Confirmation and Transcriptomic Characterization of Glufosinate-ammonium Resistance in Waterhemp (Amaranthus tuberculatus) Populations from Illinois
This study investigated four suspected cases of glufosinate-ammonium (GA) resistance in Amaranthus tuberculatus (waterhemp) populations from Illinois using greenhouse, field, and transcriptomic approaches. Greenhouse assays revealed 2.2-3.4-fold resistance based on survival and 1.3-2.8-fold based on biomass compared to a susceptible population. Transcriptomic analyses of plants surviving GA at the recommended field rate revealed no resistance-associated mutations or differential transcript abundance in the plastid and cytosolic isoforms of glutamine synthetase. Among the four resistant populations, there were 182 genes differentially expressed relative to two susceptible populations. Different sets of genes were differentially expressed among the populations studied, with only one gene (upregulated relative to sensitive populations) shared among all four. Many of the differentially expressed genes, including cytochrome P450s, glutathione S-transferases, glycosyltransferases, transporters, and transcriptional regulators, are commonly associated with metabolic resistance. Gene ontology enrichment analyses indicated significant overrepresentation of stress response, defense regulation, and secondary metabolism categories across the populations. Together, these findings provide initial evidence for the evolution of GA resistance in populations of A. tuberculatus in Illinois. While more in-depth studies are needed to fully characterize the underlying mechanisms, the consistent differential expression of metabolism-related genes and no indication of target-site mechanisms points to a potential metabolic basis for resistance.