Ithaca, New York
September 30, 2004
Postdoctoral researcher Brian Ayre
was listening attentively to a
Cornell University seminar on flower development when he
asked what seemed an obvious question: "What is the signal that
controls flowering?" The seminar speaker laughed. "They've been
trying to figure that out for a hundred years," he said. More
laughter followed, as one of Ayre's colleagues shouted from the
back of the room: "Florigen!"
No one's laughing now. Ayre,
currently a faculty member at the
University of North Texas, went on to publish a provocative
report in the August 2004 issue of Plant Physiology along
with his postdoctoral adviser Robert Turgeon, a Cornell
professor of plant biology. Their paper recounts the
serendipitous discovery that the plant protein, CONSTANS, may be
the signal -- "florigen" -- that causes plants to flower. Or at
least, the researchers say, CONSTANS plays an important role in
generating the signal.
Trying to understand flowering
is a popular pursuit because of its importance in agriculture.
Flowers are the precursor of fruit, and if flowering can be
controlled, plants can be manipulated to remain in a vegetative
or flowering state. Accelerated flowering could lead to a much
shorter growing season -- an important advance for both growers
and plant breeders. And the significance for the floriculture
industry is equally huge.
Textbooks predating the 1970s
dedicated entire chapters to this elusive signal. More recently,
though, florigen has become an example of a dead-end pursuit in
plant biology -- one more likely to prompt sarcastic grins than
scientific inquiry to find this crucial puzzle piece in the
understanding of plant development.
Turgeon's research focus at
Cornell is on understanding how molecules move in the phloem,
the "bloodstream" of plants that carries food, nutrients and
signaling molecules. When Ayre joined his group, they were by no
means setting out to discover the signal that induces
flowering.But as sometimes happens in science, "people from
outside of the field end up making significant contributions
because they have different tools and different perspectives,"
says Turgeon, picking up the story: "We were coming at the study
from a transport perspective. We got into this when we got a
hold of the promoter of the galactinol synthase gene, a genetic
factor that drives expression of genes specifically in the vein
of the leaf so that they can enter the phloem. I saw this as a
tool to study the transport of large molecules through the
phloem. Once we got the tool, we began to design experiments to
use it. We applied it and got a very interesting result."
The researchers took two
approaches that led them to the conclusion that CONSTANS is a
signal involved in flowering. First, using Arabidopsis
plants in which all CONSTANS protein had been abolished, they
introduced a copy of the CONSTANS gene under the control of the
galactinol synthase promoter, which causes the protein to be
synthesized only in the leaf. Despite this precise expression
pattern, they saw that the signal had a dramatic effect on
flowering. This suggests that either CONSTANS is moved
throughout the plant to the site of flowering through the
phloem, or it interacts with another factor inside the phloem
that is transported to the site of flowering.
They provided further evidence
for CONSTANS' role in floral signaling when they grafted
Arabidopsis plants that contained no CONSTANS protein onto
plants synthesizing CONSTANS in their leaves. This elegant
experiment showed that CONSTANS, or another factor that it
interacts with, was able to move through the graft junction to
signal flowering in the parts of the plant that formerly were
devoid of any of the protein.
Turgeon credits the late
Russian plant physiologist M.H. Chailakhyan for some of the
earliest work in trying to understand the nature of the
flowering signal. In 1937 Chailakhyan named and defined florigen
as a graft-transmissible signal that induces flowering. Ayre's
and Turgeon's work appears to fit this historical definition of
the flowering hormone, they say.
However, it is not clear
whether CONSTANS is in fact the flowering hormone. More likely,
Ayre says, "It may be interacting with another downstream factor
that moves to the site of flowering action. It is clear now that
CONSTANS is an important factor in generating this signal."
Comments Jan Zeevaart, an
emeritus professor of plant biology at Michigan State University
who has dedicated much of his research career to florigen and
other plant hormones: "It is gratifying to see that there are
finally molecular approaches to the problem. For quite some
time, some people have ridiculed the concept of florigen, but
those of us who have worked on the physiological aspects always
knew that it could not be dismissed so easily."
"The exciting thing is that it
appears that people are finally closing in on the identity of
florigen," Turgeon responds. Ayre adds: "I suspect that CONSTANS
and downstream components, such as a protein called FT, are
going to be pretty hot topics in the next couple of years."The
article in Plant Physiology was titled "Graft
Transmission of a Floral Stimulant Derived from CONSTANS."
Ayre's and Turgeon's work was supported by the U.S. Department
of Agriculture and the National Science Foundation.
At Cornell, Peter Davies, a
professor in the Department of Plant Biology since 1969, has
spent his career working on other plant hormones. Excited by the
finding, he recently recalled a quotation from a fellow plant
physiologist in the 1970s: "Flowering is a religion based on the
totally unfounded dogma of florigen."
As it turns out, the "religion"
may be about to get some new followers.
Related World Wide Web
sites:
The following sites provide
additional information on this news release. Some might not be
part of the Cornell University community, and Cornell has no
control over their content or availability.
oTurgeon laboratory:
http://www.plantbio.cornell.edu/people.php?netID=ert2#research
This article was prepared by
Sarah Nell Davidson, a graduate student in plant biology and
science-writing intern in the Cornell News Service. |
