| Summary: | <p>Abstract</p> <p>Background</p> <p>When photosynthetic organisms are exposed to harsh environmental conditions such as high light intensities or cold stress, the production of reactive oxygen species like singlet oxygen is stimulated in the chloroplast. In <it>Chlamydomonas reinhardtii </it>singlet oxygen was shown to act as a specific signal inducing the expression of the nuclear glutathione peroxidase gene <it>GPXH/GPX5 </it>during high light stress, but little is known about the cellular mechanisms involved in this response. To investigate components affecting singlet oxygen signaling in <it>C. reinhardtii</it>, a mutant screen was performed.</p> <p>Results</p> <p>Mutants with altered <it>GPXH </it>response were isolated from UV-mutagenized cells containing a <it>GPXH</it>-arylsulfatase reporter gene construct. Out of 5500 clones tested, no mutant deficient in <it>GPXH </it>induction was isolated, whereas several clones showed constitutive high <it>GPXH </it>expression under normal light conditions. Many of these <it>GPXH </it>overexpressor (<it>gox</it>) mutants exhibited higher resistance to oxidative stress conditions whereas others were sensitive to high light intensities. Interestingly, most <it>gox </it>mutants produced increased singlet oxygen levels correlating with high <it>GPXH </it>expression. Furthermore, different patterns of altered photoprotective parameters like non-photochemical quenching, carotenoid contents and α-tocopherol levels were detected in the various <it>gox </it>mutants.</p> <p>Conclusions</p> <p>Screening for mutants with altered <it>GPXH </it>expression resulted in the isolation of many <it>gox </it>mutants with increased singlet oxygen production, showing the relevance of controlling the production of this ROS in photosynthetic organisms. Phenotypic characterization of these <it>gox </it>mutants indicated that the mutations might lead to either stimulated triplet chlorophyll and singlet oxygen formation or reduced detoxification of singlet oxygen in the chloroplast. Furthermore, changes in multiple protection mechanisms might be responsible for high singlet oxygen formation and <it>GPXH </it>expression, which could either result from mutations in multiple loci or in a single gene encoding for a global regulator of cellular photoprotection mechanisms.</p>
|
|---|
