ArabidopsisPhytochrome B Promotes SPA1 Nuclear Accumulation to Repress Photomorphogenesis under Far-Red Light

Xu Zheng,Haiyang Wang,Peng Zhou,Meifang Song,Jianping Yang,Zhiyong Li,Yulin Xi,Li Yang,Liang Su,Fanhua Meng,Suowei Wu,Huqu Zhai,Yingfan Cai

doi:10.1105/tpc.112.107086

Abstract

Phytochrome A (phyA) is the primary photoreceptor mediating deetiolation under far-red (FR) light, whereas phyB predominantly regulates light responses in red light. SUPPRESSOR OF PHYA-105 (SPA1) forms an E3 ubiquitin ligase complex with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), which is responsible for the degradation of various photomorphogenesis-promoting factors, resulting in desensitization to light signaling. However, the role of phyB in FR light signaling and the regulatory pathway from light-activated phytochromes to the COP1-SPA1 complex are largely unknown. Here, we confirm that PHYB overexpression causes an etiolation response with reduced ELONGATED HYPOCOTYL5 (HY5) accumulation under FR light. Notably, phyB exerts its nuclear activities and promotes seedling etiolation in both the presence and absence of phyA in response to FR light. PhyB acts upstream of SPA1 and is functionally dependent on it in FR light signaling. PhyB interacts and forms a protein complex with SPA1, enhancing its nuclear accumulation under FR light. During the dark-to-FR transition, phyB is rapidly imported into the nucleus and facilitates nuclear SPA1 accumulation. These findings support the notion that phyB plays a role in repressing FR light signaling. Activity modulation of the COP1-SPA E3 complex by light-activated phytochromes is an effective and pivotal regulatory step in light signaling.

Highlights

As sessile organisms, plants have evolved a high degree of developmental plasticity to optimize their growth and reproduction
To investigate the role and regulatory mechanism of Arabidopsis phyB in FR light signaling, we generated transgenic plants that overexpress the full-length PHYB gene fused with green fluorescent protein (PHYB-GFP) under the control of the constitutive cauliflower mosaic virus 35S promoter
Seedlings were grown in the dark (Dk) or FR light (2.5 μmol$m–2$s–1) for 4 d. (A) Nuclear accumulation of GUS-CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) in the parental No-0 GUS-COP1 plants but not in the spa1-3 GUSCOP1 plants under FR light

Summary

Introduction

Plants have evolved a high degree of developmental plasticity to optimize their growth and reproduction. Light is one of the most important factors modulating many developmental processes of plants, from seed germination to the time of flowering (Deng and Quail, 1999; Li et al, 2011). Plants possess a series of photoreceptors that monitor light quality, quantity, and duration (Briggs and Olney, 2001; Lin, 2002; Christie, 2007; Rizzini et al, 2011). Prominent among these are the red (R)/far-red (FR) reversible photoreceptors, the phytochromes. The phyA phyB double mutant has hypocotyl elongation similar to that of the phyA mutant under FR light, it exhibits less unhooking than the phyA mutant

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Conclusion