Replacement of α-Tocopherol by β-Tocopherol Enhances Resistance to Photooxidative Stress in a Xanthophyll-Deficient Strain of Chlamydomonas reinhardtii

Tocopherols (vitamin E) comprise a class of lipid-soluble antioxidants synthesized only in plants, algae, and some cyanobacteria. The majority of tocopherols in photosynthetic cells is in the α form, which has the highest vitamin E activity in humans, whereas the β, γ, and δ forms normally account for a small percentage of total tocopherols. The antioxidant activities of these forms of tocopherol differ depending on the experimental system, and their relative activities in vivo are unclear. In a screen for suppressors of the xanthophyll-deficientnpq1 lor1 double mutant ofChlamydomonas reinhardtii , we isolated avte3 mutant lacking α-tocopherol but instead accumulating β-tocopherol. Thevte3 mutant contains a mutation in the homolog of a 2-methyl-6-phytyl-1,4-benzoquinone methyltransferase gene found in plants. Thevte3 npq1 lor1 triple mutant with β-tocopherol survived better under photooxidative stress than did thenpq1 lor1 mutant, but thevte3 mutant on its own did not have an obvious phenotype. Following transfer from low light to high light, the triple mutant showed a higher efficiency of photosystem II, a higher level of cell viability, and a lower level of lipid peroxide, a marker for oxidative stress, than did thenpq1 lor1 mutant. After high-light transfer, the level of the photosystem II reaction center protein, D1, was also higher in thevte3 npq1 lor1 mutant, but the rate of D1 photodamage was not significantly different from that of thenpq1 lor1 mutant. Taken together, these results suggest that the replacement of α-tocopherol by β-tocopherol in a xanthophyll-deficient strain ofChlamydomonas reinhardtii contributes to better survival under conditions of photooxidative stress.