Arbuscular mycorrhizas (AM) are symbiotic associations involving up to 90% of terrestrial plants and symbiotic fungi belonging to Glomeromycota. AM fungi enhance the absorption of plant nutrients and give them resistance against pathogens by colonizing the root with inter- and intracellular hyphae and arbuscules, the highly branched structures where the nutrient exchange takes place. A symbiotic interface compartment is developed by the host cells around intracellular fungal structures and is considered as a hallmark of the biotrophic condition of AM fungi, and enables fungal development inside the plant cell space, while preserving its integrity. This presentation focuses on the plant perception of AM fungi and their accommodation within the host cell. Our results, largely based on in vivo confocal microscopy, demonstrate that the process of interface construction takes place upon recognition of the AM fungus and adhesion of a hyphopodium to the root epidermis. Epidermal cells contacted by the fungus show repetitive oscillations (spiking) of nuclear calcium concentration. These oscillations are a central element in the signaling pathway that controls the symbiosis. Activation of this pathway leads to the assembly of the prepenetration apparatus (PPA), a columnar cytoplasmic aggregation, containing all the elements of the secretory pathway. By taking advantage of a range of fluorescent protein markers we show that the proliferation of the host plasma membrane takes place within the PPA, leading to the assembly of the perifungal membrane and symbiotic interface, in advance of hyphal tip growth.