Flavonoid-based inhibitors of the Phi-class glutathione transferase from black-grass to combat multiple herbicide resistance

The evolution and growth of multiple-herbicide resistance (MHR) in grass weeds continues to threaten global cereal production. While various processes can contribute to resistance, earlier work has identified the phi class glutathione- S -transferase ( Am GSTF1) as a functional biomarker of MHR in black-grass ( Alopecurus myosuroides ). This study provides further insights into the role of Am GSTF1 in MHR using a combination of chemical and structural biology. Crystal structures of wild-type Am GSTF1, together with two specifically designed variants that allowed the co-crystal structure determination with glutathione and a glutathione adduct of the Am GSTF1 inhibitor 4-chloro-7-nitro-benzofurazan (NBD-Cl) were obtained. These studies demonstrated that the inhibitory activity of NBD-Cl was associated with the occlusion of the active site and the impediment of substrate binding. A search for other selective inhibitors of Am GSTF1, using ligand-fishing experiments, identified a number of flavonoids as potential ligands. Subsequent experiments using black-grass extracts discovered a specific flavonoid as a natural ligand of the recombinant enzyme. A series of related synthetic flavonoids was prepared and their binding to Am GSTF1 was investigated showing a high affinity for derivatives bearing a O -5-decyl-α-carboxylate. Molecular modelling based on high-resolution crystal structures allowed a binding pose to be defined which explained flavonoid binding specificity. Crucially, high binding affinity was linked to a reversal of the herbicide resistance phenotype in MHR black-grass. Collectively, these results present a nature-inspired new lead for the development of herbicide synergists to counteract MHR in weeds. Nature inspired flavonoid derivatives bind to AmGSTF1 and overcome herbicide resistance in multiple herbicide resistant (MHR) Black Grass.