Cell death regulation and function in plant-fungal symbiosis

Colonization of plant roots by the beneficial fungal endophyte Serendipita indica (syn. Piriformospora indica) follows a biphasic strategy. After an early biotrophic phase the interaction switches to a host cell death phase restricted to the root epidermal and cortex layers. This host cell death is required for fungal accommodation and the establishment of a long-lasting beneficial interaction in barley and Arabidopsis. Using genomics, transcriptomics and proteomics we have identified two fungal symbiosis factors that act synergistically in the apoplast, the enzymatically active ecto-5-nucleotidase (SiE5NT) and nuclease A (SiNucA). The action of these two enzymes at later colonization stages leads to production of deoxyadenosine (dAdo). Extracellular dAdo, but not Ado, activates a previously undescribed cell death mechanism in A. thaliana. dAdo also triggers cell death in the liverwort Marchantia polymorpha, suggesting that this fungal-elicited cell death is conserved across plant lineages. dAdo-induced cell death in planta resembles the mammalian host immune cell death induced by bacterial pathogens such as Staphylococcus aureus during degradation of neutrophil extracellular DNA traps into dAdo. In planta dAdo induces activation of plant cell death marker genes, electrolyte leakage and 26S proteasome activity. The plant stress-signalling metabolite MEcPP (methylerythritol cyclodiphosphate) additionally accumulates in the apoplast which is involved in the regulation of expression of stress-responsive genes. In this project, we propose to dissect plant immunity and cell death mechanisms induced by dAdo during fungal accommodation and the associated signalling pathways by genome mining and manipulation of host and fungal pathways.