XVI International Botanical Congess

In protonemata of the moss Ceratodon purpureus, the phototropic response is mediated by phytochrome. The aphototropic mutant ptr116 which lacks the phytochrome chromophore phytochromobilin (PFB) was used as a target for microinjection. Expression plasmids that encode for a soluble rat heme oxygenase or the plastid-targeted heme oxygenase of Arabidopsis, HY1, rescued phototropism. This indicates that ptr116 is defective in heme oxygenase, which catalyses the formation of biliverdin, the precursor of PFB. When filaments of ptr116 were grown on phycoerythrobilin, the cells displayed a fluorescence characteristic for phytochrome. The adduct was found in the cytosole. Bleaching experiments point to a high mobility. It is suggested that a less mobile membrane associated subfraction of phytochrome mediates the phototropic response.