Figure 1.

Mechanism of state transitions in plants. (A) The plant photosynthetic thylakoid membrane is divided into stacked grana and unstacked stromal lamellae domains. PSII and LHCII are concentrated mostly in the grana, whereas PSI and ATP synthase are exclusively found in the stromal lamellae. cytb₆f is found in both domains. (B) When light conditions favor PSII excitation (see text), the plastoquinone electron carrier pool becomes reduced (PQH₂). This activates the kinase STN7, which phosphorylates LHCII. (C) Phosphorylation of LHCII (forming LHCII-P) weakens the lateral and stacking interactions that sustain the grana structure and leads to increased association between PSI and LHCII. (D) Association of LHCII-P with PSI rebalances the relative excitation rates of PSI and PSII, allowing reoxidation of the plastoquinone pool (to PQ). The state transition is reversed by the phosphatase TAP38, which dephosphorylates LHCII when PSI excitation is favored. To see this figure in color, go online.