The Arbuscular Mycorrhiza Fungal Extraradical Mycelium : Developmental Dynamics in the Formation of Functionally Different Types of Extraradical Hyphae and Implications for Costs and Benefits of the Endomycorrhizal Symbiosis.

It has frequently been observed that AMF form different types of extraradical hyphae (Bago et al., 2004; Hart and Reader, 2005). Coarse, thick-walled, long-lived runner hyphae probably function in the spread of infection and in the extension of the fungal colony (Friese and Allen, 1991). Finely branched, thin-walled hyphae seem to function mainly in mineral element uptake from the soil (Bago et al., 1998a, b). When AMF spores are formed, the number of finely branched structures decreases in favor of spore carrying hyphae (Declerck et al., 2001). Despite these observations, the architecture of soil-grown AMF ERM has never been investigated in detail. To date, also nothing is known about what factors may trigger the formation of different types of extraradical hyphae and how differences in the AMF ERM architecture are related to the contribution of the symbiosis to plant P nutrition and its demand for plant C. Knowledge on these aspects could help to better predict the outcome of endomycorrhizae in different developmental stages of the symbiosis and/or under different environmental conditions. Such information could also promote the understanding of the ecological niches, different AMF occupy in ecosystems (Pringle and Bever, 2002), and allow for a better management of AMF populations in agricultural and horticultural plant production systems.