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Glomeromycota

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Glomeromycota
Gigaspora margarita in association with Lotus corniculatus
Scientific classification Edit this 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 as glomeromycetes, as they include only one class, Glomeromycetes) are one of eight currently recognized divisions within the kingdom Fungi,[3] with approximately 230 described species.[4] 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 pyriformis, is known not to do so. Instead, it forms an endocytobiotic association with Nostoc cyanobacteria.[5] 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.[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 (>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.[7] 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 (occasionally sparsely septate) mycelia and reproduce asexually through blastic development of the hyphal tip to produce spores[2] (Glomerospores) with diameters of 80–500 μm.[8] In some, complex spores form within a terminal saccule.[2] Recently it was shown that Glomus species contain 51 genes encoding all the tools necessary for meiosis.[9] Based on these and related findings, it was suggested that Glomus species may have a cryptic sexual cycle.[9][10][11]

Colonization

New colonization of AM fungi largely depends on the amount of inoculum present in the soil.[12] 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. Spores are commonly dispersed by fungal and plant burrowing herbivore partners, but some air dispersal capabilities are also known.[13] 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.[12] 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.[13]

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 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 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 association allows the host plant to respond better to environment stresses, and the non-photosynthetic fungi to obtain carbohydrates produced by photosynthesis.[13]

Phylogeny

Initial studies of the Glomeromycota were based on the morphology of soil-borne sporocarps (spore clusters) found in or near colonized plant roots.[14] Distinguishing features such as wall morphologies, size, shape, color, hyphal attachment and reaction to staining compounds allowed a phylogeny to be constructed.[15] Superficial similarities led to the initial placement of genus Glomus in the unrelated family Endogonaceae.[16] Following broader reviews that cleared up the sporocarp confusion, the Glomeromycota were first proposed in the genera Acaulospora and Gigaspora[17] before being accorded their own order with the three families Glomaceae (now Glomeraceae), Acaulosporaceae and Gigasporaceae.[18]

With the advent of molecular techniques this classification has undergone major revision. An analysis of small subunit (SSU) rRNA sequences[19] indicated that they share a common ancestor with the Dikarya.[2] Nowadays it is accepted that Glomeromycota consists of 4 orders.[20]

Glomeromycota

Several species which produce glomoid spores (i.e. spores similar to Glomus) in fact belong to other deeply divergent lineages[21] and were placed in the orders, Paraglomerales and Archaeosporales.[2] This new classification includes the Geosiphonaceae, which presently contains one fungus (Geosiphon pyriformis) that forms endosymbiotic associations with the cyanobacterium Nostoc punctiforme[22] and produces spores typical to this division, in the Archaeosporales.

Work in this field is incomplete, and members of Glomus may be better suited to different genera[23] or families.[8]

Fossil Glomeromycota are known back 2200 million years as Diskagma, and also by Mesoproterozoic Horodyskia and Ordovician to Devonian Prototaxites.

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.[24] The first mycorrhizal gene to be sequenced was the small-subunit ribosomal RNA (SSU rRNA).[25] 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).[26] 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.[27]

