Plants possess a highly sensitive photosensor phytochrome. Red wavelength light is perceived by the Pr form of photosensor molecule, which is photochemically converted to the Pfr form. The latter then triggers two types of cellular signal transduction in plants, one involving expression of several genes that are nt transcribed in the dark and another involving organelle movement and morphological/circadian rhythms such as flowering, etc. The Pfr form can be reverted to the Pr form by far-red wavelength light. The molecular mechanism of the phytocrome-mediated photosignal responses in plants still has not been fully elucidated. The long term objective of the proposed study is, therefore, is to understand the mechanisms of phytochrome action at the structural-mechanistic level. Our specific aim of the proposed research is to elucidate the structure-function relationship of the photochromic light sensor in plants and in model systems such as protoplasts. Since the gene regulatory role and the photosensory transduction process mediated by phytochrome are likely to reveal a useful contrast and/or similarity to the hormonal gene regulation and sensory transduction (e.g.m vision) in animals, the proposed study is biomedically relevant in enhancing our understanding of the basic principles of regulatory biology and sensory physiology.