A Short Amino-Terminal Part of Arabidopsis Phytochrome A Induces Constitutive Photomorphogenic Response

Phytochrome A （phyA） is the dominant photoreceptor of far-red light sensing in Arabidopsis thaliana, phyA accumulates at high levels in the cytoplasm of etiolated seedlings, and light-induced phyA signaling is mediated by a com- plex regulatory network. This includes light- and FHY1/FHL protein-dependent translocation of native phyA into the nucleus in vivo. It has also been shown that a short N-terminal fragment of phyA （PHYA406） is sufficient to phenocopy this highly regulated cellular process in vitro. To test the biological activity of this N-terminal fragment of phyA in planta, we produced transgenic phyA-201 plants expressing the PHYA406-YFP （YELLOW FLUORESCENT PROTEIN）-DD, PHYA406- YFP-DD-NLS （nuclear localization signal）, and PHYA406-YFP-DD-NES （nuclear export signal） fusion proteins. Here, we report that PHYA406-YFP-DD is imported into the nucleus and this process is partially light-dependent whereas PHYA406-YFP-DD-NLS and PHYA406-YFP-DD-NES display the expected constitutive localization patterns. Our results show that these truncated phyA proteins are light-stable, they trigger a constitutive photomorphogenic-like response when localized in the nuclei, and neither of them induces proper phyA signaling. We demonstrate that in vitro and in vivo PHYA406 Pfr and Pr bind COP1, a general repressor of photomorphogenesis, and co-localize with it in nuclear bodies. Thus, we conclude that, in planta, the truncated PHYA406 proteins inactivate COP1 in the nuclei in a light-independent fashion.