The functional analysis of late effectors in the maize pathogen Ustilago maydis

The biotrophic basidiomycete fungus Ustilago maydis causes smut disease in maize. Hallmarks of the disease are large tumors that develop on all aerial parts of the host in which dark pigmented teliospores are formed. Ros1, a member of the WOPR family of transcription factors, is a central regulator of the late infection stages in U. maydis and is essential for karyogamy, hyphal aggregation, formation of a mucilaginous polysaccharide matrix and spore production. Ros1 also triggers a major switch in the effector repertoire and a set of 70 late effectors is upregulated by Ros1. We are speculating that late effectors could be used as defense against other microbes which colonize mature tumor tissue when it ruptures. To analyze this, single gene deletion mutants of the eight of candidate late effector genes regulated by Ros1 were created in compatible haploid strains. Seven of these mutants showed no significant defects in virulence and spore formation. One of the mutants showed reduced virulence and formed hyphal aggregates, but produced fewer spores. We are currently isolating plant proteins interacting with this late effector and discuss our results. For the effectors which do not contribute to virulence, we are testing a possible antimicrobial function towards microbes isolated from corn plants grown outside of the laboratory. In addition, we are determining the microbiome composition of uninfected plants and plants infected with wild type and ros1 mutant strains to investigate whether they are difference in community structure. This would indicate an involvement of late effectors in controlling microbial colonization of U. maydis infected plants.