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.