A plastid-derived signal plays an important role in the coordinated expression of both nuclear-and chloroplast-localized genes that encode photosynthesis-related proteins. Arabidopsis GUN (genomes uncoupled) loci have been identified as components of plastid-to-nucleus signal transduction. Unlike wild-type plants, gun mutants have nuclear Lhcb1 expression in the absence of chloroplast development. We observed a synergistic phenotype in some gun double-mutant combinations, suggesting there are at least two independent pathways in plastid-to-nucleus signal transduction. There is a reduction of chlorophyll accumulation in gun4 and gun5 mutant plants, and a gun4gun5 double mutant shows an albino phenotype. We cloned the GUN5 gene, which encodes the ChlH subunit of Mg-chelatase. We also show that gun2 and gun3 are alleles of the known photomorphogenic mutants, hy1 and hy2, which are required for phytochromobilin synthesis from heme. These findings suggest that certain perturbations of the tetrapyrrole biosynthetic pathway generate a signal from chloroplasts that causes transcriptional repression of nuclear genes encoding plastid-localized proteins. The comparison of mutant phenotypes of gun5 and another Mg-chelatase subunit (ChlI) mutant suggests a specific function for ChlH protein in the plastid-signaling pathway.A number of components required for plastid structure and development are encoded by the nuclear genome. There is a considerable body of evidence that suggests that the proper and timely expression of these genes depends in part on the functional state of the chloroplast. For example, nuclear mutations that result in developmentally arrested chloroplasts also result in the reduced expression of nuclear-localized photosynthetic genes (1). These results tie the functional state of the chloroplast to nuclear function and suggest that the chloroplast signals the nucleus in a retrograde fashion (2).Such retrograde signaling between chloroplast and nucleus has been studied primarily by using carotenoid-deficient plants induced either by mutations or by carotenoid biosynthesis inhibitors such as Norflurazon (Nf; refs. 3 and 4). Carotenoids prevent the production of reactive oxygen species by excited triplet states of chlorophyll. Carotenoid-deficient plants thus suffer a rapid photooxidation of most chloroplast components under intense light. Although most nuclear-encoded cytoplasmic enzymes are present at normal levels in photooxidatively damaged plants, a small set of nuclear-encoded chloroplast enzymes is absent (3, 4). Accordingly, it was hypothesized that plastids send an unknown signal(s) to the nucleus that regulates the expression of a small subset of nuclear-localized photosynthetic genes (2, 5).The molecular nature of the plastid signal(s) and the mechanism by which it is relayed to the nucleus remain ambiguous. Although both plastid transcription and translation are necessary for the production of the plastid signal (4, 6), the plastid signal is not a direct translational product of a plastid gene. Lig...

Plant responses to red and far-red light are mediated by a family of photoreceptors called phytochromes. In Arabidopsis thaliana, there are genes encoding at least five phytochromes, and it is of interest to learn if the different phytochromes have overlapping or distinct functions. To address this question for two of the phytochromes in Arabidopsis, we have compared light responses of the wild type with those of a phyA null mutant, a phyB null mutant, and a phyA phyB double mutant. We have found that both phyA and phyB mutants have a deficiency in germination, the phyA mutant in far-red light and the phyB mutant in the dark. Furthermore, the germination defect caused by the phyA mutation in farred light could be suppressed by a phyB mutation, suggesting that phytochrome B (PHYB) can have an inhibitory as well as a stimulatory effect on germination. In red light, the phyA phyB double mutant, but neither single mutant, had poorly developed cotyledons, as well as reduced red-light induction of CAB gene expression and potentiation of chlorophyll induction. The phyA mutant was deficient in sensing a flowering response indudive photoperiod, suggesting that PHYA participates in sensing daylength. In contrast, the phyB mutant flowered earlier than the wild type (and the phyA mutant) under all photoperiods tested, but responded to an inductive photoperiod. Thus, PHYA and PHYB appear to have complementary fundions in controlling germination, seedling development, and flowering. We discuss the implications of these results for possible mechanisms of PHYA and PHYB signal transduction.

