Regulatory photoreceptor which exists in two forms that are reversibly interconvertible by light: the Pr form that absorbs maximally in the red region of the spectrum and the Pfr form that absorbs maximally in the far-red region. Photoconversion of Pr to Pfr induces an array of morphogenetic responses, whereas reconversion of Pfr to Pr cancels the induction of those responses. Pfr controls the expression of a number of nuclear genes including those encoding the small subunit of ribulose-bisphosphate carboxylase, chlorophyll A/B binding protein, protochlorophyllide reductase, rRNA, etc. It also controls the expression of its own gene(s) in a negative feedback fashion. Involved in the flowering time regulation. Involved in light-regulated circadian phase control that triggers stomatal aperture, stomatal conductance, and CO(2) assimilation. Implicated in red light perception, and, to a lower extent, in blue light signaling (PubMed:12177480). Regulates temperature responses by associating with the promoters of key target genes in a temperature-dependent manner and subsequently repressing their expression probably in a PIF4-dependent manner. Thermal timer that integrates temperature information over the course of the night (PubMed:27789797). Homodimer (PubMed:24982198). Interacts with ADO1 and PKS4. Stabilized by interactions with PAPP5 and FYPP3 which are enhanced in the phosphorylated Pfr form. Interacts with VOZ1 and VOZ2 (PubMed:11260718, PubMed:12468726, PubMed:15707897, PubMed:18390804, PubMed:22904146). Binds, via its photosensory domain, to PTAC12/HMR when photoactivated; this interaction stimulates its localization to photobodies (PubMed:22895253). Interacts with CRY1 specifically when in the dark/far-red (Pr) state, but not when red light-activated (Pfr) (PubMed:22577138). Interacts with PIF4 AND PIF5 in response to low blue light (LBL) (PubMed:26724867). Component of a red light-dependent nuclear complex made of PHL, PHYB and CO. Interacts directly with PHL (PubMed:24127609). Cytoplasmic in darkness, but translocated to the nucleus upon illumination, when associated with PAPP5 into speckles. Inactivation is proportional to temperature in the dark. Contains one covalently linked phytochromobilin chromophore. In oop1, defects in circadian timing with altered phase; early timing of the peak (acrophase) of multiple circadian rhythms such as leaf movement, CO(2) assimilation and light-induced gene expression. Strong photoperception defect in red light leading to long hypocotyls; this phenotype is increased when blue lights are combined to red lights. Increased sensitivity to SO(2). Elongated internodes before the transition to flowering when grown in short day conditions. Belongs to the phytochrome family.