Regulation of light harvesting in multimeric and monomeric photosystem II supercomplexes

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.