Understanding the role of antimicrobial effector proteins secreted by the lichen-forming fungus Peltigera rufescens

Symbiotic associations are found across the fungal kingdom, among which lichens are one of the most successful mutualistic fungal symbioses. By classical dual definition, lichens are comprised of two primary partners. While one is a lichen-forming fungus, termed the mycobiont, the other is a photosynthetic partner, called the photobiont. We believe effector mediated microbiome manipulation previously discovered for pathogenic fungi is also fundamental to fungi with different lifestyles and therefore aim to understand the role of antimicrobial proteins for lichen-forming fungi. Initial analyses of a high quality Peltigera rufescens genome identified potential effectors with antimicrobial activity that might play a role in shaping the community structure of the lichen microbiota. A candidate, for which RNA-sequencing counts support the expression of the protein coding gene in the natural lichen, displays antimicrobial activity in vitro on bacteria and fungi from the lichen"s natural environment. Beyond we recorded a reduction in photosynthetic activity when the protein is applied to phototrophic organisms. The selected ~18 kDa protein contains a ricin B lectin domain which in studies on the ricin protein, a plant toxin, has been shown to bind simple sugars. In the future we will gather evidence for the secretion of our protein in the natural Peltigera lichen and uncover its mode of action.