Role of the histone variant H2A.Z in phytopathogenic fusaria

Fusaria are among the most important group of phytopathogenic fungi infecting various economically important host plants worldwide. Besides enormous crop losses caused by these fungal attacks, fusaria are able to produce a diverse spectrum of natural compounds, referred to as secondary metabolites. These compounds include mycotoxins that frequently contaminate food and feed, thereby posing a serious health threat to animals and humans when consumed. A crucial step towards the development of efficient and durable strategies against fungal infections and contaminations with mycotoxins is to understand the regulatory network that orchestrates pathogenesis and secondary metabolite biosynthesis. Gene expression in eukaryotes functions within the context of chromatin. This includes histone posttranslational modifications that do not alter the DNA sequence, but affect the read out thereof, i.e. inducing or silencing expression of the underlying genes. These histone marks emerge more and more as key factors in regulating fungal virulence and secondary metabolism. Our working hypothesis is that during fungal development and during infection of the plant, the chromatin structure is dynamic and driven by changes in the histone marks deposited on the genome. These changes allow the expression of virulence- and secondary metabolite-related genes hitherto silent as optionally embedded in repressive chromatin. Among known eukaryotic histone marks, although regularly found as decorating transcriptionally active genes, the role of the variant H2A.Z remains to date a riddle, with conflictual roles often described for the same organisms. The function of H2A.Z in fungi has, to date, received very little attention. HISTOVAR proposes to focus on the chromatin dynamics in the two prominent Fusarium spp., Fusarium fujikuroi and Fusarium graminearum, infecting rice and wheat, respectively, and to study the role of so far overlooked but likely essential mechanisms involving H2A.Z during secondary metabolism and pathogenesis. HISTOVAR is a collaborative project between an Austrian and a French research group who both aim at, ultimately, finding the Fusariums Achilles heel that could serve as preferential target(s) for efficient, durable, and environment-friendly fighting strategies against fungal infections and mycotoxin contamination. By a combination of reverse genetics and whole genome approaches (analyses of the transcriptome, the metabolome and the epigenome), HISTOVAR will provide groundbreaking knowledge regarding the function of H2A.Z in fungal development, pathogenicity, and secondary metabolism.

The histone variant H2A.Z is highly conserved and, thus probably the most well-studied non-canonical histone. However, several studies in model organisms such as the yeast Saccharomyces cerevisiae, the plant Arabidopsis thaliana, or the fly Drosophila melanogaster revealed largely controversial data on H2A.Z incorporation in the nucleosome structure, even within the same organism. Thus, the overall function of H2A.Z still is poorly understood. Within this joint project, we explored the role of H2A.Z incorporation in the phytopathogen Fusarium fujikuroi, a major pathogen of rice. Here we revealed that H2A.Z is essential in F. fujikuroi as shown for several other higher eukaryotes. Next to this, we show that H2A.Z predominantly localizes to the +1-nucleosome which highly correlates with gene transcription, especially at loci associated with gene regulatory processes. Next, we determined that H2A.Z positioning is largely exclusive to the presence of H3K4me3 a histone mark associated with positive gene transcription. However, this is not true for the histone mark H3K27me3, a hallmark of facultative heterochromatin, which is important for gene silencing in Fusarium. These results are unlike as shown for most other eukaryotes, implying a different H2A.Z-dependent regulatory network. Next to this, by correlating this data to the transcriptional output we could demonstrate that the incorporation of H2A.Z in H3K27me3 marked regions counteracts gene silencing mediated by facultative heterochromatin. Lastly, making use of an H2A.Z depletion and an H2A.Z overexpression strain allowed the first functional characterization of the histone variant H2A.Z in F. fujikuroi. Here, we showed that H2A.Z is vital for regular fungal development, but largely dispensable for natural product biosynthesis in this fungus. Overall, this project revealed novel insights into the complex chromatin regulatory network in Fusarium and thereby is important to further develop sustainable and efficient strategies to constrain fungal plant diseases.