title: Plants Actively Avoid State Transitions upon Changes in Light Intensity: Role of Light-Harvesting Complex II Protein Dephosphorylation in High Light.
abstract: Photosystem II (PSII) core and light-harvesting complex II (LHCII) proteins in plant chloroplasts undergo reversible phosphorylation upon changes in light intensity (being under control of redox-regulated STN7 and STN8 kinases and TAP38/PPH1 and PSII core phosphatases). Shift of plants from growth light to high light results in an increase of PSII core phosphorylation, whereas LHCII phosphorylation concomitantly decreases. Exactly the opposite takes place when plants are shifted to lower light intensity. Despite distinct changes occurring in thylakoid protein phosphorylation upon light intensity changes, the excitation balance between PSII and photosystem I remains unchanged. This differs drastically from the canonical-state transition model induced by artificial states 1 and 2 lights that concomitantly either dephosphorylate or phosphorylate, respectively, both the PSII core and LHCII phosphoproteins. Analysis of the kinase and phosphatase mutants revealed that TAP38/PPH1 phosphatase is crucial in preventing state transition upon increase in light intensity. Indeed, tap38/pph1 mutant revealed strong concomitant phosphorylation of both the PSII core and LHCII proteins upon transfer to high light, thus resembling the wild type under state 2 light. Coordinated function of thylakoid protein kinases and phosphatases is shown to secure balanced excitation energy for both photosystems by preventing state transitions upon changes in light intensity. Moreover, proton gradient regulation5 (PGR5) is required for proper regulation of thylakoid protein kinases and phosphatases, and the pgr5 mutant mimics phenotypes of tap38/pph1. This shows that there is a close cooperation between the redox- and proton gradient-dependent regulatory mechanisms for proper function of the photosynthetic machinery.
authors.affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku, 20014 Turku, Finland.
authors.forename: Nageswara Rao
authors.identifier: http://orcid.org/0000-0002-6309-5532
authors.initials: NR
authors.lastname: Mekala
authors.affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku, 20014 Turku, Finland.
authors.forename: Marjaana
authors.initials: M
authors.lastname: Suorsa
authors.affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku, 20014 Turku, Finland.
authors.forename: Marjaana
authors.initials: M
authors.lastname: Rantala
authors.affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku, 20014 Turku, Finland evaaro@utu.fi misati@utu.fi.
authors.forename: Eva-Mari
authors.initials: EM
authors.lastname: Aro
authors.affiliation: Molecular Plant Biology, Department of Biochemistry, University of Turku, 20014 Turku, Finland evaaro@utu.fi misati@utu.fi.
authors.forename: Mikko
authors.identifier: http://orcid.org/0000-0002-2753-9597
authors.initials: M
authors.lastname: Tikkanen
mesh_terms: D000064:Acclimatization / Q000528:radiation effects; D017360:Arabidopsis / Q000502:physiology* / Q000528:radiation effects*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D015151:Immunoblotting; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism*; D008954:Models, Biological; D046912:Multiprotein Complexes / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D010766:Phosphorylation / Q000528:radiation effects; D045331:Photosystem I Protein Complex / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000378:metabolism; D011522:Protons; D020524:Thylakoids / Q000378:metabolism / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; D045342:Light-Harvesting Protein Complexes; D046912:Multiprotein Complexes; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D011522:Protons