Identification and functional characterization of arbuscular mycorrhizal fungi effector candidates

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Arbuscular mycorrhiza fungi (AMF) live in obligate symbiosis with roots of the majority of land plants. This symbiosis persisted morphologically unchanged throughout the evolutionary development of the plant phylum. One AMF species can infect different plant species, and one plant species can be mycorrhized by various AMF. Therefore, the mechanisms of plant infection and colonization seem to be ancient and conserved within AMF. Recent studies of pathogenic and symbiotic plant-microbe interactions showed similarities during infection. While in the intensely studied pathogenic systems effector proteins were identified as playing a key role in this process, in symbiotic systems evidence for a similar role is limited to a few examples. Pathogen effector proteins either suppress the plant immune system, or they can be recognized by the plant cell, which results in the induction of effector-triggered immunity. We hypothesize that effectors play an equivalent role during the conserved process of AM symbiosis establishment. From the genomes of two AMF, Rhizophagus irregularis and Rhizophagus clarus, we have identified more than 200 homologous genes encoding potential effector proteins. Based on similarity level we divided the candidates into conserved (>90%) and less-conserved ones. In this project we focus on the functional characterization the most conserved potential effectors, since they are the most likely to play a role during the conserved process of symbiosis establishment. In a first step we will select candidates that are secreted, show nuclear localization, and suppress hypersensitive responses in heterologous systems. Only validated candidates will be further characterized with respect to their effect on AM symbiosis using the Lotus japonicus hairy root expression system. Transcript analysis of plant genes during AM symbiosis will be used to identify targets of effector action. In a separate assay we will investigate the impact of effector proteins in a pathogenic system by heterologous expression in the oomycete plant pathogen, Phytophthora palmivora.This study will provide a better understanding of the molecular processes underlying symbiotic interactions. Of special interest will be the similarities and differences of effector action between pathogenic and symbiotic systems. Effector genes in pathogens tend to be under diversifying selection, and it will be interesting to explore the evolutionary forces acting on symbiont effector genes.