Functional Analysis of Tocopherols in Plants and their Integration within the Plastid Antioxidant Network

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During its lifecycle, a plant is exposed to a wide range of abiotic and biotic stresses that can cause production of reactive oxygen species (ROS) in excess of that normally generated by metabolism and for signaling in the absence of stress. Although all subcellular compartments produce ROS, chloroplasts are the major ROS source in photosynthetic tissues. In order to avoid oxidative damage and death, plants have evolved an interconnected, complementary network of compounds (e.g., ascorbate, tocopherols, carotenoids, glutathione), enzymes (e.g., superoxide dismutases, peroxidases, catalases) and biochemical responses (e.g., non-photochemical quenching) that serve to minimize plastid ROS production under stressful conditions or rapidly detoxify excess ROS species or byproducts that are produced. Tocopherols, more commonly known as Vitamin E, are the major lipid soluble antioxidant in plastids and are proposed to be an essential component of the plastid antioxidant network. Studies in mammals and artificial membranes indicate tocopherols are important for membrane structure, quenching of singlet oxygen and other ROS, scavenging of chain-propagating lipid peroxy radicals and other "non-antioxidant" functions related to signaling and transcriptional regulation. In plants, tocopherols are assumed to function similarly though experimental evidence for this is lacking. During the past five years, the DellaPenna laboratory has cloned all enzymes of the tocopherol biosynthetic pathway in Arabidopsis and developed mutant/transgenic lines whose leaves are completely deficient in tocopherols or accumulate tocopherols at 4-5 times wild type levels. These tocopherol-modified lines will be used:1. To define the role(s) of tocopherols in scavenging specific ROS species and control of lipid peroxidation. 2. In genetic combinations with mutations that affect other components of the antioxidant network to assess functional compensation and redundancy of antioxidant network components.3. To study a novel germination phenotype in tocopherol deficient lines.4. In whole genome expression analysis to assess the response of individual antioxidant network genes and other cellular components to tocopherol modifications. Tocopherols, more commonly known as Vitamin E, are the major lipid soluble antioxidant in chloroplasts of all plants and are proposed to be an essential component of the plastid antioxidant network. The research being undertaken will provide a comprehensive understanding of tocopherol function(s) in photosynthetic tissues of Arabidopsis. This work will also define and characterize the integration and complementation of tocopherols with and by other components of the plastid antioxidant network (Vitamim C, carotenoids and enzymes that scavenge reactive oxygen). Because of the evolutionary conservation of the antioxidant network in chloroplasts, the data obtained will be of direct relevance to a wide range of plant species and crops.