The anatomy of phytochrome, a unique photoreceptor in plants

Red and far-red light control of plant growth and development is mediated by the photoreceptor phytochrome. The way plants utilize red and far-red light is unique in nature, as are the molecular properties of phytochrome, the molecule that provides the mechanistic basis for this type of light perception. Much of what we know about how plants perceive red light has come from research on the structure and function of this photoreceptor. This review discusses the main structural features of phytochrome and some new ideas concerning the relationship between phytochrome structure and function. We propose that phytochrome functions as a dimer and that receptor recognition of phytochrome depends on its gross conformation. We also describe a conserved amino acid repeat within the phytochrome molecule and propose that this repeat is important for dimerization and/or phototransformation.