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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.