The genetic basis of glyphosate (RoundUp®) resistance in the Central Valley agricultural weed hairy fleabane, and possible interactions with drought resistance
Agricultural weeds cause crop yield loss by competing with crop plants for resources, and chemical herbicides are one of the most effective ways to control these pest plants. The herbicide glyphosate (RoundUp®) targets the 5-enolpyruvulshikimate-3-phosphate synthase (EPSPS) gene, inhibiting its normal activity and causing plant death by preventing the synthesis of aromatic amino acids. Since the 1990s, many weed species have evolved resistance to this chemical, including mutations in the target EPSPS gene or non-target-site mechanisms of resistance. In 2007, the first glyphosate-resistant population of hairy fleabane (Erigeron (Conyza) bonariensis) was discovered in California’s Central Valley. Previous research on fleabane and the related species horseweed (E. canadensis) suggests that the resistance mechanism in this species may be due to transcriptional upregulation of the EPSPS gene and/or ATP-binding cassette (ABC) transporter genes responsible for sequestering glyphosate in the leaf mesophyll cell vacuoles, to prevent its translocation around the plant and subsequent toxicity. Although much molecular work has been performed on horseweed, very little has been performed on fleabane, which has a more limited geographical distribution as a problematic agricultural weed primarily in the Central Valley. Population genetic work performed on California fleabane indicates that glyphosate resistance has evolved multiple times in the Valley, and could have different genetic bases in different populations. Using a combination of field collection, greenhouse herbicide screening, Sanger sequencing, quantitative real-time PCR for candidate genes, and RNA-Seq, students in my lab are undertaking a comprehensive investigation of possible genetic changes underlying target and/or non-target resistance mechanisms in Valley populations of fleabane. Dr. Anil Shrestha and his students in the College of Agriculture at CSU, Fresno are partners on this project.
The genetic work involves germination of fleabane seeds in Petri plates (L: seedlings from Valley Population 7), followed by transplanting and spraying with glyphosate to screen for resistance. Among the candidate genes we are investigating is an ABC transporter gene, M10 (R: gel electrophoresis of gradient PCR products from a glyphosate-sensitive and glyphosate-resistant individual, to determine optimal annealing temperature).
A related graduate student project involves examining the interaction of herbicide resistance evolution and drought adaptations in Central Valley hairy fleabane, using common garden experiments. This question is particularly important given the current and future expected effects of climate change on Central Valley agriculture: California droughts are intensifying in duration and severity, and predicting how weeds will respond is crucial from an applied perspective, as well as interesting from an evolutionary perspective.