job of toxic cleanup. Take, for example, wild grasses already growing on test firing ranges. Scientists could genetically modify the grass to knock out the gene and then sow the hardy seeds.

"It comes down to just relatively straightforward plant breeding," Bruce says.

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