Glutathione metabolism and free amino acid profile in Amaranthus palmeri with non-target site-based resistance to sulfonylureas

Although the majority of Amaranthus palmeri populations resistant to ALS inhibitors are due to alterations in the enzyme (target-site resistance mechanism), there are some cases of resistance due to herbicide metabolization (which is a non-target site (NTS) mechanism). In this study, we investigated the physiology of a sensitive (S) population and a sulfonylurea-resistant population due to NTSR of A. palmeri (R). Both populations were grown hydroponically and treated with the ALS inhibitor nicosulfuron, alone or combined with NBD-Cl (GST inhibitor). Seven days later, free amino acid profile, glutathione (GSH) content and GST activity and expression were measured in the leaves. Only S population leaves showed the typical amino acid profile previously described after sulfonylureas; R population showed no change, suggesting that such physiological disruption is prevented as ALS activity is maintained through herbicide metabolism. In the S population, the herbicide-induced oxidative stress triggered an increase in GST activity and accumulation of glutathione-related compounds (GSH and its precursors). R population showed a higher basal GST activity than S population and GST activity was induced by the herbicide. Three Phy and three Tau GSTs were evaluated, but none were clearly linked to increased activity. Root length in agar plates was used as a marker of herbicide sensitivity in the presence or absence of NBD-Cl and malathion (P450s inhibitor). The results provide evidence that P450s are involved in NTSR in the population of A. palmeri of this study, as previously reported, and they suggest, for the first time in a dicot weed, the involvement of GSTs in sulfonylurea metabolism.