Is the Pr form of phytochrome biologically active in the nucleus?

The sessile nature of plants has promoted the evolution a wide range mechanisms that enable to adapt changing environment. In particular, light environment is highly dynamic and, therefore, have evolved several families photoreceptors detect almost all facets light, such as quality, quantity, direction, and duration. Phytochromes are red (R) far-red (FR) in play fundamental roles modulating adaptive growth response their (Li et al., 2011Li J. Li G. Wang H. Deng X.W. Phytochrome signaling mechanisms.Arabidopsis Book. 2011; 9: e0148Crossref PubMed Google Scholar). model plant Arabidopsis thaliana there five phytochrome photoreceptors, named A (phyA) phyE. PhyA photolabile, whereas phyB phyE photostable. phyB, two most abundant phytochromes, account for ~95% total proteins etiolated seedlings important role regulating seedling photomorphogenesis exist vivo distinct but photointerconvertible forms, Pr Pfr, after color they maximally absorb: form an absorption peak R (660 nm) while Pfr FR (730 Light triggers conversion between both directions, depending on wavelength intensity Pr?Pfr Pfr?Pr photoconversion rates different. Due dominates over reversion at this wavelength, makes dominant under These properties molecular basis R/FR-reversible, toggle-switch-like behavior phytochromes. synthesized localized cytosol dark; upon irradiation, convert into translocate nucleus, where alter expression numerous light-responsive genes, which ultimately leads responses Light-controlled translocation phytochromes from cytoplasm nucleus proved be key regulatory step pathway (Klose 2015Klose C. Viczian A. Kircher S. Schafer E. Nagy F. Molecular mediating light-dependent nucleo/cytoplasmic partitioning photoreceptors.New Phytol. 2015; 206: 965-971Crossref Scopus (53) Translocation phyA very rapid process (within minutes) depends transport facilitators FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) FHY1-LIKE (FHL) (Hiltbrunner 2006Hiltbrunner Tscheuschler Kunkel T. FHY1 FHL act together mediate nuclear accumulation photoreceptor.Plant Cell Physiol. 2006; 47: 1023-1034Crossref (132) contrast, no dedicated transporter yet been identified, it possible transported bound transcription factors (Pfeiffer 2012Pfeiffer Nagel M.-K. Popp Wüst Bindics Viczián Hiltbrunner Schäfer Interaction with can import B.Proc. Natl. Acad. Sci. U S 2012; 109: 5892-5897Crossref (59) Scholar; Klose similar spectra when expressed yeast, action planta different (Shinomura 1996Shinomura Nagatani Hanzawa Kubota M. Watanabe Furuya Action A- B-specific photoinduction seed germination thaliana.Proc. 1996; 93: 8129-8133Crossref (412) Scholar, Shinomura 2000Shinomura Uchida K. Elementary processes photoperception by high-irradiance hypocotyl elongation Arabidopsis.Plant 2000; 122: 147-156Crossref (138) Eichenberg 2000Eichenberg Bäurle I. Paulo N. Sharrock R.A. Rüdiger W. C E spectral characteristics those B.FEBS Lett. 470: 107-112Crossref (65) Since form, hitherto prevailing assumption only biologically active, should light. This indeed true photostable not whose profile exhibits spectrum It was proposed phyA-specific properties, degradation R-light exposure its interaction shuttle FHL, shift (Rausenberger 2011Rausenberger Nordmeier Timmer Fleck Photoconversion trafficking cycles determine A's light.Cell. 146: 813-825Abstract Full Text PDF (111) Moreover, expressing constitutively nuclear-localized fhy1 fhl mutant background hypersensitive still show full indicating FHY1/FHL-independent must shape (Menon 2020Menon required signal transduction nucleus.Plant Commun. 2020; 1: 100007Abstract (5) Despite general inactive, experimental data also suggest may some biological activity, especially cycled through Recently, studies provided complementary lines evidence supporting notion non-negligible activity. One line came yeast three-hybrid assays report Sheerin 2015Sheerin D.J. Menon zur Oven-Krockhaus Enderle B. Zhu L. Johnen P. Schleifenbaum Stierhof Y.D. Huq Light-activated B interact members SPA family promote reorganizing COP1/SPA complex.Plant Cell. 27: 189-201Crossref (172) Scholar. Binding domain-fused COP1 interacted strongly activation SPA1; however, binding SPA1, co-expressed phyA-NLS or phyB-NLS (a localization [NLS] fused facilitate cells) dramatically inhibited SPA1 (Sheerin Surprisingly, completely COP1–SPA1 (as indicated level reporter gene expression), brought down ~10%–15% absence Consistent observation, drastic inhibition observed independent study (Zhou 2018Zhou Y. Yang Duan Cheng Shen X. Han R. Z. al.Hinge region plays function.Proc. 2018; 115: E11864-E11873Crossref (10) able inhibit view (and possibly other downstream factors) bind higher affinity than Constitutively versions containing NLS terminus (sometimes followed preceded GFP YFP) were often generated phosphorylation, degradation, body formation phyA, functions (Genoud 2008Genoud Schweizer Debrieux D. Casal J.J. Fankhauser mediates light-activated photoreceptor.PLoS Genet. 2008; 4: e1000143Crossref (84) Toledo-Ortiz 2010Toledo-Ortiz Kiryu Kobayashi Oka Kim Nam H.G. Mochizuki Subcellular sites A.Plant 2010; 51: 1648-1660Crossref (23) Rausenberger Zhou phyA-NLS-expressing used these mostly indistinguishable wild-type grown dark, although Scholar mentioned weak cop (constitutively photomorphogenic) phenotype phyA-GFP-NLS. showed abundance transgenic phyA-NLS-YFP certain level, induced surprisingly strong, dose-dependent dark-grown seedlings, including unfolding cotyledons repression growth. Although long believed findings could activity sufficiently high targeted NLS. However, much attention paid interpretation active. First, shown tagged fluorescent protein exclusively dark (Kircher 2002Kircher Gil Kozma-Bognar Fejes Speth V. Husselstein-Muller Bauer Adam Nucleocytoplasmic A, B, C, D, regulated differentially diurnal rhythm.Plant 2002; 14: 1541-1555Crossref (241) FHY1/FHL Thus, FHY1/FHL-dependent reliably restrict dark—where solely FHY1/FHL—but efficiently Scholar) (Figure 1). Nuclear less tightly Scholar), relatively slow takes hours kinetics transport, early (Tepperman 2006Tepperman J.M. Hwang Y.S. Quail P.H. red-light signals rapidly responding genes initiation de-etiolation.Plant 48: 728-742Crossref Second, noted active form. For example, displayed ~10- 30-fold inhibitory compared toward cells Y242H-NLS-YFP, locked Pfr-like triggered more pronounced did Third, date induce obvious (Toledo-Ortiz Overexpression seems cause visible (i.e., slight opening apical hook most) (Lyu 2019Lyu Shi Kuang Chen Kou Zhong Oligomerization photo-deoligomerization HOOKLESS1 controls differential cell growth.Dev. 2019; 78-88Abstract (11) Endogenous does contain functional NLS, FHY1/FHL-mediated prevents Therefore, reasonable ask whether pool exists finding any relevance mere curiosity. With regard questions, note major portion being state, photoconverts monochromatic low R/FR ratio, deep canopy shade even plants, conditions activate signaling. function triggering 1) needs tested experimentally future.