Fig. 4.

Schematic diagram of events taking place in the grana and stroma thylakoids under excessive illumination. This diagram shows a top view of a thylakoid (light blue), which has a rounded shape and is stacked with other thylakoids to form the grana. Here, two grana-forming thylakoid domains are depicted (left, dark control; right, illuminated with high light), which are interconnected by a stroma thylakoid. PSII core complexes (blue) are surrounded by LHCII complexes (green). When the thylakoids are illuminated with high light, the PSII/LHCII complexes are mobilized, probably by phosphorylation, and LHCII complexes form reversible aggregates to dissipate the excess light energy as heat. Simultaneously, free spaces are generated in the grana and the damaged PSII complexes move to the grana margin regions to react with proteases such as FtsH (orange) using these free spaces. However, when the light intensity is extremely high, ¹O₂ molecules are produced in PSII/LHCII complexes through photochemical reaction or in the lipid matrix through lipid peroxidation, and the reactive oxygen species damage the proteins and lipids to form irreversible aggregates. MDA may also participate in this process. These protein and lipid aggregates hinder the movement of PSII/LHCII complexes on the membranes, and the function of the thylakoids may finally deteriorate.