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

• Published in: Plant and cell physiology Vol. 62; no. 11; pp. 1770 - 1785
• Main Authors: Strom, Seth A, Hager, Aaron G, Concepcion, Jeanaflor Crystal T, Seiter, Nicholas J, Davis, Adam S, Morris, James A, Kaundun, Shiv S, Riechers, Dean E
• Format: Journal Article
• Language: English
• Published: UK Oxford University Press 10.12.2021
• Subjects: Acetamides - pharmacology; Amaranthus - drug effects; Amaranthus - metabolism; Herbicide Resistance; Herbicides - pharmacology; Metabolic Networks and Pathways; Plant Weeds - drug effects; Plant Weeds - metabolism; Regular Paper; Zea mays - drug effects; Zea mays - metabolism; Xenobiotic detoxification; Herbicide metabolism; Weed resistance; transferase; Cytochrome P450; Glutathione; Glutathione S-transferase; Amaranthus
• ISSN: 0032-0781; 1471-9053
• DOI: 10.1093/pcp/pcab132

Abstract 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.