Gigaspora gigantea is one of a group of obligate symbionts called Arbuscular-Mycorrhizal fungi. The term mycorrhiza refers to a symbiosis between a fungus and a plant root. The association has been traditionally understood as a mutualism where the fungus receives photosynthetically derived carbon compounds from the plant in exchange for phosphorus and/or nitrogen. With this added resource efficiency, the plant’s tolerance for drought and nutrient-poor soils increases, as does its resistance to pathogens. The relationship can turn parasitic, however, if the soil environment is heavily fertilized – that is, under circumstances in which the plant does not need mycorrhizal nutrient assistance. A mycorrhiza is not always a one-to-one association; a fungus may associate with multiple plants, or vice versa.
There exist several mycorrhizal types within Kingdom Fungi. Arbuscular-Mycorrhizal (AM) fungi, all of which belong to the phylum Glomeromycota, are endomycorrhizal – that is, they physically penetrate plant cortical cell walls and form arbuscules along the surfaces of cell membranes. Ectomycorrhizal fungi, in contrast, belong to the Basidiomycota or Ascomycota and are confined to extracellular spaces. Ectomycorrhizal (EM) fungi typically form sheaths around root tips, from which hyphae may enter the root cortex but never traverse cortical cell walls. For EM fungi, nutrient exchange occurs outside root cells. Other mycorrhizal types associate with specific plant groups; orchid mycorrhizas and ericoid mycorrhizas colonize the Orchidaceae and Ericaceae, respectively.
To establish a mycorrhiza, Gigaspora gigantea relies on chemical signals to find its way to a host root. When it does, it punctures the epidermis of a plant root tip and enters the root cortex. After penetrating the root cortical cell wall, the fungus can form a tree-like arbuscule, which provides the surface for nutrient exchange between fungus and plant. After the arbuscules are established, the fungus can send runner hyphae back into the soil to assist the plant with nutrient uptake. The fungus, in return, absorbs carbon molecules from the plant either through its arbuscules or along its intraradical hyphae – that is, the hyphae inside the root.
The suborder Gigasporineae, to which Gigaspora gigantea belongs, does not have intraradical vesicles. Vesicles are oily-looking compartments that are believed to function as storage sites for lipids. While none have been observed in Gigaspora gigantea, the species does have lipid-storing structures outside the root called auxiliary cells. It has been suggested that these stores can be tapped during periods of nutrient deprivation, or during periods of regrowth following a traumatic event.
Gigaspora gigantea is the type species for Gigaspora, a genus known for its exceptionally large spores. Aptly named, Gigaspora gigantea is the giant of these giant-spored fungi. Its huge spores were originally termed “azygospores” because they resemble zygospores but do not result from the union of gametangia. Broadly speaking, all AM fungi are thought to be asexual.
The spore is the principal feature used to recognize the species. Curiously, although its spores are the largest of the genus, they also have the thinnest spore walls. They are bright yellow with a greenish tint, a color that is not observed elsewhere in the order Glomerales. This color is intrinsic to the spore cytoplasm, not to the spore wall as is the case with other species from the family Gigasporaceae. For this reason, it is thought that Gigaspora gigantea’s spore cytoplasm and wall have unique biochemical properties.
The spores of Gigaspora gigantea are almost perfectly round, and their contents glisten under transmitted light. The spore wall has three layers: a smooth, glass-like outer layer that does not react to Melzer’s reagent; a layer of thin sheets, called laminae, which is also unreactive to Melzer’s reagent; and a bumpy layer that forms right before germination. The spore is connected to the rest of the fungal body with a hypha that contains very few septa, called the sporophore.
During germination, a germ tube sprouts from the innermost of the three cell wall layers and penetrates directly through the spore wall. Spores are capable of germinating multiple times.
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