Dynamic Mechanical Responses of Arabidopsis Thylakoid Membranes during PSII-Specific Illumination

Abstract

Remodeling of thylakoid membranes in response to illumination is an important process for the regulation of photosynthesis. We investigated the thylakoid network from individual chloroplasts of Arabidopsis thaliana using atomic force microscopy to capture dynamic changes in height, elasticity, and viscosity of thylakoid membranes caused by changes in illumination. We also correlated the mechanical response of the chloroplast with membrane ultrastructure using electron microscopy. We find that the elasticity of the thylakoid membranes increases immediately upon PSII-specific illumination, followed by a delayed height change. While the change in stiffness depends primarily on the transmembrane pH gradient, the height change requires both a pH gradient, and the STN7-kinase-dependent phosphorylation of LHCII. Direct visualization by electron microscopy and image analysis further indicate that there is a significant change in the packing repeat distance of the membrane stacks. We propose that the stiffness change is due to a pH-dependent lumen expansion, while the height change may additionally require the disruption of stromal interactions between membranes by phosphorylation. Our studies indicate that lumen expansion in response to illumination is not simply a result of the influx of water, and we propose a model in which protein interactions within the lumen drive these changes.