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

Phytochromes are evolutionarily conserved photoreceptors in bacteria, fungi, and plants. The prototypical phytochrome comprises an N-terminal photosensory module and a C-terminal histidine kinase signaling-output module. However, the plant phytochrome has been postulated to transduce light signals by interacting with a group of nodal Phytochrome-Interacting transcription Factors (PIFs) and triggering their degradation via the N-terminal photosensory module, while its C-terminal output module, including a Histidine Kinase-Related Domain (HKRD), is thought not to participate directly in signaling. Here, we show that the C-terminal module of Arabidopsis phytochrome B (PHYB) is unexpectedly sufficient to mediate the degradation of PIF3 and to induce a distinct set of PIF-regulated photosynthetic genes. These signaling functions require the HKRD and particularly its dimerization. A D1040V mutation, which disrupts the dimerization of HKRD and the interaction between the C-terminal module and PIF3, abrogates the early light signaling functions of PHYB in nuclear accumulation, photobody biogenesis, and PIF3 degradation. In contrast, disruption of the interaction between PIF3 and PHYB's N-terminal photosensory module has little effect on PIF3 degradation. Together, this study provides novel insight into the central mechanism of early phytochrome signaling that the C-terminal signaling-output module of PHYB interacts with PIF3 in the nucleus to mediate PIF3 degradation by light. Overall design: Whole seedling mRNA profiles of 100h dark-grown phyB-9 mutant and BCY overexpression line were generated by deep sequencing, in triplicate, using Illumina NextSeq 500

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Qiu Y, Pasoreck EK, Reddy AK, Nagatani A, Ma W, Chory J, Chen M