| Gaeumannomyces graminis var. graminis is a filamentous soil ascomycete that synthesizes 1,8-dihydroxynapthalene (DHN) melanin as a secondary metabolite. We isolated two mutant strains affected in the melanin biosynthetic pathway, using either chemical or UV mutagenesis. One of these mutants, JH4300, was unable to synthesize DHN-melanin. Because it accumulated 2-hydroxyjuglone, a melanin pathway shunt product, this mutant was most likely defective in the reductase that catalyzes the conversion of 1,3,8-trihydroxynapthalene to vermelone, the penultimate reaction in DHN synthesis. Genetic crosses with our wild-type strain indicated that this deficiency was the result of a single mutation. Another slow-growing mutant, JH4301, constitutively synthesized DHN-melanin and produced more mucilage surrounding the cell wall than our wild-type strain. Genetic crosses with our wild-type strain suggested that the heavily melanized mutant had a single mutation responsible for its phenotype.;The melanized wild-type and dark mutant strain JH4301 were more resistant to the lytic enzymes chitanase and glucanase, and to UV damage than the unmelanized mutant. The heavily melanized mutant JH4301 secreted fewer lytic enzymes, and tricyclazole inhibition of melanin restored its secretory ability. Both mutants were unaltered in pathogenicity to rice compared to the wild-type strain, but the wild-type was a better competitor in mixed rhizosphere communities of rice.;The light mutant JH4300 absorbed less copper than our wild-type as determined by inductively-coupled plasma atomic emission spectroscopy analysis and silver staining, while the dark mutant bound significantly more copper when grown with copper or when exposed to copper following growth. The heavily melanized mutant was more sensitive to the toxic effects of copper than either the wild-type or unmelanized strains. All three strains bound significant amounts of copper, lead, zinc and iron from tailings material. Inoculation of range grass rhizospheres with the three strains did not affect the uptake of copper, iron or zinc from tailings into plant tissue. However, melanized strains increased the uptake of lead from tailings into shoot tissue. These results suggest that melanized G. graminis var. graminis may have application in phytoremediation of lead-contaminated sites. |
