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Division Glomeromycota

Asexual Spores

All species in this Division are known to reproduce asexually. The fungi form blastospores usually at the terminus of a hypha. The primary separation of groups within the division is associated with the development of blastospores. Minor morphological variation exists between blastospores of each species within each family. Minor and continous molecular separation is found between isolates making identification of species particularly problematic.

Classes

Class: Glomeromycetes

All but one member of the class form arbuscules in cortical cells of the roots of angiosperm plants. The taxon, Geosiphon, forms an interesting symbiosis with algae. PLEASE NOTE, the taxonomy of this group is undergoing massive change. We will incorporate these changes as we come to understand their implications.

Orders: Archaeosporales, Diversisporales, Glomales, Paraglomales (more to come).

See Schussler et al (2001) Mycol Research 105: 1413-1421.

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Cultural Conditions

The division is united in having an association with photosynthetic hosts. All but one taxon of those examined so far form arbuscular mycorrhiza. They have only been cultured with a living plant growing in adequate light intensities or in axenic conditions on transformed roots accessing sucrose from the medium.

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Ecology

Glomeromycota are symbionts found only in terrestrial habitats. Those that form arbuscular mycorrhiza (AM) with plants constitute at least half the living fungal biomass in soils. They are at a competitive advantage over other soil-borne fungi in that they have access to a constant source of organic nutrients supplied by their host while living in a largely oligotrophic medium. The fungi benefit their host by functioning as an extension of the root system. In addition, they ramify through soil, where they release glycoprotein, including glomalin, which may function in gluing together fine soil fragments. Hyphae also physically aggregate soil.

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Hyphal Characteristics

Mycelia of AM fungi are characteristically aseptate with, septa only being found to delimit spores or dying portions of hyphae. The hyphae can anastomose, thereby forming interconnected mycelia. The walls of hyphae contain chitin, chitosan and polyglucuronic acid.

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Physiology

In general, AM fungi produce enzymes that enable them to colonise plant roots. These enzymes can be recovered from spores or hyphae. However, enzymes to enable digestion of complex carbohydrates are unknown. Energy from the host plant is transferred probably as glucose at the interface between the symbionts, and then transformed into fungal sugars thereby preventing its return to the host. Carbon is moved about the fungal thallus, and may be stored in the root of plants, though only minor transfer into plant tissues seems possible (mycoheterotrophic plants being the exception).

AM fungi also access large quantities of dissolved minerals from soil. Minerals are also transferred around the thallus, but they can be transferred to the host via an as yet unknown mechanism. Transfer of minerals such as phosphate, copper and zinc appear to be essential to the function of the host in soils where these minerals are deficient. While the fungi also access nitrates and other small minerals, their transfer to the host is rarely greater than that supplied by simple diffusion.

Because of their mutual benefit, it seems clear that some form of recognition is necessary for the initiation of each symbiosis. The genes involved include some well characterised in the formation of nitrogen fixing nodules in legumes.

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Pores

While the fungi are known to be aseptate not a great deal is known about the formation of pores between separate compartments. Multiperforate septa are found in some members. It was assumed that these would be found in all members of the division. However, as the Paraglomaceae is deeply divided phylogenetically from the other orders, further examination is warranted.

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Sexual Spores

Zygospores have been found in only one species of Gigaspora. The fungus was shown to be heterothallic. Molecular studies indicate that members of Glomus are asexual. Further studies are needed to clarify this situation.

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Further Reading

Alexopoulos, Mimms & Blackwell 1996 Introductory Mycology (4th edition), Ch 5/6.

Schussler, Schwartzott & Walker 2001 A new phylum, the Glomeromycota: phylogeny and evolution. Mycological Research 105: 1413 - 1421.

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