Carotenoids Involved in Antioxidant System of Chloroplasts

Carotenoids are tetraterpenoid pigments that are present in the chloroplasts and chromoplasts of the photosynthetic organisms. Carotenoids are divided into two classes: carotenes, composed of carbon and hydrogen and xanthophylls, containing additionally oxygen. In chloroplasts carotenoids play two key roles: they absorb light energy for use in the photosynthesis, and they are also important components of the antioxidant system of chloroplasts protecting photosynthetic apparatus from photodamage. In this chapter properties of carotenoids as effective nonenzymatic plant antioxidants are described. At the beginning the chemical structure of carotenoids in relation to their antioxidant properties is explained. Next, the photoprotective role of two all-trans β-carotene molecules existing in PSII reaction center will be described. In the last part of the chapter the significance of xanthophylls in photoprotection will be discussed. Special attention is given to the role of carotenoids involved in several types of the xanthophyll cycle. Furthermore, the meaning of the xanthophyll cycles as antioxidant systems is discussed. Four essential mechanisms clarifying the protective role of xanthophyll cycles will be presented. Three of these mechanisms show an indirect and one a direct participation of xanthophylls in the photoprotection. The mechanisms based on indirect participation of pigments created as products of the light phase of xanthophyll cycle in quenching of overexcitation include: (I) aggregation-dependent LHCII quenching; (II) light-driven mechanisms in LHCII and (III) charge transfer quenching between Chl a and Zx. Selected results of research on the antioxidant properties of xanthophyll cycle pigments in model systems is also shown. Finally, the role of ascorbate as an antioxidant and as a reductant required to carry out de-epoxidation is also considered.