Manganese oxidation by plant pathogenic fungi

The ability to oxidize manganese is a potential virulence factor in fungal phytopathogenic interactions. The purpose of this study was to identify and characterize the Mn oxidizing factor (MOF) produced by the ascomycete fungi, Gaeumannomyces graminis var. tritici (Ggt) and Magnaporthe grisea, and to confirm the role of Mn oxidation in fungal virulence. Culture assays and spectrophotometric analysis of crude culture extracts derived from Ggt support the involvement of a multicopper oxidase such as laccase in Mn oxidation. The importance of laccase in the phytopathogenic fungal biology is supported by the identification of twelve putative laccase genes in the M. grisea genome, and the strong expression of a specific laccase, lcc15, under Mn oxidizing conditions. Two genes with homology to Mn peroxidase were identified in M. grisea. Targeted deletion of these genes did not abolish Mn oxidation activity but resulted in mutants with distinct Mn oxidation phenotypes. Further, these mutants appeared to be reduced in virulence causing smaller lesions on susceptible rice line CO-39 than wild-type isolates. The research presented herein suggests the participation of two separate enzymes, a multicopper oxidase and a peroxidase, in Mn oxidation by Ggt and M. grisea. Further, reduced virulence in M. grisea gene deletion mutants with an altered Mn oxidation phenotype supports the role of Mn oxidation in fungal virulence