Action spectrum of the redox state of the plastoquinone pool defines its function in plant acclimation

SUMMARY The plastoquinone (PQ) pool mediates electron flow and regulates photoacclimation in plants. Here we report the action spectrum of the redox state of the PQ pool in Arabidopsis thaliana, showing that 470–500, 560 or 650–660 nm light favors Photosystem II (PSII) and reduces the PQ pool, whereas 420–440, 520 or 690 nm light favors Photosystem I (PSI) and oxidizes PQ. These data were used to construct a model predicting the redox state of PQ from the spectrum of any polychromatic light source. Moderate reduction of the PQ pool induced transition to light state 2, whereas state 1 required highly oxidized PQ. In low‐intensity PSI light, PQ was more oxidized than in darkness and became gradually reduced with light intensity, while weak PSII light strongly reduced PQ. Natural sunlight was found to favor PSI, which enables plants to use the redox state of the PQ pool as a measure of light intensity. Significance Statement The redox state of the chloroplast plastoquinone (PQ) pool regulates state transitions and gene expression in response to illumination conditions. We show how wavelengths and intensity of photosynthetically active light determine the redox state of the PQ pool in such a way that plants can measure the intensity of sunlight using the PQ pool as a sensor.