The Properties and Biological Action of Phytochrome: Prologue

  • B. Thomas
Conference paper
Part of the NATO ASI Series book series (volume 50)


Plants possess sophisticated mechanisms for detecting light quality and quantity and adapting their patterns of growth and development depending upon their environment. This process of photomorphogenesis requires photoreceptor pigments which absorb light and provide the initial biochemical signal to trigger the plant’s photoresponse. The major photoreceptor in plants is phytochrome. This chromoprotein is found in all green plants including algae, mosses and ferns. It exists, characteristically, as two photoisomers called Pr and Pfr, each with a distinct absorbance spectrum. Typically, Pr has a major absorbance maximum (λmax) at about 660 nm and a secondary maximum at about 380 nm. Pfr on the other hand has a λmax at about 730 nm and a secondary peak at about 400 nm. When either form absorbs light a series of photochemical and protein conformational changes is initiated leading within milliseconds to the formation of the other photoisomer. For many responses red light at about 660 nm is the most effective part of the spectrum and the effect of red can be prevented by a subsequent far-red irradiation at about 730 nm. In such cases sequentially antagonistic actions of red and far-red can be seen over several cycles of irradiation. The effectiveness of Pfr-forming irradiation and reversal by Pr-forming irradiation leads inevitably to the conclusion that Pfr is the biologically active isomer and Pr an inactive form of phytochrome. It is worth bearing in mind that short alternating treatments with narrow waveband light, as used in these experiments is not necessarily comparable with natural irradiation. Nevertheless, the distinction between the biological activity of Pr and Pfr has provided the rationale underlying four decades of phytochrome research.


Phytochrome Gene Specific mRNA Level Phytochrome Control Chromophore Attachment Etiolate Plant 
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Copyright information

© Springer-Verlag Berlin Heidelberg 1991

Authors and Affiliations

  • B. Thomas
    • 1
  1. 1.Horticulture Research InternationalLittlehampton, West SussexUK

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