| Glomeromycota | |
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| Gigaspora margarita in association withLotus corniculatus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Fungi |
| Division: | Glomeromycota |
| Subdivision: | Glomeromycotina C.Walker & A.Schuessler (2001)[2] |
| Class: | Glomeromycetes Caval.-Sm. (1998)[1] |
| Orders | |
Glomeromycota (often referred to asglomeromycetes, as they include only one class, Glomeromycetes) are one of eight currently recognizeddivisions within thekingdomFungi,[3] with approximately 230 described species.[4] Members of the Glomeromycota formarbuscular mycorrhizas (AMs) with the thalli ofbryophytes and the roots ofvascular land plants. Not all species have been shown to form AMs, and one,Geosiphon pyriformis, is known not to do so. Instead, it forms an endocytobiotic association withNostoccyanobacteria.[5] The majority of evidence shows that the Glomeromycota are dependent on land plants (Nostoc in the case ofGeosiphon) for carbon and energy, but there is recent circumstantial evidence that some species may be able to lead an independent existence.[6] Thearbuscular 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 inwetlands, including salt-marshes, and associated with epiphytic plants.
According to multigene phylogenetic analyses, this taxon is located as a member of the phylumMucoromycota.[7] Currently, the phylum name Glomeromycota is invalid, and the subphylum Glomeromycotina should be used to describe this taxon.[8]
The Glomeromycota have generallycoenocytic (occasionally sparselyseptate)mycelia and reproduce asexually through blastic development of the hyphal tip to producespores[2] (Glomerospores,blastospore) with diameters of 80–500 μm.[9] In some, complex spores form within a terminal saccule.[2] Recently it was shown thatGlomus species contain 51 genes encoding all the tools necessary formeiosis.[10] Based on these and related findings, it was suggested thatGlomus species may have a cryptic sexual cycle.[10][11][12]
New colonization of AM fungi largely depends on the amount ofinoculum present in the soil.[13] Although pre-existing hyphae and infected root fragments have been shown to colonize the roots of a host successfully, germinating spores are considered to be the key players in new host establishment.Spores are commonly dispersed by fungal and plant burrowing herbivore partners, but some air dispersal capabilities are also known.[14] 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.[13] In addition, new data also suggests that AM fungi host plants also secrete chemical factors that attract and enhance the growth of developing spore hyphae towards the root system.[14]
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 fungalmycelium. Colonization is accomplished by the interactions between germinating spore hyphae and the root hairs of the host or by the development ofappressoria between epidermal root cells. The process is regulated by specialized chemical signaling and changes in gene expression of both the host and AM fungi. Intracellular hyphae extend up to thecortical cells of the root and penetrate the cell walls but not the inner cellular membrane creating an internalinvagination. The penetrating hyphae develop a highly branched structure called anarbuscule, which has low functional periods before degradation and absorption by the host's root cells. A fully developed arbuscular mycorrhizal structure facilitates the two-way movement of nutrients between the host andmutualistic fungal partner. Thesymbiotic association allows the host plant to respond better to environmental stresses, and the non-photosynthetic fungi to obtain carbohydrates produced by photosynthesis.[14]
Initial studies of the Glomeromycota were based on the morphology of soil-borne sporocarps (spore clusters) found in or near colonized plant roots.[15] Distinguishing features such as wall morphologies, size, shape, color,hyphal attachment and reaction to staining compounds allowed a phylogeny to be constructed.[16] Superficial similarities led to the initial placement of genusGlomus in the unrelated familyEndogonaceae.[17] Following broader reviews that cleared up the sporocarp confusion, the Glomeromycota were first proposed in the generaAcaulospora andGigaspora[18] before being accorded their own order with the threefamilies Glomaceae (nowGlomeraceae), Acaulosporaceae and Gigasporaceae.[19]
With the advent of molecular techniques this classification has undergone major revision. An analysis ofsmall subunit (SSU) rRNA sequences[20] indicated that they share a common ancestor with theDikarya.[2] Nowadays it is accepted that Glomeromycota consists of 4 orders.[21]
| Glomeromycota | |
Several species which produce glomoid spores (i.e. spores similar toGlomus) in fact belong to other deeply divergent lineages[22] and were placed in the orders,Paraglomerales andArchaeosporales.[2] This new classification includes theGeosiphonaceae, which presently contains one fungus (Geosiphon pyriformis) that forms endosymbiotic associations with thecyanobacteriumNostoc punctiforme[23] and produces spores typical to this division, in theArchaeosporales.
Work in this field is incomplete, and members ofGlomus may be better suited to different genera[24] or families.[9]
The biochemical and genetic characterization of the Glomeromycota has been hindered by theirbiotrophic 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.[25] The first mycorrhizal gene to be sequenced was the small-subunitribosomal RNA (SSU rRNA).[26] This gene is highly conserved and commonly used inphylogenetic studies so was isolated fromspores of eachtaxonomic group before amplification through thepolymerase chain reaction (PCR).[27] A metatranscriptomic survey of theSevilleta 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.[28]
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