Phototropin 2 mediates daily cold priming to promote light responses in Arabidopsis.

Organisms adapt to predictable environmental changes via a biological mechanism called priming. Phototropin (phot) is a plant-specific blue light photoreceptor that mediates daily light-induced responses, such as chloroplast relocation, stomatal opening, and phototropism, to optimize photosynthesis. Phot also functions as a thermosensor for chloroplast relocation that may sense daily temperature decreases at night, thereby modulating light-induced responses at dawn; however, this hypothesis has not yet been fully explored. Here, we reveal that phot mediates daily cold priming to promote stomatal opening and phototropism in Arabidopsis (Arabidopsis thaliana) under dawn-mimicking conditions. A cold pretreatment in the dark enhanced subsequent blue light-induced stomatal opening and phototropism at normal temperatures, suggesting that daily cold priming is involved in these physiological responses. Arabidopsis has two phot proteins (phot1 and phot2), and we show that phot2 clearly mediates the cold priming of stomatal opening and phototropism. Cold priming appears to be based on phot-mediated thermosensing just before dawn, which plants use to optimize their light-induced responses in anticipation of dawn.