Phytochrome Signaling Mechanisms

Abstract

Phytochromes are red (R)/far-red (FR) light photoreceptors that play fundamental roles in photoperception of the light environment and the subsequent adaptation of plant growth and development. There are five distinct phytochromes in Arabidopsis thaliana, designated phytochrome A (phyA) to phyE. phyA is light-labile and is the primary photoreceptor responsible for mediating photomorphogenic responses in FR light, whereas phyB-phyE are light stable, and phyB is the predominant phytochrome regulating de-etiolation responses in R light. Phytochromes are synthesized in the cytosol in their inactive Pr form. Upon light irradiation, phytochromes are converted to the biologically active Pfr form, and translocate into the nucleus. phyB can enter the nucleus by itself in response to R light, whereas phyA nuclear import depends on two small plant-specific proteins FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) and FHY1-LIKE (FHL). Phytochromes may function as light-regulated serine/threonine kinases, and can phosphorylate several substrates, including themselves in vitro. Phytochromes are phosphoproteins, and can be dephosphorylated by a few protein phosphatases. Photoactivated phytochromes rapidly change the expression of light-responsive genes by repressing the activity of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), an E3 ubiquitin ligase targeting several photomorphogenesis-promoting transcription factors for degradation, and by inducing rapid phosphorylation and degradation of Phytochrome-Interacting Factors (PIFs), a group of bHLH transcription factors repressing photomorphogenesis. Phytochromes are targeted by COP1 for degradation via the ubiquitin/26S proteasome pathway.

The Arabidopsis ZINC FINGER PROTEIN3 Interferes with Abscisic Acid and Light Signaling in Seed Germination and Plant Development

“…Light signals are perceived by different receptors, of which phytochromes are dominant photoreceptors of red and far-red light, playing important roles in the regulation of germination and photomorphogenesis, including the inhibition of hypocotyl elongation (Franklin and Quail, 2010;Li et al, 2011). Our data show that ZFP3 and the closely related ZFPtype factors inhibit hypocotyl elongation, displaying short hypocotyls in light and in darkness.…”

COP1 mediates the coordination of root and shoot growth by light through modulation of PIN1- and PIN2-dependent auxin transport in Arabidopsis

“…Two distinct developmental programs, known as photomorphogenesis and skotomorphogenesis (or etiolation) in light and in darkness, respectively, control these radically different growth patterns (Nemhauser and Chory, 2002;Quail, 2002). Genetic analysis of the CONSTITUTIVE PHOTOMORPHOGENIC/DE-ETIOLATED/FUSCA (COP/DET/ FUS) loci of Arabidopsis thaliana (hereafter Arabidopsis) led to substantial insights into the molecular mechanisms regulating lightmediated morphogenesis (Yi and Deng, 2005;Li et al, 2011). The cop/det/fus mutants display a de-etiolated phenotype when grown in darkness and most of them are defective in the components of a macromolecular complex named the COP9 signalosome (CSN).…”

“…The CSN regulates the 26S proteasome-mediated degradation of photomorphogenesis-promoting transcription factors, a central step in the regulation of developmental responses to light (Yi and Deng, 2005;Li et al, 2011). The RING E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) ubiquitinates these transcription factors, allowing their recognition by the CSN and their subsequent degradation by the 26S proteasome (Yi and Deng, 2005;Henriques et al, 2009;Lau and Deng, 2010).…”

Phytochrome controls alternative splicing to mediate light responses in Arabidopsis

“…The N-terminal domain is sufficient for transducing light signals in the nucleus to induce photomorphogenesis (3,4). Pfr is translocated from the cytoplasm to the nucleus (5,6), where it inhibits two major negative regulators of photomorphogenesis-that is, phytochrome-interacting factors (PIFs) and CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1)-to elicit light responses in plants (7). PIFs are basic helix-loop-helix transcription factors that directly interact with Pfr (8).…”