Site of inhibition by tricyclazole in the melanin biosynthetic pathway of Verticillium dahliae

Abstract

The effect of tricylazole (5-methyl-1,2,4-triazolo-(3,4- b)-benzothiazole) on melanin biosynthesis in Verticillium dahliae was studied using a wild-type strain and brown (brm-1) and albino (alm-1 and alm-3) mutants deficient in normal pigmentation. Polyketide precursors of melanin and related compounds were isolated and identified and the effect of tricyclazole on their production was determined. Tricyclazole at 1 μg/ml primarily inhibited the reduction of 1,3,8-trihydroxynaphthalene to vermelone. This inhibition resulted in enhanced accumulation of scytalone, 2-hydroxyjuglone, and 3,4-dihydro-4,8-dihydroxy-1(2 H)-naphthalenone. At 10 μg/ml, tricyclazole inhibited not only reduction of 1,3,8-trihydroxynaphthalene to vermelone, but also other reactions of secondary sensitivity, leading to enhanced accumulation of scytalone, flaviolin, and 2-hydroxyjuglone and to suppressed production of 3,4-dihydro-4,8-dihydroxy-1(2 H)-naphthalenone. Tricyclazole inhibited the conversion of scytalone to melanin by V. dahliae albino mutants, but not the conversion of vermelone or 1,8-dihydroxynaphthalene to melanin. Similar fungal responses were exhibited when Pyricularia oryzae was tested for melanin precursor activity. The primary site of action of tricyclazole in the melanin biosynthetic pathway of V. dahliae is between 1,3,8-trihydroxynaphthalene and vermelone. The data indicate that tricyclazole mainly interferes with a reductive step at this site in the melanin biosynthetic pathway in both V. dahliae and P. oryzae. The possible relationship between inhibition of polyketide metabolism and protection of rice plants from infection by P. oryzae is discussed.