Occurrence and environmental stress responses of two plastid terminal oxidases in Haematococcus pluvialis (Chlorophyceae)
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The plastid terminal oxidase (PTOX) is a plastoquinol oxidase involved in carotenoid biosynthesis in higher plants, and may also represent the elusive oxidase in chlororespiration. Haematococcus pluvialis is a green alga that has the ability to synthesize and accumulate large amounts of the red carotenoid astaxanthin (ca. 2% of dry weight) under various stress conditions. However, the occurrence and function of PTOX in astaxanthin synthesis and the stress response in this organism is unknown. In this study, two ptox cDNAs were cloned and sequenced from H. pluvialis and were designated as ptox1 and ptox2. Genome sequence analysis and database searching revealed that duplication of PTOX gene occurred in certain eukaryotic algae, but not in cyanobacteria and higher plants. The physiological and biochemical evidence indicated that PTOX is involved in astaxanthin synthesis and plays a critical protective role against stress. Analysis of the transcriptional expression of the PTOXs and phytoene desaturase gene further suggests that it may be PTOX1 rather than PTOX2 that is co-regulated with astaxanthin synthesis. The fact that the changes in transcripts of ptoxs in response to high light and other stressors and the differential expression of ptox1 and ptox2, suggests that PTOX, coupled with astaxanthin synthesis pathway, exerts broad, yet undefined functions in addition to those identified in higher plants.
KeywordsAstaxanthin Plastid terminal oxidase Haematococcus pluvialis Environmental stress Gene expression
We thank Dr. Marcel Kuntz (Université Joseph Fourier, France) and Drs. Steven Rodermel and Aigen Fu (Iowa State University, USA) and Allison McDonald (University of Toronto, Canada) for communications of unpublished data. We also acknowledge the technical assistance of Drs. Moulun Luo and Carol Foster at Arizona State University.
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