Photosystem II Supercomplex Remodeling Serves as an Entry Mechanism for State Transitions in Arabidopsis

Within dense plant populations, strong light quality gradients cause unbalanced excitation of the two photosystems resulting in reduced photosynthetic efficiency. Plants redirect such imbalances by structural rearrangements of the photosynthetic apparatus via state transitions and photosystem stoichiometry adjustments. However, less is known about the function of photosystem II (PSII) supercomplexes in this context. Here, we show in Arabidopsis thaliana that PSII supercomplex remodeling precedes and facilitates state transitions. Intriguingly, the remodeling occurs in the short term, paralleling state transitions, but is also present in a state transition-deficient mutant, indicating that PSII supercomplex generation is independently regulated and does not require light-harvesting complex phosphorylation and movement. Instead, PSII supercomplex remodeling involves reversible phosphorylation of PSII core subunits (preferentially of CP43) and requires the luminal PSII subunit Psb27 for general formation and structural stabilization. Arabidopsis knockout mutants lacking Psb27 display highly accelerated state transitions, indicating that release of PSII supercomplexes is required for phosphorylation and subsequent movement of the antenna. Downregulation of PSII supercomplex number by physiological light treatments also results in acceleration of state transitions confirming the genetic analyses. Thus, supercomplex remodeling is a prerequisite and an important kinetic determinant of state transitions.