The signaling mechanisms involved in the dimorphic phenomenon of the Basidiomycota fungus Ustilago maydis

In the present manuscript, we describe the mechanisms involved in the yeast-to-hypha dimorphic transition of the plant pathogenic Basidiomycota fungus Ustilago maydis. During its life cycle, U. maydis presents two stages: one in the form of haploid saprophytic yeasts that divide by budding and the other that is the product of the mating of sexually compatible yeast cells (sporidia), in the form of mycelial dikaryons that invade the plant host. The occurrence of the involved dimorphic transition is controlled by the two mating loci a and b . In addition, the dimorphic event can be obtained in vitro by different stimuli: change in the pH of the growth medium, use of different carbon sources, and by nitrogen depletion. The presence of other factors and mechanisms may affect this phenomenon; among these, we may cite the PKA and MAPK signal transduction pathways, polyamines, and factors that affect the structure of the nucleosomes. Some of these factors and conditions may affect all these dimorphic events, or they may be specific for only one or more but not all the processes involved. The conclusion reached by these experiments is that U. maydis has constituted a useful model for the analysis of the mechanisms involved in cell differentiation of fungi in general.