Characterization of 2D PSII Crystalline Arrays in Thylakoid Membranes Highly Efficient in Photo-Protective Energy Dissipation by Atomic Force Microscopy

Non-photochemical quenching (NPQ) is an important mechanism by which photosynthetic organisms balance the light energy necessary for carbon fixation while preventing photo-damage. NPQ requires elaborate dynamic reorganization of the photosystem II (PSII) supercomplex and its closely associated light-harvesting complex II (LHCII). Currently, structural details directly related to NPQ and PSII-LHCII arrangements are scarce and in debate. The suppressor-of-quenching protein SOQ1 has recently been shown to prevent the formation of a slowly relaxing quenching pathway when chloroplasts are exposed to high light. We have used atomic force microscopy (AFM) to characterize the PSII organization in grana thylakoid membranes isolated from Arabidopsis thaliana wild type (wt) and soq1 mutant plants acclimated to different illumination conditions. To analyze the AFM data, we developed sensitive algorithms that automatically detect and discriminate between different types of crystalline packing observed in the membranes. We find that PSII density, extent of PSII array formation, and crystalline packing type depend strongly on illumination and mutant status. Surprisingly, pair correlation function and nearest-neighbor distribution analyses indicate that high-light-acclimated crystalline arrays from wt membranes are similar to those found in low-light-acclimated membranes from soq1 membranes. These different PSII arrangements can be correlated with the high levels of NPQ characteristic of the mutation. A potentially new type of 2D crystalline packing present in the low-light-acclimated soq1 membranes could be associated with slow relaxation of NPQ.