Metribuzin resistance via enhanced metabolism in a multiple herbicide resistant Lolium rigidum population.

Background: The photosystem II (PSII)-inhibiting herbicides are important for Australian farmers to control Lolium rigidum Gaud. and other weed species in trazine tolerant (TT)-canola fields. A L. rigidum population (R) collected from a TT-canola field from Western Australia showed multiple resistance to PSII, acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors. The mechanisms of multiple resistance in this R population were determined.

Results: The R population showed a low-level (about 3.0-fold) resistance to the PSII-inhibiting herbicides metribuzin and atrazine. Sequencing of the psbA gene revealed no differences between the R and susceptible (S) sequences. Furthermore, [ C]-metribuzin experiments found no significant difference in metribuzin foliar uptake and translocation between the R and S plants. However, [ C]-metribuzin metabolism in R plants was 2.3-fold greater than in S plants. The cytochrome P450 monooxygenase inhibitor piperonyl butoxide (PBO) enhanced plant mortality response to metribuzin and atrazine in both R and S populations. In addition, multiple resistance to ALS and ACCase inhibitors are due to known resistance mutations in ALS and ACCase genes.

Conclusion: The results demonstrate that enhanced metribuzin metabolism likely involving cytochrome P450 monooxygenase contributes to metribuzin resistance in Lolium rigidum. This is the first report of metabolic resistance to the PSII-inhibiting herbicide metribuzin in Australian Lolium rigidum. © 2020 Society of Chemical Industry.