The phytochromes: photosensory perception and signal transduction.

Phytochromes are regulatory photoreceptors which primarily absorb red (R) and far-red (FR) light. A great deal is known about the spectroscopic properties, primary structure, gene regulation and gross structure of phytochromes, and about the set of developmental changes which they control, but the early steps in signal transduction from phytochrome which result in these changes are still mysterious. In angiosperms, phytochromes are encoded by a small gene family, and as a result of recent work with mutants and transgenic overexpressors it is possible to assign distinct functions to some of the individual types of phytochrome. For two of these, phytochrome A and phytochrome B, overexpression of chimeras has revealed that the determinants for their photosensory specificity and the light-promoted degradation of phytochrome A reside on the amino-terminal halves of the molecules. The interchangeability of the C-terminal halves suggests that they may share a common signal transduction mechanism. These results also invite a reappraisal of the various models that have been proposed over the years to explain the complexity of phytochrome sensitivity to various light regimes.