Mating in the Smut Fungi: Fromatobto the Downstream Cascades

Smut fungi induce disease only in their dikaryotic stage, which is generated by mating. Mating is regulated by two loci, which harbor conserved genes. In the a locus these genes specify pheromones and receptors, while in the b locus two transcription factors are encoded. This chapter focuses on the signaling cascades, which coordinate cyclic AMP (cAMP) and mitogen-activated protein kinase (MAPK) signaling in response to pheromone, as well as the transcriptional cascade triggered by the products of the b locus. The first part of this chapter highlights variations in the organization of the mating-type loci in Ustilago maydis, Sporisorium reilianum, and U. hordei, their consequences for mating, and finally evolutionary implication. The chapter reviews the signaling pathway underlying pheromone perception as well as the regulatory cascade triggered by the homeodomain heterodimer. Pheromone perception in smut fungi elicits the formation of conjugation hyphae. The connection between mating and cell cycle in U. maydis may also provide an explanation for the observation that the pheromone-induced formation of conjugation tubes is prf1 independent. One of the driving forces for many of the studies described in this chapter has been the assumption that finding the complete set of targets for the bE/bW heterodimer will explain sexual development and its connection to pathogenesis.