Identification of seedling-specific effectors in the Ustilago maydis – maize interaction : From organ to cell type specificity

Ustilago maydis is a basidiomycete fungus that causes smut disease on maize. As tumor formation occurs on all aerial parts of the plant, the fungus has to adapt to different types of host tissues which differ in structure and physiology. U. maydis therefore deploys an organ-specific set of adaptable virulence-related proteins (effectors) to circumvent host immunity and induce tumor-formation (Skibbe et al., 2010). Individual effector genes that are required for virulence in an organ-specific manner were identified in this study (Schilling et al., 2014). To understand, how organ-specific effectors contribute to fungal virulence, the seedlingspecific effector Um01690 (Sts1) was analyzed for its host interactor by yeast-2-hybrid analysis. This identified a maize carboxypeptidase as a putative interactor of Sts1 which is described to be involved in growth and developmental control. In the second approach, tissue- and cell type-specific activity of U. maydis was investigated. A cell type-specific gene expression analysis as well as a microscopic cell size measurement was performed to analyze leaf tumor formation on the cellular level. Tracking of cell development, together with in-vivo visualization of plant DNA synthesis identified bundle sheath cells as the origin of hyperplasic cell division, while mesophyll cells turn into hypertrophic tumor cells showing endoreduplication. In parallel to that, cell wall component analyses in tumors have elucidated U. maydis-induced cell wall reformations. Histological stainings have visualized interactions of fungal and host factors in a cell type-specific and temporal context and shown physiological changes of seedling leaf tissue during tumor development. Furthermore, the U. maydis effector See1 (Redkar et al., 2015a) was identified in this study as the first effector that acts in a cell type-specific context upon leaf tumor induction. RNAseq analysis of micro-dissected tumor cells showed that the fungus deploys cell typespecific effector gene expression in leaves upon tumor induction in addition to generally transcribed core effector genes. In summary this study elucidated cell type-specific processes leading to U. maydis-induced tumor formation in maize leaves.