First and third authors: Department of Plant Pathology, University of Georgia, Athens 30602-7274; and second author: Instituto de Hortofruticultura Subtropical y Mediterránea “La Mayora”, Universidad de Málaga-Consejo Superior de Investigaciones Científicas (IHSM-UMA-CSIC), Estación Experimental “La Mayora”, 29760 Algarrobo-Costa, Málaga, Spain.
Many fungal plant pathogens are known to produce extracellular enzymes that degrade cell wall elements required for host penetration and infection. Due to gene redundancy, single gene deletions generally do not address the importance of these enzymes in pathogenicity. Cell wall degrading enzymes (CWDEs) in fungi are often subject to carbon catabolite repression at the transcriptional level such that, when glucose is available, CWDE-encoding genes, along with many other genes, are repressed. In Saccharomyces cerevisiae, one of the main players controlling this process is SNF1, which encodes a protein kinase. In this yeast, Snf1p is required to release glucose repression when this sugar is depleted from the growth medium. We have employed a reverse genetic approach to explore the role of the SNF1 ortholog as a potential regulator of CWDE gene expression in Ustilago maydis. We identified U. maydis snf1 and deleted it from the fungal genome. Consistent with our hypothesis, the relative expression of an endoglucanase and a pectinase was higher in the wild type than in the Δsnf1 mutant strain when glucose was depleted from the growth medium. However, when cells were grown in derepressive conditions, the relative expression of two xylanase genes was unexpectedly higher in the Δsnf1 strain than in the wild type, indicating that, in this case, snf1 negatively regulated the expression of these genes. Additionally, we found that, contrary to several other fungal species, U. maydis Snf1 was not required for utilization of alternative carbon sources. Also, unlike in ascomycete plant pathogens, deletion of snf1 did not profoundly affect virulence in U. maydis.