Glomeromycota

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Glomeromycota
Scientific classification
Kingdom: Fungi
Phylum: Glomeromycota
C. Walker & A. Schuessler 2001[1]
Class: Glomeromycetes
Caval.-Sm., 1998[2]
Orders

Glomerales
Diversisporales
Paraglomerales
Archaeosporales

Glomeromycota (informally glomeromycetes) is one of six currently recognized phyla within the kingdom Fungi[3], with approximately 200 described species.[4] With the exception of Geosiphon pyriformis, which forms an endocytobiotic association with Nostoc cyanobacteria[5], 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[6]. 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.


Reproduction

The Glomeromycota have generally coenocytic (occasionally sparsely septate) mycelia and reproduce asexually through blastic development of the hyphal tip to produce spores[1] with diameters of 80-500μm[7]. In some, complex spores form within a terminal saccule.[1]

Phylogeny

Initial studies of the Glomeromycota were based on the morphology of soil-borne sporocarps (spore clusters) found in or near colonized plant roots.[8] Distinguishing features such as wall morphologies, size, shape, color, hyphal attachment and reaction to staining compounds allowed a phylogeny to be constructed.[9] Superficial similarities led to the initial placement of genus Glomus in the unrelated family Endogonaceae.[10] Following broader reviews that cleared up the sporocarp confusion, the Glomeromycota were first proposed in the genera Acaulospora and Gigaspora[11] before being accorded their own order with the three families Glomaceae (now Glomeraceae), Acaulosporaceae and Gigasporaceae.[12]

With the advent of molecular techniques this classification has undergone major revision, laying to rest doubts in the group's monophyly.[13] An analysis of small subunit (SSU) rRNA sequences[14] indicated that they share a common ancestor with the Dikarya while the Endogonaceae are late branchers off the chytrid lineage.[1]

Several species which produce glomoid spores (i.e. spores similar to Glomus) in fact belong to other deeply divergent lineages[15] and were placed in the orders, Paraglomerales and Archaeosporales.[1] This new classification includes the Geosiphonaceae in the Archaeosporales which presently contains one fungus that forms endosymbiotic associations with the cyanobacterium Nostoc punctiforme[16] and produces spores typical to this phylum.

Work in this field is incomplete, and members of Glomus may be better suited to different genera[17] or families.[7]

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. The first mycorrhizal gene to be sequenced was the small-subunit ribosomal RNA (SSU rRNA).[18] 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.[7] The data enforces the long-held theory that they were instrumental in the colonization of land by plants.[19]

See also

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References

  1. 1.0 1.1 1.2 1.3 1.4 Schüßler, A. et al. (Dec 2001). "A new fungal phlyum, the Glomeromycota: phylogeny and evolution.". Mycol. Res. 105 (12): 1413-1421.
  2. Cavalier-Smith, T. (1998). "A revised six-kingdom system of Life". Biol. Rev. Camb. Philos. Soc. 73: 246. (as "Glomomycetes")
  3. Hibbett, D.S., et al. (Mar 2007). "A higher level phylogenetic classification of the Fungi". Mycol. Res. 111 (5): 509-547.
  4. http://www.lrz-muenchen.de/~schuessler/amphylo/amphylogeny.html
  5. http://www.lrz-muenchen.de/~schuessler/geosiphon/geosiphon.html
  6. Hempel, S., Renker, C. & Buscot, F. (2007). "Differences in the species composition of arbuscular mycorrhizal fungi in spore, root and soil communities in a grassland ecosystem". Environmental Microbiology 9 (8): 1930-1938.
  7. 7.0 7.1 7.2 Simon, L., Bousquet, J., Levesque, C., Lalonde, M. (1993). "Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants". Nature 363 (6424): 67-69.
  8. Tulasne, L.R., & C. Tulasne (1844). "Fungi nonnulli hipogaei, novi v. minus cogniti auct". Giornale Botanico Italiano 2: 55-63.
  9. 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.
  10. Thaxter, R. (1922). "A revision of the Endogonaceae". Proc. Am. Acad. Arts Sci. 57: 291-341.
  11. J.W. Gerdemann & J.M. Trappe (1974). "The Endogonaceae in the Pacific Northwest". Mycologia Memoirs 5: 1-76.
  12. J.B. Morton & G.L. Benny (1990). "Revised classification of arbuscular mycorrhizal fungi (Zygomycetes): a new order, Glomales, two new suborders, Glomineae and Gigasporineae, and two new families, Acaulosporaceae and Gigasporaceae, with an emendation of Glomaceae". Mycotaxon 37: 471-491.
  13. Morton, J.B. (2000). in C.W. Bacon & J.H. White (eds): Microbial Endophyes. New York: Marcel Dekker. 
  14. Schüßler, A. et al. (Jan 2001). "Analysis of partial Glomales SSU rRNA gene sequences: implications for primer design and phylogeny". Mycol. Res. 105 (1): 5-15. doi:10.1017/S0953756200003725.
  15. Redeker, D. (2002). "Molecular identification and phylogeny of arbuscular mycorrhizal fungi". Plant and Soil 244: 67-73.
  16. Schüßler, A. (2002). "Molecular phylogeny, taxonomy, and evolution of Geosiphon pyriformis and arbuscular mycorrhizal fungi". Plant and Soil 224: 75-83.
  17. Walker, C. (1992). "Systematics and taxonomy of the arbuscular mycorrhizal fungi (Glomales) - a possible way forward". Agronomie 12: 887-897.
  18. Simon, L. Lalonde, M. Bruns, T.D. (1992). "Specific Amplification of 18S Fungal Ribosomal Genes from Vesicular-Arbuscular Endomycorrhizal Fungi Colonizing Roots". American Society of Microbiology 58: 291-295.
  19. D.W. Malloch, K.A. Pirozynski & P.H. Raven (1980). "Ecological and evolutionary significance of mycorrhizal symbiosis in vascular plants (a review)". Proc. Natl Acad. Sci. USA 77 (4): 2113-2118.

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