Signaling and infection events in the arbuscular mycorrhiza– Medicago truncatula symbiosis: introduction

The most wide-spread beneficial plant-mycorrhizal association is the symbiosis between land plants and arbuscular mycorrhizal fungi (AMF). AMF assist plants in the assimilation of water and nutrients (mainly Pi and nitrate), in exchange the host plant provides carbohydrates. In angiosperms the establishment of this symbiosis requires five steps. First, Medicago truncatula and AMF produce diffusible signals, including strigolactones and mycorrhizal lipochitooligosaccharides, respectively, which activate the symbiotic program of the plant partner. Then the fungal hyphae enter in contact with the root epidermis and form structures called hyphopodia. In response to these chemical and mechanical stimuli, the plant cell underneath the hyphopodium develops a pre-penetration apparatus by remodeling the cytoskeleton allowing fungal penetration. Within the root the AMF grows inter- and intra-cellularly to reach the cortical cells where it develops highly branched structures called arbuscules. Arbuscules are surrounded by the plant membrane leading to the formation of optimized interfaces for nutrient exchange.