Metabolic Pathways for S-Metolachlor Detoxification Differ Between Tolerant Corn and Multiple-Resistant Waterhemp

Seth A. Strom, Aaron G. Hager, Jeanaflor Crystal T. Concepcion, Nicholas J. Seiter, Adam S. Davis, James A. Morris, Shiv S. Kaundun, Dean E. Riechers

Research output: Contribution to journalArticlepeer-review


Herbicide resistance in weeds can be conferred by target-site and/or non-target-site mechanisms, such as rapid metabolic detoxification. Resistance to the very-long-chain fatty acid-inhibiting herbicide, S-metolachlor, in multiple herbicide-resistant populations (CHR and SIR) of waterhemp (Amaranthus tuberculatus) is conferred by rapid metabolism compared with sensitive populations. However, enzymatic pathways for S-metolachlor metabolism in waterhemp are unknown. Enzyme assays using S-metolachlor were developed to determine the specific activities of glutathione S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) from CHR and SIR seedlings to compare with tolerant corn and sensitive waterhemp (WUS). GST activities were greater (∼2-fold) in CHR and SIR compared to WUS but much less than corn. In contrast, P450s in microsomal extracts from CHR and SIR formed O-demethylated S-metolachlor, and their NADPH-dependent specific activities were greater (>20-fold) than corn or WUS. Metabolite profiles of S-metolachlor generated via untargeted and targeted liquid chromatography-mass spectrometry from CHR and SIR differed from WUS, with greater relative abundances of O-demethylated S-metolachlor and O-demethylated S-metolachlor-glutathione conjugates formed by CHR and SIR. In summary, our results demonstrate that S-metolachlor metabolism in resistant waterhemp involves Phase I and Phase II metabolic activities acting in concert, but the initial O-demethylation reaction confers resistance.

Original languageEnglish (US)
Pages (from-to)1770-1785
Number of pages16
JournalPlant and Cell Physiology
Issue number11
StatePublished - Nov 1 2021


  • Amaranthus
  • Cytochrome P450
  • Glutathione S-transferase
  • Herbicide metabolism
  • Weed resistance
  • Xenobiotic detoxification

ASJC Scopus subject areas

  • Physiology
  • Plant Science
  • Cell Biology


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