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

Area of Science:

• Agricultural Science
• Plant Science
• Biochemistry

Background:

• Herbicide resistance in weeds is a growing agricultural problem.
• Non-target-site resistance mechanisms, like metabolic detoxification, are crucial for weed survival.
• S-metolachlor, a very-long-chain fatty acid inhibitor, is widely used but faces resistance challenges.

Purpose of the Study:

• To elucidate the enzymatic pathways responsible for S-metolachlor metabolism in herbicide-resistant waterhemp (Amaranthus tuberculatus).
• To compare the metabolic activities of glutathione S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) in resistant and sensitive waterhemp populations.

Main Methods:

• Enzyme assays were conducted using S-metolachlor to measure GST and P450 specific activities.
• Microsomal extracts from resistant (CHR, SIR) and sensitive (WUS) waterhemp, along with tolerant corn, were analyzed.
• Metabolite profiling of S-metolachlor was performed using liquid chromatography-mass spectrometry (LC-MS).

Main Results:

• GST activities were moderately increased in resistant waterhemp compared to sensitive populations.
• P450s in resistant waterhemp exhibited significantly higher ( >20-fold) O-demethylation activity of S-metolachlor compared to sensitive waterhemp and corn.
• Metabolite analysis confirmed greater abundance of O-demethylated S-metolachlor and its glutathione conjugates in resistant waterhemp.

Conclusions:

• S-metolachlor resistance in waterhemp involves a coordinated effort of Phase I (P450-mediated O-demethylation) and Phase II (GST-mediated conjugation) metabolic pathways.
• The initial O-demethylation step catalyzed by P450s is the primary mechanism conferring S-metolachlor resistance in waterhemp.