We have examined the seed germination in Arabidopsis thaliana of wild type (wt), and phytochrome A (PhyA)-and B (PhyB)-mutants in terms of incubation time and environmental light effects. Seed germination of the wt and PhyA-null mutant (phyA) was photoreversibly regulated by red and far-red lights of 10-1,000 ,umol m-2 when incubated in darkness for 1-14 hr, but no germination occurred in PhyB-null mutant (phyB). When wt seeds and thephyB mutant seeds were incubated in darkness for 48 hr, they synthesized PhyA during dark incubation and germinated upon exposure to red light of 1-100 nmol m-2 and far-red light of 0.5-10 ,umol m-2, whereas the phyA mutant showed no such response. The results indicate that the seed germination is regulated by PhyA and PhyB but not by other phytochromes, and the effects of PhyA and PhyB are separable in this assay. We determined action spectra separately for PhyA-and PhyB-specific induction of seed germination at Okazaki large spectrograph. Action spectra for the PhyA response show that monochromatic 300-780 nm lights of very low fluence induced the germination, and this induction was not photoreversible in the range examined. Action spectra for the PhyB response show that germination was photoreversibly regulated by alternate irradiations with light of 0.01-1 mmol m-2 at wavelengths of 540-690 nm and 695-780 nm. The present work clearly demonstrated that PhyA photoirreversibly triggers the germination upon irradiations with ultraviolet, visible and farred light of very low fluence, while PhyB controls the photoreversible effects of low fluence.

Phytochrome, a red/far-red-light photoreceptor protein of plants, is encoded by a small gene family. Phytochrome A (PHYA), the product of the PHYA gene, is the predominant molecular species of phytochrome in etiolated tissue and has been best charaderized biochemically. To define a role for PHYA, we isolated new mutants, designated frel (far-red elongated), in Arabidopsis thaliana that were specifically deficient in PHYA spectral activity and protein accumulation. These mutants were identified on the basis of their long hypocotyl phenotype under continuous far-red light. Although the frel mutants lacked the hypocotyl response to continuous far-red light, their responses to continuous white light and to end-of-day far-red-light treatments were normal. Thus, PHYA appears to play only a minor role in the regulation of hypocotyl elongation under natural conditions. In contrast, the frel mutation affected greening; a frel mutant was less able than the wild type to deetiolate after growth in the dark. However, the potentiation effect of a red-light pulse on accumulation of chlorophyll was not changed significantly in the frel mutants. Thus, the function of PHYA might be highly specialized and restricted to certain phases of Arabidopsis development.

Phytochrome is a ubiquitous photoreceptor of plants and is encoded by a small multigene family. We have shown recently that a functional nuclear localization signal may reside within the COOH-terminal region of a major member of the family, phytochrome B (phyB) (Sakamoto, K., and A. Nagatani. 1996. Plant J. 10:859–868). In the present study, a fusion protein consisting of full-length phyB and the green fluorescent protein (GFP) was overexpressed in the phyB mutant of Arabidopsis to examine subcellular localization of phyB in intact tissues. The resulting transgenic lines exhibited pleiotropic phenotypes reported previously for phyB overexpressing plants, suggesting that the fusion protein is biologically active. Immunoblot analysis with anti-phyB and anti-GFP monoclonal antibodies confirmed that the fusion protein accumulated to high levels in these lines. Fluorescence microscopy of the seedlings revealed that the phyB-GFP fusion protein was localized to the nucleus in light grown tissues. Interestingly, the fusion protein formed speckles in the nucleus. Analysis of confocal optical sections confirmed that the speckles were distributed within the nucleus. In contrast, phyB-GFP fluorescence was observed throughout the cell in dark-grown seedlings. Therefore, phyB translocates to specific sites within the nucleus upon photoreceptor activation.

A plant modulates its developmental processes in response to light by several informational photoreceptors such as phytochrome. Phytochrome is a dimeric chromoprotein which regulates various aspects of plant development from seed germination to flowering. Upon absorption of red light, phytochrome translocates from the cytoplasm to the nucleus, and regulates gene expression through interaction with transcription factors such as PIF3 (refs 5-7). The phytochrome polypeptide has two domains: the amino-terminal photosensory domain with a chromophore and the carboxy-terminal domain which contains signalling motifs such as a kinase domain. The latter is widely believed to transduce the signal to downstream components. Here we show that the C-terminal domain of Arabidopsis phytochrome B (phyB), which is known as the most important member of the phytochrome family, is not directly involved in signal transduction. The N-terminal domain isolated from phyB, when dimerized and localized in the nucleus, triggered full phyB responses with much higher photosensitivity than the full-length phyB. These findings indicate that the C-terminal domain attenuates the activity of phyB rather than positively transducing the signal.

