Gaeumannomyces graminis var. graminis produces
1,8-dihydroxynaphthalene (DHN) melanin in its hyphal and hyphopodial cell
walls.
We isolated G. graminis mutants that were affected in their melanin
biosynthesis. One was unable to synthesize DHN-melanin and,
because it accumulated 2-hydroxyjuglone, a DHN melanin pathway shunt product,
it is most likely to be defective in the reductase
that catalyzes the conversion of 1,3,8-trihydroxynaphthalene to vermelone,
the penultimate reaction in DHN synthesis. Genetic
crosses with our wild-type strain indicated that this trihydroxynaphthalene
reductase deficiency was the result of a single mutation.
Another mutant constitutively synthesized DHN melanin and genetic crosses
with our wild-type strain suggested that this heavily
melanized mutant had a single mutation responsible for its phenotype. This
mutant produced more melanin than the wild-type strain
as measured by Azure A binding to melanin. The wild type and constitutively
melanized mutant hyphae were more hydrophobic
and more resistant to lytic enzymes, benomyl, restrictive temperature,
and uv light than the non-melanized mutant, which also
autolysed more readily. The non-melanized mutant was not more sensitive
to heavy metal than the melanized strains. In addition,
the non-melanized mutant was unaltered in pathogenicity to rice, whereas
the constitutively melanized mutant was less pathogenic.
The constitutively melanized mutant produced less extracellular lytic enzymes
than the wild-type and the non-melanized mutant,
which may explain its reduced virulence.