ABSTRACT
All oxygenic photosynthetic organisms possess a variety of mechanisms for regulation of light harvesting efficiency in response to variable light intensity. Among these, excess energy dissipation (EED) transforms excitation from light absorbed in excess into heat. This process is usually named from an operational definition: Nonphotochemical quenching (NPQ) of chlorophyll fluorescence. Heat dissipation has a photoprotective effect since it prevents the formation of reactive oxygen species (ROS), and yet it can be reversed for photosynthesis to resume when light intensity is brought back to normal intensity. Although present among all oxygenic photosynthetic organisms, NPQ is activated through distinct molecular effectors depending on the taxa. In cyanobacteria, it is operated by the orange carotenoid protein (OCP), which is directly activated by light. In unicellular eukaryotes, such as green algae and other algal groups, NPQ activity depends on a Light-Harvesting Complex (LHC)-like protein, called LHC stress-related (LHCSR). In land plants, such as Arabidopsis thaliana, NPQ depends on a related protein: Photosystem II subunit S (PSBS). LHCSR and PSBS respond to lumenal pH and are activated via protonation of acidic residues essential for activity. The mode through which protonation of the trigger proteins is translated into NPQ is the matter of lively debate. In the case of LHCSR, the protein binds both chlorophylls (Chl) and carotenoids (Cars) and its lifetime is reduced by acidification, suggesting that its interaction with Photosystem II (PSII) antenna system is likely to cause excitation energy trapping and dissipation within the LHCSR protein itself. The case of PSBS is more obscure since this polypeptide does not have pigment binding sites that would allow for direct energy dissipation, although it does have protonatable sites essential for NPQ activity. This suggests the quenching site might be induced within PSBS-interacting proteins belonging to PSII antenna system. Transduction of the pH signal into quenching has been proposed to occur through PSBS-induced reorganization of thylakoid membrane complexes. NPQ is of crucial importance for the control of productivity of crops and algae, thus the understanding of molecular mechanisms underlying this function is critical for domesticating unicellular algae for food and fuel production and further increasing crop productivity.
