Glomeromycetes
   HOME

TheInfoList



OR:

Glomeromycota (often referred to as glomeromycetes, as they include only one class, Glomeromycetes) are one of eight currently recognized divisions within the kingdom Fungi, with approximately 230 described species. Members of the Glomeromycota form arbuscular mycorrhizas (AMs) with the thalli of bryophytes and the roots of vascular land plants. Not all species have been shown to form AMs, and one, ''
Geosiphon ''Geosiphon'' is a genus of fungus in the family Geosiphonaceae. The genus is monotypic, containing the single species ''Geosiphon pyriformis'', first described by Kützing in 1849 as ''Botrydium pyriforme''. In 1915, Von Wettstein characterized ...
pyriformis'', is known not to do so. Instead, it forms an endocytobiotic association with ''
Nostoc ''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in vari ...
''
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
. The majority of evidence shows that the Glomeromycota are dependent on land plants (''Nostoc'' in the case of ''Geosiphon'') for carbon and energy, but there is recent circumstantial evidence that some species may be able to lead an independent existence. The arbuscular mycorrhizal species are terrestrial and widely distributed in soils worldwide where they form symbioses with the roots of the majority of plant species (>80%). They can also be found in wetlands, including salt-marshes, and associated with epiphytic plants. According to multigene phylogenetic analyses, this taxon is located as a member of the phylum
Mucoromycota Mucoromycota is a division within the kingdom fungi. They include a diverse group of various molds, including the common bread molds ''Mucor'' and ''Rhizopus''. It is a sister phylum to Dikarya. It consists of mainly mycorrhizal fungi, root end ...
. Currently the phylum name Glomeromycota may be invalid, and the subphylum Glomeromycotina or the class Glomeromycetes is preferable to describe this taxon.


Reproduction

The Glomeromycota have generally
coenocytic A coenocyte () is a multinucleate cell which can result from multiple nuclear divisions without their accompanying cytokinesis, in contrast to a syncytium, which results from cellular aggregation followed by dissolution of the cell membranes insid ...
(occasionally sparsely septate) mycelia and reproduce asexually through blastic development of the hyphal tip to produce
spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s (Glomerospores) with diameters of 80–500 
μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
. In some, complex spores form within a terminal saccule. Recently it was shown that ''Glomus'' species contain 51 genes encoding all the tools necessary for meiosis. Based on these and related findings, it was suggested that ''Glomus'' species may have a cryptic sexual cycle.


Colonization

New colonization of AM fungi largely depends on the amount of
inoculum In biology, inoculum refers to the source material used for inoculation. ''Inoculum'' may refer to: * In medicine, material that is the source of the inoculation in a vaccine * In microbiology, propagules: cells, tissue, or viruses that are used t ...
present in the soil.Zangaro, Waldemar, Leila Rostirola, Vergal Souza, Priscila Almeida Alves, Bochi Lescano, Ricardo Rondina, Luiz Nogueira, and Eduardo Carrenho. "Root Colonization and Spore Abundance of Arbuscular Mycorrhizal Fungi in Distinct Successional Stages from an Atlantic Rainforest Biome in Southern Brazil." Mycorrhiza 23.3 (2013): 221-33. Web. Although pre-existing hyphae and infected root fragments have been shown to successfully colonize the roots of a host, germinating spores are considered to be the key players in new host establishment.
Spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s are commonly dispersed by fungal and plant burrowing herbivore partners, but some air dispersal capabilities are also known. Studies have shown that spore germination is specific to particular environmental conditions such as right amount of nutrients, temperature or host availability. It has also been observed that the rate of root system colonization is directly correlated to spore density in the soil. In addition, new data also suggests that AM fungi host plants also secrete chemical factors which attract and enhance the growth of developing spore hyphae towards the root system. The necessary components for the colonization of Glomeromycota include, the host's fine root system, proper development of intracellular arbuscular structures, and a well-established external fungal mycelium. Colonization is accomplished by the interactions between germinating spore hyphae and the root hairs of the host or by development of
appressoria An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable ...
between epidermal root cells. The process is regulated by specialized chemical signaling and by changes in gene expression of both the host and AM fungi. Intracellular hyphae extend up to the cortical cells of the root and penetrate the cell walls, but not the inner cellular membrane creating an internal invagination. The penetrating hyphae develop a highly branched structure called an
arbuscule An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'', a.k.a. ''endomycorrhiza'') is a type of mycorrhiza in which the symbiont fungus (''AM fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. (N ...
which have low functional periods before degradation and absorption by host's root cells. A fully developed arbuscular mycorrhizal structure facilitates the two-way movement of nutrients between the host and mutualistic fungal partner. The
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
association allows the host plant to respond better to environment stresses, and the non-photosynthetic fungi to obtain carbohydrates produced by photosynthesis.


