Love and Flower Power
Scripps Research Scientists Discover Protein
that Senses Daylight and Regulates Flowering
By Jason Socrates
Bardi
A group of scientists at The Scripps Research Institute
are publishing a paper in which they describe a new class
of proteins that regulates the timing of the flower cycle
in one small leafy weed, a relative of the mustard plant called
Arabidopsis thaliana.
The protein, called FKF1, regulates flowering in Arabidopsis
by targeting and degrading other proteins that are involved
in the flower cycle. Interestingly, FKF1 is itself regulated
by light.
Like many large protein complexes, FKF1 is a mixture of
several different protein domains—proteins or pieces
of protein that have distinct functions. FKF1 combines several
domains in a unique way that has never been seen before.
FKF1 has a domain called an "F-box" that degrades other
proteins, and FKF1 uses this domain to control the flowering
cycle of plants. FKF1 also has another domain, known as a
"LOV" domain, that it uses to sense light. These domains are
common in plant, fungal, and bacterial cells, where they sense
external cues such as light, oxygen, and voltage.
"We have discovered [in the FKF proteins] a completely novel
photoreceptor family," says Scripps Research Cell Biology
Professor Steve Kay, Ph.D., who is the lead author of the
study. "These proteins are regulating the degradation of other
proteins in a light-dependent fashion."
The work should have special relevance to agriculture because
the appropriate seasonal control of flowering is a major determinant
of crop productivity. The same technology might be used to
make certain crops bear fruit faster and in larger and more
nutritious yields.
The paper appears in the latest issue of the journal Nature
and is featured on the cover.
Light Switches for Protein Degradation
Kay's Scripps Research laboratory has for several years
been studying the way that plants use "circadian rhythms"
to follow the solar day, using Arabidopsis thaliana.
Arabidopsis is a good model organism for several
reasons. Its genome was solved a few years ago, and many of
its genes have been identified. It is tiny and has a fast
generation time, both of which fit well in the modern tight-on-space-and-time
laboratory. It also produces an overabundance of seeds at
the end of its reproductive cycle. Finally, Arabidopsis
is easily grown.
Members of the laboratory vary the plants' environment—the
amount of light, for instance—then ask how the plants
adjust their own clocks to keep abreast of these changes,
which genes are turned on and off, and what other molecules
are persistently present.
Anticipating the seasons is but one of the strategies plants
have evolved as a means to cope with the various challenges
of their environment. Because of the stark seasonal differences
in weather in many climates—with long days of burning
sunlight in summer and wet, dark, and freezing conditions
in winter—plants with the ability to flower at the best
possible time have had an advantage in evolution.
Scientists have known since the 1920s, when researchers
first began experimenting with growing plants under artificial
light, that plants flower following a "photoperiodic response"—they
flower when they detect a certain day length indicating season.
However, until recently, nobody understood the precise molecular
tools plants use to accomplish this.
Kay is one of several scientists at Scripps Research and
elsewhere who have been exploring how plants control their
flowering cycle on the level of individual cells, and how
they use circadian "clock" genes that follow the solar day.
These clock genes ebb and flow throughout the day and year
as they are needed, and comprise a complicated set of feedback
switch "clockworks" that turn on and off other genes as needed.
One gene involved in these clockworks encodes a protein
called "CONSTANS," which triggers the flowering of the plant,
but only when the timing is right. The expression of CONSTANS
varies throughout the day, and it must be abundant at the
end of the day for flowering to occur.
Kay and his colleagues found that the FKF1 protein is a
key regulator of the levels of CONSTANS and of flowering because
it can control how much CONSTANS protein accumulates. If CONSTANS
levels are too low at the end of the day, flowering will not
occur.
Interestingly, FKF1 is itself regulated by light. If there
is not enough sunlight in the late afternoon, FKF will alter
the level of CONSTANS, and delay flowering. FKF1 does this
by sensing how much blue light is in the environment. Blue
light is a good sensor because it is not absorbed by plant
chlorophyll as other types of light are.
This discovery opens the possibility that we could learn
to boost food production by manipulating day length sensitivity
of different crops and increasing our capacity to grow them
efficiently at different latitudes at different times of the
year. "The danger of running out of arable land is very real,"
says Kay, "and we have to solve the problem of feeding a rapidly
increasing population in the next 10 years."
The discovery of the FKF1 protein is also significant to
scientists, because this light-activated switch could be put
into other types of cells—possibly introducing a gene-regulating
switch into mammalian cells that can be activated by light,
for instance. The article, "FKF1 is essential for photoperiodic-specific
light signaling in Arabidopsis" was authored by Takato
Imaizumi, Hien G. Tran, Trevor E. Swartz, Winslow R. Briggs,
and Steve A. Kay and appears in the November 20, 2003 issue
of the journal Nature.
This work was supported by the National Institutes of Health,
the National Science Foundation, and by a grant from the Japan
Society for the Promotion of Science Postdoctoral Fellowships
for Research Abroad.
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