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 effectorssmall 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.
MiSSP7 is required for root cell remodeling and hyphal penetration into
the root apoplastic space. In the interaction between the
ectomycorrhizal fungus Laccaria bicolor and poplar roots, the
fungus surrounds the root and induces morphological changes in the
epidermal cells and loosens the connections between the root cells such
that fungal hyphae are able to penetrate in between the cells as seen in
(A; arrow). When the production of the small secreted effector protein
MiSSP7 of L. bicolor is reduced the same alterations to root
epidermal cells are not observed nor are the connections between root
cells loosened such that the fungal hyphae are not able to grow into the
apoplastic space (B; arrows). Alterations to plant cell morphology and
hyphal ingrowth are restored when MiSSP7 is expressed heterologously in
poplar roots (C; arrow). All experiments were performed in triplicate to
ensure reproducibility. Scale bar = 20 µm.
Poplar root exudates contain compounds that induce the expression of MiSSP7 in Laccaria bicolor.
Plett JM, Martin F.
Plant Signal Behav. 2012 Jan;7(1):12-5. doi: 10.4161/psb.7.1.18357