Phytochrome

A plant grown in the dark appears long and spindly, is pale yellow, and has unexpanded leaves. When transferred to light, the growth rate of the stem slows, chloroplasts begin to develop and accumulate chlorophyll, and the primary leaves begin to expand and develop. Many of these dramatic changes are the result of activation of light receptors (photoreceptors) called phytochromes. Phytochromes are proteins with an attached pigment molecule that allows them to detect light, especially in the red and far-red region of the spectrum. Depending on the light conditions, a phytochrome molecule may be converted to an active form or reconverted to an inactive form.

Most plants have more than one gene coding for different phytochromes, and these different products of the phytochrome gene family frequently control different responses to the light environment. Phytochromes regulate many aspects of plant growth and development by measuring the duration, intensity, and wavelengths of light. From the information gathered through phytochromes, a plant can determine the season, time of day, and whether it is growing beneath other plants versus in an open field. These photoreceptors control numerous functions throughout the life of the plant, including whether seeds germinate, how rapidly cells expand and divide, which genes are expressed, what shape and form a plant will take, and when the organism will flower and produce new seeds.

see also Photoperiodism; Rhythms in Plant Life.

Timothy W. Short

Bibliography

Sage, Linda C. Pigment of the Imagination: A History of Phytochrome Research. San Diego, CA: Academic Press, 1992.

Quail, Peter H., Margaret T. Boylan, Brian M. Parks, Timothy W. Short, Yong Xu, and Doris Wagner. "Phytochromes: Photosensory Perception and Signal Transduction." Science 268 (1995): 675-80

phytochrome A plant pigment that can detect the presence of absence of light and is involved in regulating many processes that are linked to day length (photoperiod), such as seed germination and initiation of flowering. It consists of a light-detecting portion, called a chromophore, linked to a small protein and exists in two interconvertible forms with different physical properties, particularly in the ability to bind to membranes. When exposed to daylight, inactive phytochrome absorbs red light (wavelength 660 nm), causing the protein part of the molecule to alter its folding (conformation) and making it bind more readily to plasma membranes. Also, the absorption spectrum shifts, so that it now absorbs light in the far-red region of the spectrum, at about 730 nm. This physiologically active form of phytochrome is called P_fr (for ‘far red’). In darkness, P_fr reverts over several hours to the inactive P_r (‘red’) form. See also photomorphogenesis; photoperiodism.

phytochrome A photoreversible pigment, which occurs in every major taxonomic group of plants. It exists in 2 interchangeable forms with respect to absorption, a red and a far-red form. This phenomenon is utilized in plants as a switch mechanism, linked to environmental light signals, to control many activities. These include flowering, germination, vegetative growth, circadian rhythms, and many more physiological activities.