Item – Theses Canada

OCLC number
Gerhardt, Karen E.(Karen Ellen),1963-
Ultraviolet-B (UVB) stress and acclimation in higher plants : reactive oxygen species (ROS)-mediated photomodification of ribulose-1,5-bisphosphate carboxylaseoxygenase (rubisco) and protection via accumulation of quercetin derivatives.
Ph. D. -- University of Waterloo, 2003
Ottawa : National Library of Canada = Bibliothèque nationale du Canada, [2004]
3 microfiches.
Includes bibliographical references.
Levels of ambient ultraviolet-B radiation (UVB, 290-320 nm) are increasing due to depletion of stratospheric ozone. Plant proteins are potential targets for UVB damage because tryptophan (Trp) residues absorb radiation between 290-300 nm with reasonable quantum efficiency. Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), a chloroplast stromal enzyme, contains numerous Trp residues. When the rubisco holoenzyme is exposed to UVB, a covalent crosslink can be formed between a large and small subunit (Gerhardt et al. 1999). This results in the accumulation of a 66 kDa photoproduct, and a corresponding loss of the Trp fluorescence signal (Gerhardt et al. 1999). Because reactive oxygen species are often involved in the pathway of UVB photodamage, various antioxidants were added to an 'in vitro' system of purified rubisco, irradiated with UVB (300 nm) radiation. Ascorbate was the only antioxidant to prevent both the accumulation of rubisco photoproduct, and the loss of Trp fluorescence. Based on the protein and fluorescence data, as well as 1O2 quenching rates and characteristics of the antioxidants, it was postulated that 1O2 is involved in the mechanism of rubisco photoproduct formation. To protect sensitive mesophyll targets such as rubisco, many plants accumulate flavonoids in the epidermis. In response to UVB radiation, 'Brassica napus' leaves accumulated high levels of quercetin (Q) derivatives. Interestingly, the presence of far red radiation in the light regime depressed UVB-mediated accumulation of the these flavonoids. This implies a phytochrome response in the regulatory pathway of the UVB-inducible flavonoids of 'B. napus '. In response to UVB, the Q derivatives accumulated in both the upper and lower epidermis. An increase in these flavonoids, in UVB-acclimated plants, was concomitant with a decrease in rubisco photoproduct formation, thus suggesting an acclimatory role for the Q derivatives of 'B. napus'.