Target site of a new antifungal compound KC10017 in the melanin biosynthesis of Magnaporthe grisea

A new experimental fungicide KC10017 (3-[4'-bromo-2',6'-dimethylphenoxy]met hyl-4-[(3"-methylphenyl) aminocarbonyl]methyl-1,2,4-oxadiazol-5-one) comple tely controlled rice blast disease at more than 1.0 mu g/ml without curativ e activity. When the chemical was applied after the inoculation was made wi th injury, the fungicide had no effect on controlling rice blast. The fungi cide at concentrations of more than I mu g/ml blocked appressorial melaniza tion and penetration of cellophane membrane by Magnaporthe grisea P-2 witho ut affecting fungal growth, spore germination, and appressorial formation. To elucidate the target site of KC10017 in melanin biosynthesis inhibition, M. grisea P-2 was grown in potato dextrose broths treated with both KC1001 7 and tricyclazole. The mycelia and culture media of cultures treated with both KC10017 and tricyclazole at a concentration of 5 mu g/ml each became c olorless like as the cultures treated with KC10017 alone, whereas the tricy clazole-treated cultures exhibited red color. Shunt products of 3,4,8-trihy droxy-1-(2H)-naphthalenone, 3,4-dihydro-4,8-dihydroxy-1-(2H)-naphthalenone 3,4-dihydro-4,6,8-trihydroxy-1-(2H)-naphthalenone, and 4-hydroxyscytalone w ere absent or present in only trace amounts in the cultures treated with KC 10017 alone or both KC10017 and tricyclazole. However, they were detected a t much higher levels in the cultures treated with tricyclazole alone. On th e other hand, appressorial melanization and penetration ability in M. grise a treated with KC10017 was recovered by the addition of scytalone. These re sults demonstrate that the target site of KC10017 in melanin biosynthesis i nhibition is at the pentaketide synthesis step and for at the 1,3,6,8-tetra hydroxynaphthalene formation. (C) 1998 Academic Press.