The Hydrophobin-Like OmSSP1 May Be an Effector in the Ericoid Mycorrhizal Symbiosis

Mutualistic and pathogenic plant-colonizing fungi use effectormolecules to manipulate the host cell metabolism to allow plant tissue invasion. Some small secreted proteins (SSPs) have been identified as fungal effectorsin both ectomycorrhizal and arbuscular mycorrhizal fungi, but it is currently unknown whether SSPs also play a role as effectorsin other mycorrhizal associations. Ericoid mycorrhizais a specific endomycorrhizal type that involves symbiotic fungi mostly belonging to the Leotiomycetes(Ascomycetes) and plants in the family Ericaceae. Genomic and RNASeq data from the ericoid mycorrhizal fungus Oidiodendron maius led to the identification of several symbiosis-upregulated genes encoding putative SSPs. OmSSP1, the most highly symbiosisup-regulated SSP, was found to share some features with fungal hydrophobins, even though it lacks the Pfam hydrophobindomain. Sequence alignment with other hydrophobinsand hydrophobin-like fungal proteinsplaced OmSSP1 within Class I hydrophobins. However, the predicted features of OmSSP1 may suggest a distinct type of hydrophobin-like proteins. The presence of a predicted signal peptide and a yeast-based signal sequence trap assay demonstrate that OmSSP1 is secreted during symbiosis. OmSSP1 null-mutants showed a reduced capacity to form ericoid mycorrhizawith Vaccinium myrtillusroots, suggesting a role as effectorsin the ericoid mycorrhizal interaction.