When the alarm goes off, many people hit the snooze button.
But eventually they get up and go to work or school. Most plants, too,
have an alarm clock. In fact, one of several such clocks goes off every
morning. Circadianor biologicalclocks have a 24-hour cycle
and "wake up" the processes that make plants operate properly.
According to Autar Mattoo, a plant physiologist in ARS'
Vegetable Laboratory in Beltsville, Maryland, the clock is located in
the nuclei of plant cells. The clock controls an enzyme that modifies
a protein, D1, which is critical to photosynthesis.
Mattoo explains that every morning, D1 binds phosphorus.
This process peaks before noon, providing a plant with a biotiming signal
that tells it to adjust its metabolism to face the onset of the day's
brightest light. This adjustment likely helps plants fare better when
faced with the highest light intensities, which hinder photosynthesis.
The experiments were conducted at different times of the
year and in different climates, and the theory almost always proved
true. (See "Cool
Nightlife Bad for Tomatoes," Agricultural Research,
October 1996, p. 15, for more information on how cold temperature affects
circadian clocks.) Mattoo points out that the one thing that can block
the accumulation of phosphorus on D1 is the concentration of triazine
and urea-type herbicides, such as atrazine and diuron.
D1 is also a target of ultraviolet-B (UV-B) radiation
damage to plants. The plants cope with this by putting on a "sunscreen,"
which is made up of a set of phenolic compounds that protect plants
from low doses of UV-B. At higher doses of UV-B, this photoprotection
could be compromised. "It is therefore very crucial for every plant
cell to keep time to protect itself and perform correctly, and it does
this by a biotiming mechanism," Mattoo says.
Mattoo worked on this project with researchers and students
from the United States and in collaboration with Marvin Edelman of the
Weizmann Institute in Israel. As a result of their 22 years of collaborative
research, these scientists were the first to determine the whole life
cycle of the D1 protein.
Scientists have reported several other plant circadian
clocks. Humans also have circadian clocks. Mattoo uses the example that
some people are day people and some people are night people. Jet lag
is an example of a person's biological clock being out of synch with
the actual time.
Mattoo's research appeared in the December 2002 issue
of Plant Physiology.By David Elstein, Agricultural
Research Service Information Staff.
This research is part of Plant Biological and Molecular
Processes, an ARS National Program (#302) described on the World Wide
Web at www.nps.ars.usda.gov.
Autar K. Mattoo
is with the USDA-ARS Vegetable
Laboratory, 10300 Baltimore Ave., Bldg. 010-A, Beltsville, MD 20705-2350;
phone (301) 504-7380, fax (301) 504-5555.
"Plants' Biological Clocks Help Them Prepare for the Day"
was published in the April
2003 issue of Agricultural Research magazine.