Reduced Absorption and Impaired Translocation Endows Glyphosate Resistance in Amaranthus palmeri Harvested in Glyphosate-Resistant Soybean from Argentina

Amaranthus palmeri S. Watson is probably the worst glyphosate-resistant (GR) weed worldwide. The EPSPS (5-enolpyruvylshikimate-3-phosphate-synthase) gene amplification has been reported as the major target-site-resistance (TSR) mechanism conferring resistance to glyphosate in this species. In this study, TSR and non-target-site-resistance (NTSR) mechanisms to glyphosate were characterized in a putative resistant A. palmeri population (GRP), harvested in a GR soybean crop from Argentina. Glyphosate resistance was confirmed for the GRP population by dose-response assays. No evidence of TSR mechanisms, as well as glyphosate metabolism, was found in this population. Moreover, a susceptible population (GSP) that absorbed about 10% more herbicide than the GRP population was evaluated at different periods after treatment. The GSP population translocated about 20% more glyphosate to the remainder of the shoots and roots at 96 h after treatment than the control, while the GRP population retained 62% of herbicide in the treated leaves. This is the first case of glyphosate resistance in A. palmeri involving exclusively NTSR mechanisms.