1. Chapter VII Plant Photomorphogenensis
（植物光形态建成）

2. In the dark
In light

4. Comparison of dark-grown (etiolated) and light-grown (de-etiolated) seedlings
Etiolated characteristics
De-etiolated characteristics
Distinct "apical hook" (dicot) or coleoptile (monocot)
No leaf growth
No chlorophyll
Rapid stem elongation
Limited radial expansion of stem
Limited root elongation
Limited production of lateral roots
Apical hook opens or coleoptile splits open
Leaf growth promoted
Chlorophyll produced
Stem elongation suppressed
Radial expansion of stem
Root elongation promoted
Lateral root development accelerated

5. Bean (Phaseolus vulgaris) seedlings grown under different light conditions for 6 days. Five minutes of dim red light per day is sufficient to prevent some of the symptoms of etiolation that appear under conditions of total darkness, such as reduced leaf size and maintenance of the apical hook. (Photo courtesy of H. Smith.)

6. Photomorphogenensis and Skotomorphogenensis
SEEDLINGS GROWN IN DARKNESS have a pale, almost ethereal appearance. This phenomenon is caused by skotomorphogenensis(暗形态建成）.
SEEDLINGS GROWN IN LIGHT have a stockier, green appearance. The regulation of plant growth and development by light is called photomorphogenensis（光形态建成）.
The driving force for the transitions of the two distinct appearance is LIGHT as a signal.

7. I. Photoreceptors involved in photomorphogenensis
Phytochrome（光敏色素）: Red/Far-red (660 and 735 nm)
Cryptochrome（隐花色素）: Blue ( nm)
Phototropin （向光素）: UV-A ( nm)
UV-B receptor: ( nm) has not been characterized.

8. A: Phytochrome (Red/Far-Red receptor)
Discovery:
1936 Lewis Flint: the germination of lettuce seeds is stimulated by red light and inhibited by far-red light
1952，H Borthwick(Botanist), S Hendricks(Physical chemist): the effects of red light （660nm) and far-red light (735nm) is reversible. Supposing there was a single pigment that could exist in two interconvertible forms, a red light–absorbing form and a far-red light–absorbing form.
1959, WL Butler extracted this pigment and proved Borthwick’s prediction.
1960, Borthwick et al., named “phytochrome”.

9. PP1702b.jpg
Red light

10. PP1702c.jpg
Far-red light

11. PP1702d.jpg
Red light

12. PP1702e.jpg
Far-red light

14. Phytochrome has two forms: Pr and Pfr

15. 2. Structure, synthesis & distribution
Phytochrome is a dimer, each consists of two
dormain (photosensory and regulatory dormain)

16. Phytochrome consists of chromophore and apoprotein
Pigment (chromophore: 生色团). 
blue-green
open chain tetrapyrolle; called phytochromobilin(植物胆色素）
made in the plastids.
Transported into cytosol and combined with apoprotein to phytochrome
Protein (apoprotein：脱辅基蛋白).
glycoprotein
soluble
dimer (MW 240,000 D = 240 kD); each of the two peptides are identical with a MW ca. 125,000 D and comprised of ca amino acids.  
gene(s) have been cloned and the amino acid sequence is known; large proportion of hydrophobic amino acids; suggests phytochrome is associated with membranes.
tetrapyrolle is covalently-bonded to the protein via a thioether linkage involving a cysteine. 

18. Synthesized in plastids
PP17040.jpg

19. Both the Chromophore and the Protein Undergo Conformational Changes
Since the chromophore is what absorbs the light, conformational changes in the protein are initiated by changes in the chromophore.
Upon absorption of light, the Pr chromophore undergoes a cis–trans isomerization by rotation around the double bond between carbons 15 and 16.
This change results in a more extended conformation of the tetrapyrrole.

20. Both the Chromophore and the Protein Undergo Conformational Changes
During the conversion of Pr to Pfr, the protein moiety of phytochrome (the apoprotein) also undergoes a subtle conformational change.
Pr and Pfr differ in their susceptibilities to proteases and in their phosphorylation by exogenous protein kinases.

21. Distribution of phytochrome

22. 1）Phytochrome is photoreversible
Degradation
Precursor
Responses
Dark-return
Pr and Pfr forms of phytochrome can change to the other form when expose to red or far-red light, respectively.

23. Pfr is relatively unstable, with a half life (t1/2) of 1~1.5hr
Figure 17.5
Pfr is relatively unstable, with a half life (t1/2) of 1~1.5hr
declines because Pfr is declining.

25. Note:
The absorbance spectra of Pr and Pfr overlap significantly in the red region of the spectrum (<700nm), and the Pr form of phytochrome absorbs a small amount of light in the far-red region
As a consequence, a dynamic equilibrium exists between the two forms. The proportion of phytochrome in the Pfr form after saturating irradiation by red light is only about 85%. Similarly, an equilibrium of 97% Pr and 3% Pfr is achieved after saturating irradiation by far-red light . This equilibrium is termed the photostationary state（光稳定态）and Pfr percentage over total phytochrome(Pfr and Pr) is called photostationary equilibrium（: 光稳定平衡） .
e.g. in red light, =0.8, while in far-red light, =0.03.

