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Metabolism of pyroxasulfone in a tolerant crop, wheat and a susceptible plant, rigid ryegrass, was studied using 14 C-pyroxasulfone. The main metabolites were a cysteine conjugate of the isoxazoline ring (M-26), deaminated M-26 (M-29) and a glucose conjugate of M-29, suggesting that the main metabolic route in both plants was the cleavage of the methylenesulfonyl linkage caused by glutathione conjugation. The difference in the metabolic activity was assumed to be one of the factors in determining the selectivity of pyroxasulfone between wheat and rigid ryegrass.

Pyroxasulfone has been shown to be a potent inhibitor of very-long-chain fatty acid elongase (VLCFAE) of plants; therefore, it is categorized within the K3 group of herbicides. In this paper, we studied the properties of pyroxasulfone as a VLCFAE inhibitor in more detail by examination of its inhibitory effects on VLCFAEs from some plants. Pyroxasulfone inhibited microsomal VLCFAE activities of rice, Italian ryegrass, barnyard millet, wheat, corn and soybean with time-independent reversible inhibition. There were differences in the inhibitory potencies against VLCFAEs between pyroxasulfone-susceptible plants (rice, Italian ryegrass and barnyard millet) and pyroxasulfone-tolerant plants (wheat, corn and soybean). This result confirmed that the difference in the sensitivities of VLCFAEs to pyroxasulfone was one of the factors involved in the selectivity of pyroxasulfone between crops and weeds. We succeeded in preparing recombinant VLCFAEs of Arabidopsis (FAE1) and rice (Q6F365) using yeast and rice cultured cells and constructed each assay system. VLCFAE activities of FAE1 and Q6F365 were potently inhibited by pyroxasulfone; however, a critical difference was found in the inhibition manner between FAE1 and Q6F365. Pyroxasulfone inhibited FAE1 in a time-dependent manner, whereas it inhibited Q6F365 in a time-independent manner. The time-independent reversible inhibition of rice Q6F365 and microsomal VLCFAEs of plants proposes a new inhibition mechanism of VLCFAEs by pyroxasulfone.

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