Resistance to Amino Acid Biosynthesis Inhibiting-Herbicides in <i>Amaranthus palmeri</i> Populations from Aragon (Spain)

<i>Amaranthus palmeri</i> is a highly problematic agricultural weed due to its rapid growth, high seed production, and strong tendency to develop herbicide resistance. In Spain, the initial colonization of <i>A. palmeri</i> began in 2007, when populations were detected at various locations in the province of Lleida (Catalonia). Since then, new infestations have been reported in other regions of the country, primarily infesting maize fields. Although resistance to glyphosate or to acetolactate synthase (ALS) inhibitors has been documented in several populations from Catalonia and Extremadura, little is known about the resistance profile of populations from Aragon. The main objective of this study was to characterize the putative resistance of five populations from Aragon to 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitors (glyphosate) and ALS inhibitors (nicosulfuron and imazamox). Sensitivity to both mechanisms of action was measured by root growth in vertical plates and shikimate accumulation for glyphosate. Target-site resistance was evaluated by analyzing <i>EPSPS</i> and <i>ALS</i> gene copy numbers and <i>ALS</i> gene mutations. The populations showed high variability, with no multiple resistance detected. The Bujaraloz population showed moderate resistance to glyphosate due to <i>EPSPS</i> gene amplification. In three populations, mutations in the <i>ALS</i> gene conferring resistance were detected. The Trp574Leu mutation was detected in approximately half of the individuals from the Albelda, Tamarite de Litera, and Caspe populations. In the latter, the Pro197Thr mutation was also present. This study reveals significant genetic variability within each population and provides evidence for the spread of herbicide resistance across different regions of Spain.