Time-dependent formation of Pfr-mediated signals for the interaction with blue light in Mesotaenium chloroplast orientation

Fast chloroplast orientation from edge position to face position in Mesotaenium caldariorum is controlled by an interaction of phytochrome with a blue-light photoreceptor; blue amplifies the phytochrome message. The time-dependent formation of phytochrome-mediated signals in the transduction chain and the following decline in their physiological activity were investigated. A single red flash of about 4 ms is sufficient to induce the generation of signals, as indicated by the effectiveness of following B. A strong FR flash applied 5 s after the inducing R flash prevents signal formation by photoreversion of Pfr to Pr and demonstrates clearly the involvement of phytochrome. The time-dependent formation of the Pfr-mediated signals was studied by inserting a variable dark period between the R Bash and the FR Bash. Within 1 min the generation of signals is saturated at a level, which is determined by the Pfr-gradient. Half-maximal response can be obtained after about 20 a. The physiological competence of the signals to interact with the activated B-photoreceptor decreases within approximately 5 min, as investigated by a variable dark interval between the FR flash and the onset of B. As the memory of a R pulse lasts also for about 5 min in the presence of Pfr, it is concluded that a newly formed Pfr-gradient is effective in Mesotaenium only for a very short time to generate a distinct lever of signals in the transduction chain, i.e. for about 1 min.