Rhizophagus aggregatus (N. C. Schenck & G. S. Sm.) C. Walker 2016

Glomus aggregatumis an arbuscular mycorrhizal fungal species belonging to the phylum glomeromycota¹. Like other species in its genus, G.aggregatumfunctions ecologically by forming mutualistic symbiotic relationships with the roots of plant species. It was first described asGlomusaggregatumby N.C. Schenck and George S. Smith in 1982²after being found in the roots of citrus trees in a citrus grove near Haine's City, Florida. However, R.E. Koske³(1985) emended the description of the species to includeRhizophagites Butleri, discovered and described in 1939 by Ediwin John Butler⁴but named by C.O. Rosenthal⁵in 1943.G. aggregatum is nested within the kingdom fungi, the phylum glomeromycota, the class glomerales, the order glomeraceae, and the genus glomus. Glomus is the largest genus within glomeromycota¹. In 2001 it was analyzed and reorganized by Schwarzott, Walker, and Schußler¹, who discovered that the genus was not monophyletic. As a result, many members of glomus were migrated to a new family separated from glomaceae and the creation of three other genera was prompted. G. aggregatum, however, was not placed elsewhere in this analysis nor any since. Schußler’s most recent (2010)¹⁶ phylogeny places G. Aggregatum as sharing most recent common ancestor with a polytomy formed by G. luteum, G. claroideum, G. lamellosum, and G. etunicatum within the family Glomeraceae.

Arbuscular mycorrhizal fungi are defined as fungi that form mutualistic symbiotic relationships with plants inside the roots⁷. These fungi, all of which are in the phylum glomeromycota, are obligate symbionts. They are distinct from other types of mycorrhizal fungi in that the hyphae of arbuscular mycorrhizal fungi like G. aggregatum penetrate into not only the root cortex but into the cells of the root parenchyma as well. They also have structures called arbuscules, which are branching clumps of hyphae within cells, used to increase surface area and nutrient exchange. The fungal component of any mycorrhizal symbiosis contributes inorganic nutrients, primarily phosphorus compounds, to the plant. The fungus takes advantage of organic carbon photosynthates produced by the plant⁸. In this way, mycorrhizal associations accelerate a plant’s ability to obtain nutrients by expanding the absorptive area of the root. The presence or absence of arbuscular mycorrhizal fungus can have profound effects on the physiology and activity of a plant¹². A 2013 study found that cucumber plants (Cucumis sativus) grow in different ways when inoculated with different types of arbuscular mycorrhizal fungi. For instance, inoculum obtained from both organic and conventional farms lowered the amount of flower production per cucumber plant relative to a control, but the conventional farm inoculum increased the amount of sativus’ primary defensive chemical, which would likely deter insect grazing on the host plant. It is estimated that mycorrhizal associations of some kind are present in 90% of plant roots^7,10, making mycorrhizal associations the most ecologically significant mutualistic relationship on earth. Around 80% of plants on earth have arbuscular mycorrhizal interactions¹⁰. Many plant species are unable to grow or have limited growth without a mycorrhizal component⁹. Plant dependency on mycorrhizal fungi for optimized growth can depend on a number of factors, including soil nutrient content, soil type, and resource-gathering capabilities of the plant species⁹. The relationship between arbuscular mycorrhizal fungi and plants is at least 450 million years old ¹⁰. As a result, both fungi and plants are extremely well-adapted to facilitating symbiotic mycorrhizal mutualisms. Many of the mechanisms by which these associations form is yet unknown¹⁰. Arbuscular mycorrhizal associations are cosmopolitan in both habitat and geographic range. There is good evidence that G. aggregatum and other members of glomeromycota form important links in soil detoxification processes and ecosystem-level metabolic pathways. For example, an experiment in 2010 showed that root-mycorrhizal interface was significantly more successful at detoxifying arsenic-laced soils than non-associated roots. The study showed that the presence of G. aggregatum methylated the arsenic in the soil, but that indigenous soil microorganisms were responsible for further detoxification of dimethlarsinic acid into trimethylarsine oxide¹¹. The importance of G. aggregatum in the arsenic detoxification pathway suggests other, larger, interspecies ecosystem processes that may be occurring in the rhizosphere of colonized plants. The details of these processes are still being teased out, and can change based on plant, fungus, or microbial community dynamics. The most critical feature of arbuscular mycorrhizal symbioses, from a plant perspective, is the augmented ability of mycorhizzal associated plant roots to obtain organic nitrogen and phosphorus from the soil. This is particularly vital in systems with heavily leached and/or low-nutrient soil compositions like tropical rainforests. This feature of root interactions creates more producer biomass which translates to more herbivore and predator biomass in a complicated ecosystem.

