Metamifop resistance in Echinochloa glabrescens via glutathione S-transferases-involved enhanced metabolism.

Background: Echinochloa glabrescens Munro ex Hook. f. is one of the main Echinochloa spp. seriously invading Chinese rice fields and has evolved resistance to commonly used herbicides. Previously, an E. glabrescens population (LJ-02) with suspected resistance to the acetyl-CoA carboxylase (ACCase)-inhibiting herbicide metamifop was collected. This study aimed to determine its resistance status to metamifop and investigate the internal molecular mechanisms of resistance.

Results: Single-dose testing confirmed that the LJ-02 population had evolved resistance to metamifop. Gene sequencing and a relative expression assay of ACCase ruled out target-site based resistance to metamifop in LJ-02. Whole-plant bioassays revealed that, compared with the susceptible population XZ-01, LJ-02 was highly resistant to metamifop and exhibited cross-resistance to fenoxaprop-P-ethyl. Pretreatment with the known glutathione S-transferase (GST) inhibitor, 4-chloro-7-nitrobenzoxadiazole (NBD-Cl), largely reversed the resistance to metamifop by approximately 81%. Liquid chromatography-tandem mass spectrometry analysis indicated that the metabolic rates of one of the major metabolites of metamifop, N-(2-fluorophenyl)-2-hydroxy-N-methylpropionamide (HPFMA), were up to 383-fold faster in LJ-02 plants than in XZ-01 plants. There were higher basal and metamifop-inducible GST activities toward 1-chloro-2,4-dinitrobenzene (CDNB) in LJ-02 than in XZ-01. Six GST genes were metamifop-induced and overexpressed in the resistant LJ-02 population.

Conclusion: This study reports, for the first time, the occurrence of metabolic metamifop resistance in E. glabrescens worldwide. The high-level metamifop resistance in the LJ-02 population may mainly involve specific isoforms of GSTs that endow high catalytic activity and strong substrate specificity. © 2023 Society of Chemical Industry.