Switchgrass seems to benefit from a number of interactions with soil bacteria and fungal mutualisms. Switchgrass forms essentially symbiotic relationships with arbuscular mycorrhizal fungi, which grow into the plant’s roots. This relationship enhances nutrient and water uptake, drought tolerance, and protection against pathogens and toxic contaminants and can lead to greater plant growth (Koslowsky and Boerner 1989; Brejda et al. 1998; Clark 2002; Clark et al. 2005; Ghimire et al. 2009; Ghimire and Craven 2011) although the success of these relationships can vary by strain and source (Koslowsky and Boerner 1989; Clark 2002). These associations may play an important role in switchgrass’ adaptation to marginal sites, as Clark (2002) reported that switchgrass plants grown in acidic soils (pHCa 4 and 5) with mycorrhizal fungus Glomus etunicatum had greater P, N, S, K, Mg, Zn, and Cu uptake with reduced uptake of toxic minerals such as Al.

Work by Brejda et al. (1998) showed that rhizosphere microflora from native prairies in Nebraska, Kansas, Iowa, Missouri, Virginia, and North Carolina were effective in enhancing (up to 15-fold increase) switchgrass seedling shoot and root growth, as well as up to 6- and 36-fold increases in N and P recoveries. Switchgrass also appears to be somewhat indiscriminant as a fungal host. For example, Ghimire et al. (2009) reported that switchgrass roots formed association with the ectomycorrhizal fungus Sebacina vermifera [Serendipita vermifera (Oberw.) P. Roberts, comb. nov]. This association enhanced germination of Kanlow switchgrass seed by 52%. In three harvests, S. vermifera increased shoot biomass of NF/GA-993 (EG1101) by 75, 113, and 18% over that of un-inoculated control plants, with no consequent reduction in root biomass. Ghimire and Craven (2011) have also reported large increases in shoot length and shoot and root mass when inoculated with strains of S. vermifera under both stressed (drought) and unstressed growing conditions. Associations of these ectomycorrhizal fungi also altered root architecture. Intriguingly, these fungi can have bifunctional lifestyles, acting as insect pathogens as well as endophytes (Sasan and Bidochka 2012).

While more effort has been given to studying grass-fungal associations with switchgrass, there are ongoing efforts to improve production with bacterial endophytes. Kim et al. (2012) reported greater root and shoot length, increased tillering, and greater mass (about 50%) of lowland (cv. Alamo) switchgrass seedlings when seed were first inoculated with Burkholderia phytofirmans (strain PsJN). Success (as greater plant growth) occurred under both normal and drought-stressed conditions, but was cultivar-specific.

Ker et al. (2012) isolated bacterial strains from the roots of Cave-In­Rock switchgrass that had grown for several years without fertilization. The isolated bacteria included a strain of Paenibacillus polymyxa, a N2- fixing bacterium, as well as bacteria capable of solubilizing phosphate or producing plant hormones (auxins) or both. When tested in field studies, seeds treated with the inoculum "cocktail" produced more tillers and about 40% greater total biomass.

These results suggest there are significant potential opportunities to improve establishment success and yields using various types of fungal or bacterial inocula. Greater understanding of the interactions among host plants and their microbial colonizers may lead to ways that further improve the adaptability of switchgrass to marginal sites or low input systems. For now, however, we are aware of no commercial inocula produced to capitalize on this potential.