| Trees comprise over 90% of the terrestrial biomass of the earth and serve as a primary feedstock for biofuel, fiber, solid wood products, and various natural compounds. Most commercially important tree crops produce heartwood. The presence of heartwood is the major determining factor for wood quality and influences the way in which specific woods are utilized. Understanding the molecular mechanisms of heartwood formation is of great commercial and keen scientific interest. Despite the long history of study on wood formation, our knowledge of this unique biological process is limited. It is difficult to experimentally observe the processes in sapwood that lead to heartwood formation. Forward genetic approaches are hampered by long generation times and poorly defined tree populations. Comparative molecular genetic studies have limited use because most model organisms do not undergo secondary woody growth. The changes during the transition from sapwood to heartwood are complex and involve integration of many metabolic processes in wood cells. A genomics-based approach provides a unique opportunity for understanding the molecular biology of heartwood and the processes by which its extractives are produced and stored.; In order to gain insight on the molecular regulations of heartwood and its extractive formation, I carried out global examination of gene expression profiles across the trunk wood of black locust (Robinia pseudoacacia L.) trees. Of the 2,915 expressed sequenced tags (ESTs) that were generated and analyzed, 55.3% showed no match to known sequences. Cluster analysis of the ESTs identified a total of 2,278 unigene sets, which were used to construct cDNA microarrays. Microarray hybridization analyses were then performed to survey the changes in gene expression profiles of trunk wood. In addition to the regional gene expression profiles in trunk wood, seasonal gene expression changes were studied in the sapwood-heartwood transition zones that are in charge of heartwood formation. Furthermore, in this study, plant allantoinase genes involved in ureide pathway were investigated; in general, plants have the pathway, and these genes are differentially expressed in developmental and environmental conditions. Therefore, the impact of this study is expected to expand our knowledge of heartwood formation far beyond a single hardwood species. |
