Host infection by the grass-symbiotic fungus Epichloë festucaerequires catalytically active H3K9 and H3K36 methyl transferases

Recent studies have identified key genes in  Epichloe festucae that control the symbiotic interaction of this filamentous fungus with its grass host. Here we report on the identification of specific fungal genes that determine its ability to infect and colonise the host. Deletion of  setB , which encodes a homolog of the H3K36 histone methyltransferase Set2/KMT3, specifically reduced histone H3K36 trimethylation and led to severe defects in colony growth and hyphal development. The  E. festucae Δ clrD  mutant, which lacks the gene encoding the homolog of the H3K9 methyltransferase KMT1, displays similar developmental defects. Both mutants are completely defective in their ability to infect the host grass, with mutational studies of key residues in the catalytic SET domains of these proteins showing that these phenotypes are dependent on the methyltransferase activities of SetB and ClrD. A comparison of the differences in the host transcriptome between seedlings inoculated with wild-type versus these mutants suggests that their inability of these mutants to infect the host was not due to an aberrant host defence response. Co-inoculation of either Δ setB or Δ clrD with the wild-type strain enables these mutants to colonise the host, but successful colonisation by the mutants resulted in death or stunting of the host plant. Transcriptome analysis at the early infection stage identified four fungal candidate genes that appear to be required for infection, three of which encode small-secreted proteins, that are differentially expressed in these mutants compared to wild-type. Deletion of one of these genes,  crbA , which encodes a putative carbohydrate binding protein, resulted in significantly reduced host infection rates by  E. festucae .