Extraradical mycelium (ERM)
The extraradical mycelium (ERM) is composed of two types of hyphae:
unbranched runner hyphae, which run parallel to the root length to
initiate secondary colonization, and highly branched absorptive hyphae
responsible for nutrient uptake from the soil and subsequent
translocation to the host (Friese & Allen, 1991). (Bago et al. ,
1998a) and Dodd et al. (2000) observed the formation of ’branched
absorbing structures’ - small groups of dichotomous hyphae - within the
ERM in species of Glomeraceae. The ERM is also accountable for the
formation of spores and auxiliary cells in the soil. Phenotypic
variables associated with the ERM, such as hyphal length and density,
interconnectedness, and hyphal diameter, have been studied in some AM
fungal species (Dodd et al. , 2000; Avio et al. , 2006). The
extraradical mycelium can form Common Mycorrhizal Networks (CMNs), where
a single AM fungus associates with multiple plant hosts, interconnecting
them within a shared mycelial network. These networks may serve as
biological bridges, facilitating resource exchange and, possibly,
communication among different plant hosts (Barto et al. , 2011;
Babikova et al. , 2013). However, more research is needed to
determine the role of CMNs in natural environments and to what extent
the capacity to form CMNs can be considered a fungal trait (Karstet al. , 2023).