Science 349, 1072–1075 (2015)

Credit: ELIZABETH RYLOTT

2,4,6-Trinitrotoluene (TNT) is a known explosive and environmental pollutant that inhibits plant root growth and development. Plants can partially detoxify TNT through the upregulation of glutathione transferases U24 and U25, which mediate the denitration of TNT into 2-glutathionyl-4,6-dinitrotoluene, a metabolite more amenable to biodegradation. Although plants have the potential to manage TNT toxicity, the identity of the enzymes that mediate this process have not been characterized. Johnston et al. performed a genetic screen for Arabidopsis thaliana mutants that were capable of growing in the presence of TNT. The sequencing of one TNT resistant mutant strain revealed a deletion that introduces a stop codon in mdhar6-1, which encodes a member of the monodehydroascorbate reductase (MDHAR6) family. MDHARs are flavin adenine dinucleotide–dependent oxidoreductases that act as antioxidants through the regeneration of ascorbate. A majority of the MDHARs are expressed in the cytosol or peroxisome, whereas MDHAR6 was expressed specifically in the mitochondria and plastids. Electron paramagnetic resonance (EPR) spectral analysis using MDHAR6 and TNT with a 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap revealed that TNT is reduced to a nitro radical that reacts with oxygen to produce superoxide. The authors verified that wild-type plants exposed to TNT showed elevated levels of hydrogen peroxide, whereas mdhar6-1 mutants contained reduced levels. Although it is unclear how MDHAR6 is able to mediate both pro-oxidant and antioxidant activities, these findings may inspire new strategies to engineer bacteria and other organisms to stimulate TNT degradation.