Characterization of ALS- and EPSPS-inhibitors resistance in Spanish populations of Amaranthus palmeri

Abstract

Amaranthus palmeri is an invasive weed present in Spanish territory. Populations resistant to ALS and EPSPS inhibitors have been described. It is suspected that the accumulation of multiple resistance to both modes of action (MoA) within the same population. In this study we evaluate the herbicide responses in two roadside populations of A.palmeri from Catalonia. In a dose-response essay, most of the plants treated with 540 g a.i./ha of glyphosate survived in both populations, and some individuals even survived the field dose of 1080 g/ha. Single-dose treatments with ALS inhibitors reveal high resistance to thifensulfuron-methyl (6 g a.i. ha−1) and low resistance to imazamox (40 g a.i. ha−1) in both populations. Target-site resistance mechanisms are identified as the primary cause of survival for both modes of action in both populations. Glyphosate survivors show increased EPSPS gene copies but no mutations in the target protein. Surprisingly, survivors of ALS inhibitor treatment have multiple allelic mutations at codon: Pro-197-Ser, Pro-197-Thr, Asp-376-Glu, and Trp-574-Leu. Given the selection pressure history and the high incidence of detected mutations, it is reasonable to assume an ex situ selection of these populations to these MoA. This scenario presents a significant challenge for Spanish agriculture, indicating the ongoing introduction of A. palmeri populations with multiple resistance, complicating field management.