Behavior of amyloplasts in photo- and gravitropism of the moss protonema.

The protonema of mosses Ceratodon purpureus and Pottia intermedia is negatively gravitropic in darkness and grows on the substrate surface under illumination. However, the putative mechanisms of these growth responses are not well understood so far. For gravitropism, sedimentation of amyloplasts has been widely assumed to be the first step of the signal transduction chain. This model was supported by numerous observations where amyloplasts' number or size in a protonema apical cell correlated with its gravisensitivity. Unlike multicellular graviperceptive organs, a protonema apical cell is the same site for both of gravity and light perception and realization of growth movements. In addition, red light is known to change the cell responses to gravity. Therefore, we analysed the influence of red light on the events associated with graviperception and growth movements of protonema apical cells, namely: plastid behavior, size, and number, starch content, chlorophyll fluorescence intensity and alpha-amylase activity, under gravistimulation of dark-grown protonema.