Involvement of esterase in phytotoxicity of a new pyrrolinone compound, methyl 1-[1-(3,5-dichlorophenyl)-1-methylethyl]-2,3-dihydro-4-methyl-2-oxo-3-phenyl-1H-pyrrole-3-carboxylate, to early watergrass (Echinochloa oryzicola) and rice (Oryza sativa)

A new pyrrolinone compound, methyl 1-[1-(3,5-dichlorophenyl)-1-methylethyl]-2,3-dihydro-4-methyl-2-oxo-3-phenyl-1H-pyrrole-3-carboxylate (PC-1), has herbicidal activity against early watergrass (Echinochloa oryzicola). This compound showed good selectivity between rice (Oryza sativa) and early watergrass due to the introduction of a methoxycarbonyl group into the pyrrolinone structure. To elucidate the mechanism of PC-1's phytotoxicity, the effects of esterase inhibitors (triphenyl phosphate, tributyl phosphate and fenitrothion) and a cytochrome P450 monooxygenase inhibitor (1-aminobenzotriazole) on the actions of PC-1 were examined. Triphenyl phosphate and fenitrothion reduced the phytotoxicity of PC-1 in hydroponics and greenhouse tests. Porcine liver esterase converted PC-1 to 1-[1-(3,5-dichlorophenyl)- 1-methylethyl]-4-methyl-3-phenyl-3-pyrrolin-2-one (PC-2) and this reaction was inhibited by triphenyl phosphate. The PC-2 content of early watergrass treated with PC-1 was 4 times that of rice. These results suggest that PC-1 is a prodrug and is converted to PC-2 by esterase, and that the amount of PC-2 produced in PC- 1-treated plants is related to the sensitivity of early watergrass and rice to PC-1.