Protection against oxidative stress by vitamin E
Vitamin E is composed of a group of substances, the tocopherols and the tocotrienols, which are synthesised solely by photosynthetic organisms (plants, algae and cyanobacteria). This lipid-soluble vitamin is considered to be a major antioxidant of animal and human membranes. Its protective role against oxidative stress is well known in animals, but remained to be demonstrated in plants.
These researchers have, for the first time, successfully demonstrated this anti-oxidative function in vivo in mutants of the plant model Arabidopsis thaliana exposed to photo-oxidative stress conditions.
Vitamin E acts at two levels in chloroplastic membranes, corresponding to the two main singlet oxygen production sites: it specifically protects photosystem II against photo-inhibition, and it has a more general protecting action that limits membrane lipid peroxidation. Another lipophilic antioxidant, zeaxanthin (a carotenoid synthesised rapidly when light exposure reaches a certain threshold), has the same functions, but uses different mechanisms. The suppression of zeaxanthin and vitamin E in double mutants of Arabidopsis produces a phenotype that is hypersensitive to photo-oxidative stress.
The results obtained show that vitamin E is integrated in a complex network of antioxidant systems. The identification of the antioxidant properties of vitamin E in plants suggests means of developing plants that are more resistant to environmental stresses, and of using chloroplastic membranes as a model to study the mode of action of this vitamin and its interactions with carotenoids.
wt : unmodified plant
vte1: vitamin E-deficient plant
npq1 : zeaxanthin-deficient plant
vte1, npq1 : vitamin E- and zeaxanthin-deficient plant
Plants exposed to a 'strong light' stress: the double mutant vte1 npq1, deficient in both vitamin E and zeaxanthin is photosensitive; its leaves bleach and eventually die.