Identification and complementation of a mutation to constitutive filamentous growth in Ustilago maydis.

Pathogenicity of the corn smut fungus Ustilago maydis involves the formation of a filamentous, infectious dikaryon by fusion of compatible, yeastlike haploid cells. The mating-type loci, a and b, regulate cell fusion and establishment of the dikaryotic cell type, respectively. On solid medium, compatibility at the mating-type loci, in particular heterozygosity at the b locus, is manifested by the formation of aerial hyphae on colonies formed by mating cells. We have employed this "fuzzy" phenotype to identify haploid mutants that constitutively form hyphal filaments and forego cell division by budding. A total of 125 such mutants have been isolated; characterization of one mutant (termed rem1-1) revealed that it can participate in infection of the host plant, although it must be paired with a compatible, wild-type mating partner. That is, mutation to the mycelial phenotype is not sufficient to allow a haploid strain to be pathogenic by itself. A cosmid has been isolated that restores the ability of an rem1-1 mutant to grow with a budding phenotype. Localization of the complementing region on cosmid DNA allowed the construction of an additional mutation by gene disruption. Coinoculation of plants with two compatible strains, each carrying the disruption mutation, gave greatly reduced disease symptoms. The analysis of the rem1 gene should contribute to an understanding of dimorphic growth and pathogenesis in U. maydis.