The contribution of plastids and endoplasmic reticulum to calcium-mediated signalling in arbuscular mycorrhizal symbiosis

Most land plants improve their mineral nutrition thanks to the interaction with arbuscular mycorrhizal (AM) fungi. During the establishment of this mutualistic symbiosis, plants recognize their fungal partners by perceiving chitin-derived oligosaccharides that AM fungi release in the soil. In plant cells, these molecules activate an evolutionarily conserved signalling cascade that ultimately promotes root colonization by mycorrhizal fungi. Calcium is a crucial second messenger in this signal transduction pathway. Indeed, Ca2+ elevations evoked by chitin oligomers have been recorded in both the nucleus and cytosol of root cells. However, the role of other intracellular compartments in Ca2+-mediated signalling during AM symbiosis is still largely unknown. In particular, despite plastids and the endoplasmic reticulum (ER) are known to play key roles in beneficial plant-microbe interactions, organellar Ca2+ changes in this context have been poorly investigated so far. The aim of this thesis work was to evaluate how plastids and the ER are involved in the intracellular Ca2+ signalling network underlying plant-fungus interactions in the AM symbiosis. To this purpose, Lotus japonicus roots expressing differentially targeted aequorin-based probes were challenged with microbial signalling molecules, and variations in Ca2+ levels were monitored in the plastid stroma and in the ER lumen. The analysis of different genetic backgrounds was instrumental in determining the signalling pathways in which the recorded Ca2+ changes are involved. In addition, the integration of pharmacological, genetic and imaging approaches allowed getting first insights into the complex interplay of different intracellular compartments during plant-fungus communication. Overall, the results support the hypothesis that plant symbiotic signalling is intertwined with immunity-related signalling, and show that multiple organelles interact to shape intracellular Ca2+ dynamics during the response to chitin-based signals.