Are there Specific In Vivo Roles for alpha- and gamma-Tocopherol in Plants?

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 alpha-tocopherol in leaves and by gamma-tocopherol in seeds. This observation rises the question, whether alpha-tocopherol and gamma-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 gamma- for alpha-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 gamma-tocopherol is more potent than alpha-tocopherol in protecting against lipid peroxidation in both, seeds and leaves, although its natural abundance is in seeds only. If so, why has alpha-tocopherol accumulation in leaves been favoured during the evolution of land plants and does the abundance of gamma-tocopherol in leaves conceal a disadvantage for plant fitness?