Gigaspora margarita forms globose spores in the soil.Each spore is 260-480um in diameter, formed singly and terminally on subtending hypha, which are often septate below the suspensor-like cell in age (Becker and Hall 1976) though cytoplasmic streaming was observed between the suspensor-like cell and the spore body when young (Sward 1981a). Spore bearing suspensor-like cells are 27-58 um broad, thickening at the attachment point with the spore, with 1-5um thick cell walls and are hyaline to light brown and smooth. The walls of the spores are smooth and hyaline with 4-10 fused laminations each 1.5-4um thick giving the wall a total thickness of 5-24 um when hydrated. To age spores of G. margarita, generally the greater the number of laminations the greater the age of the spore. The internal structure of the spore consists of many small oil-like droplets, which coalesce near the germination region with age and generally appear white in color (Becker and Hall 1976). The outer wall of the spores was found to be absent of chitin and consist mainly of polysaccharides, lipids, and protein, which stained dark-purple or blue with Toluidine blue (Sward 1981a).
The closest species to resemble G. margarita is Gigaspora gilmorei (Becker and Hall 1976). Both have white azygospores and laminated spore walls while other species of the genus Gigaspora have colored azygospores and often with a single layered spore wall. For example: G. margarita is distinct from G. gilmorei by having white vesicles, rather than brown as in G. gilmorei. Additionally, G. gilmorei has a cell wall with five layers of non-uniform thickness, which easily separate, while G. margarita has up to ten layers, which do not readily separate when the spore is damaged. G. margarita is distinguished from G. calospora by having generally larger spores and laminated walls (Becker and Hall 1979).
Gigaspora margarita has been shown to enhance the growth of a number of plants by forming arbuscular mycorrhizal (AM) associations with plant roots. Additional growth enhancement by inoculation with G. margarita has been shown in tobacco (Csinos 1981) and cotton where it essentially reversed the stunting caused by the nematode Meloidogyne incognita (Roncadori and Hussey 1977). Additionally, it has been shown to form AM associations with corn (Zea maysL.), sudangrass (Sorghum sudanense(PIper) Staph.), and onion (Allium cepaL.). In the eudicots, it has been shown to form AM associations with soybean (Glycine maxL.) and tomato (Solanum lycopersicumL.) among many others (Becker and Hall 1976).
Gigaspora margarita is known to harbor an endosymbiotic bacteria. Originally classified in the genus Burkholderia (Bianciotto et al. 1996) the bacterium has now been officially described as the novel species ‘Cadidatus Glomeribacter gigasporarum’ (Bianciotto et al. 2003), this bacterium was first observed in the late-1970s to early-1980s, as a “type of unusual organelle” but recognized as a ‘bacteria-like organism’ that had been identified in other species in the then-termed Endgonaceae (Sward 1981a). Attempts were made to isolate and culture this bacterium outside of its host spore but were unsuccessful until relatively recently when Jargeat et al. (2004) developed a protocol to isolate and keep these bacteria alive for up to 4 weeks. Though the bacteria have been isolated and kept alive, efforts to culture these bacteria and have them reproduce remain unsuccessful. ‘Ca. Glomeribacter gigasporarum’ is a rod-shaped, gram-negative bacteria which contains no flagella or pili and numerous ribosomes (Jargeat et al. 2004). No isolates of G. margarita have been found without endosymbiotic bacteria populations in nature (Ruiz-Lozano and Bonfante 2001), however, isolates of G. margarita ‘cured’ of the bacteria have been generated in vitro (Alessandro Desiro, personal communication). It is unknown how this association came to be, however certain bacterial genes have been shown to be involved in host cell colonization. One such gene, vacB, was first isolated in Shigella flexneri (Tobe et al. 1992) and has been found in the Burkholderia strain that is symbiotic with G. margarita (Ruiz-Lozano and Bonfante 2000).
Bacteria of many different families have been shown to be associated with the spore surface of G. margarita more than the substrate within which these spores grow. These bacterial families, discovered through sequence and clustering analysis belong to two main phyla, the Proteobacteria and the Actinobacteria (Long et al. 2008).
