Phototropin can trigger the chloroplast accumulation response from multiple subcellular locations in Marchantia...

Chloroplasts relocate within plant cells in response to various environmental stimuli. They accumulate in areas irradiated with low light intensity to maximize capture of light energy for photosynthesis (accumulation response) and move away from areas receiving high light intensity to limit photodamage (avoidance response). These chloroplast movements are mediated by a blue-light receptor, phototropin, in a wide range of plant species. Phototropin localizes to multiple subcellular compartments, namely the plasma membrane, cytosol, and chloroplast outer membrane. However, which subcellular phototropin pool mediates chloroplast relocation remains to be determined. In our previous study, we genetically modified phototropin to target the photoreceptor to the plasma membrane (pm-phot) or the cytosol (cy-phot) in a phototropin-knockout mutant of the liverwort Marchantia polymorpha. Here, we generated a phototropin variant targeted to the chloroplast outer membrane (cp-phot) by adding a targeting peptide from Arabidopsis thaliana to the N-terminus of cy-phot. Comparisons among transgenic M. polymorpha lines expressing pm-PHOT, cy-PHOT, or cp-PHOT in a phototropin-knockout background revealed that only pm-phot mediates the avoidance response, whereas all localization variants mediate the accumulation response. We quantified the accumulation response by measuring chloroplast behavior and signal transfer rates in the transgenic lines, determining that these processes were comparable in all cells. This finding suggests that phototropin in M. polymorpha exhibits spatial redundancy, with the phototropin pools at the plasma membrane, cytosol, and chloroplast outer membrane all being capable of triggering the accumulation response.