As plants are sessile organisms, the wide diversity of mutually beneficial plant-microbe interactions represents an ancient evolutionary partnership, helping the host plant survive and thrive, even in some of the harshest environments on the planet. Mechanisms of growth promotion by bacterial and fungal endophytes as well as AM fungi have been investigated in grasses for decades, and various mechanisms play roles in promoting plant growth and development. Bacterial endophytes are capable of producing or regulating plant hormones, helping acquire vital nutrients, and bio-control of pathogens (Sturz et al. 2000). Plant associated fungi, both endophytic and mycorrhizal, also confer a range of growth promotion benefits to their host plant including nutrient acquisition. Furthermore, a particular bacterial or fungal endophyte may utilize one or more mechanisms to promote plant growth and may even utilize different mechanisms at various points during the life cycle of plants. While it is clear that endophytes can benefit the host plant in many ways, establishing clear-cut growth promotion in the field can be difficult due to a number of factors including the diversity of native microorganisms in the soil and soil conditions. A more profound understanding of these mechanisms is allowing scientists to discover new ways to integrate their use into increasing yields of bioenergy crops like switchgrass. Also, by utilizing tools of modern molecular biology and functional genomics to understand the complexity of growth promotion at the genetic level, additional light will be shed on these complex interactions. As more is learned about the biochemistry, molecular biology, and physiology of microbe-plant interactions, it is evident that bacterial and fungal microorganisms will be important components for sustainable bioenergy feedstock production in the future.

Plant growth promotion can generally be achieved directly by interactions between the microorganism and host and/or indirectly through antagonistic activity against plant and environmental pathogens (Berg 2009). In this section, we will discuss both mechanisms and how different beneficial microbes may work together to benefit the host plant simultaneously (Muller et al. 2009), as well as how microorganisms, especially bacteria, may share mechanisms of actions genetically through horizontal gene transfer.