During State 1 to State 2 Transition in Arabidopsis thaliana, the Photosystem II Supercomplex Gets Phosphorylated but Does Not Disassemble

Background: State transition balances the excitation pressure between the two photosystems of plants. Results: The organization of photosystem II supercomplexes and megacomplexes is the same in state 1 and state 2. Conclusion: Phosphorylation is not sufficient to induce the disassembly of the supercomplexes. Significance: This work helps to understand how plants optimize light harvesting under ever changing light conditions. Plants are exposed to continuous changes in light quality and quantity that challenge the performance of the photosynthetic apparatus and have evolved a series of mechanisms to face this challenge. In this work, we have studied state transitions, the process that redistributes the excitation pressure between photosystems I and II (PSI/PSII) by the reversible association of LHCII, the major antenna complex of higher plants, with either one of them upon phosphorylation/dephosphorylation. By combining biochemical analysis and electron microscopy, we have studied the effect of state transitions on the composition and organization of photosystem II in Arabidopsis thaliana. Two LHCII trimers (called trimers M and S) are part of the PSII supercomplex, whereas up to two more are loosely associated with PSII in state 1 in higher plants (called extra trimers). Here, we show that the LHCII from the extra pool migrates to PSI in state 2, thus leaving the PSII supercomplex and the semicrystalline PSII arrays intact. In state 2, not only is the mobile LHCII phosphorylated, but also the LHCII in the PSII supercomplexes. This demonstrates that PSII phosphorylation is not sufficient for disconnecting LHCII trimers S and M from PSII and for their migration to PSI.