Project: Modulation of transcription factors by Clp1 specifies mating-type dependent signaling during biotrophic development of Ustilago maydis

Ustilago maydis is a dimorphic Basidiomycete and pathogen of corn. Prerequisite for a successful infection process is the switch from yeast-like budding to filamentous growth. This dimorphic transition is accompanied by a G2 cell cycle arrest, and ultimately controlled by the heterodimeric homeodomain transcription factor bE/bW encoded by the b-mating type locus. We could show that the majority of the bE/bW dependently expressed genes are regulated via Rbf1, a bE/bW dependently regulated transcription factor. Rbf1 is required and sufficient for filament formation and G2 cell cycle arrest. The bE/bW and Rbf1 induced cell cycle arrest is released via physical interaction of both transcription factors with Clp1. clp1 is a directly b-regulated gene and required for biotrophic development. Simultaneous induction of bE/bW and Clp1 blocks the b-induced dimorphic switch, and array analysis revealed the complete blocking of b-dependent transcriptional regulation. Interestingly, simultaneous expression of Clp1 and Rbf1 leads to specific downregulation of the U. maydis pheromone pathway. We hypothesize that the interaction of Clp1 with Rbf1 and bE/bW orchestrates pheromone and b-dependent signalling pathways to coordinate mating, the dimorphic transition and cell cycle to facilitate in planta development of U. maydis.