Enhanced metabolism and target gene overexpression confer resistance against acetolactate synthase‐inhibiting herbicides in Bromus sterilis

Abstract BACKGROUND Intensive application of acetolactate synthase (ALS)‐inhibiting herbicides has resulted in herbicide‐resistance in many weeds, including Bromus sterilis . The present study was conducted to identify the mechanisms conferring resistance to ALS‐inhibiting herbicides in a Bromus sterilis biotype. RESULTS Dose–response studies revealed the resistant biotype to be 288 times less sensitive to pyroxsulam than the susceptible biotype. Furthermore, experiment with a single‐dose, proved this biotype was also cross‐resistant to propoxycarbazone, iodosulfuron plus mesosulfuron and sulfosulfuron. Prior treatment with malathion, a known inhibitor of cytochrome P450s, reduced the level of resistance to pyroxsulam. No mutations were detected from the partial ALS gene sequencing. Flow cytometry and chromosome counting rejected ploidy level variation between the susceptible and resistant biotypes. Relative copy number variation ruled out gene amplification. Quantitative real‐time polymerase chain reaction (PCR) detected a significant difference in ALS gene expression between the susceptible and resistant biotypes. CONCLUSIONS Target gene overexpression and enhanced metabolism by cytochrome P450s are likely mechanisms of resistance to pyroxsulam in Bromus sterilis . The current findings highlight the need to monitor additional brome populations for herbicide resistance in Europe and endorse the need for alternate herbicides in integrated weed management to delay the possible evolution of herbicide resistance in these species. © 2020 Society of Chemical Industry