The Phytochromes, a Family of Red/Far-red Absorbing Photoreceptors*

Plants Possess Multiple Photoreceptors Because plants are photo-auxotrophic they are particularly sensitive to their light environment. To fine-tune their development according to light intensity, direction, spectral quality, and periodicity they possess a multiplicity of light sensors (1). In Arabidopsis there are eight identified photoreceptors, but this list is still incomplete. It includes three UV-A/blue light receptors (phototropin, a photoreceptor to sense light direction, and two cryptochromes that mediate many photomorphogenic responses (2, 3)) and five phytochromes (phy) named phyA–phyE that absorb mainly red/far-red light, with phyA also responding to broad-spectrum light (UV-A to far-red) of very low intensity (4). All these photoreceptors bind to a chromophore, which for the phytochromes is a linear tetrapyrrole (phytochromobilin) (5). Because many light effects are induced by the co-action of several photoreceptors and because some photoreceptors regulate multiple aspects of photomorphogenesis, a genetic approach was instrumental for dissecting the specific roles of individual photoreceptors (1). As a consequence, research has concentrated on a few species that are particularly well suited for molecular genetic studies, in particular Arabidopsis (6).