Clock Genes Keep Time to Daily Rhythm
By Jason Socrates
Bardi
For a flowering plant, life is no bed of roses.
Burning sunlight during the day, freezing cold at night,
and too much shade from the competition offer various threats
that plants must overcome to stay alive and reproduce. A single
blade in a field of grass has no other choice but to work
round the clock to stay alive.
Plants have evolved the means to cope with the various challenges
of their environment by reaching the point where they can
alter or even predict their adaptive response to their niches
day by day, as nature demands.
Plants survive by expressing certain genes and proteins
at optimal times of the day—as they are needed to protect
against the sun’s damaging UV radiation or the night’s
cold air, for instance.
This so-called circadian rhythm follows the solar day, and
a group of researchers at The Scripps Research Institute (TSRI)
have been studying the rhythm in one small, leafy, weed-like
relative of the mustard plant, Arabidopsis.
“When you can’t go indoors when it’s cold,
or find some shade when it’s sunny,” says Cell Biology
Professor Steve Kay, “you organize your stress responses
on a daily basis. Plants need to anticipate changes in their
environment, not just respond to them.” Kay says what
they do is make their own equivalent of sunscreen in the morning
and warm clothes at night.
Arabidopsis uses all the genetic tools at its disposal
to do the daily work of adapting to the varying stresses of
its environment in order to stay alive.
Timing on the Nano-Scale
Unlike chronometers, which keep the same time continuously,
plant clocks have to change continuously. Plants must sense
changes in their environment, such as length of the day, temperature,
and the foliage around them that may be blocking the sun and
entrain their clocks to adjust accordingly.
All such timing takes place on the level of individual cells,
with molecules ebbing and flowing throughout the day and year
as they are needed. Photoreceptors, for instance, need to
be assembled in plant cells in the morning and afternoon,
but not in the evening. Plant cells also need protection from
freezing at night but not often during the day. Circadian
rhythms can be identified, then, by observing this pendulum
swing of molecular expression throughout the day.
But that’s only the beginning of the story—call
it the loud ticking and turning hands that keeps the time
on the face of the clock. The real mechanism that drives the
circadian rhythm is the intricate and elegant genetic machinery
that expresses the molecules that control the ticking of the
ticking genes. The clockworks.
Clockworks comprise a number of complicated feedback switches
involving the expression of sometimes large numbers of genes,
mostly transcription factors, which regulate the ticking genes
by binding to their promoter regions, their mRNA, and otherwise
turning them on and off as needed.
Kay’s laboratory is particularly interested in these
clockworks and has been tinkering with and studying them in
Arabidopsis for some time. Members of the laboratory
vary the plants’ environments—the amount of light
these test plants receive, for instance—then ask how
the plants adjust their own clocks to keep abreast of these
changes, looking for which genes are turned on and off when
and what other molecules are persistently present.
“We’ve begun to identify some of those transcription
factors,” Kay says. His laboratory identified several
genes that are under tight circadian control in a recent issue
of the journal Science.
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