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.