Mechanism of clomazone resistance in Leptochloa fusca spp. fasicularis to clomazone

Bearded sprangletop (Leptochloa fusca spp. fasicularis) is a problematic weed in California rice (Oryza sativa) production. Historically, bearded sprangletop is controlled with clomazone in California rice fields. The continuous use of clomazone and lack of crop rotation in rice fields resulted in resistance to clomazone in several bearded sprangletop. The objective of this research was to determine the clomazone mechanism of resistance of two bearded sprangletop populations in California rice by investigating clomazone absorption, translocation, and metabolism under controlled environmental conditions in two resistant (CRBS1 and CRBS2) and one susceptible (S) populations. Absorption and translocation of 14C-clomazone were similar in R and S. Clomazone metabolism, as determined by inhibition of cytochrome P450 enzymes with malathion and determining clomazone metabolites profile, was different between S and R 3 days after treatment. Bearded sprangletop pretreated with malathion was 2-fold more sensitive to clomazone than when treated with clomazone alone, suggesting that cytochrome P450-mediated clomazone metabolism might be involved in the mechanism of resistance. An HPLC-MS/MS analysis revealed differences in clomazone metabolism between R and S biotypes. Hydroxymethylclomazone was the most abundant metabolite found in R plants with three and five-fold more abundant in CRBS1 and CRBS2, respectively, when compared to S plants. 5-ketoclomazone, the known toxic metabolite of clomazone, accumulated 2-fold more in S plants than in R plants at 72 h after treatment. This research shows that clomazone is metabolized differently between R and S populations of bearded sprangletop and that P450 monooxidation is likely involved in the mechanism of resistance.