does phytochrome work?
Many angiosperms flower at about the same
time every year. This occurs even though they may have started growing at
different times. Their flowering is a response to the changing length of
day and night as the season progresses. The phenomenon is called
photoperiodism. It helps promote cross pollination.
In 1920 two employees of the U. S.
Department of Agriculture, W. W. Garner and H. A. Allard, discovered a
mutation in tobacco - a variety called Maryland Mammoth - that prevented
the plant from flowering in the summer as normal tobacco plants do.
Maryland Mammoth would not bloom until late December.
Experimenting with artificial lighting in
winter and artificial darkening in summer, they found that Maryland
Mammoth was affected by photoperiod. Because it would flower only when
exposed to short periods of light, they called it a short-day
plant. Some other short-day plants are
Some plants such as
- chrysanthemums (bloom in the fall)
- the cocklebur
flower only after exposure to long days and
hence are called long-day plants.
- sugar beets and the
Still other plants, e.g. the tomato, are
day neutral; that is, flowering is not regulated by photoperiod.
Photoperiodism also explains why some plant
species can be grown only in a certain latitude.
Experiments with the cocklebur have shown that the term short-day is a
misnomer; what the cocklebur needs is a sufficiently long night.
- Spinach, a long-day plant, cannot flower
in the tropics because the days never get long enough (14 hours)
- Ragweed, a short-day plant, fails to
thrive in northern Maine because by the time the days become short
enough to initiate flowering, a killing frost in apt to occur before
reproduction and the formation of seeds is completed.
- Cockleburs (adapted to the latitude of
Michigan) will flower only if they have been kept in the dark for at
least 8.5 hours - the critical period. (A and B).
- Interruption of an otherwise long night
by red (660 nm) light prevents flowering. (C)
- it is followed by irradiation with far
red (730 nm) light (D).
- An intense exposure to far red light at
the start of the night reduces the dark requirement by 2 hours (E).
These response are mediated by phytochrome.
The behavior of phytochrome explains the
experimental results with the cocklebur.
- Phytochrome is a homodimer: two
identical protein molecules each conjugated to a light-absorbing
molecule (compare rhodopsin)
- Plants make 5 phytochromes: PhyA,
PhyB, as well as C, D, and E.
- There is some redundancy in function of
the different phytochromes but there also seem to be functions that
are unique to one or another.
- Phytochromes exist in two
- PR because it
absorbs red (R; 660 nm) light
- PFR because it
absorbs far red (FR; 730 nm) light
- These are the relationships:
- Absorption of red light by PR
converts it into PFR
- Absorption of far red light by PFR
converts it into PR.
- In the dark, PFR
spontaneously converts back to PR.
- Sunlight is richer in red (660 nm) than
far red (730 nm) light so at sundown, all the phytochrome is PFR.
- During the night, the PFR
converts back to PR.
- The PR form is needed for the
release of the flowering signal.
- Therefore, the cocklebur needs 8.5 hours
of darkness in which to
- convert all the PFR
present at sundown into PR
- carry out the supplementary
reactions leading to the release of the flowering signal
- If this process is interrupted by a
flash of 660-nm light, the PR is immediately reconverted to
PFR and the night's work is undone (C)
- A subsequent exposure to far red (730
nm) light converts the pigment back to PR and the steps
leading to the release of florigen can be completed (D)
- Exposure to intense far red light at the
beginning of the night sets the clock ahead about 2 hours or so by
eliminating the need for the spontaneous conversion of PFR
to PR (E).
These plants are also misnamed. Spinach and
some other members of the group will bloom successfully on a short-day
schedule if the night periods are interrupted by a brief exposure to
light. So these plants are really short-night plants. They can bloom only
if the nights are not too long.
The story is beginning to unfold. In the
etiolation response of Arabidopsis, which is mediated by phytochrome B,
The studies of the role of phytochrome in
etiolation indicate that PFR is the active form; PR
- When sunlight (660 nm) converts PR
into PFR, the PFR moves from the cytoplasm into
- There it binds to a protein called PIF3
("phytochrome-interacting factor 3").
- PIF3 is a helix-loop-helix protein as
are many transcription factors.
- The complex of the two binds to and
turns on promoters containing the sequence
- These promoters are found in genes that
themselves encode other transcription factors.
- These other transcription factors, in
turn, initiate transcription of a variety of genes that are expressed
when the plant is exposed to light.
- Exposure to far red light converts the PFR
back to PR which
- dissociates from PIF3 and
- returns to the cytoplasm.
However, flowering of long-night (short
day) plants like the cocklebur requires PR. Could it be that PFR
is the active form here as well, but acting to promote vegetative growth,
while the "inactive" PR form releases a
"default" pathway of floral induction?
This idea is supported by the finding that
interfering with the metabolism of some plants by
overcomes the effect of an incorrect
photoperiod and allows the plant to flower.
- removing some of the leaves or
- chilling the plant or
- placing it in an anaerobic atmosphere