The smut fungus, Usti/ago maydis is an obligate parasite on maize plants in order to complete its sexual life cycle. The haploid form is self-sterile but compatible mated cells undergo a dimorphic switch from yeast like to infectious dikaryotic filaments. Mating types are determined by two unlinked loci, the a locus and the b locus. The a locus is involved in self and non-self cell recognition through production and sensing of pheromone. The b locus of compatible mating types encodes a functional transcription factor that is involved in regulation of expression of genes responsible for filamentation, virulence and pathogenicity.Both loci co-ordinate through a conserved mitogen activated protein kinase (MAPK) signaling pathway for mating and the protein kinase A (PKA) pathway in response to environmental conditions. If genes contributing to these signaling pathways are altered, some associated phenotypes appear to be either mating type or mating locus specific. Mating locus specific effects were evident in mutants of the p21-activated kinase (PAK), Smu1. Data from charcoal plate mating assays, plant infections and colony morphology in an ammonium deficient environment supports involvement of b locus with function of smu1. Phenotypes associated with mutants of another PAK like kinase, Cla4 and over-expression of a vital small GTPase, Rh01, are dependent upon genetic background of the strain, but independent of the mating loci, as evident from charcoal plate mating assay. In addition, for c/a4 mutants, cell morphology on rich media and colony morphology under low nitrogen condition were also studied and used to hypothesize that even in haploid cells, homeodomain proteins encoded by the b locus may act as a functional transcription factor that interacts with certain components of signaling pathways so as to control various aspects of cell morphology and mating behavior.
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Specific b alleles matter in phytopathogenic Ustilago maydis.
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