Comparative profiling of lipid-soluble antioxidants and transcripts reveals two phases of photo-oxidative stress in a xanthophyll-deficient mutant of Chlamydomonas reinhardtii

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Abstract
Excess light can impose severe oxidative stress on photosynthetic organisms. We have characterized high-light responses in wild-type Chlamydomonas reinhardtii and in the npq1 lor1 double mutant. The npq1 lor1 strain lacks two photoprotective carotenoids, lutein and zeaxanthin, and experiences acute photo-oxidative stress upon exposure to excess light. To examine the ability of npq1 lor1 cells to respond to photo-oxidative stress, we measured changes in lipid-soluble antioxidants following a shift from low light to high light in the wild type and the double mutant. The size of the xanthophyll cycle pool increased in both the wild type and mutant during the first 6 h of exposure to high light levels, but then decreased in the mutant during photo-oxidative bleaching. The level of α-tocopherol (vitamin E) was constant in the wild type and mutant during the first 6 h; then it increased by three-fold in the wild type but declined in npq1 lor1 cells. We also used cDNA microarrays and RNA gel-blot analysis to monitor differences in gene expression. Both strains showed an initial light-stress response in the form of a transient increase in expression of (1) GPXH, a glutathione peroxidase gene that has been shown to respond specifically to singlet oxygen and lipid peroxidation; (2) SMT1, a gene for a putative sterol C-methyltransferase; and (3) LI818r, a stress-responsive member of the light-harvesting complex superfamily. These transient changes in gene expression in high light were followed by a second series of changes in npq1 lor1, coincident with declines in lipid-soluble antioxidants but preceding detectable photo-oxidative damage to proteins and lipids. Thus, the response of npq1 lor1 to high light is unexpectedly complex, with initial changes in lipid-soluble antioxidants and RNA levels that are associated with acclimation in the wild type and a second wave of changes that accompanies photo-oxidative bleaching.

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