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

The widespread occurrence of anastomoses and nuclear migration in intact ex traradical arbuscular mycorrhizal (AAA) networks is reported here. Visualization and quantification of intact extramatrical hyphae spreading f rom 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 mos seae 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) roo t length, respectively. In mycelium spreading from colonized roots of P. ce rasifera and T. vulgaris, contacts leading to hyphal fusion were 64% and 78 %, with 0.46 and 0.51 anastomoses mm(-1) of hypha, respectively. Histochemi cal localization of succinate dehydrogenase activity in hyphal bridges demo nstrated protoplasmic continuity, while the detection of nuclei in the hyph al 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 mig ht also be active in the network.