1. Chapter 17.
Phytochrome and Light Control
of Plant Development

2. Corn and bean seedlings grown in the light or the dark
De-etiolated
etiolated
De-etiolated
etiolated
Light-grown corn
Dark-grown corn
Light-grown bean
Dark-grown bean
Developmental changes by light (photomorphogenesis)
(transformation from the etiolated to the green state)
A decrease in the rate of stem elongation
The beginning of apical-hook straightening
The initiation of the synthesis of pigments

3. Phytochrome: red light
development
photosynthesis
Function of light
stomata opening
flowering
Phototropin: blue light
photoreceptor
Cryptochrome: blue light
Phytochrome: red light

4. The photochemical and biochemical properties of phytochrome

5. Typical photoreversible responses induced by phytochrome in variety of higher and lower plants

7. Lettuce seed germination is a typical photoreversible response controlled by phytochrome
Red, nm
Far-red, nm
dark

8. Phytochrome can interconvert between Pr and Pfr forms
Pr : red – light absorbing form
Pfr: far- red - light absorbing form

9. Absortion spectra of purified oat phytochrome
in the Pr and Pfr forms overlap
Red light Pr
:15%
Pfr
:85%
Far-red light
Pfr
: 3% Pr
: 97%

10. Pfr is the physiologically active forms of Phytochrome
Wild type:
short hypocotyl by white light
hy mutant (phytochrome mutant):
long hypocotyl by white light
If the phytochrome-induced response to white light is caused
by the absence of Pr, such phytochrome-mutants should have
short hypocotyl in both darkness and white light.
Instead, the opposite occurs.

11. De-etiolated
etiolated
De-etiolated
etiolated
Pr of Pfr form?

12. Phytochrome is a dimer composed of two polypeptides
Native phytochrome is a soluble protein with MW about 250 kDa.
It occurs as a dimer made up of two equivalent subunits.
Each subunit consists of two components:
light absorbing pigment: chromophore
a polypeptide chain: apoprotein (125 KDa)

13. Structure of the Pr and Pfr forms of the chromophore

14. Structure of the phytochrome dimer

15. Phytochromobilin is synthesized in plastids
Transported into cytosol by a passive process
Assembly of the phytochrome apoprotein (autocatalytic)
4. Consists of holoprotein
hy1 and hy2 mutant: fail to supress hypocotyl elongation.
chromophore biosynthesis mutant, normal apoprotein.

16. Both chromphore and protein undergo conformational changes
Absorption of light
The Pr chromophore undergoes cis-trans isomerization
The protein moiety of phytochrome holoprotein
also undergoes a subtle conformational change
( both N-terminal and C-terminal regions)

17. Two types of phytochromes have been identified
Dark 9 : 1
Light 5 : 5
Existence
Etiolated plant
Green plant and seed

18. Phytochrome is encoded by a multigene family
5 structurally related phytochrome genes in Arabidopsis
(PHYA, PHYB, PHYC, PHYD, PHYE)

19. PHY genes encode two types of phytochrome
Phytochrome gene family (PHY)
PHYA
PHYB
PHYC
PHYD
PHYE
Type I
Type II

20. PHYA PHYB PHYC PHYD PHYE red mRNA Pr Pfr Response far-red
ubiquitin ATP
ubiquitin
degradation
degradation
PHYB
PHYC
red
mRNA Pr
Pfr
Response
PHYD
far-red
PHYE