Alternative sources of fuel such as bioethanol offer benefits such as energy independence and stability, reduction in green house gas emissions, and are a renewable resource. The C4 grasses are large contributors of fermentable carbohydrates for bioenthanol production. However, fermentable carbohydrate yields from grass stems have plateaued in recent years. Transgenic manipulation may provide solutions to engineering increased nonstructural carbohydrate yields, increase cell wall digestibility and accessibility, and reduce agricultural input costs. Sorghum bicolor is a prime candidate as a biofuel feedstock, but lacks the molecular characterization essential for transgenic manipulation. Therefore, the focus of this research was to conduct a comprehensive characterization of the biochemical, molecular and genetic dynamics of nonstructural carbohydrate accumulation and cell wall biogenesis in the sorghum stem. To facilitate this investigation a high biomass accumulating elite sweet sorghum named Della was selected as a model since this sorghum accumulates high biomass and therefore closely approximates a sweet energy sorghum.
Analysis of dry biomass accumulation revealed that Della invests heavily in stem growth starting at the beginning of the vegetative phase and ending at anthesis, subsequently accumulating sugar in its stem during flowering. To identify genes involved in cell wall biogenesis and carbohydrate accumulation, a time-course of Della internode transcriptome dynamics was conducted beginning at floral initiation and ending ~20 days after grain maturity encompassing the time course of carbohydrate accumulation in the stem. This analysis identified candidate genes involved in cell wall biosynthetic processes and nonstructural carbohydrate partitioning and accumulation were identified.
Della synthesizes starch in its stem after anthesis. Since starch is more stable than sucrose, has favorable osmotic properties, and is already present in the sorghum stem, this pathway was selected as the focus for further analysis. This analysis revealed candidate genes that will be useful for future transgenic studies. Additionally, stem biomass from Della was observed to have accumulated starch up to ~15% of the total stem dry biomass and ~30% of total nonstructural carbohydrate content. Therefore, utilization of existing starch stores in sorghum stem biomass would increase fermentable carbohydrate yield significantly.
Lastly, genetic analysis of the grain*sweet sorghum cross RTx436*Della identified loci that underlie stem volume traits when mapping quantitative trait loci for internode carbohydrate content. This suggests that stem carbohydrate concentration reached a maximum level and was limited by volume. Also, carbohydrate was found to be correlated with internode volume and not stem juiciness and leaf area. Finally, radiation use efficiency analysis revealed that at certain points during development the canopy may be underutilized and that increasing the sink strength of the sorghum stem may benefit nonstructural carbohydrate yield
