Sensory photobiology

The presentation covers phytochrome, cryptochrome and phototropin mediated signaling responses in plants.

1. Sensory photobiology
•Plants utilize a wide range of sensory systems to perceive and transduce
specific incoming environmental signals
•light controls multiple developmental processes in the plant life cycle
Photosynthesis
Photomorphogenesis
seed germination,
seedling de-etiolation,
leaf expansion, stem elongation,
phototropism,
stomata and chloroplast movement,
shade avoidance,
circadian rhythms, and flowering time
Soumitra Paul, Dept. of Botany, CU

2. Four major classes of photoreceptors
Phytochromes (phys): primarily responsible for absorbing the red (R) and
far-red (FR) wavelengths (600–750 nm)

Cryptochromes (crys)
Phototropins (phot)
LOV/F-box/Kelch-repeat proteins ZEITLUPE (ZTL)
FLAVIN-BINDING KELCH REPEAT F -BOX (FKF)
 LOV KELCH REPEAT PROTEIN 2 (LKP2)
blue /ultraviolet-
A (UV-A) region
of the spectrum
(320–500 nm)
Soumitra
Paul, Dept. of Botany, CU

3. Soumitra Paul, Dept. of Botany, CU

4. Photomorphogenesis, Phototropism,
Photoperiodism
Phytochrome responses (red/far red)
flowering and dormancy; branch
patterns; root growth
Blue light responses
stomatal opening; phototropism;
chloroplast orientation
Plants Respond to Light
Soumitra Paul, Dept. of Botany, CU

5. What Is Phytochrome ?
Phytochrome is a pigment found in some
plant cells that has been proven to control plant
development.
 This pigment has two forms or “phases” in
can exist in. P-red light sensitive (Pr) and P –far
red light sensitive (Pfr) forms.
The actual plant response is very specific to
each specie, and some plants do not respond at
all.
Soumitra
Paul, Dept. of Botany, CU

6. Light Responses of Phytochromes
Soumitra Paul, Dept. of Botany, CU

7. cis-trans
isomerisation at
C15-C16
Toggle model
Β-hairpin changes
conformation to an α-
helix after rotation
Photoreversion
Soumitra
Paul, Dept. of Botany, CU

8. Protein Structure of Phytochrome
N-terminal domain 70 kda C-terminal domain 55 kda
Red light induces the rotation of the D ring of chromophore which causes
β-hairpin to become helical a nd exert a tug on the helical spine

Conversion of Pr to Pfr by red light may expose the functional NLS of the
PRD domainof Phy B facilitating PhyB’s import into the nucleus

For phyA , FHY1 is required for nuclear transport as PRD domain is not
associated with nuclear transport Soumitra Paul, Dept. of Botany, CU

9. Mechanism of action
Ion fluxes change in membrane potential and rapid turgor responses
Altered gene expression, which typically results in slower, long term responses
Phytochrome signalling
1. PIF mediated: PIF can act as transcriptional activator or suppressor in dark
SPF45 is an splicing factor for phytochrome signaling and plays regulatory role of PIFs
Soumitra Paul, Dept. of Botany, CU

10. 2. Phytochrome signaling involves protein phosphorylation and dephosphorylation
 (Phytochrome kinase substrate) PKS1 interacts with phyA and phyB in both the
active pfr and inactive Pr forms
Soumitra Paul, Dept. of Botany, CU

11. 3. Photomorphogenesis by protein degradation
COP/DET/FUS complex:
COP1 supressor of PHYA (COP1-SPA) complex
COP9 signallosome : CSN complex
Complex protein for ubiquitination
Proteasomal degradation
Soumitra Paul, Dept. of Botany, CU

12. Cryptochrome
CRY1 and CRY2 : inhibits hypocotyl
elongation; stem elongation is
inhibited by both red and blue
photoreceptors (Stem elongation
during night: seedling emergence)
Blue light changes membrane
potential by depolariziation of
membrane of hypocotyl cells
CRY1: blue light induced inhibition
of hypocotyl elongation; increased
production of anthocyanin

CRY2: degraded under blue light;
Cotyledon expansion, flowering
induction along with phytochrome
Soumitra Paul, Dept. of Botany, CU

13. Regulation of transcription
via light-dependent
interaction of CRYs with CIB
transcription factor, and post-
translational regulation of
proteolysis via light-
dependent interaction of
CRYs with COP1 andSPA1.
Conformational changes
during cryptochrome
activation. On blue light
exposure, CRY
homodimerizes via PHR
domain, followed by
phosphorylation of C-
terminal domain and
subsequent conformational
changes, favoring its
interaction with signaling
proteins
Mechanism of Action
Soumitra Paul, Dept. of Botany,
CU

14. Phytochrome and cryptochrome duel
for photomorphogenesis and flowering
Soumitra Paul, Dept. of Botany, CU

15. Phototropin
Phot1, Phot2: Blue light mediated chloroplast movement, stomatal opening, leaf
expansion
1. Auto-phosphorylation:
2. Auxin mobilization: Activation of phototropin kinases triggers phototropism
LOV: Light-oxygen-voltage
Soumitra
Paul, Dept. of Botany, CU

16. Stomata opening/closing
Phototropin absorbs blue light & autophosphorylated Phosphorylate BLUS1
Regulate PRSL unit of PP1 Regulate protein kinase Binding of 14-3-3 to H
+
ATPase
Soumitra Paul, Dept. of Botany, CU

17. Chloroplast movement
By actin polymerization: Redistribution of
chloroplast depends on the F-actin filament
assembly
Soumitra Paul, Dept. of Botany, CU