Phytochrome

A pigment that controls most photomorphogenic responses in higher plants. Mechanisms have evolved in plants that allow them to adapt their growth and development to more efficiently seek and capture light and to tailor their life cycle to the climatic seasons. These mechanisms enable the plant to sense not only the presence of light but also its intensity, direction, duration, and spectral quality. Plants thus regulate important developmental processes such as seed germination, growth direction, growth rate, chloroplast development, pigmentation, flowering, and senescence, collectively termed photomorphogenesis.

To perceive light signals, plants use several receptor systems that convert light absorbed by specific pigments into chemical or electrical signals to which the plants respond. This signal conversion is called photosensory transduction. Pigments used include cryptochrome, a blue light-absorbing pigment; an ultraviolet light-absorbing pigment; and phytochrome, a red/far-red light-absorbing pigment.

Phytochrome consists of a compound that absorbs visible light (chromophore) bound to a protein. The chromophore is an open-chain tetrapyrrole closely related to the photosynthetic pigments found in the cyanobacteria and similar in structure to the circular tetrapyrroles of chlorophyll and hemoglobin. Phytochrome is one of the most intensely colored pigments found in nature, enabling phytochrome in seeds to sense even the dim light present well beneath the surface of the soil and allowing leaves to perceive moonlight. See Chlorophyll, Hemoglobin

Phytochrome can exist in two stable photointerconvertible forms, P_r or P_fr, with only P_fr being biologically active. Absorption of red light (near 666 nanometers) by inactive P_r converts it to active P_fr, while absorption of far-red light (near 730 nm) by active P_fr converts phytochrome back to inactive P_r. Plants frequently respond quantitatively to light by detecting the amount of P_fr produced. As a result, the amount of P_fr must be strictly regulated nonphotochemically by precisely controlling both the synthesis and degradation of the pigment.

Phytochrome has a variety of functions in plants. Initially, production of P_fr is required for many seeds to begin germination. This requirement prevents germination of seeds that are buried too deep in the soil to successfully reach the surface. In etiolated (dark-grown) seedlings, phytochrome can measure an increase in light intensity and duration through the increased formation of P_fr. Light direction also can be deduced from the asymmetry of P_fr levels from one side of the plant to the other. Different phytochrome responses vary in their sensitivity to P_fr; some require very low levels of P_fr (less than 1% of total phytochrome) to elicit a maximal response, while others require almost all of the pigment to be converted to P_fr. Thus, as the seedling grows toward the soil surface, a cascade of photomorphogenic responses are induced, with the more sensitive responses occurring first. This chain of events produces a plant that is mature and photosynthetically competent by the time it finally reaches the surface. Production of P_fr also makes the plant aware of gravity, inducing shoots to grow up and roots to grow down into the soil. See Plant movements, Seed

In light-grown plants, phytochrome allows for the perception of daylight intensity, day length, and spectral quality. Intensity is detected through a measurement of phytochrome shuttling between P_r and P_fr; the more intense the light, the more interconversion. This signal initiates changes in chloroplast morphology to allow shaded leaves to capture light more efficiently. If the light is too intense, phytochrome will also elicit the production of pigments to protect plants from photodamage.

Temperate plants use day length to tailor their development, a process called photoperiodism. How the plant measures day length is unknown, but it involves phytochrome and actually measures the length of night. See Photoperiodism

Finally, phytochrome allows plants to detect the spectral quality of light, a form of color vision, by measuring the ratio of P_r to P_fr. When a plant is grown under direct sun, the amounts of red and far-red light are approximately equal, and the ratio of P_r to P_fr in the plant is about 1:1. Should the plant become shaded by another plant, the P_r/P_fr ratio changes dramatically to 5:1 or greater. This is because the shading plant's chlorophyll absorbs much of the red light needed to produce P_fr and absorbs almost none of the far-red light used to produce P_r. For a shade-intolerant plant, this change in P_r/P_fr ratio induces the plant to grow taller, allowing it to grow above the canopy.

