Photoprotection in Photosynthesis: From plant to protein

Oxygenic photosynthesis is the process in plants, algae and cyanobacteria that transforms sunlight into stable chemical energy. The initial reactions are carried out by two large multi-subunit pigment-protein complexes, Photosystem II (PSII) and Photosystem I (PSI). Light is absorbed by pigments associated with the photosystems, and the resulting excitation energy is rapidly transferred from pigment to pigment until it reaches the reaction centre. Here the excitation energy drives photochemical charge separation, setting off a series of downhill redox reactions that ultimately generate ATP and NADPH. To ensure sufficient light is absorbed for the photosynthetic reactions to result in a net energy surplus, the two photosystems are surrounded by a peripheral light-harvesting antenna system. In plants, this is made up of a family of Light-Harvesting Complexes (LHCs), which bind many chlorophylls and carotenoids. However, photosynthesis runs on an energetic tightrope. If the amount of absorbed light surpasses the capacity of the photosynthetic reactions, the excess energy can damage photosynthetic machinery, resulting in a net energy deficit. To achieve this balancing act, plants employ a photoprotective mechanism called non-photochemical quenching (NPQ). Within minutes of exposure to high light intensities, NPQ causes the light-harvesting antenna of PSII to switch to a dissipative state, safely releasing excess absorbed energy as heat. In this thesis, the location and mechanism of NPQ are investigated at the plant level, all the way down to the protein level. First, the thesis details the construction and characterisation of a mutant of Arabidopsis thaliana lacking LHCII, the main antenna complex of plants. The mutant is characterised using PAM fluorometry, time-resolved fluorescence and various biochemical techniques. The characterisation identifies LHCII as the predominant site of quenching in plants. Second, the thesis details the construction of a proteoliposome system using proteins purified from the thylakoid membrane. This in vitro system successfully replicates NPQ observed in vivo, allowing us to determine the minimal components necessary to trigger quenching in LHCII. Overall, the results provide detailed insight into a complex and important mechanism. Inherent inefficiencies in NPQ result in an estimated 30% loss of daily carbon uptake in crops, making it an important target for synthetic biologists hoping to improve crop yield.