In oxygen-evolving photosynthesis, the balanced excitation of photosystem (PS) I and PSII complexes is required for safe and efficient utilization of light energy. During the course of evolution, plants have acquired dynamic regulatory mechanisms to control the excitation energy transfer and distribution as well as the photosynthetic electron transport. Concerted action of the regulatory mechanisms is required for the optimization of photosynthetic efficiency under changing light conditions.

Short-term changes in light intensity alter the phosphorylation pattern of the PSII core and its light harvesting antenna (LHCII) proteins. Reversible phosphorylation of the LHCII proteins mediates the excitation energy distribution between the two photosystems in the so called state transitions, which are known to be dependent on the redox status of the plastoquinone pool. In this study, the analysis of the kinase mutants (stn7 and stn8) and the phosphatase mutants (pph1/tap38 and pbcp) revealed that the PPH1/TAP38 phosphatase is required for the dephosphorylation of LHCII in high light (HL). In pph1/tap38 mutant, both the PSII core and LHCII proteins are simultaneously phosphorylated upon transfer to high light, which lead to increased excitation energy distribution towards PSI, thus mimicking the effect of the state 2 light. It is also shown that the PGR5 protein, which is essential for the generation of a transthylakoid proton-gradient, is likely to be involved in the regulation of the thylakoid protein phosphorylation upon increasing light intensities. Indeed, a close cooperation between the redox and proton-gradient dependent regulatory mechanisms is required for maintaining functionality of the photosynthetic machinery.

Novel information on the effects of PSII photoinhibition on the dissipation of excitation energy in the thylakoid membrane is likewise provided. It is also demonstrated that the proton gradient-dependent and PGR5-mediated control of electron transfer via the Cyt b f complex together with controlled photoinhibition of PSII limit the electron flow from PSII to PSI, thereby providing protection for the PSI complex against photodamage. Further, PSII photoinhibition is shown to lead to the phosphorylation-independent loss of thylakoid membrane lateral heterogeneity, thus allowing the oxidized P700 to act as an energy quencher. In addition, the PsbS-dependent non-photochemical quenching of excess excitation energy is revealed to enhance the spillover of the excitation energy towards PSI at low lumenal pH. Collectively, the evidence provided in this thesis has highly improved our knowledge concerning photoprotective mechanisms inside the plant cell.