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Concurrent common fungal networks involving different guilds of fungi
Is this the reality and should we be focusing on this going forward?
Fungal networks, in particular common fungal networks, have been capturing the imagination of mycologists. Common fungal networks are such networks between at least two plants that are formed by any type of fungus, according to a recent paper in which we propose a new concept and nomenclature for such links. The types of fungi that can form such networks are pathogens/parasites, endophytes, saprobes, and mycorrhizal fungi. All we have apparently studied so far are the common networks formed by mycorrhizal fungi, the common mycorrhizal networks.
Most likely, in reality, all kinds of such networks between plants are formed concurrently and are acting at the same time. So, common mycorrhizal networks are just part of the story. Are they the most interesting part of the story? They certainly are very interesting, but we don’t know about the others. Are they the most important part of the story? That is what we have been implicitly assuming by focusing our research attention on them, but it’s unclear (to me) if this is really the case.
In terms of common fungal networks to study, mycorrhizal fungi are of course an obvious option. By virtue of their well-studied direct interaction with plant roots, growing inside them, any effects they transmit are likely to be relevant for the plant. But other fungi also grow inside the plant, like endophytes or parasitic fungi. They would by this logic also be interesting to study. What do we know about such connections by these other guilds of fungi? Not much. In a recent symposium by the Mycological Society of America, Beatrice Bock (a PhD student from Northern Arizona University) spoke about her work on endophytic fungi connecting among two plants. It does seem such connections are indeed occurring, and the functional significance is currently being explored. In the same symposium, Martin Bidartondo reported on networks formed by mycoheterotrophic plants and their mycorrhizal fungi, supplying them with carbon coming from a host plant, serving as an intermediary between the parasite (the mycoheterotrophic plant) and the host (for example, a tree). This may be the only other case of a common fungal network that has been studied in some detail, even though the fluxes of carbon in this case are reversed and the connection is still formed by mycorrhizal fungi (ectomycorrhizas and arbuscular mycorrhizas), making these connections like a ‘positive control’ for common mycorrhizal fungal networks, as argued in a recent paper.
It thus seems worthwhile to pursue the question if common fungal networks involving other groups of root-colonizing fungi are formed and have functional significance for the fungus or the hosts. Endophytic fungi of various kinds (including dark septate endophytes, Sebacinales and many others) and and parasitic fungi should be on the radar.
But even saprobic fungi could be interesting. In common fungal networks, it is not necessary to have direct hyphal connections between plants (so-called hyphal continuity), as we have recently argued. It is thus conceivable that also saprobic fungi are involved in such networks among plants. Water, infochemicals and other microbes could still flow along their hyphae between plants even though they don’t colonize them. Perhaps they grow near the root or on the root surface. And of course the transition from saprobe to endophyte is not as clear as the categories might suggest.
The vision is that between two (or more) plants there are common fungal networks formed by mycorrhizal fungi, endophytic fungi, parasitic fungi and saprobic fungi. Perhaps some are more important than others, and it doesn’t always need to be mycorrhizal fungi that are the champions of importance. Maybe the relative importance for the host plants of the common fungal networks formed by different guilds changes over time, and it very likely depends on environmental parameters, such as moisture and nutrients. It is also likely that these networks interact with each other, perhaps they complement each other functionally, or maybe they interfere with each other.
Studying this situation of common fungal networks involving different guilds of fungi will be exceptionally difficult, but I believe this is the direction we should be heading. Many of the same techniques developed for mycorrhizal networks are applicable, so we wouldn’t completely have to reinvent the wheel.
What do you think?
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I believe it would be beneficial to consider mycelial turnover within the mycelial network. Some studies suggest that the turnover of mycelium in the soil is quite rapid (For example, Staddon et al., 2003), and a mycelial network with stable functions should ideally be relatively stable.
What about taking a more eco-evolutionary stance and ask how competitive/selfish the fungal mycelia are in their interaction - from a plant’s and fungus’ perspective?