this presentation describes phytochrome structure , function , types

2. CHAPTER 17

3. 1.provides a major source of
information from the
environment during plant
growth and development.
2. In plants, light-dependent
responses are controlled by a
series of photoreceptors that
can be classified into three
known groups—the
phytochromes, cryptochromes
and phototropins.

5. There are 4 classes of photoreceptors :
1. Phytochromes (red and far-red)
2. Cryptochrome (blue and UV-A): seedling development
and flowering
3. Phototropin (blue and UV-A): differential growth in a
light gradient
4. UV-B receptors
photo receptor reads the information
contained in the light by selectively absorbing different
wave length of light.

6. Phytochrome:
• Phytochrome is a photoreceptor, a pigment that
plants, and some bacteria and fungi, use to detect
light
• It is sensitive to light in the red and far-red region of
the visible spectrum.
• In other word a blue-green pigment found in many
plants, in which it regulates various developmental
processes.
It alters plant behavior

8. Biosynthesis of
phytochrome..
• Phytochrome is produced in
different parts of the cell and
assembled from those parts.
• The phytochrome binding protein
is coded in nuclear genes,
transcribed in the nucleus and
translated on cytosolic ribosomes.
• The phytochrome chromophore
is produced in the plastid. These
are assembled in the cytosol

9. Phytochrome Is a Dimer Composed of Two Polypeptides
• Native phytochrome is a soluble protein
• It occurs as a dimer made up of two equivalent subunits.
• Together, the apoprotein and its chromophore make up the
holoprotein.
• In higher plants the chromophore of phytochrome is a linear
tetrapyrrole termed phytochromobilin.
There is only one chromophore per monomer of apoprotein, and it
is attached to the protein through a thioether linkage to a
cysteine residue

13. 1. The Pr form:
•Absorbs at a peak of 660nm
•Is the form synthesized in dark-grown seedlings.
•When Pr absorbs red light, it is converted to the Pfr
form
2. • The Pfr form:
•Absorbs at a peak of 730 nm.
•The Pfr form is the active form that initiates biological
responses.
•When Pfr absorbs far red light, it is converted to the Pr
form.

15. What is strange about these pigments ????
When these pigments absorbed light (photon),
they change chemically into the OTHER form.
The two forms of phytochrome differ in their
absorption spectra.

16. Phytochrome Is Encoded by a Multigene Family
•cloning of phytochrome genes – helps in the detailed comparison of the
amino acid sequences of the related proteins
•phytochromes are soluble proteins
•phytochrome gene family is named PHY
•PHYA, PHYB, PHYC, PHYD, and PHYE
•The apoprotein by itself (without the chromophore) is designated
PHY
•holoprotein (with the chromophore) is designated phy
•Different phytochrpmes are expressed in different tissues at different
times in development and mediate different light responses
•PHY A and PHY B mediates red and far red responses
•Phytochromes also interact with each other and also with other
receptors and devolopmentala stimuli

17. PHY Genes Encode Two Types of
Phytochrome
•On the basis of their expression patterns, the products
of members of the PHY gene family can be classified as
•Type I
•Type II phytochromes
•PHYA is the only gene that encodes aType I
phytochrome.
•Additional studies of plants that contain mutated
•forms of the PHYA gene (termed phyA alleles) have
confirmed this conclusion and have given some clues
about the role of this phytochrome in whole plants

18. • Light can be absorbed only when the polypeptide is
covalently linked with phytochromobilin to form the
holoprotein.
Phytochromobilin is synthesized inside plastids and is
derived from 5-aminolevulinic acid via a pathway that
branches from the chlorophyll biosynthetic pathway
. • Because the chromophore 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 of the double
bond between carbons 15 and 16 and rotation of the
C14–C15 single bond.

20. • The amount of light is referred to as the fluence.
- number of photons hitting on a unit surface area
• In addition to the fluence, some phytochrome
responses are sensitive to the irradiance or fluence
rate of light .
• Each phytochrome response has a characteristic
range of light fluences over which the magnitude of
the response is proportional to the fluence

21. 1. VLFRs (very low fluence responses)
• < 10-3 mmol photons/m2 (converts 0.01% of phytochrome)
Not FR reversible
Transform 0.02% of Pr into Pfr
2. HIR (high irradiance responses)
• >1000 mmol photons/m2, continuous irradiation, dependent
on actual fluence
3. LFRs (low fluence responses)
• 1-1000 mmol photons/m2, FR light reversible
Require high exposure to high fluence

22. 1)regulate the germination of seeds (photoblasty)
2) the synthesis of chlorophyll
3) the elongation of seedlings
4)the size, shape and number and movement of leaves
and the timing of flowering in adult plants
Many flowering plants use it to regulate the time of flowering based
on the length of day and night (photoperiodism) and to set
circadian rhythms
This pigment has a regulatory role in all phases of plant growth and
development (photomorphogenesis) and is apparently ubiquitous in
all taxonomic groups of eukaryotic plants with the exception of fungi.

23. :
1.Is the process by which plant development is controlled by light .
• Plants exhibit different growth habits in dark and light
• In the dark they have elongated stems, undifferentiated chloroplasts
and unexpanded leaves. This is called skotomorphogenesis.
• Photomorphogenesis (light grown) involves the inhibition of stem
elongation, the differentiation of chloroplasts and accumulation of
chlorophyll, and the expansion of leaves.
• Thus the same stimulus causes opposite effects on cell elongation in
leaves and stems. Photomorphogenesis can be induced by red, far red
and blue light. • You know that a pigment absorbs light and is altered
electronically . • results in a change in other chemicals in the
immediate environment to drive photosynthesis.

25. Seed germination
is inhibited by far red light and stimulated by red light
Since chlorophyll in green leaves absorbs red light but not far red
light
Seeds deposited under a heavy canopy of trees don’t germinated
Some seeds germinate only in spring because they become
exposed to red light after the leaves have fallen and decomposed
and before new leaves that shade them have emerged.
The phytochrome protein is generally synthesized in the Pr form
when red light enters the plant some of the Pr is converted into
Pfr
In some plants the PFR enters the nucleus and bind to proteins
that associate with the dna either activate orinhibit transcription
of specific genes.