Gigaspora margarita

Arbuscular Mycorrhizal Fungi are characterized by their intracellular arbuscules they form within the associated plant's roots. These arbuscules can come in many different variations with no one looking identical to another. This makes it hard for them to be used as a classification tool. Arbuscules are bush-like structures where they have branches hyphae forming from a swollen hyphal base. Oftentimes, the hyphae of AMF will stain blue using trypan blue dye.[1]

Gigaspora margarita is distinguished primarily by the morphology of its spores. Young spores are often salmon colored and will become pearly white to yellow-brown at maturity. A mature spore has three cell wall layers (L1, L2, L3):

• L1 is the outermost layer which forms the rigid and smooth shell of the spore. It is also pale brownish to yellow and is 1.6 - 2.4 micrometers thick.
• L2  is the middle layer that consists of hyaline sub-layers called laminae. These layers increase in number with maturity. The thickness of the L2 layer is highly variable even within the same spore. It can range anywhere from 14 to 23 micrometers. It also has a brown to brownish yellow tint when mounted with PVLG fluid and will turn a dark reddish brown to dark reddish purple if it is stained with Melzer's reagent.
• L3 is the innermost layer that is impossible to distinguish from the L2 layer unless the ultrastructure is looked at. Basically, the L3 layer is more electron dense than the L2 layer. The L3 layer forms warts (aka papillae) on its inner surface in clusters. These clusters are an indication of where a germ tube will form.[1]

Gigaspora margarita also has auxiliary cells produced on tightly wound hyphae. These cells are spikey in appearance and are found in clusters of 4-20. Auxiliary cells are found in all species classified under the Gigasporaceae family so while they do not specifically distinguish G. margarita, they are a good indicator that a certain AMF species is in the Gigasporaceae family.[4]

Gigaspora margarita has a unique ability to associate with a diverse range of endobacteria.[7] Though rare, the ability for Gigaspora margarita to host this intrahyphal bacteria is not completely unique. Many forms of AMF have been shown to associate with bacteria. The primary species of bacteria that associates with G. margarita is named Candidatus Glomeribacter gigasporarum (italicized?). Found in the Burkholderia genus, Ca G gigasporarum has had its entire genome sequenced and found to be a lot smaller than other Burkholderia species.[8] This indicates a reliance on G. sporangium to survive. Ca G gigasporarum has never been found on its own in nature and, so far, can not be isolated in culture. Adding to this hypothesis is the fact that, Ca. G. gigasporarum is vertically transmitted with G. margarita spores. Each spore contains around 250,000 of these bacteria which may be a contributor to the unusually large spores G. margarita produces.[9]

The symbiotic relationship between G. margarita and its endobacteria is asymmetric meaning one of the species can survive without the other while the other species cannot. In this case, G. margarita has been isolated and grown without the presence of endobacteria. Still, endobacteria play an important role in optimizing the relationship between the plant and the fungus. The presence of endobacteria is correlated with a higher antioxidant metabolic rate and lipid biosynthesis in the associated plant. The synthesized lipids would end up being exchanged to G. margarita.[3] Basically, endobacteria are not required for the completion of the G. margarita life cycle however they are major contributors to the health of the interaction between plant and fungus.