The role of reactive oxygen species in the retrograde chloroplast-nucleus signalling pathway
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Photosynthetic organisms are particularly susceptible to photooxidative stress, because they are dependent on light energy for converting carbon dioxide into organic compounds and as a by-product, generate high levels of oxygen in the chloroplast. While photosynthetic eukaryotes exhibit altered nuclear gene expression in response to changes in the production of reactive oxygen species (ROS) in the chloroplast, little is known about how this signal is transmitted from the chloroplast to the nucleus. In the green alga Chlamydomonas reinhardtii, the cytosolic GLUTATHIONE PEROXIDASE 5 gene (GPX5) is known to be up-regulated at the level of transcription in response to singlet oxygen. Previous studies have shown that when the promoter region of GPX5 is fused to the ARYLSULFATASE 2 gene (ARS2), an effective reporter system can be generated and used to study GPX5 expression. This system was used in this study to generate a stably transformed C. reinhardtii strain which expresses the ARS2 protein in a singlet oxygen-dependent manner. Using the strain of C. reinhardtii harbouring a singlet oxygen-sensitive promoter gene, secondary mutagenic screen was performed. This allowed identification of mutant cell lines that were unable to up-regulate GPX5-ARS2 fusion expression, based on ARS2 activity, in response to singlet oxygen production. In one of these lines, the mutation was subsequently localized to the first exon of the PSBP-like gene (PSBP2), and this line was designated as psbP2. The PSBP2 gene is part of a small gene family in C. reinhardtii that is conserved in higher plant species. While each member of the PSBP protein family contains a similar domain to the PSBP1 protein, which is a member of the oxygen evolving complex of photosystem II (PSII), the PSBP2 protein does not appear to be involved in PSII function. While psbP2 does not produce greater ARS2 activity in response to singlet oxygen, it still accumulates both the GPX5-ARS2 and native GPX5 transcripts when challenged by photosensitizer exposure, although at lower levels than the original lines. It was demonstrated that the PSBP2 protein is involved in transmitting information related to the accumulation of singlet oxygen in the chloroplast to control the singlet oxygen-dependent GPX5 driven ARS2 expression in the nucleus and/or ARS2 activity through a post-transcriptional process in the cytoplasm.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorWilson, Ken E.; Gray, Gordon R.
CommitteeBonham-Smith, Peta; Wang, Hong; Zou, Jitao
Copyright DateNovember 2011