The Degradation of HFR1, a Putative bHLH Class Transcription Factor Involved in Light Signaling, Is Regulated by Phosphorylation and Requires COP1

Abstract

All developmental transitions throughout the life cycle of a plant are influenced by light. In Arabidopsis, multiple photoreceptors including the UV-A/blue-sensing cryptochromes (cry1-2) and the red/far-red responsive phytochromes (phyA-E) monitor the ambient light conditions. Light-regulated protein stability is a major control point of photomorphogenesis. The ubiquitin E3 ligase COP1 (constitutively photomorphogenic 1) regulates the stability of several light-signaling components. HFR1 (long hypocotyl in far-red light) is a putative transcription factor with a bHLH domain acting downstream of both phyA and the cryptochromes. HFR1 is closely related to PIF1, PIF3, and PIF4 (phytochrome interacting factor 1, 3 and 4), but in contrast to the latter three, there is no evidence for a direct interaction between HFR1 and the phytochromes. Here, we show that the protein abundance of HFR1 is tightly controlled by light. HFR1 is an unstable phosphoprotein, particularly in the dark. The proteasome and COP1 are required in vivo to degrade phosphorylated HFR1. In addition, HFR1 can interact with COP1, consistent with the idea of COP1 directly mediating HFR1 degradation. We identify a domain, conserved among several bHLH class proteins involved in light signaling , as a determinant of HFR1 stability. Our physiological experiments indicate that the control of HFR1 protein abundance is important for a normal de-etiolation response.

A Negative Feedback Loop between PHYTOCHROME INTERACTING FACTORs and HECATE Proteins Fine-Tunes Photomorphogenesis in Arabidopsis

“…In contrast, HFR1, PAR1, and PAR2 are induced in response to light Roig-Villanova et al, 2006). HFR1 is also degraded in the dark and stabilized in response to light (Duek et al, 2004;Yang et al, 2005). Therefore, HECs are expected to function more in the dark and dark-to-light transition, while HFR1, PAR1, and PAR2 are expected to function under prolonged light conditions.…”

Phytochrome‐mediated inhibition of shade avoidance involves degradation of growth‐promoting bHLH transcription factors

“…Seedlings were grown as described previously (Duek et al, 2004). All mutants are in the ecotype Columbia (Col-0).…”

“…These experiments were performed as described in previously (Duek et al, 2004), except that Roche anti-HA antibodies (3F10; http://www.roche.com) directly conjugated with peroxidase were used to detect HA-tagged proteins. For phosphatase treatments, seedlings were homogenized with 110 ll of ice-cold extraction buffer [50 mM Tris-HCl, pH 7.5, 150 mM NaCl, 1% Na deoxycholate, 0.5% Triton X-100, 1 mM DTT, 10 ll ml -1 protease inhibitor cocktail (Sigma, http://www.sigmaaldrich.com/), 50 mM MG132, 50 mM MG115, 50 mM ALLN1, 50 mM PS1).…”

Biochemical Characterization ofArabidopsisComplexes Containing CONSTITUTIVELY PHOTOMORPHOGENIC1 and SUPPRESSOR OF PHYA Proteins in Light Control of Plant Development

“…However, COP1 also acts as a positive regulator for several specific responses in tolerance toward low levels of UV-B light and possibly in a response mediated by PHYB (Boccalandro et al, 2004;Oravecz et al, 2006). Molecularly, COP1 acts as an E3 ligase to target several photomorphogenesis-promoting transcription factors, such as ELONGATED HYPOCOTYL5 (HY5) (Osterlund et al, 2000b), LONG AFTER FAR-RED LIGHT1 (Seo et al, 2003), and LONG HYPOCOTYL IN FAR-RED1 (HFR1) (Duek et al, 2004;Jang et al, 2005;Yang et al, 2005;Lin and Wang, 2007), for degradation via the 26S proteasome. In addition, the far-red light photoreceptor phyA (Seo et al, 2004) and the blue light photoreceptor cry2 (Wang et al, 2001;Shalitin et al, 2002) are also likely targets of COP1.…”