Ionic regulation of thylakoid membrane architecture: Mg2+-driven destacking and restacking visualized

The thylakoid membrane houses the complexes involved in the light-harvesting reactions of photosynthesis. In plants, this membrane is intricately folded into cylindrical grana stacks, connected by stroma lamellae. This architecture allows for the lateral segregation of photosystem II in the grana and photosystem I in the stroma lamellae. The thylakoid ultrastructure is dynamic and can change in response to light and other environmental cues, allowing for regulation of the light-harvesting reactions. Isolated thylakoid membranes in vitro can reversibly destack and restack depending on the concentration of cations such as Mg2+. However, it is currently unknown how this destacking and restacking is possible, given the complex thylakoid architecture. Here, we combine fluorescence spectroscopy with expansion and electron microscopy to investigate the reversible Mg2+-dependent stacking of Arabidopsis thaliana thylakoids in vitro. Our data suggest that the Mg2+ concentration determines the segregation of photosystem I and photosystem II in the thylakoid membrane, regardless of prior status (stacked or destacked). Furthermore, the microscopy results show that thylakoids under fully destacked conditions still retain loose grana-like structures. The loose nature of this thylakoid architecture likely allows the intermixing of the photosystems. Furthermore, our data suggest thylakoids undergo structural reorganisations upon Mg2+-induced restacking. While complete thylakoid destacking and restacking do not occur in vivo, our results offer insights into how subtle changes in ionic conditions could influence energy distribution and protein mobility through local modulation of membrane stacking.