The cold-induced switch in direction of chloroplast relocation occurs independently of changes in endogenous phototropin levels.

When exposed to fluctuating light intensity, chloroplasts move towards weak light (accumulation response), and away from strong light (avoidance response). In addition, cold treatment (5°C) induces the avoidance response even under weak-light conditions (cold-avoidance response). These three responses are mediated by the phototropin (phot), which is a blue-light photoreceptor and has also been reported to act as a thermosensory protein that perceives temperature variation. Our previous report indicated that cold-induced changes in phot biochemical activity initiate the cold-avoidance response. In this study, we further explored the induction mechanism of the cold-avoidance response in the liverwort Marchantia polymorpha and examined the relationship between changes in the amount of phot and the induction of the cold-avoidance response. The switch between the accumulation and avoidance responses occurs at a so-called 'transitional' light intensity. Our physiological experiments revealed that a cold-mediated decrease in the transitional light intensity leads to the induction of the cold-avoidance response. While artificial overexpression of phot decreased the transitional light intensity as much as cold treatment did, the amount of endogenous phot was not increased by cold treatment in wild-type M. polymorpha. Taken together, these findings show that the cold-avoidance response is initiated by a cold-mediated reduction of the transitional light intensity, independent of the amount of endogenous phot. This study provides a clue to understanding the mechanism underlying the switch in direction of chloroplast relocation in response to light and temperature.

Phototropin perceives temperature based on the lifetime of its photoactivated state.

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.

Chloroplast aggregation during the cold-positioning response in the liverwort Marchantia polymorpha.

Under low-light conditions, chloroplasts localize along periclinal cell walls at temperatures near 20 °C, but they localize along anticlinal cell walls near 5 °C. This phenomenon is known as the cold-positioning response. We previously showed that chloroplasts move as aggregates rather than individually during the cold-positioning response in the fern Adiantum capillus-veneris. This observation suggested that chloroplasts physically interact with each other during the cold-positioning response. However, the physiological processes underlying chloroplast aggregation are unclear. In this report, we characterized chloroplast aggregation during the cold-positioning response in the liverwort Marchantia polymorpha. Confocal laser microscopy observations of transgenic liverwort plants expressing a fluorescent fusion protein that localizes to the chloroplast outer envelope membrane (OEP7-Citrine) showed that neighboring chloroplast membranes did not fuse during the cold-positioning response. Transmission electron microscopy analysis revealed that a distance of at least 10 nm was maintained between neighboring chloroplasts during aggregation. These results indicate that aggregated chloroplasts do not fuse, but maintain a distance of at least 10 nm from each other during the cold-positioning response.

Cold-induced organelle relocation in the liverwort Marchantia polymorpha L.

Organelles change their subcellular positions in response to various environmental conditions. Recently, we reported that cold treatments alter the intracellular position of chloroplasts and nuclei (cold positioning) in the fern Adiantum capillus-veneris; chloroplasts and nuclei localized to the periclinal cell wall relocated to anticlinal cell wall after cold treatments. To further understand organelle positioning under cold conditions, we studied cold-induced organelle relocation in the liverwort Marchantia polymorpha L. When sporelings and gemmmalings were treated under low temperature (5 °C), chloroplast cold positioning response was successfully induced both in the sporelings and the gemmmalings of M. polymorpha. Using a genetic transformation, nuclei, mitochondria or peroxisomes were visualized with a fluorescent protein, and the transgenic gemmmalings were incubated under the cold condition. Nuclei and peroxisomes, but not mitochondria, clearly relocated from the periclinal cell wall to the anticlinal cell wall after cold treatments. Our findings suggest that several organelles concurrently change their positions in the liverwort cell to cope with cold temperature.