FIG 10.

Hierarchical regulatory network controlling different cell types of Ustilago maydis. During the fusion of two nonpathogenic, yeast-like sporidia, the binding of peptide pheromones to the pheromone receptor (Pra) activates a MAP kinase (MAPK) cascade, and environmental signals feed via a so-far-unidentified receptor into a cAMP-dependent protein kinase (PKA) pathway. Both pathways converge at Prf1, the central transcription factor required for mating-dependent gene expression. Prf1 is additionally regulated at the transcriptional level by the transcription factors Rop1 and Hap2, both of which are thought to be targets of the MAPK cascade. The phosphorylation of Prf1 results in the induction of the bE and bW genes; after the fusion of two sporidia, bE and bW form a heterodimeric transcription factor that is the master regulator to induce filamentous growth and pathogenic development. The central node for gene regulation at the onset of pathogenic development is the transcription factor Rbf1. rbf1 gene expression is already induced by Prf1 prior to cell fusion, but expression is boosted after cell fusion and the formation of the filamentous dikaryon by the bE/bW heterodimer. Rbf1 is sufficient to trigger the expression of all genes required for plant penetration. Among the Rbf1-induced genes are the transcription factors Biz1 and Hdp2, both of which are absolutely required for plant infection. Hdp2 and Biz1 are highly expressed at the later stages of in planta development, while rbf1 expression is barely detectable within the plant. biz1 and hdp2 are also regulated by physical cues on the plant surface via the plasma membrane receptor Sho1/Msb2. Two transcription factors specifically expressed during the biotrophic phase are Fox1, which is thought to contribute to the regulation of several effector genes, and Ros1, a regulator for the late developmental steps during spore formation.