Analysis of secreted protein effectors from the arbuscular mycorrhizal fungus G. intraradices

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Microbial pathogens often deliver a myriad of effector proteins to hijack the cellular program of their hosts. In the previous phase of this Focus Program, we started a research project to identify and characterize putative effectors from the fungus G. intraradices that could be featuring in the arbuscular mycorrhizal symbiosis. We have shown that the symbiotic mycorrhizal fungus G. intraradices secretes a protein, SP7, that interacts with the pathogenesis-related transcription factor ERF19 in the plant nucleus. ERF19 is highly induced in roots by infection with the fungal pathogen Colletotrichum trifolii but only transiently during early mycorrhiza colonization. SP7 constitutively expressed in planta leads to higher mycorrhization while reduces the levels of C. trifolii-mediated ERF19 induction. Furthermore, expression of SP7 in the rice blast fungus M. oryzae attenuates root decay symptoms. Taken together these results suggest that SP7 is an effector that contributes to establish/maintain the biotrophic status of arbuscular mycorrhizal fungi in roots by counteracting the early plant immune response. However, several questions are still open: How is SP7 entering the plant cell? Is ERF19 the only target of SP7? Are other effectors cooperating with SP7? Are there arbuscule-specific effectors? We propose to give answer to these questions in the next phase of the project.