It is not known how phytochrome elicits the diverse array of photomorphogenic responses, but the regulatory action must result from discrete changes in the molecule following photoconversion of P_r to P_fr. These changes must then start a chain of events in the photosensory transduction chain leading to the photomorphogenic response. Many photosensory transduction chains probably begin by responding to P_fr or the P_r/P_fr ratio and branch off toward discrete end points. See Photomorphogenesis

Phytochrome

a light blue pigment of the group of complex proteins called chromoproteids that is present in the cells of photosynthesizing organisms. Phytochrome was first detected in 1959 by the American biochemist W. Butler in the cotyledons of turnip sprouts that had been cultivated in darkness.

The involvement of phytochrome in physiological processes is due to the presence of chromophore groups called bilins (mixtures of sodium taurocholate and glycocholate), which are similar to the chromophores of phycocyanins in terms of spectral and chromatographic properties. Phytochrome exists in two interconvertible forms, Ph₆₆₀ and Ph₇₃₀, which differ in the spectrums of absorption. Under red light with a wavelength of 660 nm, inactive Ph₆₆₀ is converted to Ph₇₃₀. The opposite conversion takes place either in darkness or under illumination with red light with a wavelength of 730 nm. It is thought that the interconversions are caused by the cis-transisomerization of the chromophore of phytochrome and by conformation rearrangements of the protein.

The properties of phytochrome underlie the phenomenon of photoperiodism in plants. Moreover, the conversion time of Ph₇₃₀ to Ph₆₆₀ in darkness is apparently used as a unit of measurement in counting time in the mechanisms of biological clocks. Phytochrome is involved in the regulation of seed germination and floral induction. Thus, red light inhibits the flowering of short-day plants but stimulates that of long-day ones. Longwave red light produces the opposite effect. Phytochrome is also responsible for photomorphogenetic reactions of plants. The wide range of physiological processes controlled by phytochrome and the occurrence of phytochrome in representatives of the most diverse species indicate that the pigment is an ancient, unique regulatory system.

The mechanism underlying phytochrome has not been fully studied. According to one hypothesis, it is associated with a change in the permeability of membranes. It has been established that phytochrome controls the syntheses of biological polymers (DNA, RNA, and proteins) and the biosynthesis of chlorophyll, carotenoids, anthocyanins, organic phosphates, and vitamins. Phytochrome activates cellular respiration and oxidative phosphorylation and accelerates the catabolic breakdown of polysaccharides, fats, and reserve proteins. On the subcellular and cellular levels, the regulation by phytochrome of plastid formation and cell division and elongation have been recorded.

REFERENCES

Konev, S. V., and I. D. Volotovskii. Fotobiologiia. Minsk, 1974.
Fotoreguliatsiia metabolizma i morfogeneza rastenii. Moscow, 1975.
Smith, H. Phytochrome and Photomorphogenesis. London, 1975.

L. G. EROKHINA

phytochrome

[′fīd·ə‚krōm] (biochemistry) A protein plant pigment which serves to direct the course of plant growth and development in response variously to the presence or absence of light, to photoperiod, and to light quality.

