Most plants and animals show changes in activity over a 24-hour cycle.
Now, for the first time, researchers have shown how a plant combines signals
from its internal clock with those from the environment to show a daily
rhythm of growth.
Using time-lapse photography, postdoctoral researcher Kazunari Nozue,
with colleagues from UC Davis and the University of Lausanne, Switzerland,
found that the shoots of Arabidopsis seedlings show a spurt of growth once a
day. The timing of that growth spurt is controlled by both the plant's
internal clock and by exposure to light, acting on two genes called PIF 4
and PIF 5.
"It's a nice, elegant mechanism for how these two systems interact,"
said Julin Maloof, assistant professor of plant biology at UC Davis, who is
senior author on the paper.

When the seedlings are grown in constant light, most growth occurs in
the late afternoon. But when the plants were moved to a more natural
light/dark cycle, growth shifted several hours to occur just before dawn. In
nature, that is the time when water is usually most available.

The researchers identified the two genes, PIF 4 and PIF 5, that are
connected to plant growth and regulated by the internal clock. The PIF 4 and
5 genes are "switched on" to make protein during the day, switch off after
dark but then turn on again late in the night. But the proteins made by PIF
4 and PIF 5 break down when exposed to light. So while the internal clock
drives transcription of the genes to produce proteins, external light
removes the protein.

The PIF 4 and 5 proteins are thought to act as transcription factors
that turn on other genes involved in growth.

The work is published online in the journal Nature, and was funded
principally by the National Science Foundation and the Swiss National
Science Foundation. The other authors on the paper are postdoctoral
researcher Michael Covington and Stacey Harmer, assistant professor of plant
biology at UC Davis; and Paula Duek, Severine Lorrain and Christian
Fankhauser at the University of Lausanne, Switzerland.


[www.ucdavis.edu]