Intraradical mycelium (IRM)
The AM fungal mycelial system colonizes two distinct environments: the IRM grows inside the roots in a very constant environment while the ERM grows in the soil under highly variable environmental conditions (Smith & Read, 2008). Two broad anatomical groups of IRM can be recognized in mycorrhizal roots , the Arum -type, dominated by arbuscules, and the Paris- type, dominated by coils, although evidence suggests a continuum between these types, depending on the host plant and the fungus (Dickson, 2004). Presence of ’H’ branches in the IRM is more common in Glomeraceae taxa compared to Acaulosporaceae, while looping hyphae and hyphae with small bumped projections are prevalent in species forming gigasporoid spores (see (Dodd et al. , 2000)for a review). Arbuscules are highly branched structures with a turnover rate ranging from 7 to 16 days (Alexander et al. , 1989) or longer in woody plants (Brundrett & Kendrick, 1990), and they serve as the primary site of nutrient exchange between the fungus and the host. The main differences observed in arbuscule architecture relate to branching patterns. In Gigasporaceae, the trunk is wide and branching is abrupt, whereas the trunk is narrow and branching is gradual in Acaulosporaceae and Glomeraceae. Vesicles are thick-walled, globose to lobed structures that store lipids and contain many nuclei (Smith & Read, 2008). They are not formed by members of Gigasporaceae, and there is some evidence the same is true for basal families such as Ambisporaceae, Archaeosporaceae, and Paraglomeraceae.