The photo-thermochemical properties and functions of Marchantia phototropin encoded by an unduplicated gene in land plant evolution.

Phototropin (phot) is a blue light photoreceptor in plants and possesses two photosensory light­oxygen-voltage (LOV1 and LOV2) domains with different photo-thermochemical properties. While liverworts contain a single copy of PHOT (e.g., MpPHOT in Marchantia polymorpha), many land plant species contain multicopy PHOT genes (e.g., AtPHOT1 and 2 in Arabidopsis thaliana) due to evolutionary gene duplication. The LOV domains of duplicated phot proteins have been studied in detail, but those of single-copy phot proteins remain to be characterized. As phot has not been duplicated in liverworts, we hypothesized that Mpphot may retain the ancestral function and photo-thermochemical properties. To learn more about the unduplicated phot proteins, we analyzed chloroplast relocation movement and the photo-thermochemical properties of LOV1 and LOV2 in Mpphot (Mpphot-LOV1 and Mpphot-LOV2, respectively). The function of Mpphot-LOV1, which induced a response to move chloroplasts to weak light (the accumulation response) in the absence of photoactive LOV2, differed from that of LOV1 of the duplicated phot proteins of A. thaliana (e.g., Atphot1-LOV1 preventing the accumulation response). On the other hand, the function of Mpphot-LOV2 was similar to that of LOV2 of the duplicated phots. The photo-thermochemical properties of Mpphot were a hybrid of those of the duplicated phots; the photochemical and thermochemical reactions of Mpphot were similar to those of the phot2- and phot1-type proteins, respectively. Our findings reveal conservation and diversification among LOV domains during phot duplication events in land plant evolution.

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.