Differential gene expression of Hymenoscyphus fraxineus grown on Fraxinus excelsior and F. mandshurica supplemented media

Abstract

Ash dieback is caused by the invasive fungal species Hymenoscyphus fraxineus and leads to the loss of the common ash (Fraxineus excelsior) in many areas. In contrast, the pathogen seems to colonize the local ash species Fraxinus mandshurica (Manchurian ash) symptom-free in its native environment East Asia. In order to gain insight into the differences in the substrate dependence of the pathogen, two H. fraxineus strains (1511 and 1431) were grown in media supplemented with leave material of F. excelsior or F. mandshurica and without supplement. A total of 95 and 916 genes were upregulated in media containing F. excelsior leaf material for strain 1431, compared to F. mandshurica and media without plant supplementation. The differential expression analysis of strain 1511 revealed 483 and 875 upregulated genes. Oxidative phosphorylation is highly upregulated in strain 1431 cultivated in F. excelsior-supplemented media indicating an active metabolism but probably also linked to the release of reactive oxygen species. In both strains during growth on F. excelsior, transcripts with similarity to genes of toxin biosynthetic pathways of other Ascomycota are enriched. In strain 1511, a gene cluster with similarity to depudecin cluster is upregulated in addition. For both strains, the upregulated genes, if grown in media supplemented with leaf material from F. excelsior, include a gene of the biosynthetic cluster of phytotoxin viridiol. The results suggest increased metabolic activity and toxin production in the presence of F. excelsior tissue compared to F. mandshurica.