Living organisms detect changes in temperature using thermosensory molecules. However, these molecules and/or their mechanisms for sensing temperature differ among organisms. To identify thermosensory molecules in plants, we investigated chloroplast positioning in response to temperature changes and identified a blue-light photoreceptor, phototropin, that is an essential regulator of chloroplast positioning. Based on the biochemical properties of phototropin during the cellular response to light and temperature changes, we found that phototropin perceives temperature based on the temperature-dependent lifetime of the photoactivated chromophore. Our findings indicate that phototropin perceives both blue light and temperature and uses this information to arrange the chloroplasts for optimal photosynthesis. Because the photoactivated chromophore of many photoreceptors has a temperature-dependent lifetime, a similar temperature-sensing mechanism likely exists in other organisms. Thus, photoreceptors may have the potential to function as thermoreceptors.iving organisms perceive temperature using thermosensory molecules. Various types of thermosensory molecules have been identified in microorganisms, animals, and plants, and these molecules perceive temperature via temperature-dependent intraand intermolecular interactions (1-3). However, these thermosensory molecules and/or their mechanisms for temperature sensing differ among organisms (1).As photosynthetic organelles, chloroplasts change their intracellular position in response to light and temperature (4-7). For example, under low light conditions at 22°C, chloroplasts accumulate at the cell surface (along the periclinal cell wall) to maximize photosynthetic efficiency, in a phenomenon termed the accumulation response (4, 6). However, under the same light conditions, but at a temperature of 5°C, the chloroplasts move to the cell periphery (along the anticlinal cell wall), possibly to avoid the light, in a phenomenon termed the cold-avoidance response or the cold-positioning response (5, 6). Because the cold-avoidance response is temperature dependent (5), the response is likely controlled by a thermosensory molecule. In a previous study, we found that the blue-light (BL) photoreceptor phototropin (phot) is responsible for the cold-avoidance response in the fern Adiantum capillus-veneris, which has three types of phot molecules (phot1, phot2, and neochrome1) (5,8). In this fern, phot2 mediates the cold-avoidance response (5). Therefore, in the present study, we hypothesized that phot is involved in temperature perception and we further analyzed the cold-avoidance response in the liverwort Marchantia polymorpha, which has only a single copy of the PHOT gene (MpPHOT) (9).
Results and DiscussionFirst, we examined whether MpPHOT is essential for the coldavoidance response in M. polymorpha. When the wild type (WT), the knockout mutant (Mpphot KO ), and the complementation line (Mpphot/Mpphot KO ) (9) (Fig. S1) were treated with low white light at 5°C for 24 h to induce ...