The occurrence of anastomosis formation and nuclear exchange in intact arbuscular mycorrhizal networks.

•  The widespread occurrence of anastomoses and nuclear migration in intact extraradical arbuscular mycorrhizal (AM) networks is reported here. •  Visualization and quantification of intact extramatrical hyphae spreading from colonized roots into the surrounding environment was obtained by using a two-dimensional experimental model system. •  After 7 d the length of extraradical mycelium in the AM symbiont Glomus mosseae ranged from 5169 mm in Thymus vulgaris to 7096 mm in Prunus cerasifera and 7471 mm in Allium porrum, corresponding to 10, 16 and 40 mm mm-1 root length, respectively. In mycelium spreading from colonized roots of P. cerasifera and T. vulgaris, contacts leading to hyphal fusion were 64% and 78%, with 0.46 and 0.51 anastomoses mm-1 of hypha, respectively. Histochemical localization of succinate dehydrogenase activity in hyphal bridges demonstrated protoplasmic continuity, while the detection of nuclei in the hyphal bridges confirmed the viability of anastomosed hyphae. •  The ability of AM extraradical mycelium to form anastomosis and to exchange nuclei suggests that, beyond the nutritional flow, an information flow might also be active in the network.