Roots of most land plant species are colonized by mycorrhizal fungi. is association serves mutual benets: plant-derived carbohydrates are allocated to the fungus and in return the fungus delivers nutrients, in particular phosphorus and nitrogen to its host plant (Smith and Read 2008). It has been estimated that about 80% of the plant P and N contents have been acquired via mycorrhizas underlining their importance for plant nutrition (van der Heijden et al. 2008). However, it should not be forgotten that mycorrhizal fungi provide additional ecological benets to their hosts such as increased drought tolerance, decreased susceptibility to biotic diseases, improved tolerance to heavy metals, excess salinity, and other environmental cues (Schützendübel and Polle 2002; Polle and Schützendübel 2003; Luo et al. 2009; Beniwal et al. 2010; see also Chapter 40).