title: Photoprotection of photosystems in fluctuating light intensities.
abstract: Oxygenic photosynthetic organisms experience strong fluctuations in light intensity in their natural terrestrial and aquatic growth environments. Recent studies with both plants and cyanobacteria have revealed that Photosystem (PS) I is the potential target of damage upon abrupt changes in light intensity. Photosynthetic organisms have, however, developed powerful mechanisms in order to protect their photosynthetic apparatus against such potentially hazardous light conditions. Although the electron transfer chain has remained relatively unchanged in both plant chloroplasts and their cyanobacterial ancestors, the photoprotective and regulatory mechanisms of photosynthetic light reactions have experienced conspicuous evolutionary changes. In cyanobacteria, the specific flavodiiron proteins (Flv1 and Flv3) are responsible for safeguarding PSI under rapidly fluctuating light intensities, whilst the thylakoid located terminal oxidases are involved in the protection of PSII during 12h diurnal cycles involving abrupt, square-wave, changes from dark to high light. Higher plants such as Arabidopsis thaliana have evolved different protective mechanisms. In particular, the PGR5 protein controls electron flow during sudden changes in light intensity by allowing the regulation mostly via the Cytochrome b6f complex. Besides the function of PGR5, plants have also acquired other dynamic regulatory mechanisms, among them the STN7-related LHCII protein phosphorylation that is similarly responsible for protection against rapid changes in the light environment. The green alga Chlamydomonas reinhardtii, as an evolutionary intermediate between cyanobacteria and higher plants, probably possesses both protective mechanisms. In this review, evolutionarily different photoprotective mechanisms under fluctuating light conditions are described and their contributions to cyanobacterial and plant photosynthesis are discussed.
authors.affiliation: Molecular Plant Biology laboratory, Department of Biochemistry, University of Turku, Turku 20520, Finland.
authors.forename: Yagut
authors.initials: Y
authors.lastname: Allahverdiyeva
authors.affiliation: Molecular Plant Biology laboratory, Department of Biochemistry, University of Turku, Turku 20520, Finland.
authors.forename: Marjaana
authors.initials: M
authors.lastname: Suorsa
authors.affiliation: Molecular Plant Biology laboratory, Department of Biochemistry, University of Turku, Turku 20520, Finland.
authors.forename: Mikko
authors.initials: M
authors.lastname: Tikkanen
authors.affiliation: Molecular Plant Biology laboratory, Department of Biochemistry, University of Turku, Turku 20520, Finland evaaro@utu.fi.
authors.forename: Eva-Mari
authors.initials: EM
authors.lastname: Aro
mesh_terms: D000064:Acclimatization; D017360:Arabidopsis / Q000378:metabolism / Q000528:radiation effects; D016825:Chlamydomonas reinhardtii / Q000378:metabolism / Q000528:radiation effects; D002736:Chloroplasts; D000458:Cyanobacteria / Q000378:metabolism / Q000528:radiation effects; D004579:Electron Transport; D008027:Light; D008954:Models, Biological; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D020524:Thylakoids
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex
keywords: Cyanobacteria; PGR5; PGRL1; Photosystem I (PSI); flavodiiron proteins; green algae; photodamage; photosynthesis; plant; terminal oxidases.