While G. margarita does not reproduce sexually, diversity in the genome has been shown with multiple genomes present in the organism at one time. This is proposed to be due to the accumulation of mutations over time in individual nuclei, then segregation of these mutated nuclei in a single spore or many spores, leading to genetic variability over time (Kuhn et al. 2001).
The genome of G. margarita has yet to be sequenced, though the construction of a genomic DNA library was reported (Van Buuren et al. 1999). Additionally, the mitochondrial genome was sequenced using 454 and Illumina technologies and was the second AM fungi organellar genome to be published (Pelin et al. 2012).
Gigaspora margarita grows in a variety of soil types in arbuscular mycorrhizal association with a variety of plants including soybean, tomato, and onion (Becker and Hall 1979).
An identification key for this species was published by Becker and Hall (1976) at the time of discovery and is reproduced below:
Key to the species of Gigaspora
1a.) Azygospores white… (2)
1b.) Azygospores not white… (3)
2a.) Spore wall with distinct laminations of near equal width; spores germinate without forming peripheral compatments; vesicles white… G. margarita
2b.) Spore wall with inner and outer layers of unequal thickness; spores germinate from peripheral compartment; vesicles brown… G. gilmorei
3a.) Azygospores yellow, smooth…(4)
3b.) Azygospores brown, with warts or minute spines…(5)
4a.) Globose spores less than 300um in diameter, pale yellow; vesicles smooth to knobby, formed singly… G. calospora
4b.) Globose spores greater than 300um in diameter, bright yellow to greenish-yellow; vesicles echinulate, formed in clusters… G. gigantea
5a.) Globose spores greater than 300um in diameter, dark brown with hyaline warts; vesicles with coralloid projections… G. coralloides
5b.) Globose spores less than 300um in diameter, light brown with minute spines; vesicles smooth… G. heterogama
In 1981, the morphology of G. margarita spores and their development was the topic of a three part series of articles in the journal New Phytologist (Sward 1981a-c). Part one focused on the dormant spore morphology including the spore wall structure and composition, part two focused on the changes in morphology and structure as the spore starts to germinate while part three is focused on the emergence of the germ tube and its growth.
Using both light and electron microscopy, as well as cytological methods, Sward (1981a) showed that the composition of the lamination in the cell wall were not uniform, but were composed of four main layers: a chitinous layer, a ‘cementing’ layer, as well as an inner and outer protein/lipid/polysaccharide layer. Sward (1981b) used electron and light microscopy to show that surface sterilization with sodium hypochlorite can force germination. At the initiation of germination, a large redistribution of the cytoplasm occurs near the region where the germ tube will emerge. Sward (1981c) used light and electron microscopy to further follow the development of the germ tube and recognized that, given the differences in structure of the outer and inner wall, the germ tube seemed to utilize different strategies to penetrate these layers. It was also noted, unsurprisingly, that the primary cell wall is deposited at the apex of the germ tube as it grew out of the spore followed soon by deposition of the secondary wall.
The genus Gigaspora was once classified in the Endogonaceae by Gerdemann and Trappe (1974). In 1990 it was then moved into a new family, Gigasporaceae in the suborder Gigasporinae of the order Glomales (Morton and Benny 1990). The division Glomeromycota was then revised in 2004 by Walker and Schussler and Gigasporaceae was then placed in the order Diversisporales (Walker and Schussler 2004). G. margarita is now placed in the phylum Mucuromycota, subphylum Glomeromycotina, order Glomeromycetes, class Diversisporales, family Gigasporaceae, genus Gigaspora (Spatafora et al. 2016).
Taxonomy of Gigaspora spp. is distinguished by the morphology of both the spore (namely the spore wall structure) and how they form septa at the base of the newly derived spores (Khade 2011).
The type specimen of G. margarita was isolated by W.N. Becker on February 9, 1976 from the University of Illinois in Champaign County. It was discovered in a pot culture of soil from the Agronomy South Farm, and found in soil from field #1101 (at the time a soybean field) after this soil was planted with soybean and used to inoculate autoclaved soil (Becker and Hall 1976).
