Serial Analysis of Gene Expression Reveals Conserved Links between Protein Kinase A, Ribosome Biogenesis, and Phosphate Metabolism in <i>Ustilago maydis</i>

The switch from budding to filamentous growth is a key aspect of invasive growth and virulence for the fungal phytopathogen Ustilago maydis. The cyclic AMP (cAMP) signaling pathway regulates dimorphism in U. maydis, as demonstrated by the phenotypes of mutants with defects in protein kinase A (PKA). Specifically, a mutant lacking the regulatory subunit of PKA encoded by the ubc1 gene displays a multiple-budded phenotype and fails to incite disease symptoms, although proliferation does occur in the plant host. A mutant with a defect in a catalytic subunit of PKA, encoded by adr1, has a constitutively filamentous phenotype and is nonpathogenic. We employed serial analysis of gene expression to examine the transcriptomes of a wild-type strain and the ubc1 and adr1 mutants to further define the role of PKA in U. maydis. The mutants displayed changes in the transcript levels for genes encoding ribosomal proteins, genes regulated by the b mating-type proteins, and genes for metabolic functions. Importantly, the ubc1 mutant displayed elevated transcript levels for genes involved in phosphate acquisition and storage, thus revealing a connection between cAMP and phosphate metabolism. Further experimentation indicated a phosphate storage defect and elevated acid phosphatase activity for the ubc1 mutant. Elevated phosphate levels in culture media also enhanced the filamentous growth of wild-type cells in response to lipids, a finding consistent with PKA regulation of morphogenesis in U. maydis. Overall, these findings extend our understanding of cAMP signaling in U. maydis and reveal a link between phosphate metabolism and morphogenesis.