Expression profiling of tissues within the sugarcane maturing stalk

Sugarcane accumulates high concentrations of sucrose in the mature stalk, with numerous physiological processes in maturing stalk tissue both directly and indirectly involved. A considerable portion of carbohydrate metabolism is also devoted to cell wall synthesis and fibre production. Previously, we have identified differentially expressed transcripts correlated with sucrose accumulation and fibre production in various internodes of the sugarcane stalk. In this study, we have examined tissue-specific expression patterns to further explore gene pathways responsible for sucrose accumulation and fibre synthesis. We performed large-scale expression profiling of storage parenchyma, vascular bundles and rind dissected from a maturing stalk internode from field-grown commercial sugarcane harvested at 11 months of age. We identified 10 cellulose synthase subunit genes in sugarcane and examined significant differences in the expression of their corresponding transcripts and those of several sugar transporters between the different tissues. These were further correlated with differential expression patterns for transcripts of specific COBRA-like proteins and other proteins with acknowledged roles in cell wall metabolism. We found that the sugar transporters ShPST2a, ShPST2b and ShSUT4 were significantly up-regulated in storage parenchyma while ShSUT1 was up-regulated in vascular bundles. Two co-ordinately expressed groups of cell wall related transcripts were also identified. One group which is associated with primary cell wall synthesis (ShCesA1, ShCesA7, ShCesA9 and Shbk2l3) was up-regulated in parenchyma. The other group which is associated with secondary cell wall synthesis (ShCesA10, ShCesA11, ShCesA12 and Shbk-2) was up-regulated in rind. We also report an unexpected paucity in differential expression of cell wall-related genes in vascular bundles. Our results indicate that there is spatial separation for elevated expression of these important targets in both sucrose accumulation and cell wall synthesis, allowing for increased clarity in our understanding of sucrose transport and fibre synthesis in sugarcane. In order to identify transcripts that were differentially expressed in storage parenchyma (P), vascular bundles (V) and rind (R) dissected from maturing stalk, we performed hybridisation experiments using the GeneChip Sugarcane Genome Array with RNA amplified from total RNA derived from three biological replicates of field-grown sugarcane variety Q117. Each of the nine samples was hybridised separately to one array.