Glomus
Encyclopedia
Glomus is a genus
of arbuscular mycorrhiza
l (AM) fungi
, and all species
form symbiotic relationships (mycorrhiza
s) with plant root
s. Glomus is the largest genus of AM fungi, with ca. 85 species described, but as currently defined is non-monophyletic.
Glomeraceae, in the division Glomeromycota
. Some members of the genus were originally described as Sclerocystis species, but this genus has been entirely transferred to Glomus. However, further taxonomic changes are likely as the phylogeny of AM fungi becomes better understood.
Glomus is likely related to the fossil fungus Glomites, discovered in the Rhynie chert
deposits from the Early Devonian
(400 million years ago).
, dependent on their mycorrhizal association with plant roots to complete their life cycle
. They cannot be cultured in the laboratory in the absence of a plant host. Glomus species are found in nearly all terrestrial habitats, including arable land
, desert
s, grassland
s, tropical forests, and tundra
s.
Arbuscular mycorrhizal fungi can provide numerous benefits to their plant hosts, including improved nutrient uptake, drought
resistance, and disease resistance. However, the symbiosis is not mutualistic in all circumstances and may often be parasitic, with a detrimental effect on plant growth. Rarely, some plant species can parasitise the fungi.
. Spores are produced at the tips of hyphae either within the host root or outside the root in the soil
. Thought to be chlamydospores, these spores germinate and the germination tube that is produced grows through the soil until it comes into contact with roots. The fungus then penetrates the root and grows between root cells, or it may penetrate the cell wall
and grow within root cells. Inside the root, the fungus forms arbuscules, which are highly branched hyphal structures that serve as sites of nutrient exchange with the plant. Arbuscules are formed within plant cell walls but are surrounded by an invaginated cell membrane
, so remain within the apoplast
. The fungus may also form vesicles, swollen structures which are thought to function as food storage organs.
, are cultured and sold as mycorrhizal inoculant for agricultural soils. One species, G. macrocarpum (and possibly also G. microcarpum), causes tobacco
stunt disease.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of arbuscular mycorrhiza
Arbuscular mycorrhiza
An arbuscular mycorrhiza is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plant....
l (AM) fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
, and all species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
form symbiotic relationships (mycorrhiza
Mycorrhiza
A mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant....
s) with plant root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s. Glomus is the largest genus of AM fungi, with ca. 85 species described, but as currently defined is non-monophyletic.
Classification
Glomus is the only genus in the familyFamily (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Glomeraceae, in the division Glomeromycota
Glomeromycota
Glomeromycota is one of seven currently recognized phyla within the kingdom Fungi, with approximately 230 described species. Members of the Glomeromycota form arbuscular mycorrhizas with the roots or thalli of land plants. Geosiphon pyriformis forms an endocytobiotic association with Nostoc...
. Some members of the genus were originally described as Sclerocystis species, but this genus has been entirely transferred to Glomus. However, further taxonomic changes are likely as the phylogeny of AM fungi becomes better understood.
Glomus is likely related to the fossil fungus Glomites, discovered in the Rhynie chert
Rhynie chert
The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...
deposits from the Early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
(400 million years ago).
Ecology
As with other AM fungi, all Glomus species are thought to be obligate symbiontsSymbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
, dependent on their mycorrhizal association with plant roots to complete their life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
. They cannot be cultured in the laboratory in the absence of a plant host. Glomus species are found in nearly all terrestrial habitats, including arable land
Arable land
In geography and agriculture, arable land is land that can be used for growing crops. It includes all land under temporary crops , temporary meadows for mowing or pasture, land under market and kitchen gardens and land temporarily fallow...
, desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
s, grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
s, tropical forests, and tundra
Tundra
In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...
s.
Arbuscular mycorrhizal fungi can provide numerous benefits to their plant hosts, including improved nutrient uptake, drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
resistance, and disease resistance. However, the symbiosis is not mutualistic in all circumstances and may often be parasitic, with a detrimental effect on plant growth. Rarely, some plant species can parasitise the fungi.
Life cycle
Glomus species are entirely asexualAsexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
. Spores are produced at the tips of hyphae either within the host root or outside the root in the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
. Thought to be chlamydospores, these spores germinate and the germination tube that is produced grows through the soil until it comes into contact with roots. The fungus then penetrates the root and grows between root cells, or it may penetrate the cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
and grow within root cells. Inside the root, the fungus forms arbuscules, which are highly branched hyphal structures that serve as sites of nutrient exchange with the plant. Arbuscules are formed within plant cell walls but are surrounded by an invaginated cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, so remain within the apoplast
Apoplast
Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
. The fungus may also form vesicles, swollen structures which are thought to function as food storage organs.
Agricultural significance
Several species of Glomus, including G. aggregatumGlomus aggregatum
Glomus aggregatum is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture....
, are cultured and sold as mycorrhizal inoculant for agricultural soils. One species, G. macrocarpum (and possibly also G. microcarpum), causes tobacco
Tobacco
Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...
stunt disease.
Species
- Glomus aggregatumGlomus aggregatumGlomus aggregatum is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture....
- Glomus etunicatum
- Glomus fasciculatum
- Glomus intraradicesGlomus intraradicesGlomus intraradices is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture. In addition, it is one of the best mycorrhizal varieties of fungi available to mycoforestry, but it "has virtually no market value as an edible or medicinal mushroom" Glomus ...
- Glomus microaggregatum
- Glomus mosseae