Fig. 4.

Diagram of the mechanism of state transitions in plants and algae. In the diagram, the system is shown to be initially in ‘state 1’, with the absorption cross section (CS) of photosystem (PS) II being larger than that of PSI (it will have high Chl fluorescence yield because Chl in PSII is much more fluorescent than in PSI). During illumination, the plastoquinone (PQ) pool will be reduced by PSII because of higher absorption there. This is sensed by the Cyt b₆/f (via its PQH₂-oxidizing site, Q_p), and leads to activation of a kinase (Stt7/STN7) and phosphorylation of the mobile light harvesting complexes of PSII (LHCII), which then associate with the PSI antenna. The reverse occurs when the system is in ‘state 2’ initially, with the absorption CS of PSI being larger than that of PSII. Here, oxidation of the PQ pool by PSI during illumination will be sensed by the Cyt b₆/f, which leads to the inactivation of kinases, followed by de-phosphorylation of the mobile LHCIIs (by the phosphatases Pph1/TAB38) and their relocation to PSII. Abbreviations: A₀ and A₁, a particular Chl a molecule and a vitamin K1 molecule, respectively; Fe-S, three non-haem (heme) iron–sulfur centres; Fd, ferredoxin; Q_A and Q_B, plastoquinone electron acceptors of PSII; NADP⁺ and NADPH, nicotinamide adenine dinucleotide phosphate in oxidized and reduced state; P680 and P700, reaction centre chlorophylls/primary electron donors of PSII and PSI; PC, plastocyanin. Figure modified from Allen (2003) and Rochaix (2014).