The crosstalk between plants and their arbuscular mycorrhizal symbionts: a mycocentric view.

Arbuscular mycorrhizal fungi (AMF) are beneficial microorganisms fundamental for soil fertility and plant nutrition, which establish mutualistic symbioses (mycorrhiza) with the roots of 80% of land plants, including many agricultural crops. AMF obtain carbon compounds from plant cells, which, as chemoheterotrophs, are unable to synthesize, and provide their host plants with soil mineral nutrients, mainly phosphorus and nitrogen, absorbed and translocated to the root cells by the extensive extraradical mycelium spreading from roots to the surrounding soil. In addition, mycorrhizal plants are more tolerant to biotic and abiotic stresses, and show higher production of secondary metabolites with antioxidant properties. During symbiosis establishment a complex dialogue occurs between the two partners: AMF recognize compounds secreted by host roots, such as strigolactones, which elicit an extensive hyphal branching in the vicinity of the roots, before the formation of appressoria on the root surface. On the other hand, AMF release symbiotic signals, a mixture of sulphated and non-sulphated simple lipochitooligosaccharides (LCOs), triggering mycorrhizal formation in different plant families. Here, we will focus on (i) fungal release of symbiotic signals during the asymbiotic phase of AMF life cycle and fungal recognition responses to host-derived signals during the presymbiotic phase, (ii) appressorium formation on the root surface; (iii) the symbiont/host crosstalk during cell-to-cell interactions leading to the formation of a functional symbiosis, (iv) fungal behavior in the presence of non-host plants.