Regulation of light harvesting in multimeric and monomeric photosystem II supercomplexes

ABSTRACT An intriguing architecture called ‘semi-crystalline photosystem II (PSII) array’ has been observed in the thylakoid membranes in vascular plants. It is an array of PSII–light harvesting complex II (LHCII) supercomplexes only appears in the low-light, whose functional role has not been clarified. We identified PSII–LHCII supercomplexes in their monomeric and multimeric forms in the low-light acclimated spinach leaves and prepared them using sucrose density gradient-ultracentrifugation in the presence of amphipol A8-35. When the leaves were acclimated to high-light, however, only monomeric forms were present. Single particle electron microscopy identified that the multimeric PSII–LHCII supercomplexes were composed of two (‘megacomplex’) or three (‘arraycomplex’) units of PSII–LHCII supercomplexes, which aligned like a fraction of the semi-crystalline array. Further characterization using fluorescence analysis revealed that multimeric forms have a higher light-harvesting capability, but a lower thermal dissipation capability than the monomeric form, suggesting such a configurational conversion of PSII–LHCII supercomplexes possibly serves as a structural basis for the plants’ acclimation to environmental light.