The Use of Mosses for the Study of Cell Polarity

Moss development allows both the study of the development of cell polarity and the modification of polarity in an already-polar cell. The gametophore allows polarity studies to be extended to multicellular structures. Both spore germination and protoplast regeneration allow the study of the generation of a polar cell axis. The polarity of the axis of regenerating protoplasts of Ceratodon purpureus is influenced by light direction. The programming of the response is however complex. There is a delay before a response to a changed light direction is observed, indicating that axis polarity is fixed before asymmetrical development can be observed. However, the length of the delay is influenced by the state of the cell at the time the light direction is changed. When protoplasts regenerating in red light at 25°, are reoriented with respect to the light direction, there is a lag of about 9 hours before a response is observed. If protoplast are irradiated with farred light immediately before reorientation, the lag is shorter, indicating that protoplasts use phytochrome to “memorize” light direction, preventing a precipitous response to temporarily changed conditions. Protonemal apical cells show tropic responses to both light and gravity. Mutant studies show that the phototropic response is mediated by phytochrome, and that this photoreceptor also turns off the gravitropic response in light. Mutants with a reversal of the orientation of their gravitropic response, have been isolated in both Physcomitrella patens and C. purpureus, and it is possible that in the latter species, a single gene can mutate either to prevent the gravitropic response or to reverse its orientation.