Phytochromes are a family of red/far-red light-responsive photoreceptors that play crucial roles in plant growth and development. They are characterized by their conserved N-terminal photosensory core and C-terminal regulatory region. The photosensory core contains a bilin chromophore, which can photoconvert between red-absorbing (P_r) and far-red-absorbing (P_fr) forms, enabling light detection. Recent structural and biochemical studies have provided insights into the photochemical processes and signaling mechanisms of phytochromes. The N-terminal photosensory core consists of PAS, GAF, and PHY domains, while the C-terminal regulatory region includes histidine kinase-related domains. The chromophore structure, particularly the P_fr state, has been elucidated through crystallographic studies, revealing its conformational changes during photoconversion. Phytochromes utilize bilin chromophores, which are derived from heme and can be photoisomerized. The regulatory domains in phytochromes can be fused with various signaling modules, allowing them to integrate light signals with metabolic and environmental cues. The molecular mechanisms of phytochrome signaling vary among prokaryotic and plant phytochromes, but generally involve light-mediated changes in domain interactions and subunit dissociation. Understanding the structural and functional aspects of phytochromes is essential for elucidating their roles in plant biology and other non-photosynthetic organisms.Phytochromes are a family of red/far-red light-responsive photoreceptors that play crucial roles in plant growth and development. They are characterized by their conserved N-terminal photosensory core and C-terminal regulatory region. The photosensory core contains a bilin chromophore, which can photoconvert between red-absorbing (P_r) and far-red-absorbing (P_fr) forms, enabling light detection. Recent structural and biochemical studies have provided insights into the photochemical processes and signaling mechanisms of phytochromes. The N-terminal photosensory core consists of PAS, GAF, and PHY domains, while the C-terminal regulatory region includes histidine kinase-related domains. The chromophore structure, particularly the P_fr state, has been elucidated through crystallographic studies, revealing its conformational changes during photoconversion. Phytochromes utilize bilin chromophores, which are derived from heme and can be photoisomerized. The regulatory domains in phytochromes can be fused with various signaling modules, allowing them to integrate light signals with metabolic and environmental cues. The molecular mechanisms of phytochrome signaling vary among prokaryotic and plant phytochromes, but generally involve light-mediated changes in domain interactions and subunit dissociation. Understanding the structural and functional aspects of phytochromes is essential for elucidating their roles in plant biology and other non-photosynthetic organisms.