School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemLooking for expansins : a molecular approach to the investigation of tylose development and heartwood formation in Eucalyptus nitens( 2006)Advanced stages of stem development in many tree species, including eucalypts, are marked by the transition of conductive sapwood to non-conductive heartwood. Heartwood formation follows a characteristic sequence of events involving the accumulation of phenolic compounds in ray parenchyma cells, the occlusion of vessels by tylose and/or gum formation, cell death and the subsequent release of phenolic compounds into the surrounding tissue. These events are dependent upon the activity of ray parenchyma cells, but the molecular processes involved, particularly with regard to tylose formation, remain largely unknown. The identification of molecular pathways leading to tylose formation might yield insights into heartwood formation. A molecular approach to the investigation of tylose formation is hampered by the paucity and inaccessibility of ray parenchyma cells and the asynchronous nature of tylose formation. Based on the assumption that wound-induced tyloses and those formed during the transition of sapwood to heartwood develop via a common mechanism, these difficulties were overcome by using the wounding response of the tree (a 12-year-old Eucalyptus nitens sapling) to induce extensive and simultaneous development of tyloses. Tylose formation involves the marked extension of a primary cell wall structure. Elsewhere, such wall extension has been closely associated with the activity of expansins. These constitute a large, multi-gene family of proteins which are widely distributed throughout higher plants and which have been shown to induce relaxation and extension of primary cell walls, often in a cell- and tissue-specific manner. It is proposed that expansins are likely to be involved in tylose formation. Ray and axial parenchyma cells are the only living cells found in sapwood, and primary cell wall extension is only possible through tylose formation. Thus, gene expression associated with wall extension occurring in sapwood is likely to be associated with tylose formation. Cellular material from outer sapwood showing extensive wound-induced tylose development was successfully harvested and partial cDNA sequences displaying significant homology with a-expansins were identified. This provides circumstantial evidence that expansin gene expression is associated with tylose formation and should encourage further investigation of the molecular pathways involved in this process.
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ItemThe Cell biology of eucalypt heartwood formation( 2001)Trees are among the most successful of organisms. Despite voracious enemies, diverse and treacherous habitats, individuals may thrive for hundreds of years, or even millennia. Their success is due to adaptability, and the development of powerful multi-faceted defensive systems. The least understood of these is the heartwood/tylosis system. Once thought to be merely a metabolic waste dump, or an irrelevant consequence of homeostatic processes, heartwood is now, with the benefit of new perspectives, emerging as a crucial and dynamic defensive element. This thesis proposes that ray parenchyma cells actively produce heartwood to provide a micro-organism resistant base for the vulnerable sapwood transpiration system and carbohydrate reserve. Statistical analyses of a 20-year-old Eucalyptus nitens tree showed that heartwood existed as a central pillar within the tree, and tyloses within heartwood were optimally distributed to seal the largest and most vulnerable vessels against invasion. Ultrastructural analysis showed that ray cells in the middle and inner sapwood were essentially dormant, but were reactivated in the heartwood transition zone. Ray cells forming tyloses showed evidence of a high level of activity with the nucleus directing expansion from within tyloses. The coordinated activity and subsequent deaths of these cells suggest the possibility of programmed death as their ultimate fate. Ray cell deaths result in release of phytotoxins into the surrounding tissue. This is in contrast to animal cell apoptosis where release of toxins into the surrounding tissue is actively prevented. Research into the microstructural and molecular basis of heartwood formation in vivo is limited by the reactivity of extractives within the transition zone, the inaccessibility of cells, and the transitory and asynchronous nature of heartwood transformation. In vitro systems, in which initiation and inhibition of heartwood-like transformation can be easily manipulated, may ultimately provide solutions to these problems. The development of two of these systems is discussed.