Arabidopsis phytochrome A signal transduction: characterization and identifcation of cytoplasmic events and components

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Plants use phytochromes as red/far red sensing photoreceptors to monitor their light environment and regulate many aspects of development accordingly. Phytochrome A (phyA) is one of five phytochromes in Arabidopsis, regulating early seedling development among other responses. Following its activation, phyA translocates to the nucleus where it regulates gene expression. Two signaling intermediates, far red elongated hypocotol (FHY1) and FHY1-like (FHL), are crucial for nuclear translocation, as in their absence no phyA is detectable in the nucleus. Taking advantage of exclusively cytoplasmic phyA in the fhl/fhy1 double mutant, novel cytoplasmic functions of phyA could be revealed previously. In the follow-up project, the regulation of phyA import and the impact of FHY1 bindingon phyA properties, as well as the action of other cytoplasmatic components in early phyA signaling was further investigated. Thereby the mechanism by which the FHY1-phyA complex is translocated to the nucleus has been investigated in detail. Potential other signalling components which have already been identified in light-dependent yeast two- and three-hybrid assays with functional phyA have also been investigated. Thereby the influence of a cytoplasmic phosphatase on the phyA very low fluence response (VLFR) was discovered. Furthermore, heterologously expressed phyA wasbiochemically and biophysically analyzed concerning its interaction with the key signaling components. In the proposed extension of the project the in vivo status of FHY1s secondary modification during translocation, the structural basis of the phyA-FHY1 interaction and the function of the discovered phosphatase and small ubiquitin like modifier (SUMO) ligase will be investigated further. The proposed project will be a collaborative project with the Essen lab in Marburg which will provide the structural, biophysical and mass spectrometry tasks of the project while the Zeidler lab will work on Protein interaction, the function of the complex members and the kinetic and regulation of the nuclear translocation. The proposed project will advance the understanding of this photoreceptor’s function. The regulation of nuclear translocation is of general interest in a variety of organisms, and a detailed understanding of inducible and directional translocation of a receptor may prove valuable for targeted manipulations of gene expression in a number of applications,reaching beyond the plant field.