The roles of specific xanthophylls in photoprotection

Abstract

Xanthophyll pigments have critical structural and functional roles in the photosynthetic light-harvesting complexes of algae and vascular plants. Genetic dissection of xanthophyll metabolism in the green alga Chlamydomonas reinhardtii revealed functions for specific xanthophylls in the nonradiative dissipation of excess absorbed light energy, measured as nonphotochemical quenching of chlorophyll fluorescence. Mutants with a defect in either the alpha- or beta-branch of carotenoid biosynthesis exhibited less nonphotochemical quenching but were still able to tolerate high light. In contrast, a double mutant that was defective in the synthesis of lutein, loroxanthin (alpha-carotene branch), zeaxanthin, and antheraxanthin (beta-carotene branch) had almost no nonphotochemical quenching and was extremely sensitive to high light. These results strongly suggest that in addition to the xanthophyll cycle pigments (zeaxanthin and antheraxanthin), alpha-carotene-derived xanthophylls such as lutein, which are structural components of the subunits of the light-harvesting complexes, contribute to the dissipation of excess absorbed light energy and the protection of plants from photo-oxidative damage.

Figure 1

Carotenoid biosynthetic pathway in C. reinhardtii. The defects in xanthophyll metabolism in the npq1 and lor1 mutants are indicated.

Figure 2

Carotenoid composition of C. reinhardtii strains. Cells were grown at an incident photon flux density (PFD) of 70 μmol photons m⁻²⋅s⁻¹ (low light, LL) and then dark-adapted overnight. Carotenes (A), α-carotene-derived xanthophylls (B), and β-carotene-derived xanthophylls (C) were determined by HPLC analysis of cell extracts before (LL, dark-adapted) and after exposure to 1,160 μmol photons m⁻²⋅s⁻¹ for 15 min [LL + high light (HL)] by using the same conditions as for measurements of fluorescence. Values are the means of two independent experiments. Loro, loroxanthin; Alpha-Car, α-carotene; Zea, zeaxanthin; Anthera, antheraxanthin; Viola, violaxanthin; Neo, neoxanthin; Beta-Car, β-carotene.

Figure 3

Induction of NPQ in C. reinhardtii strains. Cells were grown at an incident PFD of 70 μmol photons m⁻²⋅s⁻¹ and dark-adapted overnight. Fluorescence was measured before, during, and after exposure to actinic light (1,160 μmol photons m⁻²⋅s⁻¹) in air, and NPQ was calculated as (F_m − F′_m)/F′_m. The white bar above the graph indicates the period of illumination with high light (HL); the black bar indicates illumination with weak far red background light.

Figure 4

Growth of C. reinhardtii strains. Cells were streaked on minimal agar medium, incubated overnight at 50 μmol photons m⁻²⋅s⁻¹, and grown photoautotrophically for 6 days at the indicated PFD.