High-light vs. low-light: Effect of light acclimation on photosystem II composition and organization in Arabidopsis thaliana

The structural response of photosystem II (PSII) and its light-harvesting proteins (LHCII) in Arabidopis thaliana after long-term acclimation to either high or low light intensity was characterized. Biochemical and structural analysis of isolated thylakoid membranes by electron microscopy indicates a distinctly different response at the level of PSII and LHCII upon plant acclimation. In high light acclimated plants, the C2S2M2 supercomplex, which is the dominating form of PSII in Arabidopsis, is a major target of structural re-arrangement due to the down-regulation of Lhcb3 and Lhcb6 antenna proteins. The PSII ability to form semi-crystalline arrays in the grana membrane is strongly reduced compared to plants grown under optimal light conditions. This is due to the structural heterogeneity of PSII supercomplexes rather than to the action of PsbS protein as its level was unexpectedly reduced in high light acclimated plants. In lowlight acclimated plants, the architecture of the C2S2M2 supercomplex and its ability to formsemi-crystalline arrays remained unaffected but the density of PSII in grana membranes is reduced due to the synthesis of additional LHCII proteins. However, the C2S2M2 supercomplexes in semi-crystalline arrays are more densely packed, which can be important for efficient energy transfer between PSII under light limiting conditions.