Evaluation of the over-expression of a <i>Miscanthus</i> x <i>giganteus</i> PPDK (C4ppdk1) in switchgrass (<i>Panicum virgatum</i>) for improved cold temperature C4 photosynthesis

Switchgrass (Panicum virgatum) has been proposed as a potential biofuel feedstock to aid in the displacement of petroleum-based combustible fuels over the course of the next several decades. Improving the yield potential of this perennial grass is therefore of economic interest. Increased net ethanol yields can be achieved in cellulosic feedstocks via two means: 1. Improved sugar release from digestible cell wall materials, 2. Increased overall biomass yield. The latter is the subject of this study.

As a C4 photosynthetic plant, switchgrass is highly productive in the hot, humid climate of the southeastern United States. By bypassing the photorespiratory pathway, C4 plants have the ability to avoid as much as 25% yield losses due to the oxygenase activity of Rubisco. The utilization of this alternate pathway exposes the plant to cold temperature growth inhibition. Early spring and higher latitudinal temperatures do not provide optimal growth conditions for most C4 plants.

One C4 plant, miscanthus (Miscanthus x giganteus), has been shown to avoid the inhibiting effects of cold temperatures through the upregulation of a particular C4 enzyme, pyruvate phosphate dikinase (PPDK). This enzyme, responsible for the rate limiting step in the C4 pathway, has been shown to increase cold temperature carbon fixation and growth in miscanthus when it is upregulated. For this reason, this enzyme is an interesting target for overexpression in transgenic switchgrass to increase cold temperature photosynthesis.

The miscanthus PPDK cDNA was synthesized and ultimately overexpressed in transgenic switchgrass, which were characterized for photosynthesis and growth under both cold and warm temperatures. There were no statistically significant increases in biomass yields or in photosynthetic capacity between transformed plants and non-transformed control plants. These data suggest that the initial hypothesis was probably too simplistic; there is likely greater complexity to understand the relationship of cold temperature C4 photosynthesis and the role of PPDK. Future experiments are needed to evaluate the underlying regulation and complexities of the C4 pathway to further understand how to specifically target and influence gene expression to accomplish higher efficiencies.