Our current knowledge about the organization and function of photosystem II (PSII) is primarily based on studies of plants grown under optimal conditions. However, in plants facing environmental stress, PSII becomes affected by various stress-induced proteins, whose physiological function is not fully understood. An example of such protein is Lhcb8, a poorly characterized light-harvesting antenna with putative photoprotective function. A recent discovery of the essential role of this protein in PSII of Norway spruce (and other Pinaceae and Gnetales) indicates that Lhcb8 is probably more than a rarely expressed protein with marginal function and that it should not be overlooked anymore. The proposed project is focused on the clarification of the role of Lhcb8 in PSII function and structural organization. We want to apply a multidisciplinary approach to investigate its effect on PSII supercomplex structure and photosynthetic performance in Norway spruce and in Arabidopsis thaliana and to provide direct evidence that Lhcb8 is responsible for the unique organization of PSII in spruce.