Biochemical and structural characterization of quizalofop-resistant wheat acetyl-CoA carboxylase

Differential Resistance to Acetyl-CoA Carboxylase Inhibitors in Rice: Insights from Two Distinct Target-Site Mutations

Weeds present a significant challenge to agricultural productivity, and acetyl-CoA carboxylase (ACCase)-inhibiting herbicides have proven to be effective in managing weed populations in rice fields. To develop ACCase-inhibiting herbicide-resistant rice, we generated mutants of rice ACCase (OsACC) featuring Ile-1792-Leu or Gly-2107-Ser substitutions through ethyl methyl sulfonate (EMS) mutagenesis. The Ile-1792-Leu mutant displayed cross-resistance to aryloxyphenoxypropionate (APP) and phenylpyrazoline (DEN) herbicides, whereas the Gly-2107-Ser mutants primarily exhibited cross-resistance to APP herbicides with diminished resistance to the DEN herbicide. In vitro assays of the OsACC activity revealed an increase in resistance to haloxyfop and quizalofop, ranging from 4.84- to 29-fold in the mutants compared to that in wild-type. Structural modeling revealed that both mutations likely reduce the binding affinity between OsACC and ACCase inhibitors, thereby imparting resistance. This study offers insights into two target-site mutations, contributing to the breeding of herbicide-resistant rice and presenting alternative weed management strategies in rice cultivation.

Current status of cyhalofop-butyl and metamifop resistance and diversity of the ACCase gene mutations in Chinese sprangletop (Leptochloa chinensis) from China

Leptochloa chinensis populations in China have evolved widespread resistance to acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicides cyhalofop-butyl (CyB) and metamifop (Met). 124 L. chinensis populations, randomly collected from rice fields in Jiangsu Province, were surveyed for CyB and Met resistance status, and all potential ACCase gene resistance-conferring mutations and effective pre-emergence herbicides for its control were investigated. Single-dose tests confirmed that 82 (66.1%) and 70 (56.4%) populations evolved resistance to CyB and Met, respectively. ACCase sequencing revealed that 56.4% of the populations contain plants with diverse target-site ACCase mutations (Ile1781Leu, Trp1999Cys, Trp2027Cys, Trp2027Ser, Ile2041Asn, Gly2096Ala, and in particular, a Leu1818Phe mutation). Notably, the Leu1818Phe mutation had been detected in 8 resistant populations, indicating this mutation was prone to occur in L. chinensis. Additionally, 9.7% of the populations may have single metabolic resistance to CyB, as these populations was susceptible to Met, and no any ACCase mutations were found. Moreover, the resistant populations with different ACCase mutations showed 6.5 to 33.6-fold resistance to CyB, and 4.4 to 82.6-fold resistance to Met. Importantly, five pre-emergence herbicides, including pretilachlor, pendimethalin, clomazone, pyraclonil, and mefenacet, all exhibited good control effect on resistant L. chinensis populations. This work confirmed the prevalence and distribution of CyB and Met resistance in L. chinensis. Target-site ACCase mutations made a major contribution to CyB and Met resistance. Pre-emergence herbicides could be valuable tools for management of resistant L. chinensis populations.

Survey of ACCase and ALS resistance in winter annual grasses identifies target-site and nontarget-site imazamox resistance in Secale cereale

BACKGROUND

Early detection of herbicide resistance in weeds is crucial for successful implementation of integrated weed management. We conducted a herbicide resistance survey of the winter annual grasses feral rye (Secale cereale), downy brome (Bromus tectorum), and jointed goatgrass (Aegilops cylindrica) from Colorado winter wheat production areas for resistance to imazamox and quizalofop.

RESULTS

All samples were susceptible to quizalofop. All downy brome and jointed goatgrass samples were susceptible to imazamox. Out of 314 field collected samples, we identified three feral rye populations (named A, B, and C) that were imazamox resistant. Populations B and C had a target-site mechanism with mutations in the Ser653 residue of the acetolactate synthase (ALS) gene to Asn in B and to Thr in C. Both populations B and C had greatly reduced ALS in vitro enzyme inhibition by imazamox. ALS feral rye protein modeling showed that steric interactions induced by the amino acid substitutions at Ser653 impaired imazamox binding. Individuals from population A had no mutations in the ALS gene. The ALS enzyme from population A was equally sensitive to imazamox as to known susceptible feral rye populations. Imazamox was degraded two times faster in population A compared with a susceptible control. An oxidized imazamox metabolite formed faster in population A and this detoxification reaction was inhibited by malathion.

CONCLUSION

Population A has a nontarget-site mechanism of enhanced imazamox metabolism that may be conferred by cytochrome P450 enzymes. This is the first report of both target-site and metabolism-based imazamox resistance in feral rye. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.