#015: Characteristics of Division Glomeromycota

The Glomeromycota are unusual and poorly understood organisms. Fungi from this division rarely produce easily visible structures and cannot be grown without a plant host, so investigating them is very difficult. Glomeromycotan fungi are some of the most important fungi on Earth because they form arbuscular mycorrhizas, which provide essential nutrients to the vast majority of terrestrial plants.¹

Ecology

The major unifying characteristic of fungi in Glomeromyocta is their ecological role. All glomeromycotans form arbuscular mycorrhizas (AM) with various terrestrial plants (see FFF#074 for a full discussion of AM). The one exception to this rule – since there is always an exception when it comes to fungi – is Geosiphon pyriformis, which is symbiotic with cyanobacteria instead. Unlike some mycorrhizal fungi, the Glomeromycota are obligate symbionts, meaning they cannot survive without their photosynthetic partner. Consequently, they cannot be grown in a lab unless they are provided with a suitable host.¹

Mycorrhizas are a type of mutualism between a fungus and a plant where the fungus gives the plant nutrients (especially phosphorous) in exchange for sugars produced through photosynthesis.^1,2 AM fungi facilitate this relationship by forming structures within the plant’s root cells while not causing a disease reaction.¹ Most land plant species form AM; the ones that do not usually form ectomycorrhizas but a few plant families are nonmycorrhizal (see FFF#073 for more on other types of mycorrhizas). AM fungi tend to be generalists, colonizing a variety of different plant species. Likewise, each AM plant is usually colonized by multiple species of Glomeromycota.¹ This results in a network that allows plants to share nutrients and even communicate with one another.^2,3 Because so many plants rely on them, the Glomeromycota are some of the most abundant soil fungi and are found in diverse habitats all across the globe.⁴

Morphology

The Glomeromycota produce very few structures. Inside mycorrhizal roots, they form hyphae, arbuscules, and sometimes vesicles. In the soil, they form distributive hyphae, absorptive hyphae, and spores. These structures all have very few observable features, but mycologists can use them to identify organisms down to species. It also helps to know the mycorrhizal partner and soil conditions such as pH.⁵

Inside an AM root, hyphae take on one of two forms: linear or coiled. Linear hyphae grow in air spaces between the root cells and are therefore as straight as the root structure permits. Coiled hyphae grow inside cells and grow directly from one cell to the next. These hyphae are not as limited by root structure and can follow a more meandering path.⁵ Glomeromycota are primarily aseptate fungi, meaning they rarely form walls (or ‘septa’) to divide the hyphae into cells.^1,5 AM fungi occasionally produce septa in older areas of roots where the fungus is no longer growing.⁵

Regardless of the type of hyphae, the AM fungus must penetrate a root cell and form a structure called an ‘arbuscule’ in order to exchange nutrients. Arbuscules are highly branched intracellular structures formed by the fungus. They have a very high surface area, which allows for an efficient exchange of nutrients and sugars. The degree and pattern of branching varies slightly among species.⁵

After the fungus has established arbuscules, some glomeromycotans form structures called ‘vesicles’. A vesicle is an inflated area of the hypha that functions as a storage structure. In some species, these resemble spores and may also function like spores. Vesicle presence or absence, position, and morphology are variable and can help identify the species of fungus.⁵

Outside the root, most fungal structures are designed for nutrient acquisition. Distributive hyphae are relatively large and branch infrequently; their primary purpose is to connect different parts of the fungus together. Absorptive hyphae are thin, short, and highly-branched and are used to extract nutrients from the soil. Neither of these types of hyphae is easily separated from the soil, so they are not useful for identification.⁵

Reproduction

Perhaps the most bizarre feature of the Glomeromycota is that they reproduce only asexually. Years of research have failed to find any sexual reproductive structures or any DNA evidence of sexual reproduction.¹ For an entire Division or Phylum to rely solely on asexual reproduction is almost unheard of among eukaryotes. Recombination of genes through the process of sexual reproduction helps evolution remove harmful genes and promote beneficial ones. That the Glomeromycota could abandon sexual reproduction hundreds of millions of years ago and yet remain some of the most successful fungi on the planet is simply astounding.

Since the Glomeromycota do not do sexual reproduction, each individual should contain only genetically identical nuclei. However, some genetic studies suggest that this is not the case and instead they have multiple genetically distinct nuclei. This has not been definitively proven, but it still could complicate efforts to use DNA to investigate reproduction.¹

The Glomeromycota reproduce primarily through the formation of spores. These spores are unusually large for fungi, ranging from 40µm to 800µm (corn pollen is 90µm to 100µm⁶). The upper end of that range is nearly a millimeter, which means some spores are visible to the unaided eye! To support such large spore sizes, each spore contains hundreds or even thousands of nuclei.¹ Some glomeromycotans go a step further and aggregate spores into truffle-like fruiting bodies. As with truffles, these fruiting bodies are eaten by animals, which then distribute the spores in their scat.⁴

Glomeromycotan spores have more morphologically variable features than any of the other structures produced by these fungi, so they are invaluable when identifying species. These spores have extremely thick cell walls that are often composed of multiple layers.¹ They all have at least one cell wall, but some species have as many as four distinct walls.⁷ The outermost wall also varies in color and texture: spores can range from smooth to bumpy and from clear to dark brown.⁵ Additionally, the processes of spore formation and germination differ between species and can be used to identify specimens.⁷ For details about morphological differences in glomeromycotan spores, see this paper.

When conditions are right, the spore germinates and grows a short hypha in an attempt to find a suitable root host. If none is found, the nascent fungus retreats back into the spore and waits to try again later.¹

Taxonomy and Evolution

Glomeromycota was only recently established as a division; it was separated from the old polyphyletic division Zygomycota in 2001.^1,8 Thanks to DNA technology, mycologists now know that the Glomeromycota are more closely related to the Dikarya (Ascomycota and Basidiomycota). The Glomeromycota probably split from Dikarya around 600 million years ago (Dikarya split into Ascomycota and Basidiomycota sometime after that).^1,9

Fossils of early glomeromycotan fungi date as early as 400 million years ago and correspond with the evolution of land plants. Indeed, taxonomic relationships suggest that the common ancestor of all land plants formed arbuscular mycorrhizas. Both of these lines of analysis suggest that the Glomeromycota were crucial in helping plants colonize the land (for more on the evolution of mycorrhizas, see FFF#078).¹

There is a surprising lack of diversity within the Glomeromycota, especially considering how widespread the organisms are. Glomeromycota contains about 150 species divided up among 4 orders, 11 families, and 27 genera.^1,10 However, species of Glomeromycota were primarily established using spore morphology until DNA analysis tools became widely available. It seems likely that genetic data will necessitate creating many new species to support hidden genetic diversity.¹

Kingdom	Fungi
Division	Glomeromycota C. Walker & A. Schüßler8
Class	Glomeromycetes10
Order (Families)	Glomerales (Glomeraceae, Claroideoglomeraceae)10
	Diversisporales (Gigasporaceae, Acaulosporaceae, Pacisporaceae, Diversisporaceae, Sacculosporaceae)10
	Paraglomerales (Paraglomeraceae)10
	Archaeosporales (Geosiphonaceae, Ambisporaceae, Archaeosporaceae)10