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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. Chlorophyll

Phytochrome can exist in two stable photointerconvertible forms, Pr or Pfr, with only Pfr being biologically active. Absorption of red light (near 666 nanometers) by inactive Pr converts it to active Pfr, while absorption of far-red light (near 730 nm) by active Pfr converts phytochrome back to inactive Pr. Plants frequently respond quantitatively to light by detecting the amount of Pfr produced. As a result, the amount of Pfr 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 Pfr 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 Pfr. Light direction also can be deduced from the asymmetry of Pfr levels from one side of the plant to the other. Different phytochrome responses vary in their sensitivity to Pfr; some require very low levels of Pfr (less than 1% of total phytochrome) to elicit a maximal response, while others require almost all of the pigment to be converted to Pfr. 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 Pfr also makes the plant aware of gravity, inducing shoots to grow up and roots to grow down into the soil. 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 Pr and Pfr; 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. Photoperiodism

Finally, phytochrome allows plants to detect the spectral quality of light, a form of color vision, by measuring the ratio of Pr to Pfr. When a plant is grown under direct sun, the amounts of red and far-red light are approximately equal, and the ratio of Pr to Pfr in the plant is about 1:1. Should the plant become shaded by another plant, the Pr/Pfr 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 Pfr and absorbs almost none of the far-red light used to produce Pr. For a shade-intolerant plant, this change in Pr/Pfr 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 Pr to Pfr. 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 Pfr or the Pr/Pfr ratio and branch off toward discrete end points. Photomorphogenesis

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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