Photosynthetic organisms, from bacteria to higher plants, possess light sensing molecules that enable adaptation to fluctuations in intensity, direction, duration, polarization and spectral quality of light from their environment (26). The most well known of these photoreceptors are the phytochromes, which sense the ambient light conditions via their ability to photointerconvert between red (Pr) and far-red (Pfr) light absorbing forms (17,47,51,57). This unique property of phytochromes is conferred by a linear tetrapyrrole (bilin) prosthetic group that is covalently linked to a large polypeptide. First discovered in plants, phytochrome-like molecules also have been identified in lower eukaryotic plant species (i.e. green algae, mosses, and ferns) (27,64) and, more recently, in cyanobacteria (21,25,66,70). It is well established that higher plants possess multiple phytochromes that are encoded by a small nuclear gene family termed PHYA-F (5, 45).