Multiple-herbicide resistant Echinochloa phyllopogon has been found in California paddy fields. The resistant line exhibits resistance to herbicides from at least five modes of action including acetolactate synthase (ALS) inhibitors and acetyl-CoA carboxylase (ACCase) inhibitors. Previously, we identified two ALS-herbicide-metabolizing cytochrome P450 genes (CYP81A12 and CYP81A21) whose overexpressions are associated with the ALS inhibitor resistance. Meanwhile, the genes involved in the resistance to ACCase inhibitors remain to be identified. To better understand resistance mechanism to ACCase inhibitors, we examined susceptibility of the resistant line to four ACCase inhibitors from three chemical classes such as fenoxaprop-P-ethyl and diclofop-methyl (FOPs), tralkoxydim (DIM) and pinoxaden (DEN). The resistant line exhibited reduced susceptibility to all the four herbicides compared to the susceptible line. Rice transformation of CYP81A12 or CYP81A21 was performed to evaluate its possible involvement in the reduced susceptibility to ACCase inhibitors. Transgenic rice expressing either of the genes grew vigorously in media containing diclofop-methyl, tralkoxydim or pinoxaden while the growth of wild type rice was severely suppressed. Interestingly, fenoxaprop-P-ethyl resistance was not observed in the P450 expressing rice. The results strongly suggest that the overexpressions of the P450 genes cause metabolic cross-resistance to ALS inhibitors and some ACCase inhibitors, diclofop-methyl, tralkoxydim and pinoxaden, while other mechanisms are suggested for fenoxaprop-P-ethyl resistance.

Mechanism of multiple-herbicide resistance in Echinochloa phyllopogon

256th National Meeting (2018)

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