Phytochromes are a family of red/far-red light-sensitive photoreceptors found in plants and other organisms. They regulate plant growth and development by sensing the ratio of red (R) to far-red (FR) light. Phytochromes exist in two interconvertible forms, P_r (red-absorbing) and P_fr (far-red-absorbing), which undergo photoconversion upon light exposure. This process involves a Z-E isomerization of a bilin chromophore, which is covalently attached to the phytochrome protein. The structure of phytochromes includes a conserved N-terminal photosensory core with domains such as P2/PAS, P3/GAF, and P4/PHY, and a C-terminal regulatory region, often containing a histidine kinase-related domain. Recent structural studies, including the crystal structure of the DrBphP photosensory core, have provided insights into the molecular mechanisms of phytochrome function, including the photoconversion process and signal transduction. Phytochromes are bilin lyases that assemble their chromophores through specific domains, and their signaling mechanisms involve light-induced conformational changes and interactions with regulatory domains. The photoconversion of P_r to P_fr and back is central to phytochrome function, with the P_fr state being less stable and more prone to dark reversion. Phytochrome signaling in plants involves dynamic nuclear localization and interactions with transcription factors, while in bacteria, phytochromes function in two-component signaling pathways. The study highlights the importance of phytochrome structure and function in light signaling, with ongoing research aiming to elucidate the molecular details of phytochrome signaling and its role in various organisms.Phytochromes are a family of red/far-red light-sensitive photoreceptors found in plants and other organisms. They regulate plant growth and development by sensing the ratio of red (R) to far-red (FR) light. Phytochromes exist in two interconvertible forms, P_r (red-absorbing) and P_fr (far-red-absorbing), which undergo photoconversion upon light exposure. This process involves a Z-E isomerization of a bilin chromophore, which is covalently attached to the phytochrome protein. The structure of phytochromes includes a conserved N-terminal photosensory core with domains such as P2/PAS, P3/GAF, and P4/PHY, and a C-terminal regulatory region, often containing a histidine kinase-related domain. Recent structural studies, including the crystal structure of the DrBphP photosensory core, have provided insights into the molecular mechanisms of phytochrome function, including the photoconversion process and signal transduction. Phytochromes are bilin lyases that assemble their chromophores through specific domains, and their signaling mechanisms involve light-induced conformational changes and interactions with regulatory domains. The photoconversion of P_r to P_fr and back is central to phytochrome function, with the P_fr state being less stable and more prone to dark reversion. Phytochrome signaling in plants involves dynamic nuclear localization and interactions with transcription factors, while in bacteria, phytochromes function in two-component signaling pathways. The study highlights the importance of phytochrome structure and function in light signaling, with ongoing research aiming to elucidate the molecular details of phytochrome signaling and its role in various organisms.