AM fungi can enhance a plant’s ability to acquire nutrients like phosphorus and nitrogen (Clark 2002; Parrish and Fike 2005; Leigh et al. 2009; Schroeder- Moreno et al. 2011), phytoremediate contaminated soil (Entry et al. 1999), and withstand acidic soil (Clark 2002). AM hyphae have the ability to extend beyond the usual nutrient absorption zone of plant roots, therefore reaching additional essential nutrients and transporting them to the plant (Clark 2002).

Mycorrhizal fungi and other rhizosphere microflora have played significant roles in switchgrass growth in nature (Parrish and Fike 2005). In field conditions, switchgrass plants are commonly associated with AM fungi and have shown growth stimulation under different conditions (Brejda et al. 1998; Parrish and Fike 2005; Schroeder-Moreno et al. 2011). Under acidic soil conditions, the inoculation of AM fungi (Glomus, Gigaspora and Acaulospora) increased the root length of switchgrass plants, as well as the uptake of minerals such as phosphorus, nitrogen, sulfur, potassium, calcium, magnesium, zinc, and copper but reduced the uptake of manganese, iron, boron, and aluminum (Clark 2002). Inoculation with the AM fungi Gisgospora margarita, Gi. Rosea, Glomus clarum, and Scutellospora heterogama significantly increased nitrogen in shoots (Schroeder-Moreno et al. 2011), which implies AM fungi play an important role in N cycling from the soil to switchgrass plants.

Microorganism diversities affect plant growth promotion because plants exist in a community of bacteria, fungi, algae and/or viruses (Rodriguez and Redman 2008), and plants could be associated with more than one microorganism. Inoculation of switchgrass seedlings with multiple types of rhizosphere microflora increased the yield of shoots and roots up to 15-fold and also increased nitrogen uptake 6-fold and phosphorus uptake 37-fold, compared with the control plants infected with rhizosphere bacteria only (Brejda et al. 1998). Environmental factors, such as nutrients and stress, also influence symbiosis between host plants and endophytes as well as AM fungi. Under high nutrient availability, symbiotic Neotyphodium occultans — Lolium multiflorum association showed higher seed weight than that of non-symbiotic plants (Gundel et al. 2012). Under greenhouse conditions, the combination of AM fungus and the fungal endophyte Epichloe elymi on growth promotion in the grass Elymus hystrix was found to be additive (Larimer et al. 2012). However, the presence and specificity of the fungal endophyte altered the interaction of AM fungus with the host plant as endophyte infection increased Glomus mosseae colonization while decreasing G. claroideums colonization (Bibi et al. 2012).