Levels of photoactivated phototropin modulate signal transmission during the chloroplast accumulation response.

Chloroplasts accumulate in regions of plant cells exposed to irradiation to maximize light reception for efficient photosynthesis. This response is mediated by the blue-light receptor phototropin. Upon the perception of blue light, phototropin is photoactivated, an unknown signal is transmitted from the photoactivated phototropin to distant chloroplasts, and the chloroplasts begin their directional movement. How activated phototropin initiates this signal transmission is unknown. Here, using the liverwort Marchantia polymorpha, we analysed whether increased photoactive phototropin levels mediate signal transmission and chloroplast behaviour during the accumulation response. The signal transmission rate was higher in transgenic cells overexpressing phototropin than in wild-type cells. However, the chloroplast directional movement was similar between wild-type and transgenic cells. Consistent with the observation, increasing the amount of photoactivated phototropin through higher blue-light intensity also accelerated signal transmission but did not affect chloroplast behaviour in wild-type cells. Photoactivation of phototropin under weak blue-light led to the greater protein level of phosphorylated phototropin in cells overexpressing phototropin than in wild-type cells, whereas the autophosphorylation level within each phototropin molecule was similar. These results indicate that the abundance of photoactivated phototropin modulates the signal transmission rate to distant chloroplasts but does not affect chloroplast behaviour during the accumulation response.