See also

References

  1. ^ Cavalier-Smith, T. (1998). "A revised six-kingdom system of Life". Biol. Rev. Camb. Philos. Soc. 73 (3): 203–266. doi:10.1017/s0006323198005167. PMID 9809012. (as "Glomomycetes")
  2. ^ a b c d e Schüßler, A.; et al. (December 2001). "A new fungal phylum, the Glomeromycota: phylogeny and evolution". Mycol. Res. 105 (12): 1413–1421. doi:10.1017/S0953756201005196.
  3. ^ Hibbett, D.S.; et al. (March 2007). "A higher level phylogenetic classification of the Fungi". Mycol. Res. 111 (5): 509–547. CiteSeerX 10.1.1.626.9582. doi:10.1016/j.mycres.2007.03.004. PMID 17572334.
  4. ^ Schüßler, Arthur (15 August 2011). "Glomeromycota phylogeny". www.lrz-muenchen.de. Archived from the original on 2012-05-29.
  5. ^ Schüßler, Arthur (10 March 2011). "The Geosiphon pyriformis symbiosis - fungus 'eats' cyanobacterium". www.lrz-muenchen.de. Archived from the original on 2012-08-05.
  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. doi:10.1111/j.1462-2920.2007.01309.x. PMID 17635540.
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  8. ^ a b 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. Bibcode:1993Natur.363...67S. doi:10.1038/363067a0. S2CID 4319766.
  9. ^ a b Halary S, Malik SB, Lildhar L, Slamovits CH, Hijri M, Corradi N (2011). "Conserved meiotic machinery in Glomus spp., a putatively ancient asexual fungal lineage". Genome Biol Evol. 3: 950–8. doi:10.1093/gbe/evr089. PMC 3184777. PMID 21876220.
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  13. ^ a b c Smith, Sally E.; Read, David J. (2008). Mycorrhizal symbiosis (3 ed.). Academic Press. ISBN 9780123705266.
  14. ^ Tulasne, L.R. & C. Tulasne (1844). "Fungi nonnulli hipogaei, novi v. minus cogniti auct". Giornale Botanico Italiano. 2: 55–63.
  15. ^ 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.
  16. ^ Thaxter, R. (1922). "A revision of the Endogonaceae". Proc. Am. Acad. Arts Sci. 57 (12): 291–341. doi:10.2307/20025921. JSTOR 20025921.
  17. ^ J.W. Gerdemann; J.M. Trappe (1974). "The Endogonaceae in the Pacific Northwest". Mycologia Memoirs. 5: 1–76.
  18. ^ 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.
  19. ^ Schüßler, A.; et al. (January 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.
  20. ^ Redecker, D.; Schüßler, A.; Stockinger, H.; Stürmer, S. L.; Morton, J. B. & Walker, C. (2013). "An evidence-based consensus for the classification of arbuscular mycorrhizal fungi (Glomeromycota)". Mycorrhiza. 23 (7): 515–531. doi:10.1007/s00572-013-0486-y. PMID 23558516. S2CID 16495856.
  21. ^ Redeker, D. (2002). "Molecular identification and phylogeny of arbuscular mycorrhizal fungi". Plant and Soil. 244: 67–73. doi:10.1023/A:1020283832275. S2CID 33894668.
  22. ^ Schüßler, A. (2002). "Molecular phylogeny, taxonomy, and evolution of Geosiphon pyriformis and arbuscular mycorrhizal fungi". Plant and Soil. 224: 75–83. doi:10.1023/A:1020238728910. S2CID 33054919.
  23. ^ Walker, C. (1992). "Systematics and taxonomy of the arbuscular mycorrhizal fungi (Glomales) – a possible way forward" (PDF). Agronomie. 12 (10): 887–897. doi:10.1051/agro:19921026.
  24. ^ Montoliu-Nerin, Merce; Sánchez-García, Marisol; Bergin, Claudia; Grabherr, Manfred; Ellis, Barbara; Kutschera, Verena Esther; Kierczak, Marcin; Johannesson, Hanna; Rosling, Anna (2020-01-28). "Building de novo reference genome assemblies of complex eukaryotic microorganisms from single nuclei". Scientific Reports. 10 (1): 1303. Bibcode:2020NatSR..10.1303M. doi:10.1038/s41598-020-58025-3. ISSN 2045-2322. PMC 6987183. PMID 31992756.
  25. ^ Simon, L.; Lalonde, M.; Bruns, T.D. (1992). "Specific Amplification of 18S Fungal Ribosomal Genes from Vesicular-Arbuscular Endomycorrhizal Fungi Colonizing Roots". American Society for Microbiology. 58 (1): 291–295. Bibcode:1992ApEnM..58..291S. doi:10.1128/aem.58.1.291-295.1992. PMC 195206. S2CID 6480019.
  26. ^ D.W. Malloch; K.A. Pirozynski; P.H. Raven (1980). "Ecological and evolutionary significance of mycorrhizal symbioses in vascular plants (A Review)". Proc. Natl. Acad. Sci. USA. 77 (4): 2113–2118. Bibcode:1980PNAS...77.2113M. doi:10.1073/pnas.77.4.2113. PMC 348662. PMID 16592806.
  27. ^ Hudson, Corey M.; Kirton, Edward; Hutchinson, Miriam I.; Redfern, Joanna L.; Simmons, Blake; Ackerman, Eric; Singh, Seema; Williams, Kelly P.; Natvig, Donald O.; Powell, Amy J. (December 2015). "Lignin-modifying processes in the rhizosphere of arid land grasses". Environmental Microbiology. 17 (12): 4965–4978. doi:10.1111/1462-2920.13020. PMID 26279186.
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