Have we been too focused on mycorrhiza in terms of plant-microbe interactions?
Talking about fungi, and some of our favorite fungi here....
Seems almost heretic, especially coming from me, who’s worked on arbuscular mycorrhiza ever since my PhD, some 30 years ago now. Mycorrhizal associations have always been fascinating, especially the obligate biotrophic arbuscular mycorrhiza (obligate biotroph means, they can’t complete their life cycle in the absence of their plant host). And they are quite well understood at the biochemical and molecular levels, so we know a lot about partner recognition, about how the symbiosis is established, and we have transporter genes that underpin our mechanistic knowledge of the nutritional side of this mutualism. In that sense, we are way ahead of other plant-fungal mutualistic symbioses.
Perhaps this advantage of knowledge has lulled us into believing that they are - when present - also generally important to study, and definitely more important than all the other things that also grow in roots. But there are a lot of other organisms inhabiting roots! Just speaking of fungi (root endophytic fungi), there are a lot of other fungal groups with symbiotic potential (so not speaking of pathogens), like MFRE (Mucoromycotina Fine Root Endophytes), who were originally counted as arbuscular mycorrhizal (AM) but are in a different phylum. Or Sebacinales, and many others. We know much less about them in terms of mechanism of the interaction (and in some cases nothing). And these interactions have not been ‘knighted’ with the attribute of being mycorrhizal and the attention that comes with that.
When we do experiments, we typically only work with one of these groups, say with AM fungi, which is difficult enough. We often add other microbes into the treatments as background, or for use in the controls (if the fungal inoculum is not sterile). Or we do experiments where there are no other microbes present. Rarely do we carry out experiments with more than one group of symbionts, mostly because the fungal isolates representing these other groups are simply not available; either they are not available for a given ecological system, or sometimes not at all.
In addition, when we do experiments, we typically only examine a certain slice of the environment, i.e. a given set of conditions, perhaps varying one or very few factors. So we typically only know what the symbiosis does for a very limited parameter space. Even though it’s clear that some fungi may become important to their hosts especially under certain conditions of stress (biotic or abiotic).
How would a range of different fungal symbionts (present at the same time) protect the plant from a range of different stress conditions? Would there be a kind of ‘division of labor’ among the different symbiont types, such that some will shelter the plant host from one particular set of adverse conditions, and others will do the same for a different set of stressors? Will they provide complementary ‘services’ to the host? How will this vary depending on the host? How would these effects scale to the level of the plant community? For example, are some of these fungal groups better at making mycelial connections among hosts - I think we know very little or nothing about this point, but even what we often assume we know for mycorrhizal associations may not be as clear (see Karst et al. 2023).
It will be difficult to do experiments addressing this question, because we will first need to get our hands on the fungal isolates, and isolates that match a given ecological context. Even when we have these isolates, doing these experiments will be challenging, since such studies will require the combination of many factors. Random draws from a pool is a method that we have applied before in another context, and this might work here as well: we could generate experimental ‘gradients’ in the number of fungal associates and ask if more putative symbionts are better for a host dealing with stress.
In the meantime, are we so sure mycorrhiza is the most important root mutualism, justifying our perhaps disproportional attention to this topic? and I haven’t even talked about bacteria….
And considering the plant as a whole there is also the phylllosphere and other microbe-inhabited compartments of the plant) that may come into play.
What do you think?