Transcriptomics and experimental proof that compound appressoria are arsenals of Fusarium graminearum

Fusarium graminearum is one of the most destructive plant pathogens. Its infection of cereals causes significant losses due to yield reduction and contamination with mycotoxins, e.g. deoxynivalenol (DON). F. graminearum infects the floral leaf by defined structures: runner hyphae are formed for colonization of the floral surface and compound appressoria, e.g. infection cushions, are developed to facilitate multiple penetrations of the plant cell wall. In this study, we created und compared RNAseq data sets of three different fungal cell types, i.e. infection cushions and runner hyphae (isolated via laser capture microdissection from the surface of paleae), and in culture grown mycelium. Validation of the data by qRT-PCR showed similar gene regulation patterns. The analysis showed that 3916 genes (about one-third of all genes) are specifically regulated during the infection of the plant compared to mycelium. In total 653 genes are exclusively transcribed: 235 in mycelium, 77 in runner hyphae and 341 in infection cushions. In particular, we show that 1. in runner hyphae, the highly expressed fungal pigment aurofusarin acts as an antibiotic agent against bacteria and fungi, 2. infection cushions are arsenals of virulence factors, harboring plant cell wall degrading enzymes, specific metabolites like DON, and putative effector proteins, 3. the putative effector protein FgEF1 is localized in the plant cell after colonization by F. graminearum