Are There Specific In Vivo Roles for α- and γ-tocopherol in Plants?

Abstract

Tocopherols belong to the Vitamin E family of amphiphilic antioxidants, together with the subfamily of tocotrienols. They are exclusively synthesized by photosynthetic organisms and consist of a polar chromanol head group and a lipophilic prenyl tail. The Vitamin E pool in dicots is commonly dominated by α-tocopherol in leaves and by γ-tocopherol in seeds. This observation rises the question, whether α-tocopherol and γ-tocopherol are functionally equivalent in protection against various kinds of oxidative stress in planta: superoxide and singlet oxygen evolution are high during oxygenic photosynthesis in leaves, while polyunsaturated fatty acid oxidation is the main target for tocopherols in seeds. We found that transgenic tobacco plants with a substitution of γ- for α-tocopherol in leaves are more tolerant than the wild type towards sorbitol and methyl viologen mediated oxidative stress, which increase lipid peroxidation in the chloroplast stroma. This suggests that γ-tocopherol is more potent than α-tocopherol in protecting against lipid peroxidation in both, seeds and leaves, although its natural abundance is in seeds only. If so, why has α-tocopherol accumulation in leaves been favoured during the evolution of land plants and does the abundance of γ-tocopherol in leaves conceal a disadvantage for plant fitness?