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Published on September 19, 2008; 10.1104/pp.108.126060

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Received July 8, 2008
Accepted September 12, 2008

Impact of PsbTc on Forward and Back Electron Flow, Assembly, and Phosphorylation Patterns of Photosystem II in Tobacco

Pavan Umate , Christine Fellerer , Serena Schwenkert , Mikael Zoryan , Lutz A. Eichacker , Abbagani Sadanandam , Itzhak Ohad , Reinhold G. Herrmann , and Jorg Meurer *

Department Biology I, Botany, Ludwig-Maximilians-University Munich, Menzingerstr. 67, 80638 Munich, Germany

* Corresponding author; email: joerg.meurer{at}lrz.uni-muenchen.de.

Photosystem II (PSII) of oxygen evolving cyanobacteria, algae and land plants mediates electron transfer from the Mn4Ca cluster to the plastoquinone pool. It is a dimeric supramolecular complex comprising more than 30 subunits per monomer of which 16 are bitopic or peripheral, low-molecular-weight (LMW) components. Directed inactivation of the plastid gene encoding the LMW peptide PsbTc in tobacco does not prevent photoautotrophic growth. Mutant plants appear normal green and levels of PSII proteins are not affected. Yet, PSII dependent electron transport, stability of PSII dimers and assembly of PSII-light harvesting complexes (LHCII) are significantly impaired. PSII light sensitivity is moderately increased and recovery from photoinhibition is delayed leading to faster D1 degradation in {Delta}psbTc under high light. Thermoluminescence emission measurements revealed alterations of mid-point potentials of QA/QB interaction. Only traces of CP43 and no D1/D2 proteins are phosphorylated presumably due to structural changes of PSII in {Delta}psbTc. In striking contrast to WT, LHCII in the mutant is phosphorylated in darkness consistent with its association with PSI indicating an increased pool of reduced plastoquinone (PQ) in the dark. Finally, our data suggest that the QB site, the properties of which are altered in {Delta}psbTc, is required for oxidation of reduced plastoquinone in darkness in an oxygen dependent manner. These data present novel aspects of plastoquinone redox regulation, chlororespiration, and redox control of LHCII phosphorylation.






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