Chapter 19 - Phytochrome

Abstract

The understanding of the molecular basis of the photochromic behavior of phytochrome elucidates the interactions of the protein with other cellular components and thus contribute to comprehend the mode of action of the pigment. Biological photoreceptors have been classified as either light transducers (photosensors or photoperceptors), sensing the quality and quantity of radiation, or as energy converters. Among the first, there are different molecular species triggering photomovement at various levels, for example, movement of motile organisms, intracellular movement, and movement of plant organs in higher plants. Another example is of phytochrome and various blue light/UV-A (320-400 rim) and UV-B (280-320 rim) photoreceptors, all of which control to different degrees plant growth and development, and lastly the visual pigments, in particular rhodopsin in vertebrates and various types of pigments in invertebrates. Among the energy converters, bacteriorhodopsin and the protein complexes of the chlorophylls and of bacteriochlorophyll are found. Most biological photoreceptor systems are photochromic, that is, after performing the photochemically triggered reaction, the system is somehow restored to its initial state and is ready to absorb another quantum of radiation in order to photoinitiate again the specific function. Few of these pigments remain photochromic when isolated from the living complex system of which they are part. This is the case for phytochrome. This chapter is a condensed exposition on the present views and thoughts on the molecular basis for the transformation between the two stable forms of the pigment.