Diverse mechanisms for photoprotection in photosynthesis. Dynamic regulation of photosystem II excitation in response to rapid environmental change

Photosystem II (PSII) of photosynthesis catalyzes one of the most challenging reactions in nature, the light driven oxidation of water and release of molecular oxygen. PSII couples the sequential four step oxidation of water and two step reduction of plastoquinone to single photon photochemistry with charge accumulation centers on both its electron donor and acceptor sides. Photon capture, excitation energy transfer, and trapping occur on a much faster time scale than the subsequent electron transfer and charge accumulation steps. A balance between excitation of PSII and the use of the absorbed energy to drive electron transport is essential. If the absorption of light energy increases and/or the sink capacity for photosynthetically derived electrons decreases, potentially deleterious side reactions may occur, including the production of reactive oxygen species. In response, a myriad of fast (second to minutes timescale) and reversible photoprotective mechanisms are observed to regulate PSII excitation when the environment changes more quickly than can be acclimated to by gene expression. This review compares the diverse photoprotective mechanisms that are used to dissipate (quench) PSII excitation within the antenna systems of higher land plants, green algae, diatoms, and cyanobacteria. The molecular bases of how PSII excitation pressure is sensed by the antenna system and how the antenna then reconfigures itself from a light harvesting to an energy dissipative mode are discussed.