Isolation and characterisation of Gaeumannomyces graminis var. graminis melanin mutants

Gaeumannomyces graminis var. graminis produces 1,8-dihydroxynaphthalene (DH N) melanin in its hyphal and hyphopodial cell walls. We isolated G. gramini s mutants that were affected in their melanin biosynthesis. One was unable to synthesize DHN-melanin and, because it accumulated 2-hydroxyjuglone, a D HN melanin pathway shunt product, it is most likely to be defective in the reductase that catalyzes the conversion of 1,3,8-trihydroxynaphthalene to v ermelone, the penultimate reaction in DHN synthesis. Genetic crosses with o ur wild-type strain indicated that this trihydroxynaphthalene reductase def iciency was the result of a single mutation. Another mutant constitutively synthesized DHN melanin and genetic crosses with our wild-type strain sugge sted that this heavily melanized mutant had a single mutation responsible f or its phenotype. This mutant produced more melanin than the wild-type stra in as measured by Azure A binding to melanin. The wild type and constitutiv ely melanized mutant hyphae were more hydrophobic and more resistant to lyr ic enzymes, benomyl, restrictive temperature, and uv light than the non-mel anized mutant, which also autolysed more readily. The non-melanized mutant was not more sensitive to heavy metal than the melanized strains. In additi on, the non-melanized mutant was unaltered in pathogenicity to rice, wherea s the constitutively melanized mutant was less pathogenic. The constitutive ly melanized mutant produced less extracellular lytic enzymes than the wild -type and the non-melanized mutant, which may explain its reduced virulence .