Are we too restrictive in our concept of common mycorrhizal networks?

Advocating for a more inclusive idea of common mycorrhizal neworks (CMNs)....a work in progress

Should we loosen up when talking about common mycorrhizal networks (CMNs); Dall-E2 image)

A common mycorrhizal network (CMN) is established when the mycelium of a fungus colonizes two plants (of the same species or different species). This means there is a continuity of mycelium from one plant to the other, a direct connection that does not involve a ‘step’ that goes through the soil rather than fungal cytoplasm. The definition is thus very clear, as recently explained in a must-read article by Karst et al. (2023).

What became clear to me after thinking about this paper, is that this topic of CMNs is also about categories. The degree of connectedness via mycorrhizal fungi is clearly viewed as two categories: CMN and non-CMN. However, as with any system of categories in biology there is always the danger of amplifying and exaggerating differences when cases are close to the border of these categories.

What does this mean here? Imagine a situation where the hyphae from one plant root extend to the surface of another root: it didn’t connect, so it’s not a CMN. As soon as a hypha enters the root, then we have a CMN established. Imagine next a situation where 1,000 hyphae grow between two root systems, not one connecting the two with mycelial continuity, hyphae just emanating from one of the two host plants involved, intermingling with the roots, growing along the surface and doing their hyphal things. Not a CMN, clearly. Now one of the hyphae grows into the other plant: we have a CMN! The question to ask is if this is so different from the situation before.

Maybe it is, maybe it isn’t.

Maybe it is really a different situation when you have this direct hyphal connection, because now carbon can flow from one plant to the other, at least in terms of subsidizing the mycorrhizal fungal structures in the target plant, if nothing else. This really only does happen with cytoplasmatic continuity. Similarly with nutrients, which are also translocated within the mycelium. Even though perhaps nutrients could also reach a target plant when the mycelium turns over, but on a different time scale.

On the other hand, maybe it isn’t all that different, because mycorrhizal fungal hyphae could ‘unload’ substances, like infochemicals, close to the root system of the target plant, and these can still have an effect without a direct mycelium connection. We have earlier made this point for allelochemicals (or infochemicals more generally). Simply getting the chemicals closer to the target root works, because it gets them there faster than diffusion through soil. Another example is the transport of bacteria that hitch a ride on the hyphae of mycorrhizal fungi: there is no direct connection required for this to work either. And water probably the same: it flows mostly along the outside of the hyphae, so also not really a necessity for a direct connection between roots.

So the real question is: do we put our research effort into demonstrating CMN - something that is exceptionally difficult, because you have to exclude a soil-based pathway of transport. Or is it enough to work with a connection of whatever kind between two or more plant root systems?

Here is a proposal - and it is meant as a basis for discussion. What if we refer to any situation where mycorrhizal fungi - which inevitably form a network, because the fungus always is a network - interact with root systems of different plants as a CMN. It includes all possible interactions, including the hyphal continuity, but also intermingling of the hyphae with the roots of a target plant. There is then a special case where there is hyphal continuity from one root to another, how about we call that CMN-HC, so a special case of a common mycorrhizal network with hyphal continuity (HC). I see no reason why the special case, that is hyphal continuity, should determine the name of the more general case of fungi unavoidable being a network that interacts with root systems, a network that is thus ‘in common’ with these root systems.

What’s the advantage? People would be encouraged to work on common mycorrhizal networks, at any level and degree of connection. Delineating the HC part is technically exceptionally challenging; and it’s probably too much to ask of researchers to simultaneously try to maximize ecological realism and mechanistic resolution in most situations (we need the realism for the network to be established in a meaningful way, ideally in the field; we need the mechanistic resolution to exclude pathways other than hyphal direct connection in the traditional definition). Using this new definition, people could specify the specific case of direct hyphal connection when they really mean it, and we would speak of the common mycorrhizal network as the normal, more general case of a fungal mycelium interacting with different roots. Adopting this change would also make it much more straightforward to communicate the role of mycorrhizal fungal networks to the public; maybe not a decisive reason, but still an important one.

What do you think? Does this make sense? Does it help? Let me know in the comments.