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 the PSII-LHCII supercomplex, respectively. These supercomplexes further form megacomplexes, like 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 PSI and PSII in rice form a megacomplex not directly but through LHCII molecule(s), which was further confirmed by the negatively stained electron microscopy 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.