The gravireaction of Ceratodon protonemata treated with gibberellic acid.

Moss protonemata exhibit negative gravitropism and the amyloplasts of the apical cell seem to play a key role in protonemal gravisensitivity. However, the mechanisms of this process are still poorly understood. Previously, we have shown that Ceratodon protonemata grown on agar-medium demonstrated greater gravicurvature than protonemata grown on medium with 11 mM glucose. In this study, we have examined whether gibberellic acid (GA), which promotes alpha-amylase expression, influences graviresponse of C. purpureus protonemata (strains WT-4 and WT-U) and how this event interacts with exogenous soluble sugars. After gravistimulation the WT-4 strain curved about twice as fast as the WT-U strain. However, responses of both strains to added substances were similar. High concentration of glucose (0.11 M) caused a decrease in protonema curvature, while the same concentration of sucrose did not significantly change the angles of curvature compared with controls. GA at 0.1 mM and higher concentrations inhibited gravitropism, and caused some apical cells to swell. The possible involvement of the carbohydrates in gravitropism is discussed.