Gigaspora margaritahas been shown to form arbuscular mycorrhizal (AM) associations associations with a wide range of plant hosts. In the monocots, it has been shown to form AM associations with corn (Zea mays L.), sudangrass (Sorghum sudanense (PIper) Staph.), and onion (Allium cepa L.). In the eudicots, it has been shown to form AM associations with soybean (Glycine max L.) and tomato (Solanum lycopersicum L.).
Gigaspora margarita was first described as a new species in 1976 by W.N. Becker from the University of Illinois and I.R. Hall from the Invermay Agricultural Research Station, Mosgiel, New Zealand. It was identified in soil sample from a soybean fieldas white spores amongst a population of yellow-colored spores typical ofanotherGigasporaspecies,G. gigantea. Originally thought to be the immature stage of the yellow-colored G. gigantea spores, these white spores were soon discovered to be morphologically distinct from the spores of G. gigantea. Additionally, it was also discovered that the immature spores of G. gigantea were also yellow in color, further confirming G. margarita as a separate species with characteristic white spores. The specific epithet margarita- or 'pearl' - was chosen by Becker and Hall to distinguish this species taxonomically.
Gigaspora margaritawas first described in 1976 by W.N. Becker from the University of Illinois and I.R. Hall from the Invermay Agricultural Research Station, Mosgiel, New Zealand. It was identified in soil sample from a soybean fieldas white spores amongst a population of yellow-colored spores typical ofanotherGigasporaspecies,G. gigantea. Originally thought to be the immature stage of the yellow-colored G. gigantea spores, these white spores were soon discovered to be morphologically distinct from the spores of G. gigantea. Additionally, it was also discovered that the immature spores of G. gigantea were also yellow in color, further confirming G. margarita as a separate species with characteristic white spores. The specific epithet margarita- or 'pearl' - was chosen by Becker and Hall to distinguish this species taxonomically.Since its discovery, G. margarita has been the subject of a variety of studies spanning a number of disciplines.
Gigaspora margarita is an Arbuscular Mycorrhizal Fungi (AMF) which means it is an obligate symbiont that creates mutualistic relationships with many different plant species. Being an AMF, G. margarita does not produce a fruiting body. All of its mycelium will be found in the soil, associating with plant roots. Though hard to distinguish between different species of AMF, microscopic distinctions can be made. A prominent morphological distinction for species in the Gigasporaceae family is their large sized spores. Gigaspora margarita is characterized by its large, white, pearl-like spores found anywhere from 260 - 400 micrometers.[1] This is where it gets its name as margarita in Latin means pearl.
Associating with many plants, Gigaspora margarita has been found in diverse regions across the globe. In culture, G. margarita has been found to associate with onion, tomato, soy beans, corn, and clover although this list is probably a lot longer.[2] Furthermore, G. margarita also associates with endobacteria making it a metaorganism that serves as a connection of three different kingdoms (plant, bacteria, fungus). Strains of G. margarita isolated without the endobacteria are possible indicating an asymmetric association between the fungi and the endobacteria. However these, ‘cured’ strains of G. margarita do not interact with its associated plants as well as strains with the endobacteria.[3]
Arbuscular Mycorrhizal Fungi can be hard to disti EM nguish since they do not produce fruiting bodies and their entire lifecycle is completed below ground. Researchers distinguish species by looking at their microscopic morphologies and genetics.
