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Mycorrhiza are defined as symbiotic (generally mutualistic) associations between plant roots and certain fungi. Typically, the benefit to the plant is nutritional but can include other benefits as well, while the fungus receives carbon in some form from the host. Recognized mycorrhizal associations, such as ectomycorrhiza and arbuscular mycorrhiza, have other things in common, such as a coevolutionary history between plant and fungus, having resulted in characteristic symbiotic structures. For example, the arbuscule, the jointly formed exchange structure diagnostic of the arbuscular mycorrhiza, an association that is over 400 million years old. The diagnostic feature of the ectomycorrhiza is the Hartig net, a network of hyphae inside the plant root. Mucoromycotina fine root endophytes, while not related to arbuscular mycorrhizal-forming glomeromycotan fungi, also form arbuscules, and thus would also be considered mycorrhizal, especially since they have recently been established as a nutritional symbiosis.

However, there are many other plant-fungal associations in roots that are not given the label ‘mycorrhiza’. The question I am examining here is: would we profit from including more groups in the mycorrhiza category?

In biology, categories are very useful but also potentially problematic, since they emphasize differences rather than continuity, and especially at the boundaries between categories. Mycorrhiza ist clearly such a category, because some associations are ‘in’ while others are not.

A beneficial association of plant roots and fungi that is deemed mycorrhizal for sure receives disproportional research attention, and also resources in terms of funding. There are conferences just for mycorrhiza (the ICOMs); there is an international mycorrhiza society (IMS). Mycorrhizal ecologist is an accepted term for a research specialization (and I would call myself one). There is an international journal called Mycorrhiza. There is a well-respected textbook Mycorrhizal Symbiosis (Smith and Read). Clearly, other beneficial plant-fungal associations do not receive the same type of focused attention.

While mycorrhiza is circumscribing a subset of symbiotic (generally mutualistic) plant root-fungal relationships, there are many other beneficial root-fungal relationships. For example, there are the Sebacinales. These fungi are widespread, and some form associations that fit the mycorrhizal categories, but others do not. Serendipita indica (syn. Piriformospora indica), for example, has many properties in common with arbuscular mycorrhiza, but works differently, and does not form arbuscules. Roots of many plants contain fungi from the Sebacinales that form no recognizable structures. There are plenty of other studies that report that inoculating a root with a certain fungal strain produces benefits for the host plant.

Many such benefits may be context dependent. But context dependence of effects is also a hallmark of mycorrhizal associations.

Very often we do not know how these other fungi work, how exactly they influence the host plant; but this is at least partly because they are not included in the mycorrhiza category, where most of the research attention is.

Should we include all these potentially beneficial associations within ‘mycorrhiza’? This would be compatible with the original meaning of the word (mycorrhiza = fungus-root). I do believe the formation of specific structures is important, though, and many of these associations do not form recognizable structures inside the root, even though they may be functionally quite similar to associations with such diagnostic structural features. We could include these associations in a new category called structurally undifferentiated mycorrhiza, capturing the root colonizers that just form unremarkable hyphae inside the plant tissue. Right now, these fungi have no real home, they are referred to collectively as root endophytes (literally meaning growing inside plants).

The other option is to avoid the classification altogether, in favor of a more trait-based approach. We have argued this before, and there are clear advantages of focusing on traits rather than descriptive categories when the goal is to understand ecological effects. But in the end, to communicate we still need to use categories. Even though this category could also be ‘potentially beneficial root-associating fungi’.

I think dissolving the term mycorrhiza is not a good option, because this label has been very successful in gathering and giving an identity to researchers working on these associations, which has led to us learning more about them from every angle, from molecular biology to ecology. This is why I think we should bring in more groups under the mycorrhiza umbrella instead. We should use the advantages of this brand, so to speak, also for other groups of beneficial root-colonizing fungi.

Probably much would be gained by adding this structurally undifferentiated mycorrhiza to our portfolio at conferences, in journals and learned societies. Especially since the fungi forming such associations are likely co-occurring with the other mycorrhizal groups in the plants we study. And: mycorrhiza would then include all potentially beneficial root-associating fungi; a meaningful category, also with a view towards applications. In terms of communicating with the public, this might also make things more straightforward: if you’re a potentially beneficial root-colonizing fungus, you’re a mycorrhiza.

What do you think? Please comment below….

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