26. 2) Pfr is the Physiologically Active Form of Phytochrome
In general, the magnitude of the physiological response to red light is proportional to the amount of Pfr produced.
In some cases the magnitude of the response is proportional to the ration of Pfr to Pr, or of Pfr to Ptot.
Phytochrome deficient (hy) Arabidopsis mutants have long hypocotyls in both darkness and white light. If the red light response were due to a lack of Pr, we would expect the opposite to be true, i.e. the hypocotyls would be short in both darkness and white light.

27. 4. Two Types of Phytochrome Have Been identified
Type I (phyA)
a) About 9X more abundant in dark-grown tissues.
b) The Pfr form is rapidly degraded.
c) The Pfr form feed-back inhibits its own synthesis.
Type II (phy B-E)
a) Present in equal amounts with Type I phytochrome in light-grown tissues.
b) The Pfr form is not degraded.
c) Synthesis of Type II phytochrome is not feed-back inhibited by Pfr.

28. Phytochrome is Encoded By a Multigene Family
Arabidopsis has five structurally related phytochrome genes: PHYA, PHYB, PHYC, PHYD, and PHYE.
PHYA is the only Type I phytochrome
PHYB - PHYE are all Type II phytochromes

30. 5. Responses related to phytochrome
Shade avoidance
De-etiolation
Seed germination
Circadian rhythms
Hook opening
Floral induction
Internode elongation
Plastid development
Leaf or stem succulence
Pigment formation such as anthocyanin
Enzyme activity (more than 60) such as glyceraldehyde-3-phosphate dehydrogenase

31. 6. PHYTOCHROME RESPONSES VARY IN THE AMOUNT OF LIGHT(energy) REQURIED
A. FLUENCE - TOTAL NUMBER OF PHOTONS IMPINGING ON A UNIT SURFACE AREA (micromoles/m2)
VLFR - VERY LOW FLUENCE RESPONSE (0.001~0.10μmol/m2)
LFR - LOW FLUENCE RESPONSE (1~1000μmol/m2)
B. IRRADIANCE - FLUENCE RATE; NUMBER OF PHOTONS IMPINGING ON UNIT SURFACE AREA PER UNIT TIME (micromoles/m2/s)
HIR - HIGH IRRADIANCE RESPONSE

33. Examples of VLFRs
In dark-grown oat seedlings, red light can stimulate the growth of the coleoptile and inhibit the growth of the mesocotyl(中胚轴）.
Arabidopsis seeds can be induced to germinate with red light in the range of 1 to 100 nmol m–2.

34. All photoreversible responses are LFRs

37. 7. Action model of phytochrome
Phytochrome induced response falls into rapid response and long-term response. The rapid responses involve changes in membrane permeability; the slower responses require alterations in gene expression.

38. (1): Phytochrome Regulates Membrane Potentials and Ion Fluxes
Phytochrome can rapidly alter the properties of membranes.
Studies have proven that phytochrome regulate of K+ channels. (rapid leaflet closure during nyctinasty)

39. (2): Phytochrome Regulates Gene Expression

40. GFP trangenic plants showed phytochrome also in nucleus

42. Blue and UV-A light responses
Cryptochrome
Phototropin

43. Cryptochromes
Cryptochromes are blue/UV-A photoreceptors mediating seedling development/flowering responses in plants.
In Arabidopsis, there are two cryptochromes, cry1 and cry2. The structure of cry2 is also similar to cry1 with two chromophores.
Cry2 has a role in determining flowering time.

44. Cryptochrome is a flavoprotein

46. Phototropin
Phototropin was orginally isolated as nph1 (nonphototropic hypocotyl 1).

47. Phototropin
Phototropin is also a flavoprotein with two flavin mononucleotide (FMN) chromophores.
FMN chromophores binds to domain called LOV (light, oxygen and voltage) domain.

48. Blue light responses Phototropism Chloroplast movement
Stomatal opening
Inhibition of stem and hypocotyl elongation
Synthesis of chlorophyll and carotenoids
Synthesis of anthocyanin.

49. PHOTOTROPINS ARE FLAVOPROTEINS WITH SER/THR PROTEIN KINASES
phototropism

50. CHLOROPLAST MOVEMENTS -LEMNA
DARK WEAK BLUE LIGHT STRONG BLUE LIGHT
DARK WEAK BLUE LIGHT STRONG BLUE LIGHT
PP0905.jpg

51. Blue light
darkness

53. C: UV-B receptor still to be identified
UV-B responses:
Inhibition of growth, dwarf stem
Destruction of chloroplast and chlorophyll
Inhibition of electron transfer
Synthesis of anthocyanin and falvonoids.

54. Interactions between Photoreceptors
100%
20%
68%

55. Hook straightening and cotyledon unfolding are controlled by all three photoreceptors
Cotyledon expansion is controlled by phyB and cry1
phyB controls hypocotyl elongation