Tutor:umar khitab

1. Phytochrome

2. Light is important for photosynthesis.But it is also important for plant growth and
development.
Photomorphogenesis:
The light mediated growth and development in the plant is
called photomorphogenesis.
Light act as a signal to initiate and regulate photomorphogenesis.
If plant receive light that means that there is some thing which receive light that is
called photoreceptor
Phytochrome are mainly photoreceptor or photomorphogenic pigment.

3. Found in Fungus,cyanobacteria etc
They change their biochemical activity in response to light environment.
Also play important role by changing the two light form red and far-red light and
cause photomorphogenesis.
Chemically phytochrome also consist of chromophore and this chromophore is called
phytochromobillin.
Also act as a temperature sensor b/c in warm condition phytochrome deactivated.

4. Also regulate growth and development of flower i.e, seed germination,and stem
elongation, elongation of leaves and timing of flower.
Structural domain in phytochrome:
◦ Phytochrome consist of protein and this protein mainly consist have two
domain
a) Photosensory domain
b) Regulatory domain

5. N──PAS-GAF-PHY ───── PRD-PRD-HKRD───C
Photosensory region Hinge region Regulatory region

6. Phytochrome found in two reversible form
a) Red light(665-668) in Pr form
b) Far-red light(730nm) in Pfr form
Pr → Pfr
Phytochrome protein initially synthesize in Pr form when a plant use
to get red light then that red light will enter to nucleus and convert the
Pr into Pfr.According to that it will either enhance the transcription pr
inhibit.

7. Some plant respond positively to a red light while some plants respond negatively to
far red light or this condition may be reverse also e.g, In some plant the seed
germination take place in the presence of of red light because chlorophyll of the leaf
absorb red light but not far red light.
In day time maximum of red light get converted into far red light and this process is
very fast .so in day time their will be high accumulation of far red light.But in night
time the process will be totally reverse and conversion of Pfr to Pr will be fast in the
presence of far red light the process is called dark reversion because in dark the Pfr
reverse to Pr form.

8. Classes of Phytochrome:
Phytochrome protein are encoded by phytochrome gene family
termed as PHY
Phytochrome have 2 classes:
Phytochrome type 1:
Type 1 phytochrome is light sensitive,so in
light transcription of PHY gene inhibited, mRNA degradation take
place.
Type 1 phytochrome mainly function in dark
e.g, PHY A

9. Type 2 phytochrome:
◦ Type 2 phytochrome is light stable, so present in both light grown and
dark grown plants.
◦ Type 2 phytochrome mainly function in light
◦ e.g, PHY B, PHY C, PHY D, PHY E.

10. Mode of action:
COP1:
Constitutive photomorphogenic 1, is a ubi-Quitin lagase which are involve
in targeting protein for 26s proteasome mediated degredation.
It inhibit the photomorphogenesis in dark by degrading the phytochrome
including factor (PIFS)
In dark COP1 is present in the nucleus but in the light it is only found in the
cytoplasm

11. HY5:
Hy5 is a transcription factor that is key regulator of photomorphogenesis in
phytochrome pathway.In light the level of HY5 protein increases and in the dark it
decline.