CO 2-induced chloroplast movement was reported in the
monograph by Gustav Senn in 1908: unilateral CO 2 supply
to the one cell-layered moss leaves induced the positively CO
2-tactic periclinal arrangement of chloroplasts.
However, from the modern criteria, several experimental settings are
unacceptable. Here, using a model moss plant Physcomitrium
patens, we examined basic features of chloroplast CO
2-tactic relocation with a modernized experimental
system. The CO 2 relocation was light-dependent and
especially the CO 2 relocation in red light was
substantially dependent on photosynthetic activity. Between the
cytoskeletons responsible for chloroplast movement of P. patens,
the microfilament mainly worked for CO 2 relocation, but
the microtubule-based movement was insensitive to CO 2.
The CO 2 relocation was induced not only by air with and
without CO 2 but also by the more realistic difference
in CO 2 concentration between the two sides. In the
leaves placed on the surface of a gel sheet, chloroplasts avoided the
gel side and positioned in the air facing surface. This was also shown
to be photosynthesis dependent. Based on these observations, we propose
a working hypothesis that the threshold light intensity between the
light-accumulation and -avoidance responses of the photorelocation would
be increased by CO 2, resulting in the CO
2-tactic relocation of chloroplast.