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Philipps-Universität Marburg
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Abstract
The coordination of complex developmental processes in multicellular organisms requires the stringent regulation and interconnection of various signaling pathways. Sexual development in yeasts and filamentous fungi is often coupled to defined cell cycle stages, to account for the synchronized development of the different cells and to guarantee a controlled cellular differentiation. In the dimorphic basidiomycete U. maydis sexual and pathogenic development are
tightly connected and depend on the host plant maize. Prerequisite for the formation of the infectious dikaryon is the cell/cell recognition and fusion that is controlled via an a-locus encoded pheromone/receptor system. The activation of this system triggers a G2 cell cycle arrest and the formation of conjugation tubes. After fusion of the conjugation tubes, further development is controlled by the b-locus. The expression of the heterodimeric bE/bW transcription factor, which is encoded by the b-locus, is required for the maintenance of the cell cycle arrest and filamentous growth. In previous studies, 345 b-dependently regulated genes have been identified, out of which more than 90% are regulated via the C2H2 zinc finger transcription factor Rbf1.
Besides this “master regulator“ of pathogenic development, clp1 represents the sole directly b-regulated gene that is, in addition to rbf1, essential for pathogenic development and expressed independently from Rbf1. Delta-clp1 strains do not form clamps and are unable to proliferate in planta, suggesting that Clp1 might be involved in the regulation of these processes. The induced co-expression of clp1 blocks the function of the b-heterodimer and prevents filamentous growth and the G2 cell cycle arrest (Scherer et al., 2006; Scherer, unpublished). In prevoius studies I have demonstrated that Clp1 interacts, among others, with both the bW-protein and Rbf1, the central regulators of pathogenic development. In this study the functional impact of Clp1-protein interactions on the pathogenic development of U. maydis were analyzed. The interaction of Clp1 with bW results in the loss of b-function resulting in the global inhibition of b-dependent gene regulation. In contrast, the interaction of Clp1 with Rbf1 leads to the specific repression of the pheromone gene mfa, and thereby represses the pheromone response on both, the morphological, as well as on the regulatory level. The further characterization of Clp1-interacting proteins led to the identification of the transcription factor Cib1. Deletion of cib1 results in a phenocopy of !clp1 strains and, similar to Clp1, Cib1 protein expression is posttranscriptionally regulated and restricted to postpenetrative stages of the pathogenic development. In contrast to Clp1, the mechanism of the posttranscriptional control of Cib1 expression has been elucidated and could be attributed to developmentally regulated alternative splicing of the cib1 mRNA. Clp1 acts as modulator of two transcription factors, of which both play a central role in the regulation of the pathogenic development and the cell cycle. Expression of either Rbf1 or the b-heterodimer trigger both a G2 cell cycle arrest that is only released after plant penetration. The function of the b-heterodimer, and most likely also of Rbf1, is required during the entire process of pathogenic development. The function of Clp1 is to allow the re-initiation of the cell cycle by the interaction with bW and Rbf1. By use of an artificial promoter system that mediates rbf1 expression prior and post penetration of the leaf surface, the formation of appressoria and the penetration of the leaf surface was demonstrated for the first time independent of an active b-heterodimer. Rbf1-regulated genes are therefore sufficient for the initial phase of pathogenic development, whereas the re-initiation of the cell cycle and the limited proliferation in planta requires the additional expression of Clp1. The further progression of the infectious development results in asynchronous nuclear divisions and elicits plant defense responses. Obviously, the b-heterodimer is required for the suppression of plant defense responses and the coordinated and careful control of the in planta development.
This approach provided new insights and a fundamental knowledge of the regulatory cross connections of the different signaling pathways and their contribution to the regulation of the biotrophic development of U. maydis.
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Created: 2010Issued: 2010-08-10Updated: 2011-08-10
Faculty
Fachbereich Biologie
Publisher
Philipps-Universität Marburg
Language
ger
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DoctoralThesis
Keywords
biotrophypathogenic developmentUstilago maydiscell cycleGene regulation
DFG-subjects
Ustilago zeaeGenregulationUstilagoPathogenitätZellzyklus
DDC-Numbers
570
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Heimel, Kai (141992778): Regulationsmechanismen der biotrophen Entwicklung von Ustilago maydis. : Philipps-Universität Marburg 2010-08-10. DOI: https://doi.org/10.17192/z2010.0474.
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This item has been published with the following license: In Copyright