Integrated Molecular and Physiological Characterization of Glyphosate Resistance in johnsongrass ( <i>Sorghum halepense</i> )

Abstract Johnsongrass [ Sorghum halepense (L.) Pers.] is a problematic, perennial weed species that is increasingly difficult to control across a range of agroecosystems; especially under intensifying pressures from climate change and herbicide resistance. Glyphosate has historically been effective at controlling S. halepense ; however, repeated and widespread use has imposed strong selection pressure resulting in resistant biotypes. A suspected glyphosate resistant biotype was identified in Texas, and this study was performed to characterize potential target and non-target site resistance mechanism(s) in this biotype. Dose-response assays revealed an observed ED 50 of 1036 g ae ha -1 , a 2.3-fold resistance ratio compared to a glyphosate sensitive biotype. Target-site resistance mechanisms were not detected; no amino acid substitutions or increased copy number of the EPSPS gene were observed. Liquid chromatography mass spectrometry was used to quantify glyphosate. At 7 and 10 days after application, glyphosate concentrations in aboveground tissue were 48 and 50% lower, respectively, in the resistant compared to the susceptible biotype; however, there were no decreases in belowground glyphosate concentrations. Collectively, these results provide putative evidence for altered glyphosate fate as a resistance mechanism in S. halepense . Additionally, this study describes the development of novel molecular markers for EPSPS amplification and copy number variation, addressing a key limitation in resistance diagnostics for this polyploid weed species.