Date Published: June 22, 2016
Publisher: Public Library of Science
Author(s): Marie Tollot, Daniela Assmann, Christian Becker, Janine Altmüller, Julien Y. Dutheil, Carl-Eric Wegner, Regine Kahmann, Yuanchao Wang.
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. We have identified a member of the WOPR family of transcription factors, Ros1, as major regulator of spore formation in U. maydis. ros1 expression is induced only late during infection and hence Ros1 is neither involved in plant colonization of dikaryotic fungal hyphae nor in plant tumor formation. However, during late stages of infection Ros1 is essential for fungal karyogamy, massive proliferation of diploid fungal cells and spore formation. Premature expression of ros1 revealed that Ros1 counteracts the b-dependent filamentation program and induces morphological alterations resembling the early steps of sporogenesis. Transcriptional profiling and ChIP-seq analyses uncovered that Ros1 remodels expression of about 30% of all U. maydis genes with 40% of these being direct targets. In total the expression of 80 transcription factor genes is controlled by Ros1. Four of the upregulated transcription factor genes were deleted and two of the mutants were affected in spore development. A large number of b-dependent genes were differentially regulated by Ros1, suggesting substantial changes in this regulatory cascade that controls filamentation and pathogenic development. Interestingly, 128 genes encoding secreted effectors involved in the establishment of biotrophic development were downregulated by Ros1 while a set of 70 “late effectors” was upregulated. These results indicate that Ros1 is a master regulator of late development in U. maydis and show that the biotrophic interaction during sporogenesis involves a drastic shift in expression of the fungal effectome including the downregulation of effectors that are essential during early stages of infection.
The basidiomycete Ustilago maydis is a biotrophic pathogen colonizing maize. The resulting disease, the so-called smut disease, is characterized by the formation of large tumors on all aerial parts of the plant. In these tumors fungal hyphae proliferate profusely and eventually produce massive amounts of dark pigmented, diploid teliospores.
In this study we demonstrate that late biotrophic development in U. maydis is coordinated by Ros1, a member of the WOPR family of fungal regulators. Ros1 does not influence the ability of U. maydis to induce tumor formation, but is the key regulator for switching from b-dependent filamentation to hyphal aggregation and spore formation. This development is accompanied by a dramatic shift in the expression of 60% of the putative effector genes without functional domain [47, 48]. The processes regulated by Ros1 are depicted schematically in Fig 11.