A naturally derived herbicide with a novel mode of action, identified by mining the genomes of fungi, is reported online this week in Nature. The discovery potentially adds a much-needed new weapon to our arsenal of weed killers, in the face of increasing herbicide resistance.

Yi Tang, Steve Jacobsen and colleagues searched the genomes of filamentous fungi. Many of these produce compounds that aid in colonizing and killing plants, making them a promising source of potential herbicides. The authors specifically looked for compounds that target dihydroxyacid dehydratase (DHAD), a metabolic enzyme which is the final step in a biosynthetic pathway that is essential to plant growth. Although this pathway is the most common target of herbicide development - as it is not present in animals - no natural DHAD inhibitor that functions in plants had been previously discovered.

The authors searched for biosynthetic gene clusters encoding a DHAD inhibitor, which they identified through the additional copy of DHAD that provides the self-resistance that the fungus would need to survive. In a number of fungal species, including the common soil mould Aspergillus terreus, they identified one such gene cluster which encodes enzymes that produce the previously known fungal product aspterric acid. The authors found that this acid is indeed an inhibitor of DHAD - and it proved effective as a spray-delivered herbicide when tested.

Furthermore, the authors confirm that the self-resistance gene they found is unaffected by aspterric acid and when transferred into plants it can confer resistance to the newly discovered herbicide. Together, these findings could help combat the problem of growing herbicide resistance in weeds, and demonstrate the potential of resistance-gene-directed approaches for the discovery of useful, natural, bioactive products.