When TNT is detonated, its toxic particles seep into the ground below. Plants that grow in this poisoned soil are stunted. Researchers have now identified a genetic mutation in plants that protects them from the hazardous impacts of the explosive. According to findings published in Science, these mutated plants can actually suck TNT out of the soil while resisting its toxic effects. 

TNT, or 2,4,6-trinitrotoluene, is especially prevalent in the soil at mines, waste sites, and military conflict zones across the planet. And it’s an environmental pollutant that accumulates in the roots of plants, significantly inhibiting growth. A team of University of York researchers discovered that this so-called phytotoxicity is caused by a damaging chemical reaction that occurs with TNT in the mitochondria, where the cell’s energy production takes place. This reaction is catalyzed by an enzyme called MDHAR6. 

The researchers then screened various lines of a small flowering plant called Arabidopsis thaliana for greater root growth in the presence of TNT. They found that Arabidopsis plants that lack the enzyme have an enhanced TNT tolerance. The enzyme has previously been linked to protecting plants from stress. Compared with wild type plants, the MDHAR6 deficient plants have longer roots and bigger shoots when grown in soil treated with TNT. 

^{MDHAR6 deficient plants display enhanced TNT tolerance compared to wild type plants. Liz Rylott}

Analysis of electron activity revealed that MDHAR6 catalyzes a one-electron alteration (or reduction) that converts TNT to a nitro radical, which then reacts with oxygen to form a reactive superoxide. The superoxide was not found when either the MDHAR6 enzyme or TNT were absent, and the MDHAR6 mutants displayed enhanced tolerance to TNT but not to other stresses. This indicates that the production of superoxide from TNT by MDHAR6 is the main cause of TNT toxicity in plants.

The researchers think that plants deficient in MDHAR6 could help with the remediation of TNT-contaminated land. In the U.S. alone, 10 million hectares of military land are contaminated with munitions constituents. 

Or, on the other hand, this plant-specific enzyme’s natural reaction with TNT could help with the development of sustainable herbicides – if degradable compounds that react with MDHAR6 in the same way as TNT can be found. "Although herbicide resistance has been increasing steadily since the 1970s, no new herbicide mode of action has been commercialized since the 1980s,” University of York’s Neil Bruce said in a statement. A new type of herbicide could be used in rotation with existing ones to limit the emergence of resistance.

Image in text: Plants deficient in MDHAR6 may contribute to the remediation of TNT-contaminated land. Phil Roberts