Light-regulated nucleo-cytoplasmic partitioning of phytochromes

Journal of Experimental Botany, Vol. 58, No. 12, pp. 3113–3124, 2007 doi:10.1093/jxb/erm145 Advance Access publication 27 September, 2007 FOCUS PAPER Light-regulated nucleo-cytoplasmic partitioning of phytochromes Eva Kevei1,2,*, Eberhard Schafer1 and Ferenc Nagy2 1 Biologie II/Institute fur Botanik, University of Freiburg, D-79104 Freiburg, Germany 2 Institute of Plant Biology, Biological Research Center, H-6726 Szeged, Hungary Abstract Phytochrome photoreceptors regulate development, growth, and fitness throughout the entire life-cycle of plants, from seed germination to flowering, by regulating expression patterns of ~10–30% of the entire plant transcriptome. Identification of components and elucidation of the molecular mechanisms underlying phytochrome-controlled signal transduction cascades have therefore attracted considerable attention. Phytochrome-controlled signalling is a complex cellular process; it starts with the light-induced intramolecular conformational change of the photoreceptor and includes regulated partitioning and degradation of signalling components and of the photoreceptors themselves. In this review, the data available about light quality- and quantity-dependent nucleo-cytoplasmic partitioning of phytochromes is summarized and the possible function of phytochrome-containing nuclear complexes, termed nuclear bodies, in red/far-red lightinduced signalling is discussed. Key words: Light signalling, nuclear import, phytochromes. Introduction Plants are sessile organisms which have established a considerable plasticity of development to respond to changes in the natural environment. The highly variable environmental factor light is used not only as the main energy source but also as an environmental cue to respond to daily light/dark cycles and to compete with neighbouring plants. To monitor changes in the ambient light environment, plants have evolved several classes of photoreceptors: the as yet unidentified UV-B photoreceptors (Beggs and Wellman, 1994), the blue/UV-A sensing cryptochromes (CRY1 and CRY2) controlling plant development (Lin and Shalitin, 2003), the phototropins regulating directional growth, chloroplast re-orientation, and stomatal opening (Briggs and Christie, 2002), and the red/far-redabsorbing photoreceptors phytochromes, controlling plant growth and development (Quail, 2002). Phytochromes are synthesized in the cytosol as ;125 kDa monomers in their inactive, Pr form (red-lightabsorbing conformer, kmax¼660 nm). Each monomer covalently binds one molecule of phytochromobilin, a linear tetrapyrrole chromophore. Phytochromes exist in vivo as dimers. Red light (R) induces an intramolecular conformational change of the molecule resulting in the formation of the physiologically active Pfr conformer (far-red light-absorbing form, (kmax¼730 nm). The photoreceptor reverts back into its inactive Pr form with subsequent far-red light (FR) treatment and this light quality- and quantity-dependent conformational change enables phytochromes to act as light-sensitive molecular switches. Higher plants contain various phytochromes, which differ in amino acid sequence by 50%. These different types of phytochromes are selectively responsible for sensing various light qualities. In the model plant Arabidopsis thaliana this gene family consists of five genes, designated PHYA, PHYB, PHYC, PHYD, and PHYE (Clack et al., 1994). Due to the overlapping absorption spectra, a light quality-dependent photo-equilibrium is established, making this photoreceptor system a very effective light sensor, especially in the red/far-red range of the spectrum (Smith, 2000). Phytochrome proteins can be divided into two classes on the basis of their mode of action and light stability. Type I phytochromes show rapid proteolytic degradation of the Pfr form, controlling Very-Low-Fluence-Responses * To whom correspondence should be addressed. E-mail: ª The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: Received 12 April 2007; Revised 25 May 2007; Accepted 30 May 2007 3114 Kevei et al. Fig. 1. Schematic illustration of phytochrome’s structure. The ;125 kDa monomer PHY apoprotein folds into two major domains: the Nterminal domain, which determines the photosensory properties of the receptor and the C-terminal, regulatory domain. Detailed description of the function of different domains and motifs are in the text. CHR, chromophore; BLD, bilin lyase domain; PHY, phytochrome domain; DIM, dimerization domain; HKRD, histidine-kinase-related domain; PAS, PER/ARNT/Sim domain, a motif, typified by the Drosophila melanogaster clock protein PER, the mammalian aromatic hydrocarbon receptor nuclear transporter ARNT, and the Drosophila single-minded protein Sim; PRD, PAS-related domain. conformation-dependent fashion. These data strongly suggest that the various phytochromes transmit the light signal, at least partly, inside the nucleus. Intracellular distribution of phytochromes in etiolated seedlings and during the early phase of photomorphogenesis phyA In etiolated seedlings the native phyA (McCurdy and Pratt, 1986; Speth et al., 1986; Pratt, 1994) and the 35S:phyA::GFP fusion protein are distributed throughout the cytosol (Kircher et al., 1999, 2002; Hisada et al., 2000). A single, brief (;5 min) FR-, R- or blue-light pulse (B) induces nuclear import of phyA and subsequent formation of phyA-containing nuclear bodies (phyA NBs) (as described by Hisada et al., 2000; Kim et al., 2000; Kircher et al., 2002). In etiolated seedlings, the nuclear import of phyA is a rapid process: it takes place within a few minutes after the inductive light pulse. An R pulse also promotes the rapid formation of phyA-containing cytosolic spots, also referred to as sequestered areas of phytochromes (SAPs) (Speth et al., 1986). The appearance of SAPs precedes nuclear transport of 35S: phyA::GFP and is thought to be the place of ubiquitination and degradation of the photoreceptor. Continuous FR light (cFR) also initiates nuclear transport and formation of phyA NBs in the nuclei. These data suggest that nuclear import of phyA correlates with phyA-mediated VLFRs and HIRs. Figure 2 shows the fluence ratedependent nuclear accumulation of 35S:phyA::GFP. In addition to FR, white (W), R, and B light illumination of etiolated seedlings is also effective in inducing (VLFR) and far-red High-Irradiance Responses (FR-HIR). Type II phytochromes are light-stable and control Low Fluence Responses (LFR) and red light High Irradiance Responses (R-HIR). Analysis of mutants deficient in various phytochromes showed that (i) type I phytochromes are encoded by the PHYA gene and type II phytochromes by PHYB–E genes (Quail, 2002) and that (ii) different members of the family have differential as well as overlapping physiological roles in controlling plant development (Smith et al., 1997; Franklin et al., 2003; (...truncated)