Plant growth-promoting bacterial endophytes can affect growth directly by providing bacterium-synthesized compounds, often plant hormones, and by facilitating the acquisition of compounds from the environment, including atmospheric nitrogen fixation. Endophytes may also act indirectly by decreasing or preventing the colonization or the deleterious effects of pathogenic organisms (Lodewyckx et al. 2002) by producing antibiotics to outcompete plant pathogens (Bibi et al. 2012).

One of the most well-studied bacterial endophyte associations is atmospheric nitrogen fixation by specific endophytes. This symbiosis is well known in leguminous plants (Stacey et al. 2006) where the soil bacteria Rhizobia infect the roots of the host plants, inducing the formation of nodules where they fix atmospheric nitrogen and provide it to the host plant in exchange for carbon compounds (Lodewyckx et al. 2002). Additionally mutualistic associations through the fixation of nitrogen can also be observed in non-leguminous plants, such as rice (Mattos et al.

2008) , maize (Montanez et al. 2009), sugarcane (Oliveira et al. 2009), wheat (Webster et al. 1997), strawberries (de Melo Pereira et al. 2012), and grasses (Reinhold-Hurek et al. 1993; Kirchhof et al. 2001).

Nitrogen-fixing bacteria have been studied extensively in the bioenergy crop sugarcane, and include Gluconacetobacter spp., Azospirillum spp., Herbaspirillum spp. and Burkholderia spp. (James and Olivares 1998; James et al. 2001; Suman et al. 2005; de Carvalho et al. 2011). In fact, in Brazil, the cultivation of sugarcane uses only a small amount of fertilizer (de Carvalho et al. 2011) without showing nitrogen deficiency symptoms (Rosenblueth and Martinez-Romero 2006), and there is evidence that a significant amount of nitrogen is obtained from plants associated with bacterial endophytes (de Carvalho et al. 2011). To date, there are no reports on nitrogen-fixing bacterial endophytes in switchgrass, and screening for diazotrophic bacteria that inhabit switchgrass is under way in our laboratory.

There have been numerous publications on plant growth promotion by bacterial endophytes (see review in Berg 2009; Mei and Flinn 2010). In switchgrass, young seedlings of the cultivar Alamo inoculated with Burkholderia phytofirmans strain PsJN, isolated from onion roots (Frommel et al. 1991), showed significant growth promotion with an increase of root and shoot length of 35.6% and 32.8%, respectively, as well as an increase of fresh weight of 83.6% compared with control plants (non-inoculated) after one month under in vitro conditions (Kim et al. 2012). The same pattern was observed under growth chamber and greenhouse conditions, where plants inoculated with the B. phytofirmans strain PsJN showed persistent growth vigor with significant increases in fresh and dry weights, and an increase in the number of early tillers (Kim et al. 2012). Also, results showed that B. phytofirmans strain PsJN has potential in the development of a low input and sustainable switchgrass feedstock production system on marginal lands as higher biomass yields were observed under sub-optimal growth conditions with PsJN inoculated plants over control (Kim et al. 2012). However, PsJN growth promotion is genotype specific in switchgrass as the upland cultivar Cave-in-Rock did not respond to inoculation. We are currently isolating bacterial endophytes from switchgrass tissues and have made progress in screening and selecting beneficial bacterial endophytes which have a broad spectrum of growth promotion in various switchgrass cultivars.