单词	phytochrome
释义	phytochrome phy·to·chrome P0279250 (fī′tə-krōm′)n. Any of several photoreceptive pigments in green plants that regulate processes such as flowering and seed germination. phytochrome (ˈfaɪtəʊˌkrəʊm) n (Botany) botany a blue-green pigment existing in two interchangeable forms, present in most plants, that mediates many light-dependent processes, including photoperiodism and the greening of leaves phy•to•chrome (ˈfaɪ təˌkroʊm) n. a plant pigment that is associated with the absorption of light in the photoperiodic response and that may regulate various types of growth and development. [1890–95] Phytochrome Phytochrome A pigment that controls most photomorphogenic responses in higher plants. Mechanisms have evolved in plants that allow them to adapt their growth and development to more efficiently seek and capture light and to tailor their life cycle to the climatic seasons. These mechanisms enable the plant to sense not only the presence of light but also its intensity, direction, duration, and spectral quality. Plants thus regulate important developmental processes such as seed germination, growth direction, growth rate, chloroplast development, pigmentation, flowering, and senescence, collectively termed photomorphogenesis. To perceive light signals, plants use several receptor systems that convert light absorbed by specific pigments into chemical or electrical signals to which the plants respond. This signal conversion is called photosensory transduction. Pigments used include cryptochrome, a blue light-absorbing pigment; an ultraviolet light-absorbing pigment; and phytochrome, a red/far-red light-absorbing pigment. Phytochrome consists of a compound that absorbs visible light (chromophore) bound to a protein. The chromophore is an open-chain tetrapyrrole closely related to the photosynthetic pigments found in the cyanobacteria and similar in structure to the circular tetrapyrroles of chlorophyll and hemoglobin. Phytochrome is one of the most intensely colored pigments found in nature, enabling phytochrome in seeds to sense even the dim light present well beneath the surface of the soil and allowing leaves to perceive moonlight. See Chlorophyll, Hemoglobin Phytochrome can exist in two stable photointerconvertible forms, P_r or P_fr, with only P_fr being biologically active. Absorption of red light (near 666 nanometers) by inactive P_r converts it to active P_fr, while absorption of far-red light (near 730 nm) by active P_fr converts phytochrome back to inactive P_r. Plants frequently respond quantitatively to light by detecting the amount of P_fr produced. As a result, the amount of P_fr must be strictly regulated nonphotochemically by precisely controlling both the synthesis and degradation of the pigment. Phytochrome has a variety of functions in plants. Initially, production of P_fr is required for many seeds to begin germination. This requirement prevents germination of seeds that are buried too deep in the soil to successfully reach the surface. In etiolated (dark-grown) seedlings, phytochrome can measure an increase in light intensity and duration through the increased formation of P_fr. Light direction also can be deduced from the asymmetry of P_fr levels from one side of the plant to the other. Different phytochrome responses vary in their sensitivity to P_fr; some require very low levels of P_fr (less than 1% of total phytochrome) to elicit a maximal response, while others require almost all of the pigment to be converted to P_fr. Thus, as the seedling grows toward the soil surface, a cascade of photomorphogenic responses are induced, with the more sensitive responses occurring first. This chain of events produces a plant that is mature and photosynthetically competent by the time it finally reaches the surface. Production of P_fr also makes the plant aware of gravity, inducing shoots to grow up and roots to grow down into the soil. See Plant movements, Seed In light-grown plants, phytochrome allows for the perception of daylight intensity, day length, and spectral quality. Intensity is detected through a measurement of phytochrome shuttling between P_r and P_fr; the more intense the light, the more interconversion. This signal initiates changes in chloroplast morphology to allow shaded leaves to capture light more efficiently. If the light is too intense, phytochrome will also elicit the production of pigments to protect plants from photodamage. Temperate plants use day length to tailor their development, a process called photoperiodism. How the plant measures day length is unknown, but it involves phytochrome and actually measures the length of night. See Photoperiodism Finally, phytochrome allows plants to detect the spectral quality of light, a form of color vision, by measuring the ratio of P_r to P_fr. When a plant is grown under direct sun, the amounts of red and far-red light are approximately equal, and the ratio of