Biofilm formation in Zymoseptoria tritici

Zymoseptoria tritici is an economically damaging fungal pathogen of wheat which is able to survive for long periods on the leaf surface. In this environment, the fungus is exposed to many stresses, including fluctuations in temperature, UV radiation, drying, and foliar fungicide applications. We observed biofilm-like cell aggregations on the surface of wheat leaves infected with Z. tritici in both compatible and non-compatible interactions. A literature search revealed few examples of plant pathogenic fungi forming biofilms, but demonstrated that such biofilms have a number of key characteristics, which are shared with other biofilmforming microbes. These include adhesion of cells to the substrate, production of extracellular matrix, altered gene expression and increased tolerance to various stresses. We used a combination of microscopy, qRT-PCR and stress tolerance assays to determine whether putative Z. tritici biofilms possessed these diagnostic characteristics. We show that Z. tritici biofilms resemble in form and function those formed by other filamentous and dimorphic fungi, producing extra-cellular matrix in which cells become embedded, creating layers of hyphal bundles and blastospores. These biofilms show increased tolerance to drying and high temperature. There is also isolate-dependent resistance to reactive oxygen stress and the fungicide carboxin. Taken together, these findings demonstrate that Z. tritici is indeed able to produce genuine biofilms.