Fungal dimorphism regulated gene expression in Ustilago maydis : II. Filament down‐regulated genes

SUMMARY Ustilago maydis displays dimorphic growth alternating between a budding haploid form and a filamentous dikaryon resulting from mating of two haploid cells. This morphological switch plays a critical role in pathogenicity because only the filamentous dikaryon can infect corn plants. Previously, we identified a role for the cAMP signal transduction pathway in dimorphism and pathogenicity. The repression of a subset of genes in filamentous cells may be critical for programming virulence. To identify these filament down‐regulated genes and to understand better the role of wild‐type budding cells in the life and disease cycle of U. maydis in nature, we used suppression subtractive hybridization. We arrayed a library of approximately 5500 cDNA clones and showed by reverse Northern blot analysis that most, as expected, are down‐regulated during filamentous growth. By an iterative sequencing and hybridization process to eliminate previously determined sequences, we showed that > 88% of the clones detected as differential in the reverse Northern blot screening harbour sequences corresponding to 48 different genes. Differential expression was confirmed for 37 of these genes by Northern blot analysis. For eight of these confirmed differential genes, expression could only be detected in budding cells. For genes expressed in both growth forms, levels of differential expression varied from as much as 65‐fold to only two‐fold higher levels in budding cells. Twenty‐seven of the 37 genes confirmed to be differential had similarity to database sequences, and fell into several putative functional categories. In future studies we will produce deletion mutants in several highly differentially expressed genes to study their roles in morphogenesis and pathogenesis.