A Role for a Light-Harvesting Antenna Complex of Photosystem II in Photoprotection

High light (beyond what is needed for maximum photosynthesis) is a major plant stress. Under extreme high-light conditions, the photosynthetic apparatus can be damaged irreversibly, but plants and algae have devised various strategies to protect themselves (photoprotection) (Björkman and Demmig-Adams, 1994). One of the strategies for survival in high light is to eliminate the excess absorbed energy as heat (thermal dissipation), which can be measured as nonphotochemical quenching (NPQ) of chlorophyll fluorescence. The process of NPQ involves acidification of the lumen of the photosynthetic membranes, operation of the xanthophyll cycle, and specific components of the antenna of photosystem II (PSII). In this issue of The Plant Cell , Elrad et al. (pages 1801–1816) characterize a mutant of Chlamydomonas reinhardtii , designated npq5 , that is defective in its ability to establish rapid, reversible NPQ of chlorophyll a fluorescence. This mutant lacks a major light-harvesting polypeptide (Lhcbm1), suggesting that Lhcbm1 is required for the cells to elicit NPQ. The relationship between specific antenna complexes and NPQ has been, and still is, hotly debated. The article by Elrad et al. (2002), which shows a strong correlation between a protein of the trimeric complexes of lightharvesting complex IIb (LHCIIb) and NPQ, adds to this excitement.