D376E, A205V and A122S substitutions recently found in A. palmeri confer cross-resistance to ALS-inhibiting herbicides

Background: Amaranthus palmeri S. Watson is a serious problem in soybean crops from Argentina. This weed has evolved high level of resistance to (ALS)-inhibiting herbicides mainly due to a target-site mechanism by an altered ALS enzyme. In an A. palmeri population from Totoras with cross-resistance to (ALS)-inhibiting herbicides, six allelic versions of the ALS enzyme were identified. Objective: The aim of this study was to evaluate plants from that resistant population carrying the ALS substitutions A122S, D376E or A205V, which had not been characterized before for this species. Methods: Subpopulations with each substitution were produced by a vegetative cloning procedure or by cross-pollination and dose-response assays and herbicide single-dose tests were performed to evaluate in vivo resistance levels to (ALS)-inhibiting herbicides. Results: Dose-response experiments showed that all the resistant subpopulations survived at the highest doses tested (32 X) for imidazolinones, triazolopyrimidines and sulfonylureas, while the susceptible population was completely controlled at considerably lower doses. Furthermore, an analysis of the novel A122S substitution showed that it provides cross-resistance to five classes of (ALS)-inhibiting herbicides, excluding the entire ALS herbicide group as an effective control tool in weed populations carrying this substitution. Conclusions: The results indicated that D376E, A205V and A122S substitutions found for the first time in A. palmeri confer cross-resistance to the most used chemical families from herbicide group 2. Interestingly, it was confirmed that the A205V substitution conferred resistance to herbicides in the triazolopyrimidines family. Data generated should be considered in management strategies for delaying the spread of resistance.