Investigating mechanism of glyphosate resistance in a common ragweed (Ambrosia artemisiifolia L.) biotype from Nebraska

Common ragweed is a weed in the midwestern United States and eastern Canada that is difficult to control due to the evolution of an important resistance to multiple herbicides including glyphosate. Recently, a common ragweed biotype with 19-fold glyphosate resistance was confirmed in Nebraska. The objective of this study was to determine the mechanism of glyphosate resistance in a common ragweed biotype from Nebraska. Both target site and non-target site based mechanisms of glyphosate resistance were investigated using glyphosate-resistant (GR) and known glyphosate-susceptible (GS) common ragweed biotypes. A lower amount of shikimate was accumulated in the GR (≤65 µg mL-1) compared with the GS (≥80 µg mL-1) biotype at all glyphosate concentrations tested. Sequencing of the conserved region of the EPSPS gene revealed no mutations at the Thr102 or Pro106 residues and no variation in EPSPS copy number was detected. A higher translocation of 14C-glyphosate in the GR compared with the GS biotype was found, although there was no difference in the amount of 14C-glyphosate absorbed. Nonetheless, analysis of 14C-glyphosate absorption or translocation data using the rectangular hyperbolic model predicted a slower rate of absorption and translocation of glyphosate in the GR compared with the GS biotype, though more research is needed. These results indicate possible involvement of a non-target site mechanism bestowing resistance to glyphosate. The possibility that a slow rate of glyphosate absorption and translocation might have a role in preventing the buildup of the minimum inhibitory concentration of glyphosate required at the target site needs further research.