Document Type

Theses, Ph.D


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Publication Details

Successfully submitted for the award of Doctor of Philosophy (Ph.D) to the Dublin Institute of Technology 2005.


Peroxidases and its effects on plants were analysed. Peroxidases are involved in many physiological processes in plants, involving responses to biotic and abiotic stresses and the biosynthesis of lignin. Liglin is a polymer responsible for rendering the plant stronger and more rigid and also making the cell walls hydrophobic. Peroxidases are involved in the polymerization of the precursors of lignin. They are also involved in the scavenging of Reactive Oxygen Species (ROS), which are partially reduced forms of atmospheric oxygen, highly reactive, and capable of causing oxidative damage to the cell. Peroxidases can be a source of hydrogen peroxide (H2O2) but also are capable of scavenging it. The overexpression of a peroxidase gene with the consequent elevated levels of the enzyme were analysed in this study. A pathogen related cell wall peroxidise gene from barley (prx8) isolated by the Thordal-Christensen et al. (1992) was cloned into a binary vector Burbridge (2001). This gene was transformed into Nicotiana tabacum L. cx. Xanthi. Also, the prx8 gene was cloned into a vector driven by the chlorophyll a/b binding promoter (cab) and transformed into Malus domestica var. Greensleeves. This plant was chosen as a woody species, due to the involvement of peroxidises in the lignification process and its importance in wood production. Transgenic plants from both species were characterised and analysed. They were submitted to abiotic stresses, such as heat and cold stress, high salinity and metal ions in high concentrations. Transgenic plants overexpressing the prx8 gene showed an increased tolerance to these abiotic stresses. In fact, germination rates of transgenic tobacco seeds in the presence of high salinity levels and metal ions in the medium were higher than wild type seeds. Electrolyte leakage of leaf discs presented with higher temperatures was reduced in transgenic plants, showing a higher tolerance to the heat stress. This was further proved on soil grown plants presented with high temperatures. The role of peroxidises in the development of plants was also studied. Plants showed acceleration in the growth rate when compared to wild type plants. Experiments on the apple plants also showed an increase in the number of xylem vessels. These transgenic plants proved the very important role of this cell wall peroxidise in the responses to environmental changes as well as the development of the plants.