Phylogeny

Initial studies of the Glomeromycota were based on the morphology of soil-borne sporocarps (spore clusters) found in or near colonized plant roots. Distinguishing features such as wall morphologies, size, shape, color,
hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...
l attachment and reaction to staining compounds allowed a phylogeny to be constructed.Wright, S.F. Management of Arbuscular Mycorrhizal Fungi. 2005. In Roots and Soil Management: Interactions between roots and the soil. Ed. Zobel, R.W., Wright, S.F. USA: American Society of Agronomy. Pp 183–197. Superficial similarities led to the initial placement of genus ''Glomus'' in the unrelated family
Endogonaceae Endogonales is an order of fungi within the phylum of Zygomycota. It contains 2 families, Endogonaceae, with four genera and 27 species and Densosporaceae, with one genera and 5 species. Genera ''Endogonaceae''; * ''Endogone'' * '' Jimgerdema ...
. Following broader reviews that cleared up the sporocarp confusion, the Glomeromycota were first proposed in the genera ''Acaulospora'' and ''Gigaspora'' before being accorded their own order with the three families Glomaceae (now Glomeraceae), Acaulosporaceae and Gigasporaceae. With the advent of molecular techniques this classification has undergone major revision. An analysis of small subunit (SSU) rRNA sequences indicated that they share a common ancestor with the Dikarya. Nowadays it is accepted that Glomeromycota consists of 4 orders. Several species which produce glomoid spores (i.e. spores similar to '' Glomus'') in fact belong to other deeply divergent lineages and were placed in the orders,
Paraglomerales The Paraglomerales are a group of exclusively hypogeous (underground) arbuscular mycorrhizal fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, ...
and
Archaeosporales Archaeosporales is an order of fungi best known as arbuscular mycorrhiza to vascular land plants (Tracheophyta). But also form free living endocyte symbioses with cyanobacteria. The free living forms have a Precambrian fossil record back 2.2 Ga ...
. This new classification includes the
Geosiphonaceae ''Geosiphon'' is a genus of fungus in the family Geosiphonaceae. The genus is monotypic, containing the single species ''Geosiphon pyriformis'', first described by Kützing in 1849 as ''Botrydium pyriforme''. In 1915, Von Wettstein characterized ...
, which presently contains one fungus (''Geosiphon pyriformis'') that forms endosymbiotic associations with the cyanobacterium ''
Nostoc punctiforme ''Nostoc punctiforme'' is a species of filamentous cyanobacterium. Under non-limiting nutritional environmental conditions, its filaments are composed of photosynthetic vegetative cells; upon nutrient limitation, some of these cells undergo diff ...
'' and produces spores typical to this division, in the
Archaeosporales Archaeosporales is an order of fungi best known as arbuscular mycorrhiza to vascular land plants (Tracheophyta). But also form free living endocyte symbioses with cyanobacteria. The free living forms have a Precambrian fossil record back 2.2 Ga ...
. Work in this field is incomplete, and members of ''Glomus'' may be better suited to different genera or families. Fossil Glomeromycota are known back 2200 million years as
Diskagma ''Diskagma'' ("disc-like fragment") is a genus of problematic fossil from a Paleoproterozoic (2200 million years old) paleosol from South Africa, and significant as the oldest likely eukaryote and earliest evidence for megascopic life on land. ...
, and also by Mesoproterozoic Horodyskia and Ordovician to Devonian
Prototaxites ''Prototaxites'' is a genus of terrestrial fossil fungi dating from the Middle Ordovician until the Late Devonian periods, approximately . ''Prototaxites'' formed small to large trunk-like structures up to wide, reaching in length, made up of ...
.


Molecular biology

The biochemical and genetic characterization of the Glomeromycota has been hindered by their biotrophic nature, which impedes laboratory culturing. This obstacle was eventually surpassed with the use of root cultures and, most recently, a method which applies sequencing of single nucleus from spores has also been developed to circumvent this challenge. The first mycorrhizal gene to be sequenced was the small-subunit ribosomal RNA (SSU rRNA). This gene is highly conserved and commonly used in phylogenetic studies so was isolated from
spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
s of each
taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
group before amplification through the polymerase chain reaction (PCR). A metatranscriptomic survey of the Sevilleta Arid Lands found that 5.4% of the fungal rRNA reads mapped to Glomeromycota. This result was inconsistent with previous PCR-based studies of community structure in the region, suggesting that previous PCR-based studies may have underestimated Glomeromycota abundance due to amplification biases.


See also

*
Prototaxites ''Prototaxites'' is a genus of terrestrial fossil fungi dating from the Middle Ordovician until the Late Devonian periods, approximately . ''Prototaxites'' formed small to large trunk-like structures up to wide, reaching in length, made up of ...
* Horodyskia *
Diskagma ''Diskagma'' ("disc-like fragment") is a genus of problematic fossil from a Paleoproterozoic (2200 million years old) paleosol from South Africa, and significant as the oldest likely eukaryote and earliest evidence for megascopic life on land. ...


References


External links


Tree of Life Glomeromycota
at the International Culture Collection of VA Mycorrhizal Fungi (INVAM)
Glomeromycota
at the University of Sydney Fungal Biology s, ite
'AMF-phylogeny'
- 'Glomeromycota database' web-site at the University of Munich * {{Authority control Fungus phyla Fungi by classification Soil biology