Haloxyfop-R-methyl resistance in Polypogon fugax: involvement of ABCG25 and ACCase gene overexpression.

Abstract

Asia minor bluegrass (Polypogon fugax) is a problematic grass weed in Chinese canola fields, and resistance to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides is increasingly widespread. Here, we characterized a haloxyfop-R-methyl-resistant P. fugax population (CZ-R) collected from Chongzhou City, Sichuan Province. Whole-plant dose-response assays showed that the CZ-R population exhibited a 15.83-fold resistance level compared with the susceptible control CZ-S. Sequencing of the plastidic ACCase gene revealed no known target-site mutations, but ACCase transcript abundance was 5.55-fold higher in the CZ-R population. Pretreatment with the cytochrome P450 inhibitor malathion or the glutathione S-transferase inhibitor NBD-Cl did not reverse resistance, and LC-MS analysis showed no difference in haloxyfop-R-methyl residues between the two populations at 24 and 48 h after treatment. Screening of ABC transporter genes identified ABCG25 and ABCI19 as upregulated candidates in CZ-R, and heterologous overexpression in rice demonstrated that ABCG25, but not ABCI19, significantly enhanced haloxyfop-R-methyl tolerance. In addition, the resistant population showed lower H2O2 and malondialdehyde (MDA) levels and higher catalase (CAT) activity, accompanied by 7.13- and 5.61-fold upregulation of CAT1 and CAT2, respectively. Collectively, these results indicate that haloxyfop-R-methyl resistance in the CZ-R population was not associated with known target-site mutations, while inhibitor and residue analyses did not support major P450- or GST-mediated degradation of the parent herbicide as the predominant resistance mechanism. Instead, resistance was associated with constitutive ABCG25 and ACCase overexpression and with an enhanced antioxidant response characterized by higher CAT activity and reduced H2O2 and MDA accumulation. These findings support the coexistence of target-related and non-target-site resistance components and broaden our understanding of the diverse mechanisms underlying ACCase-inhibitor resistance in grass weeds.