It is interesting that ambient temperature modulates the light-regulation of Arabidopsis seed germination, and different phytochromes display altered functional hierarchies at different temperatures Heschel et al. Phosphorylation by CK2 enhances the rapid light-induced degradation of Phytochrome Interacting Factor 1 in Arabidopsis. Growing evidence demonstrates that the PIF proteins, a small subset of basic helix-loop-helix bHLH transcription factors, play central roles in phytochrome-mediated light signaling networks Duek and Fankhauser, ; Castillon et al. Molecular basis of seasonal time measurement in Arabidopsis. Structural comparison of Arabidopsis phytochromes and the bacterial phytochrome Cph1 adapted from Yeh and Lagarias, The past two decades have seen dramatic progress in molecular characterization and understanding of the photobiology and photochemistry of the phytochrome photoreceptors in higher plants. The transgenic Arabidopsis plants with over-expressed or suppressed FyPP levels exhibited delayed or accelerated flowering, respectively, indicating that FyPP modulates phytochrome-mediated light signaling in the timing of flowering Kim et al.

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In addition, the law of reciprocity applies to this response, i. Transposing phytochrome into the nucleus. Coordinated transcriptional regulation underlying the circadian clock in Arabidopsis.

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Arabidopsis FHY3 specifically gates phytochrome signaling to the circadian clock. Plant development is regulated not only by the difference between light and darkness, but also by light quality, in particular the change of light quality due to shading by other plants.

All PIFs except PIF7 are rapidly phosphorylated and then ubiquitinated in response to light in vivo prior to their degradation in a phytochrome-dependent manner Al-Sady et al. The history of FHY1 essentially parallels the history of the molecular genetic analysis of phyA signaling pathway in Arabidopsis.

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Arrow, positive regulation; bar, negative regulation. The proteolysis of phyA following photoconversion from Pr to Pfr is rapid, as the Pr form has a half-life of approximately 1 week, whereas the Blje-03 form has a half-life of only 1—2 h Clough and Vierstra, It was widely accepted before the mids that phytochromes were cytoplasmic photoreceptors based on early biochemical and immunocytochemical studies Nagy and Schafer, Seedling De-etiolation Dark-grown seedlings undergo skotomorphogenesis etiolation and are characterized by long hypocotyls, closed cotyledons and apical hooks, and development of the proplastids into etioplasts.


The circadian clock controls many metabolic, developmental and physiological processes in a time-of-day-specific manner in both plants and ghj McClung, ; de Montaigu et al.

Journal List Arabidopsis Book v.

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The cytosolic degradation of phyA is also supported by the recent reports that phyA is still degraded in the fhy1 fhl double mutants, in which phyA is maintained in the cytosol, although at a slower rate than in the wild-type plants Rosler et al.

This conclusion is further confirmed by the observations that phyA phyB phyD phyE quadruple mutants display insensitivity to reductions in R: Phosphorylation of Avena phytochrome in vitro as a probe of light-induced conformational changes.

Phytochromes are synthesized in the Pr form in dark-grown seedlings. Another mechanism of gating the light input to the circadian clock, called the external coincidence model, has been proposed in the recent years.

The hfr1 laf1 double mutant has an additive phenotype compared to the two single mutants, indicating that HFR1 and LAF1 regulate largely independent pathways Jang et al. Thus, this set of cee and circadian-regulated transcription factors act directly and coordinately at the ELF4 promoter to regulate its cyclic expression Li et al. Structural basis for the photoconversion of a phytochrome to the activated Pfr form.

Phytochrome-mediated inhibition of shade avoidance involves degradation of growth-promoting bHLH transcription factors. The ability of plants to monitor their light environments and change their architecture provides them with a competitive strategy to survive and complete their life cycle in dense stands.


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Interested readers are encouraged to read the accompanying reviews on other related subjects, such as photomorphogenesis Nemhauser and Chory,cryptochromes Yu et al. The FAR1 locus encodes a novel nuclear protein specific to phytochrome A signaling. The molecular mass of the apoprotein monomer is approximately kDa. The discovery of phytochrome-like photoreceptors in bacteria, collectively called bacteriophytochromes BphPsgenerated further supporting evidence for such a view Bluw-03, ; Vierstra and Davis, Indeed, two small plant-specific proteins, FHY1 and FHL, have been shown to play an essential role in facilitating phyA nuclear translocation.

Although a recent report indicated that phyB shows some kinase activity in vitro Phee et al. The HKRD domain lacks a critical histidine residue, and thus may be an evolutionary remnant rather than an active histidine kinase Boylan and Quail, Therefore, phytochrome-induced phosphorylation and proteolysis of PIFs may represent a major biochemical mechanism of signal transfer from the photoactivated phytochromes to their interacting signaling partners in the nucleus, which rapidly alters the gene expression profiles of the genome.

Characterization cee recombinant phytochrome from the blus-03 Synechocystis.

Extensive studies have shed light blur-03 the mechanisms by which phytochromes regulate light-responsive gene expression. Definition of early transcriptional circuitry involved in light-induced reversal of PIF-imposed repression of photomorphogenesis in young Arabidopsis seedlings. In addition, phytochromes were shown to modulate cryptochrome-mediated seedling de-etiolation and phototropin-mediated phototropic curvature of Arabidopsis hypocotyls in B light Parks et al.

Decoding of light signals by plant phytochromes and their interacting proteins.