The PSI-H subunit of photosystem I is essential for state transitions in plant photosynthesis

Photosynthesis in plants involves two photosystems responsible for converti ng light energy into redox processes. The photosystems, PSI and PSII, opera te largely in series, and therefore their excitation must be balanced in or der to optimize photosynthetic performance(1). When plants are exposed to i llumination favouring either PSII or PSI they can redistribute excitation t owards the light-limited photosystem. Long-term changes in illumination lea d to changes in photosystem stoichiometry(2,3). In contrast, state transiti on is a dynamic mechanism that enables plants to respond rapidly to changes in illumination. When PSII is favoured (state 2), the redox conditions in the thylakoids change and result in activation of a protein kinase(4-6). Th e kinase phosphorylates the main light-harvesting complex (LHCII) and the m obile antenna complex is detached from PSII. It has not been clear if attac hment of LHCII to PSI in state 2 is important in state transitions. Here we show that in the absence of a specific PSI subunit, PSI-H, LHCII cannot tr ansfer energy to PSI, and state transitions are impaired.