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要旨:
The maize plant (Zea mays) is host to two closely related phytopathogenic fungi Ustilago maydis and Sporisorium reilianum. U. maydis infects all aerial parts of the plant, rapidly forming galls or tumours filled with spores. S. reilianum infects young seedlings, remains asymptomatic and grows systemically until it replaces the inflorescence with a mass of sooty spores. The identification of the factors responsible for the difference in infection specificity was the main aim of this work. As S. reilianum is known to be dimorphic it was important to characterise the mating type loci that are fundamental for the morphological changes. S. reilianum, like U. maydis, has a tetrapolar mating system. Molecular characterisation revealed that S. reilianum is an exceptional smut fungus that has three a loci: these were termed a1, a2, and a3. The a loci were found to each contain one receptor gene but two pheromone genes. Functional analysis proved that the native receptor does not recognise the two native pheromones. Instead it can only be stimulated by one pheromone, which is carried by each of the alternate a alleles. S. reilianum, similar to U. maydis, has multiple b mating type loci of which five were isolated and characterised. The b loci of S. reilianum contain two divergently transcribed open reading frames that each contains a homeobox motif. This is as is found in the b loci of U. maydis, which encode proteins that function as a transcription factor. Microscopic comparison of the early stages of infection caused by U. maydis and S. reilianum in maize demonstrated that the two fungi differ remarkably during the colonisation of the host tissue. U. maydis’ proliferative capacity is initiated as early as three dasy post infection (dpi) while S. reilianum shows only sustained hyphal growth at the same time point. Thus, the gene expression profiles produced by U. maydis and by S. reilianum infected maize using amplified fragment length polymorphism on cDNA (cDNA-AFLP) were compared. Interestingly, a nitrilase gene was identified as being differentially expressed in U. maydis tissue three dpi. As a nitrilase may be involved in the final stages of auxin biosynthesis, the auxin generating capability of the U. maydis triple deletion strains missing the nitrilase and the two indole acetaldehyde dehydrogenase genes iad1 and iad2 was investigated. Although auxin production was reduced in the mutants, plant infection assays revealed that tumour formation was unaffected. Consequently, the ability of U. maydis to produce auxin may not be directly related with its ability to produce tumours.
