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.

在光合作用电子传输中，大型的多蛋白复合物通过小的可扩散的电子载体连接，其迁移受到大分子拥挤的挑战。对于高等植物类囊体膜，长期以来一直存在的问题是，两种移动电子载体（质体醌或质体蓝素）中的哪一种介导了堆积的类体中的电子传输，其中光系统II（PSII）定位于类体的遥远未堆叠区域中PSI。在这里，我们通过使用具有不同粒度的突变体来确认质体蓝素是远程电子载体。此外，我们的结果解释了为什么较高的植物具有较窄的颗粒直径范围，因为较大的质体蓝蛋白扩散距离会危害电子传输的效率。