Regulation of plant development; mechanisms of floral induction
The major developmental change in the plant life cycle is the initiation of
flowering. Many plant species have evolved the ability to regulate
flowering in response to environmental variables such as changes in
day-length or temperature. However, the biochemical mechanisms by which
plants sense and respond to environmental cues are unknown. By genetic
analysis, we have identified several genes that are involved in the
regulation of floral induction in Arabidopsis thaliana. For example, we recently identified a gene (FLC)
that prevents flowering in biennials unless they have experienced the
cold of winter. Exposure to cold promotes flowering in biennials by a
stable epigenetic switch of FLC to a repressed state. This epigenetic state of FLC
is reset to an active state in the next generation (a review of this
topic entitled "Memories of Winter" is available as a PDF file under
List of Recent Publications). One of our long-term goals is to
elucidate the signal transduction pathway through which perception of
winter leads to an epigenetic switch of gene expression and to
flowering. We also have projects on other aspects of flowering-time
regulation such as the mechanism of day-length perception.
Flowering
work is supported by the National Science Foundation and the United
States Department of Agriculture Competitive Grants Program.
|
 Photo
left: This picture shows a cabbage plant that was grown in the
greenhouse for 5 years. (For size comparison my daughter who was the
same age as the cabbage is shown.) Cabbage is a biennial and requires
exposure to the environmental cue of prolonged winter cold in order to
flower the second spring after planting. This promotion of flowering by
cold is called vernalization. The large cabbage has never been
vernalized and cannot flower. We have discovered some of the key genes
involved in this process (see Figure below).
Photo above: wild-type Arabidopsis thaliana,
ld mutant. Both plants were grown under continuous light, which is an
inductive daylength for Arabidopsis flowering. The ld (LUMINIDEPENDENS)
mutation blocks the flowering response to inductive daylengths.
|
 |
A)
In biennial-like accessions of Arabidopsis, FRI up-regulates FLC
leading to delayed flowering (i.e. vegetative growth is prolonged). FRI
is dominant over the effects of autonomous-pathway genes (FCA, FLD, LD
and FVE) which promote flowering by down-regulating FLC.
|
 |
B) In annual accessions, the late-flowering allele of FRI is absent and the autonomous pathway down-regulates FLC.
|
 |
C)
In the absence of FRI, FLC levels rise if the autonomous pathway is
disrupted by a mutation such as the LD mutation in the illustration. In
other words, in the absence of both the positive regulator, FRI, and
the negative regulation from the autonomous pathway FLC levels increase
(thus the balloons in panels B and C). Therefore there are two routes
to biennial-like behavior in Arabidopsis: the presence of dominant
alleles of FRI (panel A) or loss-of-function mutations in
autonomous-pathway genes (panel C).
|
 |
D)
Regardless of the route to biennial-like behavior, vernalization (an
extended cold treatment such as winter) can cause an epigenetic
turn-off of FLC expression thereby alleviating the block to flowering.
(Following cold treatment, FLC transcript levels remain low for the
remainder of the plant's life, but return to high levels in the next
generation.) |