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 content of Scenedesmus cells in the light and inhibited phytoene desaturase. The relationship between substituents at various positions and inhibition of phytoene desaturase is discussed. Using very active bleaching diphenylpyrrolidinones, a 10(-5) M concentration affected neither the zeta-carotene desaturase nor the protoporphyrinogen-IX oxidase. Although some differences in their inhibitory activity were found between the in vivo and cell-free assays, it is concluded that the compounds are essentially bleachers affecting carotenoid biosynthesis in plants. Enzyme kinetics studies with recombinant 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 cofactor-binding site of phytoene desaturase.