Molecular evolution: Old branches on the phytochrome family tree

Phytochromes are a divergent family of plant photoreceptors that mediate a wide range of developmental and physiological responses to changes in light intensity and spectral quality. In the flowering plants, phytochromes regulate seed germination, circadian rhythms, leaf and stem development, and the timing of floral initiation [1]. Each phytochrome molecule exists as one of two distinct photoreceptors: one form (called Pr) has an absorption maximum in the red region of the spectrum, while the other form (Pfr) has an absorption maximum in the far-red region near the end of the visible spectrum. Light absorption by Pr causes conversion to the Pfr form, and absorption by Pfr causes conversion back to the Pr form. The Pfr form is thought to be the active signaling molecule for most developmental and physiological responses.