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What is a Phytochrome?

By Caitlin Kenney
Updated May 17, 2024
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Phytochrome is a pigment found in most plants and some bacteria that is used to monitor the color of light. Plants may use this pigment in determining photoperiods, when to germinate seeds, when to flower, and when to manufacture chloroplast, a key chemical used in photosynthesis. Photosynthesis is a process by which plants convert sunlight into nourishment. Phytochrome can also be instrumental in controlling what shape and size leaves form, the length of seeds, how many leaves form, and the optimal length of seeds to make best use of the light at hand.

A pigment is a substance that alters the color of an object by reflecting some waves of light and selectively absorbing certain others. For example, imagine that red, yellow, and blue rays were being shined on a ball. If the ball reflects blue and absorbs all other light waves, the ball will appear blue to an observer. Phytochrome is a special pigment with two forms, Pr and Pfr, which absorb red light and far red light respectively, giving off a green to blue hue. Red light and far red light are relatively low energy and frequency light sources, compared with other light waves in the electromagnetic spectrum.

Phytochrome is a photoreceptor, or a protein that senses light on an organism and elicits a response. It has both a protein component and a chromophore component, the piece responsible for the absorption of red light. The molecule begins taking in red light in the Pr form, which causes the phytochrome to undergo a chemical change to become Pfr. This Pfr state of phytochrome is the active state, or the state that begins the response processes in the plant, and prefers to absorb far-red light.

In flowering plants, this method of detecting light helps develop photoperiodism, or responses to day and night. Plants may also use phytochrome to change the shape and size of leaves and to begin synthesis of chloroplasts. This ensures that photosynthesis can make optimal use of the light at hand. It is also important to monitor light so seeds can grow successfully, without drying up or receiving too little sun.

The discovery of phytochrome began with the observation of photoperiodism in plants. Scientists began noticing that plants responded differently to day and night; some plants altered processes for longer days, some favored flowering during shorter day spans, and some stopped flowering if they were exposed to light for even a few minutes during the night. In the 1930s, at the Beltsville Agricultural Research Center, botanist Sterling Hendricks, physiologist Marion Parker, and chemist Harry Borthwick teamed up to investigate this phenomenon.

In 1948, spectrograph tests indicated that a single pigment was responsible for the photoperiodis. In 1952, tests revealed that germination was stopped when a plant was exposed to far-red light and restarted when exposed to red light. In 1959, the team performed conclusive tests on turnip seeds and named the pigment phytochrome.

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