Development and Dimorphism of the Phytopathogenic Basidiomycota Ustilago maydis

Ustilago maydis, a Basidiomycota species, is the causal agent of common smut in corn and teozintle. This fungus has a complex life cycle regulated by two mating factors: a with two alleles involved in mating, and b, multiallelic, involved in mycelial growth and pathogenesis. Two morphological stages can be recognized in the life cycle of the fungus: a yeast-like haploid, saprophytic stage (sporidia), and a mycelial dikaryotic pathogenic stage. Transition of the first one into the latter involves mating between two sporidia containing different a and b alleles, only these dikaryotic cells being infectious, and maintained only in the host, where the diploid stage, a specialized type of spore (teliospore) is formed after cytokinesis. Teliospores germinate to produce haploid sporidia. It is known that pathogenesis and morphogenesis are controlled by the formation of a heterodimer made of two gene products from compatible b genes (bx/by), that acts as a master transcription factor. Nevertheless new data have demonstrated that U. maydis haploid cells may be pathogenic to different plant species under experimental conditions, and that they may grow in the lab in the mycelial form responding to different stimuli: nitrogen starvation, use of fatty acids as C source, and incubation at acid pH. All these data suggest that both, pathogenesis and morphogenesis may occur without the involvement of the heterodimer through a bypass involving a MAPK pathway which is antagonized by a PKA pathway. Different elements such as polyamines, probably DNA methylation, cell wall alterations, but not the pH responsive Pal/Rim pathway are involved in these processes.

Keywords: Ustilago maydis, dimorphism, morphogenesis, mating type, pathogenicity, cAMP, hyphae. yeast-like, cell wall, differentiation, virulence, tumor development, teliospores.