Arbuscular Mycorrhizal Fungi are characterized by their intracellular arbuscules they form within the associated plant's roots. These arbuscules can come in many different variations with no one looking identical to another. This makes it hard for them to be used as a classification tool. Arbuscules are bush-like structures where they have branches hyphae forming from a swollen hyphal base. Oftentimes, the hyphae of AMF will stain blue using trypan blue dye.[1]
Gigaspora margarita is distinguished primarily by the morphology of its spores. Young spores are often salmon colored and will become pearly white to yellow-brown at maturity. A mature spore has three cell wall layers (L1, L2, L3):
Gigaspora margarita also has auxiliary cells produced on tightly wound hyphae. These cells are spikey in appearance and are found in clusters of 4-20. Auxiliary cells are found in all species classified under the Gigasporaceae family so while they do not specifically distinguish G. margarita, they are a good indicator that a certain AMF species is in the Gigasporaceae family.[4]
Gigaspora margarita has a peculiar genetic makeup as its genome consist of around 831 Mega base pairs (Mbp).[5] This is massive compared to the usual fungal genome size that ranges from 8.97 Mb to 177.57 Mb.[6] Furthermore, the 10 largest genomes in the kingdom of fungi belong to species that are either obligate biotrophs, endophytes, or gut fungi. This may seem like an indication that large genomes correlate with better symbiotic relationships with plants however this isn't necessarily the case. The composition of the genome of G. margarita is also unique as it is primarily made up of transposable elements (64%). Fungi usually have low levels of transposable elements often only making up 0-25% of the genome. It seems the only other fungi to have large transposable element concentrations are plant pathogens which makes sense because they allow the species to adapt quickly often as a means of overcoming plant defenses. For an AMF fungus, the reasons for having such a large repertoire of transposable elements remains unclear. The genome was also found to consist of gene sequences called Helitrons. There purpose remains unknown however these indicate that G. margarita may have captured genes from other organisms at some point in time.[5]
Gigaspora margarita has a unique ability to associate with a diverse range of endobacteria.[7] Though rare, the ability for Gigaspora margarita to host this intrahyphal bacteria is not completely unique. Many forms of AMF have been shown to associate with bacteria. The primary species of bacteria that associates with G. margarita is named Candidatus Glomeribacter gigasporarum (italicized?). Found in the Burkholderia genus, Ca G gigasporarum has had its entire genome sequenced and found to be a lot smaller than other Burkholderia species.[8] This indicates a reliance on G. sporangium to survive. Ca G gigasporarum has never been found on its own in nature and, so far, can not be isolated in culture. Adding to this hypothesis is the fact that, Ca. G. gigasporarum is vertically transmitted with G. margarita spores. Each spore contains around 250,000 of these bacteria which may be a contributor to the unusually large spores G. margarita produces.[9]
The symbiotic relationship between G. margarita and its endobacteria is asymmetric meaning one of the species can survive without the other while the other species cannot. In this case, G. margarita has been isolated and grown without the presence of endobacteria. Still, endobacteria play an important role in optimizing the relationship between the plant and the fungus. The presence of endobacteria is correlated with a higher antioxidant metabolic rate and lipid biosynthesis in the associated plant. The synthesized lipids would end up being exchanged to G. margarita.[3] Basically, endobacteria are not required for the completion of the G. margarita life cycle however they are major contributors to the health of the interaction between plant and fungus.
Little is known about the habitat and distribution other than it seems to be widespread. G. margarita is found across the globe and has been confirmed in Brazil, USA, Canada, China, Cuba, India, Japan, Mexico, New Zealand, Poland, Syria. Most extensive research on its habitat has been conducted in Brazil where it has been found in all different biomes in the country including areas with high human traffic.[10]
Further information needs to be collected in order to understand the preferable natural environment G. margarita inhabits.
{{cite web}}: CS1 maint: url-status (link) Gigaspora margarita is an Arbuscular Mycorrhizal Fungi (AMF) which means it is an obligate symbiont that creates mutualistic relationships with many different plant species. Being an AMF, G. margarita does not produce a fruiting body. All of its mycelium will be found in the soil, associating with plant roots. Though hard to distinguish between different species of AMF, microscopic distinctions can be made. A prominent morphological distinction for species in the Gigasporaceae family is their large sized spores. Gigaspora margarita is characterized by its large, white, pearl-like spores found anywhere from 260 - 400 micrometers. This is where it gets its name as margarita in Latin means pearl.
Associating with many plants, Gigaspora margarita has been found in diverse regions across the globe. In culture, G. margarita has been found to associate with onion, tomato, soy beans, corn, and clover although this list is probably a lot longer. Furthermore, G. margarita also associates with endobacteria making it a metaorganism that serves as a connection of three different kingdoms (plant, bacteria, fungus). Strains of G. margarita isolated without the endobacteria are possible indicating an asymmetric association between the fungi and the endobacteria. However these, ‘cured’ strains of G. margarita do not interact with its associated plants as well as strains with the endobacteria.
