Plant Hormones are plant regulating chemicals. Many classes of these very important chemicals exist, including the plant hormone called Phytochromes. The two main hormones in this category are called Pr and Pfr. Pr absorbs red light whereas Pfr absorbs far light. Phytochrome is not technically a hormone- it is a family of proteins with a small, covalently- bound pigment molecule.

Phytochrome is critical in absorbing light and using this light to regulate plant growth and development. Flowering and day length are controlled through the phytochrome, which induces the circadian rhythm which controls floral initiation. The proper exposure to light ensures environmentally timely flowering. Plants use the seasonal change in day length as one of the signals to flower. For example, the Morning Glory’s buds 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. Phytochrome works by absorbing absorbs red light and far red light- a form of light that is just barely visible to humans. In most plants, a suitable concentration of Pfr stimulates or inhibits physiological processes, such as those mentioned in the above examples. Since at night plants are absorbing mostly far-red light, and this converts quickly into red light, plants are being affected by the active form of phytochrome, Pfr, during the day only. Phytochrome is involved in many other plant responses besides flowering of the plant. Pfr, for example 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 un-degraded phytochrome and in 1987 the first phytochrome gene 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 to phytochrome and a plant called 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 a biochemist named 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 avoided. In other words, plants would expend less energy on growing as tall as possible and have more resources for growing seeds and expanding their root systems. The abolition of the evolutionary trait of shade-avoidance, for certain plants, could have many practical benefits. For example, grass blades that aren't wildly competing to get to the top of the canopy would grow more slowly than regular grass - and as a result wouldn't have to be mowed as often.

A fun fact about Phytochrome is that before the existence of the pigment phytochrome was proven, skeptics, who reasonably called it an illusion, called it “A pigment of the imagination.”

                                                              Eric Levin

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.