The effects of wounding response and cytosolic ascorbate peroxidase levels on ozone susceptibility in ozone-sensitive tobacco cultivar
Creators: name: Örvar, Björn Lárus
Publisher: name: University of British Columbia
Year: 2009

description: The study of signal transduction in plants in recent years has revealed that many different chemical and environmental signals can elicit similar physiological responses. We know little about how a potential ozone stress-signal is transduced into a protective response in plant cells, and within the context of this complex response, it is uncertain which components are most important for reducing cellular damage during ozone exposure. The general objective of this study was to use different tobacco genotypes to investigate the interplay between two stresses, i.e. ozone and wounding, that are both thought to involve redox perturbations in the challenged tissue. Transgenic technology was also employed to create a modified antioxidant defence system in a hypersensitive tobacco genotype, to see whether this would alter its ability to tolerate ozone stress. Both wounding and treatment with methyl jasmonate (MeJA) prior to exposure of the tissue to high concentrations of ozone (250 to 500 ppb) produced a dramatic decrease in ozone injury. A systemic pattern of increased ozone tolerance developed within 3 to 6 hr after wounding and also after local application of jasmonic acid. Ozone treatment of transgenic tobacco plants (line called NahG) expressing bacterial salicylate hydroxylase, showed that the inability of these plants to accumulate salicylic acid, which has been shown to inhibit jasmonate-mediated responses, was also accompanied by increased ozone tolerance. Expression of mRNA encoding the antioxidant enzyme ascorbate peroxidase (APX) was upregulated by ozone challenge, by wounding and by MeJA exposure within 3 to 4 hr, while levels of carbonic anhydrase mRNA were simultaneously depressed following ozone exposure and MeJA treatment. In order to investigate the effects of a modified antioxidant defence system on ozone tolerance in tobacco, a cDNA for a cytosolic APX was first cloned from tobacco and subsequently used to make transgenic tobacco plants with altered expression of cytosolic APX. The expression of an antisense construct, comprising about 45% of the 3' coding region of the cytosolic APX, significantly reduced both the endogeneous APX mRNA levels and the cytosolic APX catalytic activity in these plants, apparently without affecting the chloroplastic APX activity. These transgenic plants displayed a significantly higher level of ozone injury following very high ozone exposure, and an even more drastic difference in injury levels, following lower, prolonged exposures. In contrast, transgenic plants overexpressing APX displayed a level of ozone injury following ozone exposure similar to that seen in control plants. To my knowledge, the data presented in this thesis provide the first substantial evidence for common signalling mechanisms linking plant responses to ozone with their responses to other environmental stresses. Furthermore, the data presented in this thesis provide the first substantial evidence for the importance of cytosolic APX in ozone defence in tobacco.