Insights into the molecular mechanism and management of resistant Echinochloa crus-galli biotypes from the Philippines to acetolactate synthase inhibitor bispyribac-sodium

Rice production in the Philippines is heavily reliant on herbicide application for weed control. Acetolactate synthase (ALS) inhibitors are the mostly used herbicides for combating major weeds associated to rice, such as Echinochloa crus-galli. In separate locations from the Philippines, rice farmers expressed concerns that the application of ALS inhibitor bispyribac-sodium has been showing poor control of E. crus-galli. Here, we confirmed and quantified the level of bispyribac-sodium resistance in the putative resistant biotypes. Resistance was confirmed in R4 and R12 biotypes collected from the northern and central parts of the Philippines, respectively. The estimated GR50 (dose resulting in 50% growth reduction) revealed a 36.7-and 60.3-fold resistance in R4 and R12 biotypes (106.8 g a.i. ha-1 and 175.4 g a.i. ha-1) compared to the S biotype (2.91 g a.i. ha-1). The R4 biotype carries a single nucleotide mutation in the ALS2 gene translating to Trp-574-Leu amino acid substitution, while there was no mutation detected in any of the ALS conserved regions of R12. Pre-treatment of malathion increased the sensitivity of R12 to bispyribac-sodium, suggesting the involvement of cytochrome P450s. Sensitivity assays to alternative post-emergence herbicides showed that both R4 and R12 can be effectively controlled by metamifop [acetyl-CoA carboxylase (ACCase) inhibitor]. However, R12 survived the application of fenoxaprop-P-ethyl + ethoxysulfuron (ACCase inhibitor + ALS inhibitor). This study unravels the molecular mechanism of the first resistance case to ALS inhibitor in the Philippines and provide practical implications for efficient herbicide resistance management in the affected rice areas.