PHOTOPROTECTIVE DYNAMICS AND LIGHT ADAPTATION IN THE MOSS PHYSCOMITRIUM PATENS.

This doctoral thesis examines the regulatory mechanisms governing the light reactions of photosynthesis. Photosystem I (PSI) and Photosystem II (PSII) are pigment-protein super-complexes responsible for capturing and transferring sunlight energy to drive linear electron flow within the thylakoid membrane, leading to the production of reducing power (NADPH) and the synthesis of adenosine triphosphate (ATP). Both NADPH and ATP are essential for carbon fixation in the Calvin–Benson–Bassham cycle. In natural environments, plants face varying light conditions over both short and long timescales, making photoprotective mechanisms critical for preventing damage to the photosystems. For instance, non-photochemical quenching (NPQ) of chlorophyll-a fluorescence is crucial for PSII protection, while cyclic electron flow (CEF) and pseudo-cyclic electron transport (PCEF) help prevent over-reduction of PSI. Due to its evolutionary position between aquatic and terrestrial life, along with its comprehensive set of photosynthesis regulatory mechanisms, the moss Physcomitrium patens serves as an excellent model for studying plant adaptation to fluctuating environmental conditions. In this study, the photoprotective mechanisms of P. patens were extensively investigated through a combination of approaches. We focused on the interaction between the carotenoid zeaxanthin (Zx) and NPQ triggers in P. patens, specifically the PSBS protein (conserved in vascular plants) and the LHCSR protein (conserved in green algae). Our findings show that zeaxanthin-dependent quenching strongly relies on the LHCSR protein, but in the absence of PSBS, constitutive zeaxanthin accumulation can partially compensate. Additionally, using a base editing mutagenesis system, we identified key residues of the PSBS protein. The PCEF pathway mediated by flavodiiron proteins (FLVs) was another focus of this study. Recombinant P. patens FLVs were expressed in Escherichia coli, and we demonstrated the formation of a stable heterocomplex between the purified FLVA and FLVB proteins. This complex exhibited oxygen reductase activity in the presence of an electron donor, allowing us to calculate the kinetic parameters of FLVs. We also examined the effects of overexpressing FLVs in P. patens mutants with significantly increased FLVs accumulation—up to 20 times higher than wild-type levels. These mutants showed faster electron transport during the onset of light, though they also experienced a growth penalty compared to the wild type. Various hypotheses regarding the evolutionary trends of FLVs were critically discussed. In conclusion, this thesis provides a comprehensive overview of the interactions between different photoprotective mechanisms (NPQ, CEF, PCEF) under varying light conditions and investigates the molecular mechanisms underlying these processes.