Target-site resistance to ALS-inhibiting herbicides in Apera spica-venti populations is conferred by documented and previously unknown mutations

P>In this study, whole-plant bioassays were performed on 72 Apera spica-venti populations that have survived application of acetolactate synthase (ALS)-inhibiting herbicides in recent years. Molecular genetic analysis of the ALS gene revealed a Thr mutation at Pro(197) within 67 populations. Sequencing of the whole ALS gene from wild-type and resistant plants not carrying the above-mentioned mutation revealed the presence of a Leu mutation at Trp(574) within two populations and an Asn mutation at Pro(197) within two populations. As the Pro(197)-Asn amino acid substitution is reported for the first time in a field-selected weed population, a Cleaved Amplified Polymorphic Sequences (CAPS) marker was developed for its quick detection. In addition, one novel mutation was found within a population that coded for a His substitution at Arg(377). Enzyme assays confirmed a significant reduction in inhibition of ALS activity compared with the wild type. This population showed resistance to sulfonylureas (SUs) and cross-resistance to sulfonylaminocarbonyltriazolinones (SCTs) and triazolopyrimidines (TPs) within the whole-plant bioassays. ALS protein sequence alignments from weedy and cultural plants revealed that the Arg377 is highly conserved among known wild-type enzymes. In agreement with existing literature concerning the structure and mechanisms of inhibition of plant ALS, this mutation is probably involved in target-site resistance to ALS inhibitors. Our results suggest that further single-nucleotide polymorphisms impairing proper herbicide performance might be selected within field populations in the near future, making the short- and long-range evolution of target-site resistance difficult to predict depending solely on herbicide use history.