Formation of a stable PSI-PSII megacomplex in rice that conducts energy spillover.

In green plants, photosystem I (PSI) and photosystem II (PSII) bind to their respective light-harvesting complexes (LHCI and LHCII) to form the PSI-LHCI supercomplex and PSII-LHCII supercomplex, respectively. These supercomplexes further form megacomplexes, like the PSI-PSII and PSII-PSII in Arabidopsis (Arabidopsis thaliana) and spinach to modulate their light-harvesting properties, but not in the green alga Chlamydomonas reinhardtii. Here, we fractionated and characterized the stable rice PSI-PSII megacomplex. The delayed fluorescence from PSI (lifetime ~ 25 ns) indicated energy transfer capabilities between the two photosystems (energy spillover) in the rice PSI-PSII megacomplex. Fluorescence lifetime analysis revealed that the slow PSII to PSI energy transfer component was more dominant in the rice PSI-PSII supercomplexes than in Arabidopsis ones, suggesting that the PSI and PSII in rice form a megacomplex not directly but through LHCII molecule(s), which was further confirmed by negatively-stained EM-analysis. Our results suggest species diversity in the formation and stability of photosystem megacomplexes, and the stable PSI-PSII supercomplex in rice may reflect its structural adaptation.