Comparative analysis of stem carbon partitioning and development in Andropogoneae bioenergy grasses

Plants fix carbon during photosynthetic reactions but how this carbon is distributed and stored throughout the plant greatly depends on the species. While cellulose and lignin are the more stable forms, the plant only needs to produce enough of it to allow for basic cell wall function. Starch is the most common storage form for excess sugar accumulation; however, plants can also store carbon in the form of sucrose even though it is highly unstable and makes them a target for pests. Typically sucrose is stored in the vacuoles of plants but some species also store sucrose in the storage parenchyma. Members of the Andropogoneae tribe display a good example of this versatility in carbon storage. This tribe of grasses is known for their high levels of biomass production and includes many prominent crops such as sorghum, maize, and sugarcane and the emerging biofuel feedstock miscanthus. These crops are closely related and not only handle carbon storage in different ways but also have different carbon storage forms within each species. There are varieties in each species that store sugar with sugarcane being the most recognized for its high levels of sucrose in the stem. There are also varieties in sorghum and maize that funnel their carbon into storage for grain production. Stem fiber can also be a sink for carbon as seen by the high biomass varieties of Saccharum energy cane. Miscanthus stores large volumes of carbon as above ground cellulo-lignosic biomass during the growing season and then begins funneling carbon in the form of starch into its rhizomes during the fall for overwintering. The differences in carbon partitioning among species or genotypes are likely mediated by changes in the expression of genes that control developmental programs or carbon metabolism. Maize and sorghum have sequenced genomes, but the complex polyploid genome of Saccharum presents challenges for studies of gene function and regulation. Gene expression profiling is an efficient first step to gain an understanding of the biological pathways whose activities are associated with phenotypic variation in stem carbon partitioning within Andropogoneae grasses. This study conducts genome-scale gene expression profiling in two experimental systems that will enable a comparative genomics approach. The first system is maize hybrids that overexpress Glossy15, which exhibit delayed shoot maturation and accumulate greater stem biomass and sugars compared to near-isogenic grain hybrids. The second system is a Saccharum pedigree where progeny vary for their relative production of stem sugar, fiber, and biomass. In both systems, the differences in compositional, anatomical, and gene expression differences are compared between the high sugar type and the high fiber type