Mechanisms of resistance to bispyribac-sodium in an Echinochloa phyllopogon accession

Weeds are a major problem for rice production in California, and late water grass (Echinochloa phyllopogon (Stapf) Koss) is one of the most serious wee ds in water-seeded rice. Severe infestations can reduce yields by more than 50%. Flooding only partially controls this weed; thus, farmers rely heavil y on herbicides. Resistance to several herbicides, including bispyribac-sod ium, an acetolactate synthase (ALS) inhibitor not yet commercially used, ha s developed in late watergrass populations of California rice. knowing the mechanisms of bispyribac resistance is relevant to designing herbicide mana gement strategies for delaying resistance development to enhance the succes sful introduction of this new herbicide. We examined whether an insensitive ALS and cvt P-450-dependent detoxification were possible resistance mechan isms in a bispyribac-sodium-resistant (R) late watergrass population collec ted in California rice fields. which was previously determined to be resist ant to molinate, thiobencarb, and fenoxaprop-ethyl. ALS activity was assaye d on leaf extracts from young R and susceptible (S) plants for a range of b ispyribac-sodium concentrations, and cross-resistance to another ALS inhibi tor, bensulfuron-methyl, was evaluated using whole-plant bioassays. Resista nce was not due to reduced ALS sensitivity to bispyribac-sodium in R plants , although the R accession was highly cross-resistant to bensulfuron-methyl . Although S and R plants had similar ALS activity (mg acetoin mg protein(- 1)) without herbicide. more (P < 0.05) leaf protein was extracted from R 15 .35 mg g(-1) leaf fresh weight) than from S (3.19 mg g(-1)) plants, and gen eral ALS activity (mg acetoin g leaf fresh weight(-1)) for all herbicide co ncentrations was higher in R than in S plants. The cyt P-450 inhibitors pip eronyl butoxide and malathion were used for detection of herbicide degradat ion by cyt P-450 monooxygenation. The addition of these inhibitors strongly enhanced herbicide phytotoxicity toward R plant, suggesting that metabolic degradation of bispyribac-sodium contributed significantly to the observed resistance. (C) 2000 Academic Press.