Higher-plant phytochrome: “I used to date histidine, but now I prefer serine”

We all know that plants harvest light energy via the process of photosynthesis. Not quite so widely appreciated is that in addition to the chlorophylls and other light-absorbing pigments associated with the photosynthetic apparatus, plants use a separate class of photoreceptors to enable them to sense both the quality and quantity of light in the surrounding environment. In these respects plants can see, in a manner not totally distinct from you and I. In response to this information, they adjust their growth habits accordingly and hence maximize their capacity to use radiant energy. The primary photoreceptors of higher plants are the phytochromes and cryptochromes—quite distinct from the rhodopsins that animals use for vision. Phytochrome mediates responses to red/far-red light through a tetrapyrolle chromophore (1, 2), whereas the cryptochromes respond to blue/UV-A light through a flavin (3, 4). An important and distinguishing feature of the phytochrome light-sensing system is that it measures, not the absolute amount of red/far-red light, but the ratio of the amount of light (the number of photons) corresponding to these two regions of the spectrum. Phytochrome acts as a light-regulated molecular switch, with the capacity to undergo repeated interconversion between the red light-absorbing phytochrome (Pr) form and the far-red light-absorbing phytochrome (Pfr). Whereas it has been generally agreed that Pfr is the active form of phytochrome, the actual biochemical nature of this activity has remained unclear and a matter of some controversy. This, in spite of almost a half century of intense research. The work described by Yeh and Lagarias (5) in this issue of the Proceedings sheds some light on this question, and in the eyes of many, it will be argued that the basic question concerning the general activity of phytochrome has now been laid to rest. From early studies, it had been …