Mechanism of early light signaling by the carboxy-terminal output module of Arabidopsis phytochrome B

Abstract

Plant phytochromes are thought to transduce light signals by mediating the degradation of phytochrome-interacting transcription factors (PIFs) through the N-terminal photosensory module, while the C-terminal module, including a histidine kinase-related domain (HKRD), does not participate in signaling. Here we show that the C-terminal module of Arabidopsis phytochrome B (PHYB) is sufficient to mediate the degradation of PIF3 specifically and to activate photosynthetic genes in the dark. The HKRD is a dimerization domain for PHYB homo and heterodimerization. A D1040V mutation, which disrupts the dimerization of HKRD and the interaction between C-terminal module and PIF3, abrogates PHYB nuclear accumulation, photobody biogenesis, and PIF3 degradation. By contrast, disrupting the interaction between PIF3 and PHYB’s N-terminal module has little effect on PIF3 degradation. Together, this study demonstrates that the dimeric form of the C-terminal module plays important signaling roles by targeting PHYB to subnuclear photobodies and interacting with PIF3 to trigger its degradation.

Differential phosphorylation of the N‐terminal extension regulates phytochrome B signaling

“…1a). The OPM contains two PAS-related domains (PAS-A, PAS-B) and a Histidine Kinase Related Domain (HKRD) and is required for phytochrome dimerization (Nagatani, 2010;Qiu et al, 2017). The OPM contains two PAS-related domains (PAS-A, PAS-B) and a Histidine Kinase Related Domain (HKRD) and is required for phytochrome dimerization (Nagatani, 2010;Qiu et al, 2017).…”

Increases in ambient temperature progressively disassembleArabidopsisphytochrome B from individual photobodies with distinct thermostabilities

“…Because PB localization requires the dimeric Pfr:Pfr form of phyB 43,46 , the thermostability of a PB is likely determined by the stability of the Pfr:Pfr dimers in the PB. Consistent with this notion, our results showed that the thermosensitivity of PBs is determined by the N-terminal photosensory module of phyB.…”

“…The dynamics of PBs have been characterized extensively using transgenic lines expressing phyB-FP 36,39,40,[44][45][46]54,55 , and therefore, to examine the dynamics of phyB-FP PBs induced by temperature changes, we went back to the phyB-FP lines, including PBG (phyB-GFP), which expresses functional phyB-GFP to complement the null phyB-5 mutant in the Landsberg erecta (Ler) background 36 , and PBC (phyB-CFP), which expresses phyB-CFP to complement the null phyB-9 mutant in Col-0 46,55 . Because both PBG and PBC overexpress phyB-FP 36,55 , we first examined whether they retain the same temperature effects as their respective ecotype background.…”

“…The structural basis of the light-dependent photobody localization of phyB has been extensively investigated. PhyB is a homodimer; PB localization requires the dimeric Pfr form of phyB 40,41,43,46,59 . Each phyB monomer contains an N-terminal photosensory module and a Cterminal output module 20,21 .…”

“…Each phyB monomer contains an N-terminal photosensory module and a Cterminal output module 20,21 . PB-localization of phyB is mediated by the dimeric form of phyB's C-terminal output module, which presumably contains localization signals or sequences for targeting phyB to the nucleus and PBs 46,54,55 . PhyB's C-terminal module alone localizes constitutively to PBs 54,55 .…”