Resistance to a nonselective 4‐hydroxyphenylpyruvate dioxygenase‐inhibiting herbicide via novel reduction–dehydration–glutathione conjugation in <i>Amaranthus tuberculatus</i>
Jeanaflor Crystal T. Concepcion, Shiv Shankhar Kaundun, James A. Morris, Sarah‐Jane Hutchings, Seth A. Strom, Anatoli V. Lygin, Dean E. Riechers
Abstract
Summary Metabolic resistance to 4‐hydroxyphenylpyruvate dioxygenase (HPPD)‐inhibiting herbicides is a threat in controlling waterhemp ( Amaranthus tuberculatus ) in the USA. We investigated resistance mechanisms to syncarpic acid‐3 (SA3), a nonselective, noncommercial HPPD‐inhibiting herbicide metabolically robust to Phase I oxidation, in multiple‐herbicide‐resistant (MHR) waterhemp populations (SIR and NEB) and HPPD inhibitor‐sensitive populations (ACR and SEN). Dose–response experiments with SA3 provided ED 50 ‐based resistant : sensitive ratios of at least 18‐fold. Metabolism experiments quantifying parent SA3 remaining in excised leaves during a time course indicated MHR populations displayed faster rates of SA3 metabolism compared to HPPD inhibitor‐sensitive populations. SA3 metabolites were identified via LC‐MS‐based untargeted metabolomics in whole plants. A Phase I metabolite, likely generated by cytochrome P450‐mediated alkyl hydroxylation, was detected but was not associated with resistance. A Phase I metabolite consistent with ketone reduction followed by water elimination was detected, creating a putative α,β‐unsaturated carbonyl resembling a Michael acceptor site. A Phase II glutathione–SA3 conjugate was associated with resistance. Our results revealed a novel reduction–dehydration–GSH conjugation detoxification mechanism. SA3 metabolism in MHR waterhemp is thus atypical compared to commercial HPPD‐inhibiting herbicides. This previously uncharacterized detoxification mechanism presents a unique opportunity for future biorational design by blocking known sites of herbicide metabolism in weeds.