The mode of action of isoxaflutole I. Physiological effects, metabolism, and selectivity

Isoxaflutole (5-cyclopropyl isoxazol-4-yl-2-mesyl-4-trifluoromethylphenyl k etone) is a novel herbicide for preemergence control of a wide range of imp ortant broadleaf and grass weeds in corn and sugarcane producing characteri stic foliar bleaching activity in susceptible species prior to plant death. Following either foliar or root uptake, isoxaflutole is rapidly converted to a diketonitrile derivative (2-cyclopropyl-3-(2-mesyl-4-trifluoromethylph enyl)-3-oxopropanenitrile) by opening of the isoxazole ring. This diketonit rile undergoes degradation to a benzoic acid derivative (2-mesyl-4-trifluor omethyl benzoic acid) in treated plants and the extent of this degradation is correlated to the degree of susceptibility, being most rapid in tolerant corn and slowest in susceptible Abutilon theophrasti. Following a preemerg ence application of [phenyl(U)-C-14]isoxaflutole, equivalent to 250 g ai ha (-1) only 10% of the extracted C-14 activity was the active diketonitrile i n corn, compared to 90% in A. theophrasti. The rate of diketonitrile degrad ation was intermediate in moderately susceptible Ipomea spp. The foliar sym ptoms are associated with an accumulation of phytoene in treated leaves of Brassica kaber, typical of herbicidal inhibitors of phytoene desaturase. Ho wever, neither the diketonitrile derivative nor isoxaflutole inhibits the d esaturation of radiolabeled phytoene by a microsomal extract of carrot cell cultures. However, the diketonitrile does inhibit 4-hydroxyphenylpyruvate dioxygenase, preventing the biosynthesis of homogentisate, the precursor of plastoquinone and or-tocopherol. The content of both these prenylquinones in emerging leaves of isoxaflutole-treated plants decreases prior to the ac cumulation of phytoene and the decrease in carotenoids. The mode of action of isoxaflutole is suggested to be due to an indirect inhibition of phytoen e desaturase resulting from the absence of plastoquinone, an essential cofa ctor for the desaturase. (C) 1998 Academic Press.