Metabolism of 2,4-D in resistant Amaranthus palmeri S. Wats. (Palmer amaranth)

Amaranthus palmeri S. Wats. is one of troublesome weed species in field crops in the mid-southern USA. In our recent results of screening for herbicide resistance of various weeds collected from agricultural areas of the mid-southern USA, three A. palmeri accessions resistant to 2,4-D (R1, R2, and R3) were verified. In this study, all three resistant (R) accessions, along with two 2,4-D-susceptible (S1 and S2) standards, were subjected to dose-response experiments to characterize their sensitivity to 2,4-D. Based on the dose-response results, resistance/susceptibility ratios of the tested A. palmeri accessions were greater in the order of R2 (7.1) ≥ R1 (5.7) = R3 (5.7) > S2 (1.9) ≥ S1 (1.0). Treatment of malathion (cytochrome P450 inhibitor) or 4-chloro-7-nitrobenzofurazan [glutathione S-transferase (GST) inhibitor] followed by 2,4-D choline increased the mortality of R1 and R2 accessions, which means that enhanced metabolism of 2,4-D contributed to evolution of the herbicide resistance in such R accessions. In contrast, the mortality of R3 accession was not affected by either of the metabolic inhibitors. Absorption and translocation of 2,4-D were similar between S1 and R accessions for 48 h after the herbicide treatment. However, the 48-h metabolism of 2,4-D in R1 (48%) and R2 (68%) accessions were 2- to 3-fold greater than those in S1 (21%) and R3 (19%) accessions. Thus, enhanced metabolism by activities of cytochrome P450s and GSTs can be one of multiple mechanisms conferring 2,4-D resistance to A. palmeri. Further genetic studies are needed to verify the presence of target-site resistance mechanisms in all three R accessions.