In photosynthetic electron transport, large multiprotein complexes are connected by small diffusible electron carriers, the mobility of which is challenged by macromolecular crowding. For thylakoid membranes of higher plants, a long-standing question has been which of the two mobile electron carriers, plastoquinone or plastocyanin, mediates electron transport from stacked grana thylakoids where photosystem II (PSII) is localized to distant unstacked regions of the thylakoids that harbor PSI. Here, we confirm that plastocyanin is the long-range electron carrier by employing mutants with different grana diameters. Furthermore, our results explain why higher plants have a narrow range of grana diameters since a larger diffusion distance for plastocyanin would jeopardize the efficiency of electron transport. In the light of recent findings that the lumen of thylakoids, which forms the diffusion space of plastocyanin, undergoes dynamic swelling/shrinkage, this study demonstrates that plastocyanin diffusion is a crucial regulatory element of plant photosynthetic electron transport.

Copenhagen Plant Science Centre Department of Plant and Environmental Sciences University of Copenhagen DK 1871 Frederiksberg C Copenhagen Denmark

Institute of Biological Chemistry Washington State University Pullman WA 99164 6340

Institute of Biological Chemistry Washington State University Pullman WA 99164 6340;

Institute of Plant Molecular Biology Biology Centre of the Czech Academy of Sciences 37005 České Budějovice Czech Republic

Plant Molecular Biology Department Biology 1 Ludwig Maximilians Universität München 81252 Planegg Martinsried Germany

Plant Molecular Biology Department Biology 1 Ludwig Maximilians Universität München 81252 Planegg Martinsried Germany;

School of Biological Sciences Washington State University Pullman WA 99164 4236

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