Sundry information on mycorrhizae, particularly Glomus, in grasses

(writing in progress)
 
A typical genus of mycorrhizal fungus in grasses is Glomus (please see https://en.wikipedia.org/wiki/Glomus_(fungus) ). The kingdom is Fungi, the Class is Glomeromycetes, the Family is Glomeraceae, and the genus is Glomus. This type of fungus is incapable of living free, and is obligately dependent on plant roots.
 
Unlike ectomycorrhizal fungi associated with pines, oaks, eucalypts and other trees, which form mushrooms, Glomus has no mushroom and simply releases its spores underground, from its hyphae.
 
Glomus is called an arbuscular mycorrhizal fungus because of its arbuscules, which are in effect minuscule ‘roots’ within the grass root cells. So at one end the fungus has a network of hyphae permeating the soil more thoroughly and more rapidly than the root hairs of any plant could do, and at the other end the fungus has hyphal branches within the root cells of the grass. The fungus takes up sugar at the arbuscule and uses it to grow and to metabolise, its various functions on behalf of the grass including immunity (mainly from pathogenic fungi), uptake of nutrients, and uptake of water. The fungus has no source of energy other than the photosynthesis of the grass, in contrast to free-living saprophytic fungi and also, I think, ectomycorrhizal fungi (which certainly take much energy from the tree but also possibly get some of their energy by breaking down litter in a way similar to white-rot fungi).
 
It is hard to say which function of the fungus (immunity, nutrition, or hydration) is most expensive from the viewpoint of the grass. Possibly this varies according to the grass taxon and the habitat.
 
You’ll seldom see this written anywhere, but it’s safe to assume that the fungus is aerobic. There is little oxygen underground and as I understand it grasses are fundamentally designed to supply air to the fungus. As you know, grasses have aerenchyma, so that in e.g. aquatic rice the plant acts as a conduit of oxygen from the air down into the waterlogged root-zone. It’s easy to assume that the grass roots need this air but I think a more correct way of viewing this would be that it is the arbuscular mycorrhiza of rice (and I can confirm that Glomus occurs also in rice Oryza) that has the main need for oxygen. I suspect that, per unit biomass, the fungal cells consume oxygen an order of magnitude more rapidly than the root cells themselves.
 
So when a gardener aerates a lawn, by driving holes through the turf, I assume that it is really the fungus (often Glomus) that is really being aerated. And the particular need of lawns to keep their roots aerated has several implications.
 
Furthermore, the benefit to lawns of frequent watering (to a degree apparently far in excess of what the plant itself could possibly need for its transpiration) may be to the fungus. As you know, fungi are easily dehydrated. Many or most of the hyphae of Glomus probably permeate the topsoil, because this is where much of the organic matter and nutrients are. For lawn to thrive, there must be enough air for the fungus to breathe, and enough water for the fungus to drink. The fungus can be expected to exceed the grass in its dependence on both breath and drink, because it catabolises so much more rapidly than the grass and because its hyphae are so narrow and thus so vulnerable to desiccation.
 
Because lawns tend to be maintained by large grazers, which trample the turf, I infer that the lawn-grazer relationship would not work well without a means of aeration. Not only do lawns tend to have earthworms (and, in the tropics, termites such as Hodotermes) which continually aerate the root zone, but the herbivores themselves promote the continual burrowing of invertebrates because their faeces are consumed by e.g. dung beetles. A grazing lawn is thus not so much a two-way partnership as a three-way partnership. It is easy to forget that the mutualism between grass and grazer would possibly not work in the absence of the third partner, the burrowing invertebrates (earthworms are typical in temperate climates and Charles Darwin was on to them in a big way although I’m not sure that he thought the grazing lawn concept through).
 
What nobody seems to have thought of before – although for many decades now there has been a virtual army of researchers worldwide working on mycorrhizae – is the role of selenium for the mutualism between grass and arbuscular mycorrhizal fungi such as Glomus.
 
I think it’s safe to assume that the cells of the grass itself, including its roots cells, have such a small requirement for Se that it is to all intents and purposes both immeasurable and negligible. However, that is not to say that the grass-fungal symbiosis has a similarly negligible requirement for Se.
 
I predict that if one were to work at the minute scale necessary to sample the hyphae themselves of Glomus, and if one were to analyse these for concentration of Se, one would find a concentration of Se an order of magnitude or more greater than that in the cells of the grass plant.
 
It’s interesting that, while even the most oligotrophic and fire-prone of grasses, such as Triodia, have arbuscular mycorrhizae, the entire family Restionaceae lack this fungal assocation, having cluster roots instead. This implies that restios lack the (indirect) requirement for Se that I postulate for all grasses. Of course, I assume that a hummock grass requires less Se than a lawn grass, but it’s interesting that hummock grasses – although sharing sclerophylly with restios – differ qualitatively in the presence/absence of mycorrhizae.

(writing in progress)

Posted on July 3, 2022 04:31 AM by milewski