(H.K., M.T.)Phototropins (phot1 and phot2, formerly designated nph1 and npl1) are blue-light receptors that mediate phototropism, blue light-induced chloroplast relocation, and blue light-induced stomatal opening in Arabidopsis. Phototropins contain two light, oxygen, or voltage (LOV) domains at their N termini (LOV1 and LOV2), each a binding site for the chromophore flavin mononucleotide (FMN). Their C termini contain a serine/threonine protein kinase domain. Here, we examine the kinetic properties of the LOV domains of Arabidopsis phot1 and phot2, rice (Oryza sativa) phot1 and phot2, and Chlamydomonas reinhardtii phot. When expressed in Escherichia coli, purified LOV domains from all phototropins examined bind FMN tightly and undergo a self- The photocycle involves the light-induced formation of a cysteinyl adduct to the C(4a) carbon of the FMN chromophore, which subsequently breaks down in darkness. In each case, the relative quantum efficiencies for the photoreaction and the rate constants for dark recovery of LOV1, LOV2, and peptides containing both LOV domains are presented. Moreover, the data obtained from full-length Arabidopsis phot1 and phot2 expressed in insect cells closely resemble those obtained for the tandem LOV-domain fusion proteins expressed in E. coli. For both Arabidopsis and rice phototropins, the LOV domains of phot1 differ from those of phot2 in their reaction kinetic properties and relative quantum efficiencies. Thus, in addition to differing in amino acid sequence, the phototropins can be distinguished on the basis of the photochemical cycles of their LOV domains. The LOV domains of C. reinhardtii phot also undergo light-activated spectral changes consistent with cysteinyl adduct formation. Thus, the phototropin family extends over a wide evolutionary range from unicellular algae to higher plants.Plants use light not only as an energy source for photosynthesis but also as a signal to indicate the properties of their surrounding environment. UV-A (320-390 nm) and blue (390-500 nm) light regulate a wide variety of responses in higher plants. These responses include phototropism, chloroplast relocation, inhibition of hypocotyl elongation, circadian timing, regulation of gene expression, and stomatal opening (Briggs and Huala, 1999;Lin, 2000;Christie and Briggs, 2001;Briggs et al., 2001b). Four blue-light receptors have been identified from the model higher plant Arabidopsis: cryptochrome 1 (Ahmad et al., 1993), cryptochrome 2 (Hoffman et al., 1996;Lin et al., 1996Lin et al., , 1998, phototropin 1 (phot1; Huala et al., 1997;Christie et al., 1998Christie et al., , 1999, and phototropin 2 (phot2; Jarillo et al., 1998).In Arabidopsis, phot1 and phot2 (formerly known as nph1 and npl1, respectively; see Briggs et al., 2001a) have been shown to serve as photoreceptors mediating phototropism (Huala et al., 1997;Christie et al., 1998), blue light-mediated chloroplast movement Sakai et al., 2001;Jarillo et al., 2001), and blue light-induced stomatal opening (Kinoshita et al., 2002 Article, publicati...

We examined whether spectrally active phytochrome A (PhyA) and phytochrome B (PhyB) play specific roles in the induction of seed germination in Arabidopsis thaliana (L.) Heynh., using PhyA- and PhyB-null mutants, fre1-1 (A. Nagatani, J.W. Reed, J. Chory [1993] Plant Physiol 102: 269-277) and hy3-Bo64 (J. Reed, P.Nagpal, D.S. Poole, M. Furuya, J. Chory [1993] Plant Cell 5: 147-157). When dormant seeds of each genotype imbibed in the dark on aqueous agar plates, the hy3 (phyB) mutant did not germinate, whereas the fre1 (phyA) mutant germinated at a rate of 50 to 60%, and the wild type (WT) germinated at a rate of 60 to 70%. By contrast, seeds of all genotypes germinated to nearly 100% when plated in continuous irradiation with white or red light. When plated in continuous far-red light, however, frequencies of seed germination of the WT and the fre1 and hy3 mutants averaged 14, nearly 0, and 47%, respectively, suggesting that PhyB in the red-absorbing form prevents PhyA-dependent germination under continuous far-red light. When irradiated briefly with red or far-red light after imbibition for 1 h, a typical photoreversible effect on seed germination was observed in the fre1 mutant and the WT but not in the hy3 mutant. In contrast, when allowed to imbibe in the dark for 24 to 48 h and exposed to red light, the seed germination frequencies of the hy3 mutant were more than 40%. Immunoblot analyses of the mutant seeds showed that PhyB apoprotein accumulated in dormant seeds of the WT and the fre1 mutant as much as in the seeds that had imbibed. In contrast, PhyA apoprotein, although detected in etiolated seedlings grown in the dark for 5 d, was not detectable in the dormant seeds of the WT and the hy3 mutant. The above physiological and immunochemical evidence indicates that PhyB in the far-red-absorbing form was stored in the Arabidopsis seeds and resulted in germination in the dark. Hence, PhyA does not play any role in dark germination but induces germination under continuous irradiation with far-red light. Finally, we examined seeds from a signal transduction mutant, det1, and a det1/hy3 double mutant. The det1 seeds exhibited photoreversible responses of germination on aqueous agar plates, and the det1/hy3 double mutant seeds did not. Hence, DET1 is likely to act in a distinct pathway from PhyB in the photoregulation of seed germination.