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.