Caracterizacion de factores especificos del programa de virulencia en Ustilago maydis

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura:20-04-2007The dimorphic fungus Ustilago maydis is the causative agent of corn smut desease. The process of plant invasion involves regulated growth and highly organized morphological changes. Haploid cells (sporidia) of this fungus are unicellular, and grow saprophytically by budding. The pathogenic form, the filamentous dikaryon, is established after fusion of two sporidia that have to harbor different alleles of the a and b mating type loci of U. maydis. The a locus controls the cell fusion via a pheromone-receptor based system. Upon pheromone stimulation cells arrest budding growth and start the formation of conjugation tubes that are cell cycle arrested in the G2 phase. These mating filaments are highly polaryzed structures that undergo directed tip growth towards the pheromone source, followed by cell fusion and the formation of dikaryotic hyphae. The subsequent steps in filament formation and pathogenic development are controlled by the multiallelic b-locus that encodes two distinct homeodomain transcription factors, bE and bW. A heterodimeric complex of the two proteins is formed when they are derived from different alleles. The dikaryon formed after the fusion of compatible sporidia is arrested in the G2 phase of the cell cycle. On the plant surface, filaments differentiate appressoria and penetrate the cuticule. Finally, spores are generated and dispersed by air. Spores will germinate and upon germination, meiosis takes place and pairs of compatible haploid cells are generated. In this work we have identified two new factors involved in pathogenic development. Pcl7 is a cyclin of Cdk5 implicated in morphogenesis during pathogenic development. Δpcl7 cells have defects on polarization during the formation of the conjugative tube and the infective filament. Biz1 (b-dependent zinc finger protein), is a transcriptional regulator that plays a pivotal role during the infection process. Δbiz1 cells show a severe reduction in appressoria formation and plant penetration. In biz1 mutants, when the hypha penetrates, its pathogenic development is arrested directly after plant penetration. pcl7 is upregulated by the induction of the amating type locus and both biz1 and pcl7 are induced via the bmating type locus. High levels of biz1 or pcl7 expression induce highly polarized growth and a G2 cell cycle arrest supporting roles in morphogenesis and cell cycle regulation