Search

Cell-cell fusion in fungi is required for colony formation, nutrient transfer and signal transduction. Disruption of genes required for hyphal fusion in Epichloë festucae, a mutualistic symbiont of Lolium grasses, severely disrupts the host interaction phenotype. They examined whether symB and symC, the E. festucae homologs of Podospora anserina self-signaling genes IDC2 and IDC3, are required for E. festucae hyphal fusion and host symbiosis. Deletion mutants of these genes were defective in hyphal cell fusion, formed intra-hyphal hyphae, and had enhanced conidiation. SymB-GFP and SymC-mRFP1 localize to plasma membrane, septa and points of hyphal cell fusion. Plants infected with Δ symB and Δ symC strains were severely stunted. Hyphae of the mutants colonized vascular bundles, were more abundant than wild type in the intercellular spaces and formed intra-hyphal hyphae. Although these phenotypes are identical to those previously observed for cell wall integrity MAP kinase mutants no difference was observed in the basal level of MpkA phosphorylation or its cellular localization in the mutant backgrounds. Both genes contain binding sites for the transcription factor ProA. Collectively these results show that SymB and SymC are key components of a conserved signaling network for E. festucae to maintain a mutualistic symbiotic interaction within L. perenne. [ABSTRACT FROM AUTHOR]

E pichloё festucae is a filamentous fungus that forms a mutually beneficial symbiotic association with L olium perenne. This endophyte synthesizes bioprotective lolitrems ( ltm) and ergot alkaloids ( eas) in planta but the mechanisms regulating expression of the corresponding subtelomeric gene clusters are not known. We show here that the status of histone H3 lysine 9 and lysine 27 trimethylation ( H3 K9me3/ H3 K27me3) at these alkaloid gene loci are critical determinants of transcriptional activity. Using ChIP- qPCR we found that levels of H3 K9me3 and H3 K27me3 were reduced at these loci in plant infected tissue compared to axenic culture. Deletion of E . festucae genes encoding the H3 K9- ( ClrD) or H3 K27- ( EzhB) methyltransferases led to derepression of ltm and eas gene expression under non-symbiotic culture conditions and a further enhancement of expression in the double deletion mutant. These changes in gene expression were matched by corresponding reductions in H3 K9me3 and H3 K27me3 marks. Both methyltransferases are also important for the symbiotic interaction between E . festucae and L . perenne. Our results show that the state of H3 K9 and H3 K27 trimethylation of E . festucae chromatin is an important regulatory layer controlling symbiosis-specific expression of alkaloid bioprotective metabolites and the ability of this symbiont to form a mutualistic interaction with its host. [ABSTRACT FROM AUTHOR]