Plugging into the network: belowground connections between germlings and extraradical mycelium of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs: nevertheless, their spores can germinate in the absence of the host plants. Such inconsistent behaviour is balanced by diverse survival strategies. The ability of AM fungal hyphae to fuse might represent a fundamental survival strategy, since germlings could plug into compatible mycorrhizal networks, thus gaining access to plant-derived carbon before asymbiotic growth arrest. An in vivo experimental system was used to grow extraradical mycelium produced by Glomus mosseae colonizing three different plant species and germlings of the same isolate. After symbiotic and asymbiotic mycelia came into contact, we evidenced the ability of germlings to plug into mycorrhizal networks and to establish protoplasm connections with nuclei occurring in fusion bridges. The frequency of anastomoses was 4.9–23.9%. Pre-fusion and post-fusion incompatible responses, with protoplasm withdrawal in interacting hyphae, were evident in some hyphal contacts. In perfect fusions, the mingling of germling nuclei with those of the mycorrhizal network may allow the establishment of a flow of genetic information, which could represent a means for the maintenance of genetic diversity in the absence of sexual recombination