2.21: Glomeromycota- important mycorrhizal fungi

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The Glomeromycota are a very common, yet rarely seen, group of fungi. They are ubiquitous partners with angiosperms, forming associations called mycorrhizae, more specifically 'endomycorrhizae', also called vesicular/arbuscular (VA) mycorrhizae. Most plants (more than 80%) are mycorrhizal and most of these form endomycorrhizae with a fungal associate in the Glomeromycota.

A plant root (rectangular cells) infected by Glomeromycota filaments, showing both a vescicle, the round structure, and an arbuscule, the tree-like structure.

Taxonomy and Phylogeny

The Glomeromycota have long been considered a part of the bread mold (Zygomycota) group because of structural similarities (see below) , but recent studies, in particular molecular studies, have indicated that they are distinct from the bread molds and are closer to club fungi and sac fungi than to the bread molds. They are not a very diverse group (less than 500 species in the whole phylum) and morphologically they do not exhibit a great deal of variation.

Structure

Like most fungi, the Glomeromycota generally exist as filamentous hyphae, and like the bread molds, the hyphae have no cross-walls, i.e. the members are coenocytic. Commonly the hyphae produce relatively large spores. Unlike the club fungi and cup fungi they do not produce fruiting bodies. When inside plant cells most Glomeromycota produce characteristic structures called arbuscules that function to transfer materials between the fungus and plant. Also produced are vesicles, which are enlarged spherical parts of the hyphae where materials are stored.

Reproduction

Glomeromycota are strictly asexual. They produce spores but they do not appear to have any sexual process.

Matter and energy

Most, but apparently not all, members of the group are mycorrhizal (endomycorrhizal) and obtain carbohydrates from their photosynthetic associate (usually an angiosperm but there is one documented case of an association with a filamentous blue-green algae). All apparently obtain nutrients (i.e. minerals) from the soil and there is evidence that although most species are mycorrhizal, a few species also obtain food (i.e. carbohydrates) from the soil.

Interactions

Glomeromycota interact with many (nearly all) plant species and it is increasingly apparent that they significantly affect the behavior of their hosts, generally allowing them to grow more vigorously. The group was probably significant in the colonization of land by autotrophs and thus the origin of plants.