Different roles of alpha- and beta-branch xanthophylls inphotosystem assembly and photoprotection

Xanthophylls (oxygenated carotenoids) are essential components of the plant photosynthetic apparatus, where they act in photosystem assembly, light harvesting and photoprotection. Nevertheless, the specific function of individual xanthophyll species awaits complete elucidation. In this work we analyze the photosynthetic phenotypes of two newly isolated Arabidopsis mutants in carotenoid biosynthesis containing exclusively alpha-branch (chy1chy2lut5) or beta-branch (chy1chy2lut2) xanthophylls. Both mutants show complete lack of qE, the rapidly reversible component of non-photochemical quenching, and high levels of photoinhibition and lipid peroxidation under photooxidative stress. Both mutants are much more photosensitive than npq1lut2, which contains high levels of viola- and neoxanthin and an higher stoichiometry of light-harvesting proteins with respect to photosystem II core complexes, suggesting that the content in light-harvesting complexes plays an important role in photoprotection. In addition, chy1chy2lut5, which has lutein as the only xanthophyll, shows unprecedented photosensitivity even in low light conditions, reduced electron transport rate, enhanced photobleaching of isolated LHCII complexes and a selective loss of CP26 with respect to chy1chy2lut2, highlighting a specific role of beta-branch xanthophylls in photoprotection and in qE mechanism. The stronger photosystem II photoinhibition of both mutants correlates with the higher rate of singlet oxygen production from thylakoids and isolated light-harvesting complexes, while carotenoid composition of photosystem II core complex was ininfluential. In-depth analysis of the mutant phenotypes suggests that alpha-branch (lutein) and beta-branch (zeaxanthin, violaxanthin, neoxanthin) xanthophylls have distinct and complementary roles in antenna protein assembly and in the mechanisms of photoprotection.