Glomeromycota: Characteristics, Nutrition, Habitat, Reproduction

The Glomeromycota are forced symbiotic fungi with plant roots. They constitute the arbuscular mycorrhizae, which are a type of ectomycorrhiza. 410 million-year-old fossil records of arbuscular mycorrhizae have been found. It is considered that this symbiotic relationship was one of the characteristics that allowed the colonization of the terrestrial environment by plants.

Glomeromycota have non-septate mycelia (coenocytes). They are characterized by being generally hypogeous and have only asexual reproduction . The spores germinate in the soil until they colonize a root and later form arbuscules and vesicles. Arbuscles are branching hyphae that take up plant nutrients and vesicles are lipid reservoir structures.

Glomeromycota species are distributed throughout the planet in various climatic conditions, being symbionts of bryophytes and vascular plants. Members of the order Archaeosporales form symbionts with cyanobacteria .

Currently about 214 species of Glomeromycota are known, classified into four orders, 13 families and 19 genera. These were observed for the first time in 1842 and located in the Endogonaceae family of the Zygomycota due to the presence of thick-walled spores. Later, based on molecular studies, they were located in a new phyllum (Glomeromycota) at the beginning of the XXI century.

General characteristics

These fungi are multicellular and form hyphae nonseptate (cenocitos). These hyphae can grow within the root cells (intracellular ) or between them (intercellular).

Habitat

The Glomeromycota are distributed throughout the world, occupying practically all the biomes of the planet. They tend to be more abundant and diverse in tropical ecosystems.

The largest number of species is present in Asia, followed by South America. So far only three species have been found in Antarctica .

They can be present in disturbed environments, associated with crops and more abundantly in terrestrial natural ecosystems, from tropical forests to deserts.

More than 40% of the species in this group are cosmopolitan and only 26% are endemic, while the rest have a disjunct distribution. 

Lifestyle

Glomeromycota are obligate symbiotic fungi, that is, they require living in symbiosis with other organisms.

They associate with the roots of plants and form endomycorrhizae (with the hyphae of the fungus within the cells of the plant root). This is beneficial for both species; the fungus and the associated plant.

The fungi belonging to the phyllum Glomeromycota are not pathogenic parasites, they do not cause diseases or harmful effects to other living beings.

Reproduction

Glomeromycota fungi do not exhibit sexual reproduction. They reproduce only asexually through chlamydiospores, which are spores of resistance to unfavorable environmental conditions.

These fungi disperse through the fragmentation of their mycelium (set of filaments or hyphae), together with fragments of roots of plants that they have colonized. They are also spread by chlamydospores.

Mycelium and nutrition

The mycelium or set of filaments of the fungi Glomeromycotas is coenocytic; that is, hyphae do not have partitions or septa and cells have many nuclei.

The hyphae have cell walls with chitin, which gives them rigidity. This rigidity and toughness facilitates its penetration into the cells of the plant roots.

The mycelium of the fungus develops within the root (intraradical mycelium, forming endomycorrhizae) and also outside the root (extraradical mycelium). The symbiotic fungus-root association of plants is called mycorrhiza.

The hyphae of the Glomeromycotas fungi also have the ability to penetrate the cortical cells (or cells of the cortex, located under the epidermis) of the roots and form structures called arbuscules and vesicles.

The bushes are formed by a specialized haustorium or hypha, which absorbs nutrients from the root of the plant. This haustorian hypha is highly branched and develops intracellularly (within the root cells).

The exchange of nutrients between the two symbionts (plant and fungus) takes place in the arbuscules.

The fungus supplies the plant with macronutrients, especially phosphorus (P), which it takes from the soil efficiently. To supply the plant with these plant macronutrients, the fungus uses an extraradical mycelium, which grows in association with the root but externally to it. The plant supplies the fungus with sugars (carbohydrates) that it has produced thanks to photosynthesis.

Some Glomeromycotas fungi have vesicles, which are balloon-shaped structures where they store lipids (fats), as reserve substances.

Hyphal system

The mycelial system (set of hyphae) is composed of the internal mycelia (within the root tissues) and the external mycelia (which extend over the soil surface.

The external mycelia are branched. These form a network that interconnects the roots of plants of different species in the ecosystem .

In the internal mycelia there are two types of hyphae. The Paris type are uniquely intracellular and spiral-shaped, while those of the Arum type are primarily intercellular.

Intracellular hyphae branch forming arbuscules (branched hyphae that occupy more than 35% of the volume of the infected cell). These are short-lived and it is the site of nutrient exchange between symbionts.

In some groups of Glomeromycota there are vesicles that are structures that form at the apex of the hyphae and accumulate nutrients.

The spores are asexual with thick, multinucleate walls. The nuclei are generally genetically different (heterokaryotic).

Phylogeny and taxonomy

The first Glomeromycota were observed in the 19th century and were located in the Zygomycetes class due to the presence of thick-walled spores. During the 90s of the 20th century it was determined that all arbuscular mycorrhizal fungi were obligate symbionts, with unique morphological characteristics.

In 2001 the Glomeromycota phylum was established based on morphological, biochemical and molecular characteristics. This is a sister group to the Dikarya sub-kingdom.

Orders

It is subdivided into four orders: Archaeosporales, Diversisporales, Glomerales and Paraglomerales. These comprise 13 families, 19 genera and so far 222 species have been described.

