CO2‐induced chloroplast movement in one cell‐layer moss leaves.

CO2‐induced chloroplast movement was reported in the monograph by Gustav Senn in 1908: unilateral CO2 supply to the one cell‐layered moss leaves induced the positively CO2‐tactic periclinal arrangement of chloroplasts. Here, using the model moss plant Physcomitrium patens, we examined basic features of chloroplast CO2‐tactic relocation with a modernized experimental system. The CO2 relocation was light‐dependent and, especially, CO2 relocation in red light was substantially dependent on photosynthetic activity. In blue light, CO2 relocation was mainly dependent on microfilaments while microtubule‐based movement was insensitive to CO2, whereas in red light, both cytoskeletons contributed redundantly to CO2 relocation. The CO2 relocation was observed not only when the two leaf surfaces were exposed to CO2‐free air versus CO2‐containing air, but also by exposing them physiologically relevant differences in CO2 concentrations. In the leaves placed on the surface of a gel sheet, chloroplasts avoided the gel side and positioned in the air‐facing surface, and this phenomenon was also shown to be photosynthesis dependent. Based on these observations, we propose a hypothesis that the threshold light intensity between the light‐accumulation and ‐avoidance responses of the photorelocation would be increased by CO2, resulting in the CO2‐tactic relocation of chloroplasts. Summary Statement: Chloroplasts in one cell‐layered leaf of Physcomitrium patens showed CO2‐tactic relocation in blue and red light. This relocation was based on microfilaments in blue light, while based on microfilaments and microtubules in red light. This relocation was partly photosynthesis dependent. [ABSTRACT FROM AUTHOR]