Assessing photoprotective functions of carotenoids in photosynthetic systems of plants and green algae

Carotenes and xanthophylls act as photoreceptors in the photosystems of plants and algae by absorbing light energy which drives photosynthetic electron transport. Moreover, these carotenoid pigments protect chloroplasts from excess light and from reactive species generated during oxygenic photosynthesis. These pigments share similar spectral properties, a feature which contrasts with the extreme level of conservation of their relative composition and abundance in leaves across taxa. Such a conservation through evolution suggested each carotenoid species had a peculiar role, which indeed has been investigated by different approaches. These studies included the purification of individual carotenoid-binding proteins and their characterization in vitro. In a complementary approach, plant and algal mutants devoid of selected carotenoid species have been produced. The physiological characterization of these mutants revealed that the integrated contributions of all carotenoid species provide the most efficient response to photooxidative stress. In this chapter, we provide step-by-step guides for characterizing the in vivo antioxidant activity of carotenoids in plants and green algae, and methods for quantifying the effect of photooxidative stress in genotypes with altered carotenoid composition or impaired defense mechanisms.