A Mode of Action of Anti-blast Substances

Abstract

The effects of organophosphorus fungicides, IBP (S-benzyl O, O-diisopropyl phosphorothiolate, Kitazin P®), EDDP (O-ethyl S, S-diphenyl phosphorodithiolate, edifenphos, Hinosan®) and ESBP (S-benzyl O-ethyl phenylphosphonothiolate, Inezin®) for Pyricularia oryzae were investigated. The fungicides did not significantly inhibited on DNA, RNA, protein and chitin biosynthesis, exogeneous respiration and leakage of potassium at the ED50 concentration value for mycelial growth. On the other hand, the close correlation between growth inhibition and the inhibition of phosphatidylcholine biosynthesis by sequential methylations of phosphatidylethanolamine (Greenberg pathway) was observed, but scarcely inhibited the biosynthesis of phosphatidylcholine from choline through CDP-choline (Kennedy pathway). Phosphatidylcholine biosynthesis via the Kennedy pathway was strongly inhibited by hemicholinium-3, however, the mycelial growth was not influenced. Therefore, phosphatidylcholine biosynthesis via the Kennedy pathway was not essential for the mycelial growth of P. oryzae. Thus, we conclude from the experimental results described that the primary antifungal action of organophosphorus fungicides is the inhibition of the phosphatidylcholine biosynthesis via the Greenberg pathway. Next, the degradation of IBP, EDDP and BPA (dibutyl N-methyl-N-phenylphosphoramidate) by P. oryzae, and the degradation of IBP by a pig liver reconstitution mixed function oxidase enzyme system were investigated. From the experimental results, we conclude the degradation of IBP, EDDP and BPA by mycelial cells of P. oryzae are catalyzed by mixed function oxidase. And next, we isolated a group of novel phytoalexins (Oryzalexin A, B and C) from rice plant (Oryza sativa) leaves infected with P. oryzae, and presented the absolute configurations of these substances as (+)-sandaracopimaradiene derivatives. Now, we are interested in the biosynthesis of Oryzalexins as to elucidate the dynamic aspect of plant protection.