P_r to P_fr in the plant is about 1:1. Should the plant become shaded by another plant, the P_r/P_fr ratio changes dramatically to 5:1 or greater. This is because the shading plant's chlorophyll absorbs much of the red light needed to produce P_fr and absorbs almost none of the far-red light used to produce P_r. For a shade-intolerant plant, this change in P_r/P_fr ratio induces the plant to grow taller, allowing it to grow above the canopy. It is not known how phytochrome elicits the diverse array of photomorphogenic responses, but the regulatory action must result from discrete changes in the molecule following photoconversion of P_r to P_fr. These changes must then start a chain of events in the photosensory transduction chain leading to the photomorphogenic response. Many photosensory transduction chains probably begin by responding to P_fr or the P_r/P_fr ratio and branch off toward discrete end points. See Photomorphogenesis Phytochrome a light blue pigment of the group of complex proteins called chromoproteids that is present in the cells of photosynthesizing organisms. Phytochrome was first detected in 1959 by the American biochemist W. Butler in the cotyledons of turnip sprouts that had been cultivated in darkness. The involvement of phytochrome in physiological processes is due to the presence of chromophore groups called bilins (mixtures of sodium taurocholate and glycocholate), which are similar to the chromophores of phycocyanins in terms of spectral and chromatographic properties. Phytochrome exists in two interconvertible forms, Ph₆₆₀ and Ph₇₃₀, which differ in the spectrums of absorption. Under red light with a wavelength of 660 nm, inactive Ph₆₆₀ is converted to Ph₇₃₀. The opposite conversion takes place either in darkness or under illumination with red light with a wavelength of 730 nm. It is thought that the interconversions are caused by the cis-transisomerization of the chromophore of phytochrome and by conformation rearrangements of the protein. The properties of phytochrome underlie the phenomenon of photoperiodism in plants. Moreover, the conversion time of Ph₇₃₀ to Ph₆₆₀ in darkness is apparently used as a unit of measurement in counting time in the mechanisms of biological clocks. Phytochrome is involved in the regulation of seed germination and floral induction. Thus, red light inhibits the flowering of short-day plants but stimulates that of long-day ones. Longwave red light produces the opposite effect. Phytochrome is also responsible for photomorphogenetic reactions of plants. The wide range of physiological processes controlled by phytochrome and the occurrence of phytochrome in representatives of the most diverse species indicate that the pigment is an ancient, unique regulatory system. The mechanism underlying phytochrome has not been fully studied. According to one hypothesis, it is associated with a change in the permeability of membranes. It has been established that phytochrome controls the syntheses of biological polymers (DNA, RNA, and proteins) and the biosynthesis of chlorophyll, carotenoids, anthocyanins, organic phosphates, and vitamins. Phytochrome activates cellular respiration and oxidative phosphorylation and accelerates the catabolic breakdown of polysaccharides, fats, and reserve proteins. On the subcellular and cellular levels, the regulation by phytochrome of plastid formation and cell division and elongation have been recorded. REFERENCES Konev, S. V., and I. D. Volotovskii. Fotobiologiia. Minsk, 1974. Fotoreguliatsiia metabolizma i morfogeneza rastenii. Moscow, 1975. Smith, H. Phytochrome and Photomorphogenesis. London, 1975. L. G. EROKHINA phytochrome [′fīd·ə‚krōm] (biochemistry) A protein plant pigment which serves to direct the course of plant growth and development in response variously to the presence or absence of light, to photoperiod, and to light quality. phytochrome Fig. 252 Phytochrome . The photoconversion of the two forms. phytochrome a type of plant pigment that occurs in two forms, P₆₆₀ (P_r) which absorbs red light (600–700 nm) and P725 (Pf_r) which absorbs far-red light (700–760 nm). P₇₂ 5 is biologically active in that it stimulates enzymic reactions, whereas P₆₆₀ is biologically inert. The conversion of one form into the other occurs simultaneously. See Fig. 252 . Daylight contains a mixture of red and far-red light, with a predominance of red light. During the day the proportion of P725 builds up relative to P660, whereas during the night there is a gradual conversion of P725 to P660. Thus phytochrome provides the plant with a method of detecting day and night, with P₇₂ 5 predominant when it is day, and P₆₆₀ predominant when it is night. This mechanism is the basis of PHOTOPERIODISM.
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phytochrome

phy·to·chrome

phytochrome

phy•to•chrome

Phytochrome

Phytochrome

Phytochrome

REFERENCES

phytochrome

phytochrome

phytochrome