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.