Signalling and the Re-structuring of Plant Cell Architecture in AM Symbiosis Andrea Genre Abstract Arbuscular mycorrhizas are widespread and ancient plant symbioses that were already established by the first land plants when they abandoned the water environment. Arbuscular mycorrhizal fungi scavenge mineral nutrients and water from the soil improving the overall plant fitness, receiving in exchange carbohydrates that are indispensable to complete their life cycle. In spite of their importance in natural and agricultural ecosystems, many biological aspects of these interactions are still partially obscure, especially concerning the early stages of symbiosis establishment which involve a signal exchange between the partners. Nonetheless, recent advancements have started to shed light on plant–fungus signalling mechanisms and their relation with the cell responses that culminate in fungal accommodation in the root cells. This chapter is focused on such advances and the new views that they have suggested. 1 Introduction Arbuscular mycorrhizas (AM) are considered to be the most ancient plant symbio- sis. Fossil data report the presence of arbuscules inside the tissues of some of the first plants that abandoned the water environment to start dry land colonisation (Remy et al. 1994). The ability to scavenge mineral nutrients and water from humid soils has likely been the key to the success of AM fungi, and hence of the plants that were hosting them. Present-day soils retain their nutrients as strongly as those prehistoric shores (Leeper 1952). It is therefore not surprising that AM interactions are still so popular in the plant kingdom. Eighty to ninety percent of all plant species interact in natural and agronomical environments with AM fungi (Smith and Read 2008). A. Genre (*) Dipartimento di Biologia Vegetale, Universita ` di Torino, Viale Mattioli 25, 10125 Torino, Italy e-mail: andrea.genre@unito.it S. Perotto and F. Balu ska (eds.), Signaling and Communication in Plant Symbiosis, Signaling and Communication in Plants 11, DOI 10.1007/978-3-642-20966-6_3, # Springer-Verlag Berlin Heidelberg 2012 51