The most large scale application of arbuscular mycorrhizal fungi such as G. aggregatum to human activities is its presence in commercial agriculture as an inoculum. Mycorrhizal relationships are important in this context because long-term agriculture tends to drain nutrients like phosphorus from the soil. For this reason, modern agriculture must apply vast amounts of phosphorus and other nutrient to fields yearly. Artificial and stimulated mycorrhizal associations can help plants mobilize phosphorus from the soil and utilize it¹³. This can lead to higher yields and can also lessen the need for artificial phosphorus fertilization. It has also been suggested that selective inoculation of mycorrhiza into certain crops can increase water retention and help mitigate toxic factors in major food sources such as rice¹⁴. It has been shown that presence of G. intraradices and G. geosporum in rice growth experiments has significantly reduced concentration of toxic arsenic, as well as improving grain phosphorus uptake. Although mycorrhizal fungi is not always successful in reducing arsenic fully to safe consumption levels, it has been demonstrated that it effectively lowers accumulation. Although the artificial application of arbuscular mycorrhizal fungus in agriculture has shown success, there is evidence suggesting that a given regions agricultural products perform better with inoculants of their native mycorrhizal communities rather than a less diverse (and possibly less adapted) commercial inoculum ¹⁵.

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Glomus aggregatumis an arbuscular mycorrhizal fungal species belonging to the phylum glomeromycota¹. Like other species in its genus, G.aggregatumfunctions ecologically by forming mutualistic symbiotic relationships with the roots of plant species. These symbioses are extremely ecologically significant, affecting over 90% of plant roots^7,10 on earth. It was first described asGlomusaggregatumby N.C. Schenck and George S. Smith in 1982²after being found in the roots of citrus trees in a citrus grove near Haine's City, Florida. However, R.E. Koske³(1985) emended the description of the species to includeRhizophagites Butleri, discovered and described in 1939 by Ediwin John Butler⁴but named by C.O. Rosenthal⁵in 1943.G. aggregatum is nested within the kingdom fungi, the phylum glomeromycota, the class glomerales, the order glomeraceae, and the genus glomus. Glomus is the largest genus within glomeromycota¹. In 2001 it was analyzed and reorganized by Schwarzott, Walker, and Schußler¹, who discovered that the genus was not monophyletic. As a result, many members of glomus were migrated to a new family separated from glomaceae and the creation of three other genera was prompted. G. aggregatum, however, was not placed elsewhere in this analysis nor any since. Schußler’s most recent (2010)¹⁶ phylogeny places G. Aggregatum as sharing most recent common ancestor with a polytomy formed by G. luteum, G. claroideum, G. lamellosum, and G. etunicatum within the family Glomeraceae.

G. aggregatum has sporocarps containing spores which are not closely grouped. Spores are usually pear-shaped or spherical and measure between 40 and 85 micrometers in diameter, whereas sporocarps can be 200-1800 micrometers by 200-1400 micrometers in diameter. Spore color ranges from pale yellow to a darker yellow-brown or orange-brown. Spores can be contained in either one or two cell walls, but if there are two, the outer wall is always thicker. A second type of spore wall thickening has been observed in G. aggregatum spores wherein the wall undergoes localized thickening in one hemisphere or a smaller space. This can happen in multiple locations on a single spore and can contribute to the spore having a pear-like shape. The attached hypha can be blocked from the pore by this thickening. Spores are formed by the process of internal proliferation. In this process, a hypha extends into the interior of a formed spore and begins to expand a new spore. This may occur in fully developed spores with either one or two cell walls. The source hypha for the forming internal spore is derived from the old hypha, and the internal spore can be formed by hyphal expansion beginning at the base of the exterior spore or some distance within the old spore. This process can occur several times within the same external spore, and either type of spore formation (at the base of the old spore or extended into it) has been observed within a single nested group. Five nested spores have been observed within the same external spore. Loose sporocarps can have internal hyphae or double walls as found in the spores. Because of the variations in spore shape, color, walls, and internal proliferation character, there can be a great deal of diversity between spores even in a single sporocarp. The first sporocarps form on the outside of the root and proliferate, eventually forming appressoria and penetrating the root cortex. As is the case for all species in glomus, the mycorrhizal structure of G. aggregatum proliferates in straight lines along the cortex, branching dichotomously at cell junctions as it penetrates deeper into the root and extending in two directions at once. The mycorrhizal hyphae stain dark. Arbuscules that breach into root cells are thick and intricately branched into compact hyphal bunches.⁶

Glomus aggregatum is an arbuscular mycorrhizal fungus used as a soil inoculant in agriculture and horticulture. Like other species in this phylum it forms obligate symbioses with plant roots, where it obtains carbon (photosynthate) from the host plant in exchange for nutrients and other benefits.