1. Light and plant development
Photomorphogenesis - a change in plant
development induced by specific kinds of light
and not dependent on photosynthesis.

2. Photomorphogenesis involves special photoreceptors that
initiate developmental changes.
Photoreceptors transduce information in the environment
into appropriate developmental patterns.
Information examples
position in a layered plant canopy
seed depth in soil
presence of competitors
approach of sunrise
day length

3. Two main photoreceptors
Phytochrome (Chapter 17) - a protein pigment that absorbs red and far-red light and interconverts between two forms.
involved with many and varied responses
2. Blue-light photoreceptors (Chapter 18)
guard cell responses
phototropisms

4. Phytochrome Detects red and far-red light
Provides information the environment
Answers questions for plants:
Am I in the light?
Do I have competitors?
Is it time to flower?

5. PP17T010.jpg

6. Many phytochrome responses are reversible
Absorption of red light will cause a response that is reversed by far red light.
Red ≈ nm
Far red ≈ nm
“photoreversible”

7. "But seeds are invisible
"But seeds are invisible. They sleep deep in the heart of the earth's darkness, until some one among them is seized with the desire to awaken." —Antoine de Saint Exupery, The Little Prince (Harcourt, Brace & World Inc.)

8. The classic phytochrome system - lettuce seed germination
PP1702a.jpg
Dark = very little germination

9. Brief red light causes germination
PP1702b.jpg
Brief red light causes germination

10. Red followed by far-red = no germination
PP1702c.jpg
Red followed by far-red = no germination
first
second

11. Red then far-red then red again = germination
PP1702d.jpg
first
second
third

12. Red then far-red then red then far-red
Photoreversibility
PP1702e.jpg
first
second
third
fourth

13. Lettuce seed germination Red and far-red have opposing effects
Each can reverse the effect of the other
How does this work?
Two antagonistic receptors?
2. One receptor with two forms?

14. Lettuce seed germination Red and far-red have opposing effects
Each can reverse the effect of the other
How does this work?
Two antagonistic receptors
2. One receptor with two forms?

15. Two forms of phytochrome
“Pr” is red form, peak absorption in red
“Pfr” is far red form, peak absorption
in far-red

17. photoreversible responses
Red light converts Pr to Pfr
Far red light converts Pfr to Pr
Red light
Pr Pfr
Far-red light
This is the basis of the
photoreversible responses

18. Pr
Pfr

19. Pr Pfr Red light Far-red light Physiologically active
form of phytochrome
Red light
Pr Pfr
Chlorophyll
production
Far-red light
De-etiolation
germination

20. Phytochrome location
In meristematic regions of etiolated seedlings,
areas of active cell division and expansion.

21. Red light
Pfr Pr
de-etiolated
etiolated

22. General effect of Pfr is to reduce internode elongation.
Sun plants tend to be sensitive to amount of Pfr, while
shade plants are often insensitive.
Shaded environments
have lower red light to
far-red light ratio.
What does this do to
Pfr/Ptotal ratio?

24. Phytochrome responses vary in timing.
Slow vs. rapid phytochrome responses
Slow Rapid
Morphological responses Biochemical responses
Effects on gene Effects on ion fluxes
expression and turgor

25. Nyctinastic leaf movements involve a rapid phytochrome response.
Day Night
PP1712A.jpg

26. Folding and opening of leaflets involves changes in turgor
of two sets of ”motor” cells, the turgor changes being driven
by fluxes of K+ and Cl-. Changes in the PMF also involved.
Fig
PP17140.jpg

27. Opening and closing of leaflets can be entrained
as a circadian rhythm.
PP17130.jpg

28. Phytochrome is involved with effects on leaf movement.
1. Red light followed by darkness causes leaflets to
close, and this effect can be reversed by far-red light.
Example of a rapid response based on an immediate
biochemical change.
2. Red/far red treatments influence gene expression of
light harvesting proteins, which in turn alters leaf movement responses.
Example of a slower response requiring a change in gene expression.

29. The mechanism of phytochrome-mediated leaf movements
Phytochrome (Pfr) regulates H+ pumps and K+ channels
of motor cells.
Fig
PP17140.jpg

30. Phytochrome and plant competition: how do plants
detect the presence of neighbors that compete
for sunlight?
Web Essay 17.2
With increased density of neighbors, the R/FR
ratio perceived by
stems decreases
because of increased FR reflection from leaves and stems.

31. Plant neighbors? Far red enriched = neighbors Red absorbed
Far red reflected from other plants.
Far red enriched = neighbors

32. With increased density of neighbors,
the R/FR ratio perceived by stems decreases i
R/FR ratio
Leaf area index, m2 m-2

33. A low R/FR ratio allows internode elongation,
a favorable response to potential light competition by neighbors

34. Pr Pfr Red light Far-red light Physiologically active
form of phytochrome
Red light
Pr Pfr
Far-red light
De-etiolation
etiolation

35. The effect of FR reflection by neighboring plants can
be simulated using mirrors that selectively reflect Red
or Far Red light.
As for reflection by plants, FR reflection increases
internode elongation. Recall that this is because little of
the Pfr form is then present to inhibit internode elongation

36. Filtering out the FR light
received by the stem reduces
internode elongation at high
densities of neighboring plants.

37. Under other plants? Far red enriched = understory
Far red reflected from other plants or transmitted.
Red absorbed
Far red enriched = understory

38. Phytochrome and flowering. When is the right time to flower?
Unreliable indicators of time of year
Temperature
Moisture
Light levels
Reliable: length of day/night
Varies with season
Varies with latitude
Detected by phytochrome

39. Sunlight
Mostly red
A little far red

40. In sunlight In sunlight most P gets converted to Pfr form. Pr Pfr Pfr

41. Start of night Most P in Pfr form. Pr Pfr Pfr Pr Pfr Pr Pfr Pfr Pr Pfr

42. In the dark Pfr form changes gradually to Pr form. Pfr Pr Pfr Pr Pfr

43. After a short night Much Pfr still left. Pfr Pr Pfr Pr Pfr Pr Pfr Pfr

44. Long day plant = Short night plants
Needs short night to flower
Needs Pfr still present at end of night
Pfr promotes flowering for LDPs

45. Later in the night More Pfr changes to Pr. Pfr Pr Pr Pfr Pfr Pfr Pr

46. After a long night All the Pfr is gone. Pr Pfr Pr Pr Pfr Pfr Pr Pr Pr

47. Day dawns Most P gets converted to Pfr form again. Pr Pfr Pfr Pr Pfr

48. Short day plant = Long night plant
Needs long night
Needs Pfr gone at end of night
Pfr inhibits flowering for SDPs
Can trick a SDP into not flowering with a brief flash of red light during the long night, this resets much of its phytochrome to Pfr form.

49. LDP SDP Long day: Pfr left at end of short night.
Pfr promotes flowering for LDPs.
Pfr inhibits flowering for SDPs.
Short day: Pfr gone at end of long night.
No Pfr to promote flowering for LDPs.
No Pfr to inhibit flowering for SDPs.

50. Waiting for the right time
Plants grow leaves until it is time to flower
LDPs wait until the day is long enough
Really night short enough
Some time before June 21
SPDs wait until the day is short enough
Really night long enough
Some time after June 21
Flower opening happens later

51. Day neutral plants Flower when mature enough
Maybe other environmental signals (temp?)
Day length (dark length) doesn’t matter