At the root of the wood wide web: self recognition and non-self incompatibility in mycorrhizal networks

Abstract

Arbuscular mycorrhizal (AM) fungi are mutualistic symbionts living in the roots of 80% of land plant species, and developing extensive, below-ground extraradical hyphae fundamental for the uptake of soil nutrients and their transfer to host plants. Since AM fungi have a wide host range, they are able to colonize and interconnect contiguous plants by means of hyphae extending from one root system to another. Such hyphae may fuse due to the widespread occurrence of anastomoses, whose formation depends on a highly regulated mechanism of self recognition. Here, we examine evidences of self recognition and non-self incompatibility in hyphal networks formed by AM fungi and discuss recent results showing that the root systems of plants belonging to different species, genera and families may be connected by means of anastomosis formation between extraradical mycorrhizal networks, which can create indefinitely large numbers of belowground fungal linkages within plant communities.

Keywords: anastomosis; arbuscular mycorrhizal symbiosis; extraradical mycelium; mycorrhizal networks; non-self incompatibility; plant interconnectedness; self recognition.

Figure 1

Light micrographs showing self recognition (A) and non-self incompatibility (B) between AM fungal hyphae. (A) Visualization of complete fusions of hyphal walls and protoplasmic continuity, evidenced by formazan salt depositions in hyphal bridges (succinate dehydrogenase activity, SDH) in two compatible hyphae of the AM fungus Glomus mosseae. (B) Incompatible interaction between hyphae of two geographically different isolates of the AM fungus Glomus mosseae, visualised after SDH and Trypan blue staining, showing protoplasm withdrawal and septum formation in the approaching hypha (isolate IN101C) after contact with a branch initial (isolate SY710). Scale bar = 10 µm.

Figure 2

Visualisation of intact extraradical networks produced by the AM fungal species Glomus mosseae, spreading from mycorrhizal roots of Prunus cerasifera and uniformly colonizing the surrounding environment.

Figure 3

Visualization under the dissecting microscope (40×) of Glomus mosseae extraradical hyphae spreading from intact (A) and cut (B) mycorrhizal roots of Allium porrum.