Experiments that were conducted after Garner and Allard's discoveries prompted researchers to look for a pigment that controlled photoperiodism. They had already found that the initiation of flowering could be inhibited if plants are exposed to even a brief period of red light during the night, which suggested the existence of a light-sensitive pigment. Within a few years, such a pigment was discovered. It was not visible, however, and a special pigment-analysis instrument had to be constructed to detect it. In 1959, after the pigment had been isolated, it was named phytochrome. Since 1959, several different phytochromes have been identified.
Pr far-red light far-red light
Pfr (active form)
Pfr (active form)
Phytochromes are extraordinary pale blue proteina-ceous pigments that apparently occur in all higher plants and are associated with the absorption of light. Only minute amounts are produced, mostly in meristematic tissues. Phytochromes occur in two stable forms, either of which can be converted to the other: Pred, or Pr, is a form that absorbs red light; Pfar-red, or Pfr, is an active related form that absorbs the far-red light found at the edge of the visible light spectrum. When either form absorbs light, it is converted to the other form, so that Pr becomes Pfr when it absorbs red light, and Pfr becomes Pr when it absorbs far-red light. Pr is stable indefinitely in the dark. The normal effect of light in nature is to cause more Pr to become Pfr than vice versa. Pfr converts back to Pr in the dark over a period of several hours or else becomes inactivated, but its conversion in the presence of appropriate light is instantaneous (Fig. 11.21).
Phytochrome pigments play a role in a great variety of plant responses: in addition to photoperiodism, they are involved with several aspects of plant development, changes in plastids, the production of anthocyanin pigments, and with the detection of shading by other plants.
One of the most studied phenomena associated with phytochrome pigments involves the germination of seeds. Some seeds, for example, do not germinate in darkness, but the red part of the spectrum in sunlight converts Pr to Pfr, which, in turn, somehow unblocks the germinating mechanism. If such seeds (e.g., those of "Grand Rapids" lettuce) are given suitable moisture and oxygen conditions and then exposed only to red light, they will germinate readily, but if they are given only far-red light, they will not. Furthermore, if they are given alternating flashes of red and far-red light, the type of light in the final flash determines whether or not they will germinate. If the last flash, which may be of only a few seconds duration, is red light, the seeds will germinate, but if it is far-red light, they will not.
When a seedling first emerges from the soil, light changes the Pr to Pfr, triggering a reduction in the production of ethylene by the cells. This, in turn, allows the crooks in the hypocotyls to relax, and the plant straightens up. Elongation of stems appears to be inhibited by Pfr. The mechanism is so sensitive that as little as 3 seconds of clear moonlight may produce a shortening of internodes of etiolated oat seedlings (those that are spindly and pale from
having been grown in the dark). More far-red light reaches young growing stems of a tree that is shaded, however, causing the Pf to be converted to Pr, thus lowering the inhibition so that the young stems grow longer and out from under the shade.
A second group of blue, light-sensitive pigments known as cryptochromes also play a role in circadian rhythms and evidently interact with phytochromes in plants in controlling reactions to light. Much of the research since cryptochromes were discovered has centered around fruit flies (Drosophila), cyanobacteria, and fungi. Cryptochromes have also been identified in humans.
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