FHY1 Mediates Nuclear Import of the Light-Activated Phytochrome A Photoreceptor Thierry Genoud1, Fabian Schweizer1, Anke Tscheuschler2, Dimitry Debrieux1, Jorge J. Casal3,4, Eberhard Scha¨ fer2,5, Andreas Hiltbrunner2,5*, Christian Fankhauser1* 1 Centre for Integrative Genomics, University of Lausanne, Lausanne, Switzerland, 2 Institut fu¨ r Biologie II/Botanik, Albert Ludwigs Universita¨t, Freiburg, Germany, 3 IFEVA, Facultad de Agronom´ıa, Universidad de Buenos Aires, Buenos Aires, Argentina, 4 Consejo Nacional de Investigaciones Cient´ıficas y Te´cnicas (CONICET), Buenos Aires, Argentina, 5 BIOSS, Centre for Biological Signalling Studies, University of Freiburg, Freiburg, Germany Abstract The phytochrome (phy) family of photoreceptors is of crucial importance throughout the life cycle of higher plants. Light- induced nuclear import is required for most phytochrome responses. Nuclear accumulation of phyA is dependent on two related proteins called FHY1 (Far-red elongated HYpocotyl 1) and FHL (FHY1 Like), with FHY1 playing the predominant function. The transcription of FHY1 and FHL are controlled by FHY3 (Far-red elongated HYpocotyl 3) and FAR1 (FAr-red impaired Response 1), a related pair of transcription factors, which thus indirectly control phyA nuclear accumulation. FHY1 and FHL preferentially interact with the light-activated form of phyA, but the mechanism by which they enable photoreceptor accumulation in the nucleus remains unsolved. Sequence comparison of numerous FHY1-related proteins indicates that only the NLS located at the N-terminus and the phyA-interaction domain located at the C-terminus are conserved. We demonstrate that these two parts of FHY1 are sufficient for FHY1 function. phyA nuclear accumulation is inhibited in the presence of high levels of FHY1 variants unable to enter the nucleus. Furthermore, nuclear accumulation of phyA becomes light- and FHY1-independent when an NLS sequence is fused to phyA, strongly suggesting that FHY1 mediates nuclear import of light-activated phyA. In accordance with this idea, FHY1 and FHY3 become functionally dispensable in seedlings expressing a constitutively nuclear version of phyA. Our data suggest that the mechanism uncovered in Arabidopsis is conserved in higher plants. Moreover, this mechanism allows us to propose a model explaining why phyA needs a specific nuclear import pathway. Citation: Genoud T, Schweizer F, Tscheuschler A, Debrieux D, Casal JJ, et al. (2008) FHY1 Mediates Nuclear Import of the Light-Activated Phytochrome A Photoreceptor. PLoS Genet 4(8): e1000143. doi:10.1371/journal.pgen.1000143 Editor: Gregory P. Copenhaver, The University of North Carolina at Chapel Hill, United States of America Received April 28, 2008; Accepted June 25, 2008; Published August 1, 2008 Copyright: 2008 Genoud et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants from the Swiss National Science Foundation and HFSP to CF (SNF 3100A0-112638; HFSP RGY0016/2004-C), the Deutsche Forschungsgemeinschaft (DFG) to ES (SFB 592 and SFB 746), the Agencia Nacional de Promocio´ n Cient´ıfica y Tecnolo´ gica to JJC (BID 1728 OC-AR PICT32492) and a postdoctoral fellowship from HFSP to AH (LT00631/2003-C). Competing Interests: The authors have declared that no competing interests exist. * E-mail: andreas.hiltbrunner@biologie.uni-freiburg.de (AH); christian.fankhauser@unil.ch (CF) Introduction Plants are sessile organisms and therefore have to adapt growth and development to the environmental conditions at their site of germination. Light is one of the most important factors directing such adaptive responses and it is involved in many developmental steps throughout the life of plants [1,2]. To detect intensity, quality (wavelength) and direction of incident light plants have evolved a set of photoreceptors monitoring red/far-red (R/FR), blue/UV-A and UV-B [3–7]. The phytochrome family of red/far-red photoreceptors plays a key role in seed germination, leaf and stem development, circadian rhythms, shade avoidance and induction of flowering [8]. Although in higher plants phyto- chromes are not the primary photoreceptors controlling photot- ropism and chloroplast movements, the phytochromes modulate these responses [9–11]. Phytochromes are homodimeric chromoproteins containing the linear tetrapyrole phytochromobilin as chromophore. They photoconvert between two spectrally distinct forms: the red- light-absorbing Pr and the biologically active far-red light- absorbing Pfr form [3,12]. As the absorption spectra of the two forms overlap the photoconversion is not complete in either direction. Irradiation with light therefore results in a wavelength- specific equilibrium between the Pr and Pfr forms, with only ,2% Pfr in far-red light and ,85% Pfr in red light [13]. Under natural conditions the Pfr/Pr ratio differs dramatically depending on the position of the plant within the community (canopy shade versus open environment) [14,15]. In Arabidopsis the phytochrome gene family consists of five members (PHYA–E), among which PHYA and PHYB play the most prominent functions [16]. phyB is the major red light receptor and mediates the red/far-red reversible low fluence response (LFR). Other members of the phytochrome family contribute to responses primarily controlled by phyB. In contrast, responses to continuous far-red light (high irradiance response, HIR) and to single light pulse of very low fluence light (VLFR) depend exclusively on phyA [1,3,12]. Photoreceptor mutants have reduced fitness but only the phyA mutant is conditionally lethal, highlighting the importance of this photoreceptor [17,18]. Its functional importance is further revealed by the high degree of sequence conservation among all angiosperms [19]. phyA is also crucial for the modulation of phototropin responses such as the enhancement of phototropism [10,11]. PLoS Genetics | www.plosgenetics.org 1 August 2008 | Volume 4 | Issue 8 | e1000143