Poplar root exudates contain compounds that induce the expression of MiSSP7 in Laccaria bicolor

Communication between organisms is crucial for their survival, especially for sessile organisms such as plants that depend upon interactions with mutualistic organisms to maximize their nutrient acquisition. This communication can take the form of the exchange of volatile compounds, metabolites or effectors-small protein signals secreted from the colonizing cell that change the biology of the host cell. We recently characterized the first mutualistic effector protein from an ectomycorrhizal fungus, a small secreted protein named MiSSP7 encoded by Laccaria bicolor. Ectomycorrhizal fungi are soil-borne mutualistic organisms whose hyphae wrap around host roots and grow into the root apoplastic space where the fungus exchanges nutrients such as nitrogen and phosphorus in return for plant derived sugars. The MiSSP7 protein is induced by root exudates and is highly expressed throughout the root colonization process. Its presence was responsible for alterations to the plant transcriptomic profile, a mechanism by which MiSSP7 may aid in the formation of the symbiotic interface. Here we discuss the implications of these findings and, further, we demonstrate that the production of MiSSP7 is induced by two flavonoids, rutin and quercitin, a class of compounds normally found within the exudates of plant roots. We also consider the interesting similarities between the mechanisms of effector induction and action between pathogenic and mutualistic fungi.