Phytochrome

Phytochrome is a plant pigment involved in the detection of light. Many flowering plants (angiosperms) use phytochrome to regulate flowering.

There are two isoforms of phytochrome – Pr and Pfr. The Pr isoform absorbs red light (at 660nm) while the Pfr isoform absorbs far-red light (at 730nm). Absorbtion of light causes phytochrome to inter-convert. Since daylight has a large component of red light, during the day phytochrome is mostly in the Pfr form. At night, phytochrome will slowly convert back to the Pr form. Stimulation with far-red light will also convert Pfr back to Pr.

Many flowering plants use the seasonal change in day length as one of the signals to flower. For example, the buds of Morning Glory (Ipomoea) unfurl as the day brightens with the rising sun. Another example is the California poppy (Eschscholzia californica), which blooms from 1:00 p.m. until dusk, but only on sunny days.

Broadly, flowering plants can be classified as long day plants, short day plants, or day neutral plants. Long day plants require a certain amount of daylight to initiate flowering, so these plants flower in the spring or summer. Conversely, short day plants will flower when the length of daylight falls below a certain amount. This is called photoperiodism. Day neutral plants do not initiate flowering based on photoperiodism; some may use temperature (vernalization) instead.

In most plants, a suitable concentration of Pfr stimulates or inhibits physiological processes, such as those mentioned in the above examples. Thus Pfr is considered the active form of the pigment.

Phytochrome is involved in many other plant responses besides flowering of the plant. For example, Pfr inhibits the elongation of seedlings. Because Pfr breaks down or reverts to Pr in the dark, seedlings germinating in the darkness of the soil contain no Pfr and subsequently elongate rapidly, emerging from the soil. Other plant responses include leaf growth, chlorophyll synthesis, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.

In 1983 both the Quail and Lagarias laboratories reported the purification of phytochrome and in 1987 the first phytochrome genetic sequence was published. By 1989, it had been shown as a result of these experiments that more than one type of phytochrome existed. For example, the pea plant was shown to have two phytochromes while Arabidopsis was shown to have five.

Phytochrome was isolated in 1959 by a group of scientists working at ARS’s Pioneering Research Laboratories. This group included biophysicist Warren Butler and biochemist Harold Siegelman.

Scientists have speculated that if the structure of phytochrome can be changed through genetic engineering to absorb far-red light, shade-avoidance can be circumvented. As a result, plants would expend less energy on growing as tall as possible and have more resources for growing seeds and expanding their root systems. This would have many practical benefits: for example, grass blades that would grow more slowly than regular grass would not require mowing as frequently.

Before the existence of phytochrome was proven, skeptics called it “A pigment of the imagination.”

References

• http://www.ars.usda.gov/is/timeline/light.htm
• http://www.mobot.org/jwcross/duckweed/phytochrome.htm#tetrapyrrole
• http://ucce.ucdavis.edu/files/filelibrary/616/17562.htm
• http://www.unl.edu/scarlet/v9n26/v9n26features.html
• Biology: Life on Earth. Terry and Gerry Audesirk.