Increasing concern over foreign energy supplies, global greenhouse gas emissions and the need for rural economic development has driven the interest in sustainable biomass production for bioenergy and bio-products. It has been suggested that by 2025, the world energy demand will likely be increased by more than 50% [1, 2]. This demand, and societal concerns about the environmental impact of burning fossil fuels are key factors stimulating the development of national and regional strategies aimed at the growth of renewable energy supplies, primarily focused on biofuels. To reduce the reliance on fossil fuels, the USA, the world’s major energy consumer, released the Energy Independence and Security Act of 2007 that aims to increase the production of renewable fuels from 9.0 billion gallons in 2008 to 36 billion gallons by 2022 . The recent USDA/DOE National Biofuels Action Plan  has helped to delineate the priority areas required to accelerate sustainable biofuel industry development. Within this document, Action Area 2 was identified as feedstock production and improvement. Various feedstocks, such as perennial rhizomatous grasses, can provide sources of lignocellulosic biomass, serving as new sources of crop growth and income for regional farmers.
One of the most promising feedstocks capable of contributing to the realization of US renewable energy goals is the common perennial grass, switchgrass (Panicum virgatum L.). This native prairie grass, consisting of a diverse germplasm , can grow on marginal lands under low inputs of water and agrochemicals , so that its cultivation does not compete with food crops for land and other resources. Due to its large root system and fast stand regrowth, switchgrass has other positive environmental effects, including the prevention of surface runoff and soil erosion, carbon sequestration, and the provision of a wildlife habitat [5, 8]. Switchgrass cultivated lands also had much higher total soil organic carbon deposits than lands cultivated with annual crops, such as corn and wheat [9, 10].
The economics of biofuel production is highly dependent on feedstock cost and conversion technology [1, 6]. The development of improved switchgrass varieties for low-cost production on marginal lands is one prerequisite for the success of the bioenergy program [5, 11]. One such approach involves the use of beneficial microorganisms, such as endophytes, which form intimate associations with plants [12, 13]. Endophytes, both fungal and bacterial, have been targeted as mechanisms to enhance plant characteristics for commercial uses . The colonization of grasses by fungal endophytes for performance enhancement is well documented , including their use with switchgrass [16, 17]. However, to our knowledge, only one study has reported growth promotion of a bioenergy feedstock grass (Miscanthus x giganteous) seedlings by a bacterial endophyte (Herbaspirillum frisingense) . A key component of our bioenergy crop research program involves the utilization of beneficial bacterial endophytes that form stable and persistent associations with switchgrass, as the mechanism to improve biomass yield and enhance stress tolerance under low-input production systems . Beneficial bacterial endophytes are naturally occurring soil microorganisms that can penetrate plant roots and translocate to the above ground organs and, upon colonization, affect plant growth, health, and productivity [12, 20–22]. Although the molecular mechanisms of beneficial endophyte-host plant interactions are largely unknown, several studies have demonstrated that endophytes can promote plant growth by enhancing the plant’s capacity for nutrient acquisition, better water management, and/or resistance to abiotic and biotic stresses via regulation of hormones [12–14, 20, 21]. For instance, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase produced by endophytes lowers the ethylene levels in host plants, reducing their response to abiotic and biotic stress, and by changing root morphology, leading to stimulation of plant growth [13, 23, 24]. Many known endophytes also promote plant growth by producing gibberellic acid (GA3), indole-3-acetic acid (IAA) [18, 25], or cytokinins [26, 27].
Burkholderia phytofirmans strain PsJN has been found to be a highly effective plant growth promoting bacterial endophyte, with a broad host range including potatoes, tomatoes, and grape vines [21, 27–32]. In addition, its genome has recently been sequenced , providing the genomic resources needed to develop an understanding of the mechanisms associated with this endophyte’s ability to promote plant growth. PsJN produces a high level of ACC deaminase , enhances host plant cold  and heat  stress tolerance, improves water management  and plant resistance to pathogens [37, 38]. In this study, we report growth promotion of switchgrass cv. Alamo by Burkholderia phytofirmans strain PsJN under in vitro, growth chamber, and greenhouse conditions. To our knowledge, this is the first report detailing the switchgrass-PsJN interaction.