Glomeromycota (informally glomeromycetes) is one of six currently recognized phyla within the kingdom Fungi, with approximately 200 described species. With the exception of Geosiphon pyriformis, which forms an endocytobiotic association with Nostoc cyanobacteria, all species are thought to form arbuscular mycorrhizas with the roots of land plants. However, this has not yet been shown for all species. The majority of evidence shows that the Glomeromycota are obligate biotrophs, dependent on symbiosis with 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. They can also be found in wetlands, including salt-marshes, and associated with epiphytic plants.
The Glomeromycota have generally coenocytic (occasionally sparsely septate) mycelia and reproduce asexually through blastic development of the hyphal tip to produce spores with diameters of 80-500μm. In some, complex spores form within a terminal saccule.
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, hyphal attachment and reaction to staining compounds allowed a phylogeny to be constructed. Superficial similarities led to the initial placement of genus Glomus in the unrelated family Endogonaceae. 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, laying to rest doubts in the group's monophyly. An analysis of small subunit (SSU) rRNA sequences indicated that they share a common ancestor with the Dikarya while the Endogonaceae are late branchers off the chytrid lineage.
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 and Archaeosporales. This new classification includes the Geosiphonaceae in the Archaeosporales which presently contains one fungus that forms endosymbiotic associations with the cyanobacterium Nostoc punctiforme and produces spores typical to this phylum.
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. 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 spores of each taxonomic group before amplification through the polymerase chain reaction (PCR). A molecular clock approach, based on the substitution rates of SSU sequences, was used to estimate the time of divergence of the fungi. The molecular analysis found that they are between 462 and 353 Million years old. The data enforces the long-held theory that they were instrumental in the colonization of land by plants.
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