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.
Gigaspora gigantea, like many AM fungi, has a global distribution — it has been found to occur in every continent except Antarctica. Wind has been proposed as a long-distance dispersal mechanism. Water is another possibility, as Gigaspora gigantea spores have been shown to survive long periods of submersion in seawater. The following citations refer to survey studies where the species has been found to occur naturally, not in cultivation. The list is meant to be a representative, not an exhaustive, report of Gigaspora gigantea’s distribution — since it seems to live nearly everywhere!
In North America, Gigaspora gigantea is widespread and, because of its sanguine role in plant cultivation, is commonly an object of scientific study. One study found it associating with ferns on Atlantic dunes stretching from Quebec to Virginia. It has also been specifically identified in Champaign County, Illinois, at the University of Illinois Morrow Plots; elsewhere in southern Indiana and in South Dakota; in Benton County, Oregon; in Moonstone Beach, Rhode Island; in Durham, North Carolina; in Morgantown, West Virginia; in Maryland, Florida, Minnesota, and Wisconsin. It has even been reported in Hawaii.
In South America, Gigaspora gigantea is known to associate with tropical tree hosts in southeastern Brazil. AM fungi in general tend to associate with plants in the tropics as opposed to plants in temperate zones.
In Europe, it has been found in Hel Peninsula, Poland, and elsewhere along the coast of the Baltic Sea.
In Asia, it was observed in Tamil Nadu, India, in an ash pond and in dumps. It was also observed on a coastal strand in Karnataka, by the Arabian Sea. In Korea, it was observed in Chungeheong Province.
In Africa, it was observed in the western and northern states of Nigeria — specifically Moor Plantation, Ibadan.
In Australia, it was identified along the southern coast of New South Wales.
Though Gigaspora gigantea is known for the size of its spores, one cannot make positive identifications based on size alone because of intraspecific variation. The best way to confirm that a specimen is Gigaspora gigantea is to stain the laminate (sheet-like) layer of the spore wall with Melzer’s reagent, and/or to stain the spore cytoplasm with alkaline solution. Recall that the yellow-green spore color of Gigaspora gigantea is intrinsic to its spore contents, not to the laminae. As such, the laminae for Gigaspora gigantea do not significantly change color when stained with Melzer’s reagent, whereas the laminae of other Gigaspora species turn dark purple. Likewise, the spore cytoplasm turns red when stained with alkaline solution, whereas other Gigaspora species do not exhibit color change.
Another effective diagnostic test involves the use of transmitted light. When exposed to red light, Gigaspora gigantea hyphae and arbuscules fluoresce yellow-green, whereas Gigaspora margarita does not fluoresce.
Species-specific hyphal branching patterns can also be detected. Carrot (Daucus carota) root exudates induce branching morphologies specific to Gigaspora gigantea, which can be distinguished from the patterns of other Gigaspora species.
Niche partitioning and host specificity are described below in the Habitat section.
Because of Gigaspora gigantea’s conspicuously large spore size and the reliable diagnostic techniques mentioned above, it is not difficult to distinguish it from its close relatives, namely Gigaspora albida, Gigaspora decipiens, Gigaspora margarita, and Gigaspora rosea.
Following the first publication of the species, there was some confusion as to exactly how big these “gigantic” spores actually were. “Bulbous vacuolated spores” found Australia and New Zealand were attributed as Gigaspora gigantea. It was later confirmed that these Australasian specimens were too small to have been Gigaspora gigantea, whose mean spore size was significantly larger than the spores that were found.
AM fungi are thought to be generalists – that is, they typically are not restricted to a single host species. However, in Gigaspora gigantea, there exists some evidence to the contrary. In a controlled greenhouse experiment, Gigaspora gigantea were able to colonize only forty percent of the tropical hosts tested.
With Gigaspora gigantea, it appears that several mechanisms may be synergistically responsible for the host specificity patterns that we see. One of these is connected to seasonality. Gigaspora gigantea is active during warm seasons, and it sporulates in the winter at the end of its active phase. For this reason, it tends to associate with trees that make carbon nutrients available in their roots during warm seasons. A second mechanism has to do with the chemical signals that are secreted from plant roots. Plant root exudates are variably attractive or repugnant to the hyphae of AM fungal species, and this can influence the choice of plant host. Lastly, the effects of AM fungi are not the same on all host species. In a controlled experiment, Gigaspora gigantea helped to increase the overall biomass and plant root extent of one of its preferred hosts, an Andropogon species, whereas Glomus microcarpum was observed to have the opposite effect on Andropogon hosts. These species-specific fitness effects create feedback loops which, over time, contribute the patterns of host specificity that we observe.
Within a host population, the colonization efficiency of AM fungi depends on soil nutrient level. Plants that are nutrient-stressed tend to attract more AM fungi. One experiment demonstrated that Gigaspora gigantea enhances phosphorus uptake in its host plant when soil phosphate levels are low.
As discussed in the “Uses” section below, Gigaspora gigantea colonization has favorable effects on the performance of agricultural crops. Other viable hosts that are not necessarily of central importance to agriculture include the tulip tree (Liriodendron tulipifera_) and white ash (_Fraxinus americana).
Relevance to Humans and Ecosystems
Gigaspora gigantea is an obligate symbiont – that is, there is no way to culture AM fungi in the absence of a root. It can, however, be introduced to a host plant and monitored in a warm greenhouse. Spores can be extracted from the soil using a wet-sieving technique. One can then inoculate a test plant growing in sterilized soil or sand to propagate the species for study.
It was shown in the late 1960s that Gigaspora gigantea could promote the growth of hardwood tree seedlings. It is now clear that Gigaspora gigantea can play an important ecological role in the cultivation of agricultural crop species, such as corn (Zea mays_), carrot (Daucus carota_), grape (Vitis vinifera) and soybean (Glycine max). In the particular case of soybean, Gigaspora gigantea can be used in combination with Bradyrhizobium japonicum_, a nitrogen fixing bacterium, to promote plant growth. The fitness benefits of colonization have been proven. In an experiment where some cuttings of yew (_Taxus x media var. densiformis) were inoculated with Gigaspora gigantea, the leaves of colonized plants had higher levels of chlorophyll, and root systems were larger, longer, and more complex. Inoculation with AM fungi can also alleviate the trauma of transplantation.
The population biology of Gigaspora gigantea can be manipulated or affected in several ways. Adding sugar around the AM fungus can increase spore production. On the other hand, Gigaspora gigantea is vulnerable to soil fumigation, and sieving also has negative effects.
It has been suggested that the services of AM fungi can be harnessed as a sustainable alternative to chemical fertilizer. This argument rests on the idea that AM colonization rates increases with decreasing soil fertility, and therefore, AM fungi can be expected to make up for a reduction in topical nitrogen and phosphorus application. In addition, AM colonization is thought to increase plant hardiness and pathogen resistance.
In recent years, AM fungi have also piqued the interests of natural resource management. Because of their benefits to tropical tree growth, AM fungi have been proposed as agents of forest restoration. Also, it has been demonstrated that AM fungi can promote the extraction of heavy metal pollutants from contaminated soil. In this way, the tiny giant of giants Gigaspora gigantea could play a mighty role in the sustainability programs to come.