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 is a plant-specific blue light photoreceptor that mediates daily light-induced responses, such as chloroplast relocation, stomatal opening, and phototropism, to optimize photosynthesis. Phototropin 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 revealed that phototropin mediates daily cold priming to promote stomatal opening and phototropism in Arabidopsis 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 phototropin proteins (phot1 and phot2), and we showed that phot2 clearly mediates cold priming of stomatal opening and phototropism. Cold priming appears to be based on phototropin-mediated thermosensing just before dawn, which plants use to optimize their light-induced responses in anticipation of dawn.