Specific Light-harvesting Complexes Mediate Grana Stacking and Prevent Energy Spillover Between Photosystems in Plant Chloroplasts.

Land plant chloroplasts are characterized by a prominent morphological feature: the stacking of thylakoid membranes into grana, i.e. defining a continuous membranous structure surrounded by the stroma and folded into appressed grana and non-appressed domains, enclosing the thylakoid lumen. Photosystem II and its antenna system are concentrated, whereas Photosystem I and ATPase are located in the grana end membranes and, in the stroma, membranes connecting grana. Thylakoid stacking promotes the physical separation of Photosystems I and II, thereby ensuring a balanced flow of excitation energy between them. Through a genetic dissection of the light-harvesting complex subfamily Lhcb, we showed that removal of the antenna system serving Photosystem II completely abolished grana formation, whereas deletion of specific Lhcb subgroups reduced stacking by up to 40%. Lhcb5 alone was sufficient to partially restore stacking, whereas exclusive expression of Lhcb2 resulted in grana with an unusually large size, which disassembled upon illumination. Time-resolved fluorescence measurements revealed that the extent of excitation energy spillover from PSII to Photosystem I correlated with the level of Lhcb depletion, rather than with granal architecture. These findings provide genetic evidence for a functional link between Photosystem II antenna composition, lateral heterogeneity and excitation energy partition between photosystems.