Mode of bleaching phytotoxicity of herbicidal diphenylpyrrolidinones

A mode of action study of herbicidal diphenylpyrrolidinones was carried out through carotenoid analyses in intact Scenedesmus cells and by a cell-free plant-type phytoene desaturase assay using Escherichia coli transformants. A series of forty-eight diphenylpyrrolidinones decreased the carotenoid co ntent of Scenedesmus cells in the light and inhibited phytoene desaturase. The relationship between substituents at various positions and inhibition o f phytoene desaturase is discussed. Using very active bleaching diphenylpyr rolidinones, a 10(-5) M concentration affected neither the zeta -carotene d esaturase nor the protoporphyrinogen-IX oxidase. Although some differences in their inhibitory activity were found between the in vivo and cell-free a ssays, it is concluded that the compounds are essentially bleachers affecti ng carotenoid biosynthesis in plants. Enzyme kinetics studies with recombin ant phytoene desaturase revealed a non-competitive inhibition with respect to the substrate phytoene. A competition against the inhibitor was shown by the cofactor NADP(+), suggesting an interaction of pyrrolidinones at the c ofactor-binding site of phytoene desaturase. (C) 2001 Society of Chemical I ndustry.