Resistance to Fenoxaprop-P-ethyl in Wild Oats May Involve GST-Mediated Enhanced Antioxidant Capacity

Avena fatua (wild oat), a globally invasive weed threatening cereal production, has evolved herbicide resistance, complicating field management. A previous study identified an Australian resistant population (R1) with moderate resistance to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicide fenoxaprop-P-ethyl via nontarget-site resistance (NTSR). RNA-seq revealed that a glutathione S-transferase (GST) gene, AfGSTU16, was upregulated in R1. Transgenic rice overexpressing AfGSTU16 (AfGSTU16-OE) exhibited an enhanced tolerance to fenoxaprop-P-ethyl. However, in vitro assays of recombinant AfGSTU16 in Escherichia coli showed no metabolic activity toward fenoxaprop-P-ethyl, indicating resistance independent of direct herbicide degradation. Alternatively, AfGSTU16-OE rice accumulated lower hydrogen peroxide levels and displayed higher antioxidant enzyme activities under herbicide stress. Transcriptome analysis further showed enrichment in xenobiotic detoxification and antioxidant defense pathways. Collectively, these findings suggest that AfGSTU16 confers fenoxaprop-P-ethyl resistance in the wild oat mainly by enhancing antioxidant capacity to alleviate herbicide-induced oxidative damage rather than via direct herbicide metabolic degradation.