Archaeosporales form endosymbionts with cyanobacteria or  mycorrhiza with arbuscules and their spores are colorless. It is made up of three families and approximately five species.

The Diversisporales have arbuscules and almost never form vesicles. Eight families and about 104 species have been described.

Glomerales is the largest group. It presents arbuscules, vesicles and spores with a varied morphology. It is made up of two families and the genus Glomus is the most numerous with some 74 species.

In the Paraglomerals arbuscules are present and vesicles do not develop and the spores are colorless. It contains a family and a genus with four described species.

Nutrition

Arbuscular mycorrhizal fungi are obligate endosymbionts, so they cannot survive outside their host.

More than 90% of vascular plants and 80% of all terrestrial plants have symbiotic associations with a Glomeromycota. Arbuscular mycorrhiza fossils have been found from the early Devonian (about 420 million years ago).

It is considered that these fungi were of vital importance in the colonization of the terrestrial environment by plants. These contributed to its nutrition, mainly for the use of phosphorus and micronutrients.

Relationship between symbionts

The plant is the carbon source for the fungus. The photosynthesized are transported to the root and mobilized to the fungus through the arbuscules. Later these sugars (mainly hexoses) are transformed into lipids.

The lipids are accumulated in the vesicles and from there transported to the network of intra- and extra-radical hyphae for the nutrition of the fungus.

For its part, the fungus contributes to the absorption of inorganic phosphorus in environments poor in this nutrient for the plant. They can also take advantage of the nitrogen contained in the litter and other organic matter present in the soil.

Reproduction

Until now, asexual reproduction has only been evidenced in Glomeromycota.

Asexual spores are very thick-walled and large (40-800 µm). These can occur in a sporocarp (hyphal network) that forms directly in the root, the soil or in other structures (remains of seeds, insects or others). They are multinucleated (hundreds to thousands of nuclei) and can be genetically distinct

Host colonization

The spores fall to the ground and are carried by insects, small mammals, or water. Later they germinate, going through a very short saprophytic phase. The germ tubes can grow 20-30 mm to colonize a root.

Once the germ tube makes contact with the root, an appressorium (adhesive structure) is produced that penetrates the epidermal cells. The hyphae reach the root cortex, both intercellularly and intracellularly, and arbuscules, vesicles and the network of extraradical hyphae are formed.

Lifecycle

To explain the life cycle of the fungi of the phyllum Glomeromycota, the cycle of the fungi of the genus Glomus will be taken as an example . This genus produces its spores at the ends of its hyphae, either within the root of the plant or outside it, in the soil.

The spores of the chlamydospores type (resistant), when germinating, produce hyphae that grow through the soil until they are in contact with roots. The fungus penetrates the root and grows in the intercellular spaces or passes through the cell wall and develops within the root cells.

Once the root is penetrated, the fungus forms arbuscules (highly branched structures of hyphae). The arbuscules function as a place of exchange of nutrients with the plant. The fungus can also form vesicles that function as nutrient storage organs.

In other specialized hyphae called sporangiophores, structures called sporangia are formed at their ends, which are sac-shaped and contain spores. When the sporangium matures, it breaks and releases the spores (chlamydospores), restarting the life cycle of these fungi.

The study of the genome (set of genes) of 4 species of fungi of the genus Glomus, revealed the presence of genes that encode essential proteins for the meiosis of eukaryotic cells (with nuclei).

Since meiosis is considered a type of cell division of sexual reproduction, it would be expected that in the life cycle of these fungi, there would be a stage of sexual reproduction. To date, no sexual stage has been identified in the life cycle of fungi of the genus Glomus, despite the fact that they possess the machinery to carry it out.

Ecological and economic importance

The function of Glomeromycotas fungi in ecosystems is of vital importance. By supplying essential macronutrients to the plants with which they are associated in symbiosis, they favor the preservation of plant diversity.

Additionally, these fungi provide plants with symbionts of resistance to drought and pathogens.

From an economic point of view, by promoting the symbiosis of Glomeromycotas fungi with cultivable plants, their survival is increased, their yield is improved and production is increased. These fungi are used as soil inoculums or biofertilizers in many crops.

Examples of Glomeromycota fungi: genus Glomus

Among the Glomeromycota fungi, several species belonging to the genus Glomus can be pointed out, which is a genus of mycorrhizal arbuscular fungi (AM), with species that form symbiotic associations (called mycorrhizae) with plant roots. This is the most numerous genus of AM fungi with 85 described species.

Among the species of the genus Glomus, we can mention: Glomus aggregatum, G. mosseae. G. flavisporum, G. epigaeum, G. albidum, G. ambisporum, G. brazillanum, G. caledonium, G. coremioides, G. claroideum, G. clarum, G. clavisporum, G. constrictum, G. coronatum, G. deserticola, G. diaphanum, G. eburneum, G. etunicatum, G. macrocarpus, G. intraradices, G. microcarpus, G. tenue, among others.

References

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  3. Muthukumar T. KP Radhika, J Vaingankar, JD´Souza, S Dessai and BF Rodrigues (2009) Taxonomy of AM fungi an update. In: Rodrigues BF and T Muthukumar (ed.) Arbuscular Miycorrhizae of Goa: A manual of identification protocols. Goa University, India.
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