Phytochrome cytoplasmic signalling in lower plants

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Phytochrome photoreceptors play a central role in light-regulated development in plants. In their Pr ground state plant phytochromes are generally cytosolic but red light absorption induces the Pfr signalling state, a proportion of which is then transported into the nucleus to regulate transcription. However, some phytochrome effects are too fast for transcriptional changes to be involved. Also phytochrome in lower plants provides vectorial information for directional growth of individual cells and chloroplast relocation: this cannot be based on transcription regulation. Thus phytochrome must have another signalling mechanism within the cytoplasm. We have now identified and initially characterized this as arising from a hitherto unexpected interaction of a sub-population of phytochrome molecules with phototropin at the plasma membrane in Physcomitrella (Jaedicke et al. (2012) PNAS). The purpose of this project is to describe the complex and its signaling mechanism at the molecular level. To this end we will exploit the Physcomitrella model system physiologically (photo- and polarotropism) and genetically (genome sequence and homologous recombination) with the help of molecular genetic, cell biological, biochemical and biophysical methods. We propose to continue our work using a variety of yeast two-hybrid approaches to identify potential cytoplasmic phytochrome interactors and to characterize the associated signaling system by bimolecular fluorescence complementation, fluorescence lifetime imaging, co-immunoprecipitation, targetted knockout / complementation and other techniques. Although we have evidence that the phy-phot interaction occurs in Arabidopsis too, we do not propose to study this in the present application.