Exploratory Analysis on Herbicide Metabolism and Very-Long-Chain Fatty Acid Production in Metolachlor-Resistant Palmer Amaranth ( Amaranthus palmeri S. Wats.).

A Palmer amaranth ( Amaranthus palmeri S. Wats.) biotype resistant to S -metolachlor was confirmed from crop fields in Arkansas, USA. This study investigated the metabolic effects of malathion (cytochrome P450 inhibitor) and 4-chloro-7-nitrobenzofurazan [NBD-Cl; glutathione S -transferase inhibitor] on the S -metolachlor-resistant A. palmeri biotype. Root elongation of the resistant biotype was 20% more inhibited by treatment of NBD-Cl (50 nM) and S -metolachlor (2 μM) in mixture than by treatment of S -metolachlor alone. Metabolites of S -metolachlor were 1.4-12.1 times greater produced in the resistant biotype for 7 d than in the susceptible standard. Production of cerotic acid, one of the very-long-chain fatty acids containing 26 carbons, was more reduced in the susceptible standard (3.8-fold) than in the resistant biotype (1.8-fold) by S -metolachlor treatment. Conclusively, evolution of S -metolachlor resistance observed in this study was likely associated with improved activity of glutathione S -transferases. Further studies are needed to genetically evaluate plant endogenous enzymes involving cerotic acid production.