Genetic modification of endogenous biochemical and physiological pathways, along with traditional breeding methods, has the ability to improve the lignocellulosic feedstock quantity and quality of switchgrass. In order to decrease reliance on fossil fuels and efficiently utilize the renewable biomass produced by switchgrass, engineering new cultivars with value — added traits must be investigated. Value-added traits include, but are not limited to, enhanced taste, improved nutritional quality, or any features that would provide an additional benefit to consumers. Several value-added traits that are currently being studied include transforming switchgrass to produce bioplastics, as well as introducing cell wall degrading enzymes that will enhance conversion of the lignocellulosic feedstock into bioethanol.

Bioplastics

Bioplastics are currently being considered an alternative choice to petroleum — based polymers (Petrasovits et al. 2012). The most abundant bioplastic is polyhydroxyalanoate (PHA), a polyester that is naturally produced by microbial organisms as a reserve carbon nutrient source (Anderson and Dawes 1990). Polyhydroxybutyrate (PHB) is an extensively studied member of the PHA family that can be thermally altered to produce crotonic acid, a precursor for high demand chemicals such as propylene and butanol (Peterson and Fischer 2010; Coons 2010; Petrasovits et al. 2012).

The first report of PHB expressed in plants was published by Poirier et al. (1992). In this study, acetoacetyl-CoA reductase and PHB synthase, two enzymes from the bacterium Alicaligenes eutrophus, were expressed in Arabidopsis thaliana under the control of the cauliflower mosaic virus 35S promoter (Poirier et al. 1992). These enzymes, along with 3-ketothiolase, are essential in the conversion of acetoacetyl-CoA to PHB (Nawrath et al. 1994). In this experiment, PHB was expressed cytosolically and the plants produced 0.1 percent dry weight of PHB (Poirier et al. 1992). However, the plants displayed stunted growth, suggesting deleterious effects, along with erratic accumulation of PHB in unintended organelles, such as the nucleus and vacuole (Poirier et al. 1992). A couple of years later, Nawrath et al. (1994) expressed all three enzymes necessary for PHB production in Arabidopsis, but included a chloroplast transit peptide to target PHB production to the plastid. These plants were able to accumulate PHB up to 14 percent of their dry weight and displayed normal phenotypes (Nawrath et al. 1994). Collectively, these two studies created a platform for expressing PHB or other bioproducts, such as p-hydroxybenzoate (McQualter et al. 2005) and sorbitol (Chong et al. 2007), in plants.

Since the early 1990s, many studies have focused on engineering PHA-producing pathways into a plethora of crop species including cotton (Maliyakal and Keller 1996), tobacco (Lossl et al. 2003), maize (Poirier and Gruys 2001), sugarcane (Petrasovits et al. 2007), alfalfa (Saruul et al. 2002), and poplar (Dalton et al. 2011). As described earlier, PHB production in switchgrass was also investigated (Somleva et al. 2008).

Despite the occasional negative phenotype, this study has pioneered the way for genetically engineering switchgrass to produce functional multigene pathways. This innovation will ultimately aid in introducing value-added bioproducts in switchgrass that can be manufactured in correlation with biomass production. Further research will be necessary to optimize the expression and output of PHB without compromising plant health and viability. The next series of experiments should focus on optimizing transformation constructs (promoters, cis-acting elements, target peptide signals, etc.) with a target goal of obtaining high levels of PHB synthesis and accumulation in all tissues of the plant.