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The phytopathogenic development of Ustilago maydis is characterised by a dimorphic switch from a yeast to hyphal growth form. This transition starts when two compatible yeast like sporidia recognize each other, form conjugation hyphae and fuse. Consequently they form a dikaryotic filament which is able to penetrate the host plant maize. During all these developmental stages the fungus grows in a polar fashion. This directed growth is mediated by the delivery of cell wall- and growth components alongside the microtubule cytoskeleton in the direction of the growing tip .
The following thesis concentrates on the role of the cytoskeleton and its associated motors for polar growth of Ustilago maydis. The actin cytoskeleton is essential for polar growth of all developmental stages whereas microtubules are important for mitosis and hyphal growth, but not for conjugation tube formation. The expression of a plus-end binding protein Peb1YFP in hyphae, revealed that 86% of the plus ends are oriented towards the growing tip, suggesting that plus end-directed kinesins support polar growth. The Screening of the Ustilago maydis genome led to the identification of 10 kinesins, which belong to seven different Kinesin-families. Deletion mutants of the kinesins with a potential role in vesicle transport have no phenotype with the exception of kin2 and kin3. The deletion of these two kinesins results in bipolar growing hyphae with similar morphological defects. This might be due to a distribution defect of early endosomes. It could then be shown, that early endosomes are moved bidirectionaly by the motor proteins Kin3 and dynein in a balance within the hyphae. Kin2 on the other hand is needed for the delivery of dynein to the plus ends of the microtubules. Therefore the deletion of kin2 also results in a distribution defect of early endosomes within the hyphae. However, it seems that the delivery of dynein over short distances is supported by other motors. In an additional study it wcould be shown, that a cooperation between the actin and microtubule based transport processes exists and that this cooperation depends on Kinesin-1 and a class V myosin from Ustilago maydis. The deletion of both motors leads to an apolar morphology in sporidia and hypha, furthermore the localisation of Kin2 is Myo5 dependent.