Melanin Biosynthesis Inhibitors

Abstract

Carpropamid was the first commercial fungicide from the scytalone dehydratase inhibitors of melanin biosynthesis group (MBI-D). It has been widely used in Japan as the chemical agent for nursery-box treatment against leaf blast of paddy rice since 1998. Further MBI-Ds, diclocymet and fenoxanil, were launched in 2000 and 2001, respectively. In 2001, failure of control of rice blast was reported in southern part of Japan, where MBI-Ds have been used since 1998. The inhibitory activity of carpropamid on scytalone dehydratase (SDH) extracted from a carpropamid-resistant strain of Magnaporthe oryzae was dramatically reduced in comparison with that on SDH extracted from the sensitive strain. A single-point mutation (G to A) located at the upstream region (233 bp downstream from the ATG codon) resulting in a one-amino-acid substitution (valine [GTG] 75 to methionine [ATG]: V75M) was found in the resistant strain. To examine whether the V75M mutation is the primary reason for decreasing the sensitivity of SDH to carpropamid, the SDH cDNAs of both the sensitive and the resistant strain were cloned into a GST-fused protein expression vector system. The recombinant SDHs of both strains exhibited the same sensitivities to carpropamid as those extracted from the mycelia of the respective strains. These data clearly revealed that the V75M mutation causes the low sensitivities of the SDHs of the carpropamid-resistant strains and strongly suggest that the V75M mutation confers resistance of these strains to carpropamid.