Photoprotective Energy Dissipation Involves the Reorganization of Photosystem II Light-Harvesting Complexes in the Grana Membranes of Spinach Chloroplasts

TLDR

Plants must regulate absorbed light energy to maximize photosynthetic efficiency and protect PSII from photooxidative damage by dissipating excess energy as heat in the light‑harvesting antenna complexes. The study investigates the structural basis of light‑harvesting regulation in intact spinach chloroplasts. The authors employ freeze‑fracture electron microscopy together with laser confocal microscopy and fluorescence recovery after photobleaching to examine membrane reorganization. The photoprotective state is achieved by rapid, reversible reorganization of the thylakoid membrane, with LHCII dissociating from PSII and aggregating, reducing LHC antenna mobility, and this process occurs within 5 min, depends on ΔpH, and is amplified by violaxanthin deepoxidation to zeaxanthin.

Abstract

Abstract Plants must regulate their use of absorbed light energy on a minute-by-minute basis to maximize the efficiency of photosynthesis and to protect photosystem II (PSII) reaction centers from photooxidative damage. The regulation of light harvesting involves the photoprotective dissipation of excess absorbed light energy in the light-harvesting antenna complexes (LHCs) as heat. Here, we report an investigation into the structural basis of light-harvesting regulation in intact spinach (Spinacia oleracea) chloroplasts using freeze-fracture electron microscopy, combined with laser confocal microscopy employing the fluorescence recovery after photobleaching technique. The results demonstrate that formation of the photoprotective state requires a structural reorganization of the photosynthetic membrane involving dissociation of LHCII from PSII and its aggregation. The structural changes are manifested by a reduced mobility of LHC antenna chlorophyll proteins. It is demonstrated that these changes occur rapidly and reversibly within 5 min of illumination and dark relaxation, are dependent on ΔpH, and are enhanced by the deepoxidation of violaxanthin to zeaxanthin.