Investigating the role of transcription factors ust1 and med1 in mating, pathogenicity and sporulation of Ustilago maydis

Abstract

The plant pathogenic fungus Ustilago maydis is a model organism for the study of obligate pathogenesis. Mating, dimorphic switch, growth in planta and sporulation of U. maydis are areas of considerable interest in the context of study of pathogenicity of this fungus. In the work presented here, two transcription factors ust1 and med1 involved in these vital processes in the life cycle of U. maydis, were studied. Transcription factors ust1 and med1 are orthologs of conidiation regulators stuA and medA genes of Aspergillus nidulans. The role of Ust1 in regulation of genes differentially expressed during saprobic budding growth of U. maydis was studied using a reverse genetic approach. Several potential targets of Ust1 were identified in an earlier study of genes differentially expressed during budding growth of U. maydis. Deletion studies indicate that these genes are dispensable for mating, pathogenicity and budding growth. One exception was the thi1 gene, which was auxotrophic for thiamine in culture but dispensable for mating and pathogenicity. The ust1 mutant produces filaments and teliospore-like structures in culture. To assess the suitability of use of the ust1 mutant as a surrogate for study of sporulation of U. maydis, a transcriptome analysis was conducted. Comparison of gene expression patterns of the ust1 mutant, wild type teliospore producing plant galls, plant galls of hgl1 which produces immature teliospores to that of wild type budding cells in culture showed that there is some overlap in the gene network involved in teliospore formation and the ‘sporulation’ of ust1. The functional characterization of the U. maydis med1, a transcription factor identified in the microarray study, by gene deletion showed that this gene is required for conjugation tube formation, mating in vitro and expression of normal virulence. The expression of mating associated genes mfa1, pra1 and transcription factor prf1 was lower in med1 compared to wild type suggesting an important role of med1 in regulation of transcription of mating related genes in U. maydis.