..Photoperiodism and Phytochrome
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Photoperiodism and Phytochrome

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.

• chrysanthemums (bloom in the fall)
• poinsettias
• the cocklebur

• spinach
• Arabidopsis
• sugar beets and the
• radish

Still other plants, e.g. the tomato, are day neutral; that is, flowering is not regulated by photoperiod.

• 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.

Mechanism of Photoperiodism

Experiments with the cocklebur have shown that the term short-day is a misnomer; what the cocklebur needs is a sufficiently long night.

• 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) unless
• 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.

Phytochrome

• 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 interconvertible forms
  • P_R because it absorbs red (R; 660 nm) light
  • P_FR because it absorbs far red (FR; 730 nm) light
• These are the relationships:
  • Absorption of red light by P_R converts it into P_FR
  • Absorption of far red light by P_FR converts it into P_R.
  • In the dark, P_FR spontaneously converts back to P_R.

• Sunlight is richer in red (660 nm) than far red (730 nm) light so at sundown, all the phytochrome is P_FR.
• During the night, the P_FR converts back to P_R.
• The P_R 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 P_FR present at sundown into P_R
  • carry out the supplementary reactions leading to the release of the flowering signal ("florigen")
• If this process is interrupted by a flash of 660-nm light, the P_R is immediately reconverted to P_FR and the night's work is undone (C)
• A subsequent exposure to far red (730 nm) light converts the pigment back to P_R 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 P_FR to P_R (E).

Long-day Plants

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.

How does phytochrome work?

• When sunlight (660 nm) converts P_R into P_FR, the P_FR moves from the cytoplasm into the nucleus.
• 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

  CACGTG
  GTGCAC

• 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 P_FR back to P_R which
  • dissociates from PIF3 and
  • returns to the cytoplasm.

However, flowering of long-night (short day) plants like the cocklebur requires P_R. Could it be that P_FR is the active form here as well, but acting to promote vegetative growth, while the "inactive" P_R form releases a "default" pathway of floral induction?

This idea is supported by the finding that interfering with the metabolism of some plants by

• removing some of the leaves or
• chilling the plant or
• placing it in an anaerobic atmosphere

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