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.

Substitution of a Conserved Glycine in the PHR Domain of Arabidopsis CRYPTOCHROME 1 Confers a Constitutive Light Response

CRYPTOCHROMES （CRYs） are photolyase-like ultraviolet-A/blue light photoreceptors that mediate various light responses in plants. The signaling mechanism of Arabidopsis CRYs （CRY1 and CRY2） involves direct CRY-COP1 interaction. Here, we report that CRY1G380R, which carries a Gly-to-Arg substitution of the highly conserved G380 in the photo-lyase-related （PHR） domain of Arabidopsis CRY1, shows constitutive CRY1 photoreceptor activity in Arabidopsis. Transgenic plants overexpressing CRY1G380R display a constitutively photomorphogenic （COP） phenotype in darkness, as well as a dramatic early flowering phenotype under short-day light conditions （SD）. We further demonstrate that CRY1G380R expression driven by the native CRY1 promoter also results in a COP phenotype in darkness. Moreover, over- expression of either the Arabidopsis homolog CRY2G377R or the rice ortholog OsCRYlbG388R of CRY1G380R in Arabidopsis results in a COP phenotype in darkness. Cellular localization studies indicate that CRY1G380R co-localizes with COP1 in the same nuclear bodies （NBs） in vivo and inhibits the nuclear accumulation of COP1 in darkness. These results suggest that the conserved G380 may play a critical role in regulating the photoreceptor activity of plant CRYs and that CRY1G380R might constitutively phenocopy the photo-activated CRY1 in darkness and thus constitutively mediate CRY1 signaling.