Phototropin monitors actual temperature, not temperature difference, to regulate temperature-dependent chloroplast...

In liverworts, phototropin senses the actual temperature rather than temperature differences and switches from cis- to trans-autophosphorylation to trigger the cold-avoidance response of chloroplast movement. Blue-light (BL)-induced chloroplast movement in plant cells is temperature-dependent. At standard growth temperatures, chloroplasts move toward weak BL-irradiated regions (accumulation response), maximizing photoreception, whereas at lower temperatures they move away from the irradiated area (cold-avoidance response), reducing photodamage. This temperature-dependent switch in the chloroplast response is mediated by phototropin (phot), a BL receptor and thermosensor, which contains a kinase domain and undergoes cis- and trans-autophosphorylation in response to BL and temperature. Under weak BL conditions, phot autophosphorylates in cis at standard growth temperatures and in both cis and trans at lower temperatures. However, it remains unclear whether phot senses actual temperatures or relative temperature changes to regulate chloroplast movement via autophosphorylation. In this study, we analyzed phot-mediated chloroplast movement in the liverwort Marchantia polymorpha under varying temperature conditions. We determined that chloroplast movement responds to actual temperatures rather than temperature differences and confirmed that phot is responsible for sensing actual temperatures in planta. Phot continuously monitors the actual temperature and increases its autophosphorylation levels as temperature decreases. The threshold temperature for the transition between the accumulation response and the cold-avoidance response corresponds to that for the switch from cis- to trans-autophosphorylation of phot. Our findings reveal that phot serves as an actual temperature sensor in planta to regulate chloroplast movement through autophosphorylation mode switching.