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1. Phototropin
Dr. Brijbhan Yadav

2.  It is a Flavoprotein which act as a blue light receptor. They are light
activating serine/threonine protein kinases.
 Optimize the photosynthetic efficiency of plants.
 This include phototropism, light induced stomatal opening and
chloroplast movements in response to changes in light intensity.

3.  Phototropin is blue light (400-500nm) photoreceptor of plants.
 The chromophore for Phototropin is FMN (a Flavin), so Phototropin is a flavoprotein.
 The N-terminal half of Phototropin has 2 LOV domains each binds FMN and C-Terminal
kinase domain that has a characteristic activity of serine/threonine kinase.
 Phototropins membrane associated proteins & located at Plasma membrane & functions as
Ser/ Thr kinase.
 There are two different types of Phototropins in Arabidopsis they have been termed as
PHOT1 and PHOT2.
 Phototropin also absorbs UV-A light (320-400 nm) so Phototropin functions can also be
Phototropin

4. PHOTOTROPIN GENES
There are two different Phototropins in Arabidopsis that exhibits overlapping
function in addition to having unique physiological role.
The two Phototropins are
Phot1
Phot2
 Phot1 help in cotyledon/hypocotyl growth.
 The phot1 mutant lacks hypocotyl Phototropism in
response to low-intensity blue light but retains a
phototropic response at higher intensities.

5. STRUCTURAL DOMAIN OF PHOTOTROPIN
LOV1 LOV2
FMN FMN
Serine
threonine
kinase
N C
1. Phototropin contain two light sensing Light-oxygen-voltage(LOV) Domains, LOV1
and LOV2.
2. LOV1 and LOV2 bind a chromophore FMN.
3. Blue light irradiation/photoexcitation of
protein bound FMN cause a conformational
change of phototropin that triggers auto
phosphorylation and start the sensory
transduction cascade(phototropin signalling)

6. Photoexcitation of LOV
domain result in activation
Of C-terminal kinase
domain, Which leads to auto
phosphorylation of multiple
Serine residues.
In dark molecules
binds non covalently
to LOV domains
In presence of blue light
,FMN molecule become
covalently bound to LOV
Domain.
DARK Blue
Light
LOV1 LOV2
FMN FMN
Serine
threonine
kinase
N C
P
P

7. MM.DD.20XX
LOV1 LOV2
FMN FMN
Serine/threonine kinase
N
Jα-Helix
LOV1 LOV2
FMN FMN
Serine/threonine kinase
Jα-Helix
Substrate
Phototropin response
DARK
Dephospho
rhylation
PP2A
P
P
P
Blue Light
Auto-
phosphorhylation

8. In the dark, FMN molecules are non-covalently bound to LOV domains. LOV2 domain "cages"
and inhibits the activity of kinase domain.
Upon blue light absorption FMN molecule becomes covalently bound to LOV domains which
results in conformational change in protein & activation of LOV domains
Which results in activation of Kinase Domain due to unfolding of Ja-helix
Which leads to Autophosphorylation of kinase domain on multiple serine residues which
activates Phototropin & starts signaling pathway
A type 2A Protein Phosphatase mediates Dephosphorylation and inactivation of Phototropin in the
dark

9. CASE 1
PHOT1 mutant lacks phototropism response in
low intensity blue light but retains a
phototropic response at higher light intensities.
PHOT2 mutant lacks phototropism response at
higher light intensities but retains phototropic
response at low light intensities.
PHOT1 & PHOT2 double mutants lacks
phototropism response.
CASE 1
Stomata from PHOT1/ PHOT2 double mutants failed to
exhibit specific blue light response where as in the single
PHOT1 or PHOT2 mutant the blue light response is only
slightly affected.

10. CASE 3
 PHOT1 & PHOT2 double mutants
does not exhibits chloroplast
movements in response to blue
light.
 PHOT2 mutant does not exhibits
avoidance response in response to
high light intensities.

11. Stomata from the phot1/phot2 double mutant fail to exhibit a
specific blue-light response, whereas in the single phot1 or phot2
mutant the blue light response is only slightly affected.
The phot1 mutant lacks hypocotyl phototropism in response to low-
intensity blue light but retains a phototropic response at higher
intensities
MUTATION CASE

12. Chloroplast movement in leaves is an adaptive feature that
occurs in order to control light absorption and prevent
photodamage.
1.Phototropin regulates chloroplast movement
Case 1
Accumulation
Response
Under low light intensity
When incident radiations are weak/ low chloroplasts move at the
upper and lower surfaces of the mesophyll cells to maximize the light
absorption called accumulation response. Both PHOT1 & PHOT2
mediates the Accumulation response.
Vacuole

13. Chloroplast
Movement
Chloroplast movement in leaves is a adaptive features
that occur in order to control light absorption and
prevent photodamage.

14. High
Light
Case 2- In High Light Intensity
Under high light intensity chloroplast moves to the cell surfaces
that parallel to the incident light thus minimizing light
absorption to prevent the photodamage called Avoidance
Response only PHOT2 mediates the Avoidance Response.
Case 3- During night time under dark conditions
PHOT2 mediates chloroplast movement to the lower
surface of the mesophyll cells called dark positioning.
In Dark
Dark Positioning
Avoidance Response
Vacuole
Vacuole

15. Auxin Will
Accumulate On
shaded side
• Phototropin mediate the auxin
gradient change of plant in light.
• The plant will bend towards the light.
• The darker side will have high
concentration of auxin as compare to
light side.
• Rapid cell division and elongation at
darker side due to high concentration
of auxin.

16. 1.Both PHOT1 & PHOT2 mediates
Phototropism (Bending of plants
towards blue light).
Phototropin regulates chloroplast movements
via. F-actin Filament with help of F-actin binding
protein CHUP1(chloroplast Unusual Positioning
1). CHUP1 is localized in chloroplast envelop &
function in chloroplast movement.

17. 1.Both PHOT1 & PHOT2 Promotes stomatal opening under blue light.