Molecular genetics of xanthophyll-dependent photoprotection in green algaeand plants

The involvement of excited and highly reactive intermediates in oxygenic ph otosynthesis inevitally results in the generation of reactive oxygen specie s. To protect the photosynthetic apparatus from oxidative damage, xanthophy ll pigments are involved in the quenching of excited chlorophyll and reacti ve oxygen species, namely (1)Chl*, (3)Chl*, and O-1(2)*. Quenching of (1)Ch l* results in harmless dissipation of excitation energy as heat and is meas ured as non-photochemical quenching (NPQ) of chlorophyll fluorescence. The multiple roles of xanthophylls in photoprotection are being addressed by ch aracterizing mutants of Chlamydomanas reinhardtii and Arabidopsis thaliana. Analysis of Arabidopsis mutants that are defective in (1)Chl* quenching ha s shown that, in addition to specific xanthophylls, the psbS gene is necess ary for NPQ, Double mutants of Chlamydomonas and Arabidopsis that are defic ient in zeaxanthin, lutein and NPQ undergo photo-oxidative bleaching in hig h light. Extragenic suppressors of the Chlamydomonas npq1 lor1 double mutan t identify new mutations that restore varying levels of zeaxanthin accumula tion and allow survival in high light.