Optimizing photosynthesis under fluctuating light The role of the Arabidopsis STN7 kinase

Optimal photosynthetic performance requires that equal amounts of light are absorbed by photosystem II (PSII) and photosystem I (PSI), which are functionally linked through the photosynthetic electron transport chain. However, photosynthetic organisms must cope with light conditions that lead to the preferential stimulation of one or the other of the photosystems. Plants react to such imbalances by mounting acclimation responses that redistribute excitation energy between photosystems and restore the photosynthetic redox poise. In the short term, this involves the so-called state transition process, which, over periods of minutes, alters the antennal cross-sections of the photosystems through the reversible association of a mobile fraction of light-harvesting complex II (LHCII) with PSI or PSII. Longer-lasting changes in light quality initiate a long-term response (LTR), occurring on a timescale of hours to days, that redresses imbalances in excitation energy by changing the relative amounts of the two photosystems. Despite the differences in their timescales of action, state transitions and LTR are both triggered by the redox state of the plastoquinone (PQ) pool, via the activation of the thylakoid kinase STN7, which appears to act as a common redox sensor and/or signal transducer for both responses. This review highlights recent findings concerning the role of STN7 in coordinating short-and long-term photosynthetic acclimation responses.