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

Abstract

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.

Keywords: Arabidopsis; STN7; long-term acclimation; photosynthesis; state transitions.

Figure 1

A model illustrating the role of STN7 in state transitions and LTR. State transitions and LTR are both triggered by the STN7 kinase, the activity of which is modulated by the redox state of the PQ pool. State transitions involve the phosphorylation of the mobile pool of LHCII (pLHCII ) and its reversible association with PSI or PSII. The LTR process involves, on the other hand, a complex regulatory pathway that can be divided into two main branches. One branch impinges upon chloroplast gene expression (‘chloroplast branch’) and is responsible for the transcriptional regulation of PSI-related genes such as the psaAB operon. The ‘Chloroplast Sensor Kinase’ (CSK), reported to regulate the transcription of psaAB operon, might be a component of the chloroplast branch. A second branch is responsible for the regulated accumulation of products of nuclear photosynthesis-related genes (nucleus-cytosol branch). These regulatory processes occur both at transcriptional and post-transcriptional levels. In the latter case, the control mechanisms might operate at the level of protein translation or degradation, or upon protein import efficiency and specificity. The reorchestration of gene expression and protein accumulation during LTR is also responsible for establishing two distinct metabolic states, as determined by the different metabolite profiles observed in wild-type plants exposed to either PSI or PSII light.