Enhanced Metabolism Evolved High-Level Resistance to Fenoxaprop-P-Ethyl in Alopecurus japonicus

Alopecurus japonicus has been a serious weed across China and developed resistance to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicide. The A. japonicus ACCase gene mutations accounting for target-site resistance (TSR) have been clarified, while non-target-site resistance (NTSR) is not distinct. Here, dose–response testing indicated that an A. japonicus population (R) was resistant to four ACCase-inhibiting herbicides fenoxaprop-P-ethyl, sethoxydim, clethodim, and pinoxaden. Compared with herbicide-susceptible A. japonicus (S), no known resistant mutations for TSR in ACCase were identified in the R population using sequencing. To investigate the NTSR mechanism, increased metabolism of fenoxaprop-P-ethyl was detected in the R population using high-performance liquid chromatography (HPLC) analysis. Notably, resistance cannot be reversed by P450 and GST inhibitors. RNA-seq was performed to further explore the resistance mechanisms, and eight candidate contigs (four glycosyl transferases (GT) and four ATP-binding cassette (ABC) transporters) were chosen and their expression patterns were validated using RT-qPCR. Three GT and three ABC transporter contigs were constitutively upregulated in the R population. In short, six contigs expressed highly in the R population causing enhanced fenoxaprop-P-ethyl metabolism appear to be involved in fenoxaprop-P-ethyl resistance.