The machinery that conducts the light-driven reactions of oxygenic photosynthesis is hosted within specialized paired membranes called thylakoids. In higher plants, the thylakoids are segregated into two morphological and functional domains called grana and stroma lamellae. A large fraction of the luminal volume of the granal thylakoids is occupied by the oxygen-evolving complex of photosystem II. Electron microscopy data we obtained on dark-and light-adapted Arabidopsis thylakoids indicate that the granal thylakoid lumen significantly expands in the light. Models generated for the organization of the oxygen-evolving complex within the granal lumen predict that the light-induced expansion greatly alleviates restrictions imposed on protein diffusion in this compartment in the dark. Experiments monitoring the redox kinetics of the luminal electron carrier plastocyanin support this prediction. The impact of the increase in protein mobility within the granal luminal compartment in the light on photosynthetic electron transport rates and processes associated with the repair of photodamaged photosystem II complexes is discussed.T he primary processes of photosynthesis in cyanobacteria, algae, and higher plants are carried out within flattened vesicles, called thylakoids, which host the molecular complexes that conduct the light-driven reactions of photosynthesis and provide a medium for energy transduction. In higher plants and some green algae, the thylakoids are differentiated into two distinct morphological domains: cylindrical stacked regions ranging between 300 and 600 nm in diameter, coined grana, and unstacked membrane regions that interconnect the grana called stroma lamellae. The photosynthetic protein complexes are unevenly distributed between the two domains: most of photosystem II (PSII) and the major light-harvesting antenna complex II (LHCII) are localized in the appressed regions of the grana, whereas photosystem I (PSI) and ATP synthase are confined to nonappressed membrane regions, which include the stroma lamellae and grana end membranes and margins (refs. 1-4 and references therein).The current study focuses on the thylakoid luminal compartment. This compartment forms a continuous aqueous space encased by the thylakoid membranes, which separate it from the chloroplast stroma. The major inhabitant of the thylakoid lumen in the granal membrane domains is the oxygen-evolving complex (OEC), which stabilizes the manganese catalytic center of PSII and optimizes the ionic environment for water oxidation (ref. 5 and references therein). The very high density of PSII in the granal membranes (6) implies that the space available for movement in the lumen of these thylakoids may be limited, depending on the thickness of this compartment. This impending constraint, in turn, raises the possibility that diffusion of proteins in the granal thylakoid lumen may be modulated by reversible changes in its thickness. Such changes may occur in response to alterations in the light environment. To test this possibility, we ...

Unavoidable side reactions of photosynthetic energy conversion can damage the water-splitting photosystem II (PSII) holocomplex embedded in the thylakoid membrane system inside chloroplasts. Plant survival is crucially dependent on an efficient molecular repair of damaged PSII realized by a multistep repair cycle. The PSII repair cycle requires a brisk lateral protein traffic between stacked grana thylakoids and unstacked stroma lamellae that is challenged by the tight stacking and low protein mobility in grana. We demonstrated that high light stress induced two main structural changes that work synergistically to improve the accessibility between damaged PSII in grana and its repair machinery in stroma lamellae: lateral shrinkage of grana diameter and increased protein mobility in grana thylakoids. It follows that high light stress triggers an architectural switch of the thylakoid network that is advantageous for swift protein repair. Studies of the thylakoid kinase mutant stn8 and the double mutant stn7/8 demonstrate the central role of protein phosphorylation for the structural alterations. These findings are based on the elaboration of mathematical tools for analyzing confocal laser-scanning microscopic images to study changes in the sophisticated thylakoid architecture in intact protoplasts.confocal microscopy | macromolecular crowding | photosynthesis P hotosynthetic transformation of sunlight into metabolic energy equivalents is a dangerous venture, because toxic side products of the primary photochemical processes can lead to uncontrolled damage. Harmful photosynthetic side reactions cannot be avoided completely and become a serious problem under stress (e.g., high light stress). The main target of photoinhibition (PI) is the D1 subunit of the water-splitting photosystem II (PSII) (1, 2). The D1 subunit is buried in the massive (1,400 kDa) PSII holocomplex that is organized as a dimer and binds between two and four trimeric light-harvesting complex IIs (LHCIIs) (3, 4). Estimates predict that without repair of damaged PSII, the efficiency for photosynthetic energy conversion would drop below 5% (5). Consequently, plants would not survive in a highly dynamic and competitive natural environment. Plants address this challenge through the evolutionary invention of one of the fastest and most efficient molecular repair mechanisms in nature, the PSII repair cycle (5-7).The PSII repair cycle consist of a series of events, including phosphorylation/ dephosphorylation of PSII subunits, holocomplex disassembly/reassembly, and D1 degradation/de novo synthesis (8-10). All of these reactions are harbored in or at the thylakoid membrane system inside the chloroplast. The complex folding of the thylakoid membrane leads to a structural differentiation into stacked grana regions interconnected by unstacked stroma lamellae (11)(12)(13). This structural heterogeneity is accompanied by, and partly driven by, differential protein distributions. PSII and LHCII are concentrated in grana stacks, photosystem I (PSI) and the ATPas...