Photoperiodism & Vernalization L1-3.pdf

Photoperiodism
and
Vernalization
in
Arabidopsis thaliana
Dr. Nitin Wahi
L1-3 Lecture

CSIR-NET-JRF | ICMR-JRF | DBT-JRF | GATE-XL; BT | IIT-JAM | JGEEBILS | GAT-Bt
PHOTOPERIODISM
✓ Photoperiodism refers to the ability of the plant to measure the length of
photoperiods.
✓ Discovered by W.W. Garner & H.A. Allard in Maryland Mammoth a mutant
variety of tobacco.
✓ They concluded that “Physiological events such as flowering, seed germination
and asexual reproduction are triggered by photoperiod of certain length”.
✓ The specific photoperiod in hrs. within a day of 24 hrs. which is required for
inducing flowering in case of a long-day plant or to inhibit flowering of short-day
plant is called as Critical day length or critical photoperiod.
✓ Plants can be classified in accordance to their photoperiodic responses :

PHOTOPERIODISM
• Flowers only when day length is longer than the critical photoperiod. Ex: Wheat
• a.) Qualitative LDP: Flowering only in long day plant.
• b.) Quantitative LDP: Flowering is accelerated by long days.
Long day Plants (LDP):
Short day Plants (SDP):
• Flowers only when day length is shorter than the critical photoperiod. Ex: Tobacco
a.) Qualitative SDP: Flowering only in short day length.
b.) Quantitative SDP: Flowering is accelerated by short day length.
Day neutral plants (DNP):
• Flowering is independent to day length. Ex: Sunflower, Tomato

✓ K. Hamner and J. Bonner studied the effect of photoperiods in cocklebur (Xanthium), a SDP.
✓ Conclusion: They found that interruption of day light for a short period of darkness had little or
almost no effect on flowering but light mediated interruption of dark period referred to as night
break inhibited flowering. Thus it is not only the critical photoperiod which is important but also
the night duration or critical night.
✓ Critical night length refers to the length of the night that must be exceeded for flowering of SDP or
the length of night must be less than the critical night length in case of LDP.
Which is more Important? Night or day length

PHOTOPERIODISM: SDP V/S LDP
✓ Short day Plants: Flowers only when day length is shorter than the critical photoperiod or
when the period of darkness is more than the critical night.
✓ Long day Plants: Flowers only when day length is longer than the critical photoperiod or
when the period of darkness is less than the critical night.
SDP plants are short tempered,
Get disturbed with light
Mnemonic
Effect of
critical photoperiod
and
critical night

Practice Question

PHOTOPERIODISM: Intermittent light Exp.
✓ Light perception: The photoperiodic stimulus in plants is perceived by leaves. Plant cells uses
phytochrome to measure photoperiod.
✓ The stimulus causes changes in the pattern of gene expression in leaves induces Flowering.
Intermittent Light Exp. on SDP :
✓ A flash of red light during the dark period prevents flowering, and the
effect is reversed when a flash of far red light is used.
✓ If red and far red lights are used in sequence then the radiation used
last in the sequence will determine the response.
Intermittent LIGHT EXPERIMENT
In SDP plants,
‘F’ for ‘F’ar Red & ‘F’ for ‘F’lowering
Mnemonic

Molecular mechanism of photoperiodism
✓ In a photosensitive plant, leaves senses the light radiations, while flowering occurs at the
shoot apical meristem, thus involves long distance signal transmission.
✓ Experimental proof was provided by the grafting of shoot from photoperiodically induced
donor plant Nicotina sylvestries (LDP with exposure of light > critical photoperiod) to a non-
induced receptor plant Maryland mammoth (SDP under non inducible conditions). The signal
molecule for flowering was referred to as florigen (flower-forming).
✓ The identification of florigen was made in LDPs such as A. thaliana. Exposer to sunlight,
regulates circadian clock activates the transcription of CONSTANS (CO) gene to form CO
protein and vice versa for SDPs.
✓ CO protein is stable only in light, so level stay low than the threshold required for signal
transduction if day length is shorter than the critical photoperiod.
✓ CO protein is a Transcription factor (TF) with a zinc finger motif, expressed in the
companion cells of phloem of leaves and stem during sunlight.

Downstream Signaling Pathway of CO gene
✓ CO protein will activate the transcription of FT (Flowering Locus T) gene producing FT protein,
which moves to the SAM (Shoot apical meristem).
✓ In SAM FT protein, enters into the nucleus and forms a complex with Flowering D (FD)
gene. The [ ] gene complex then activates floral meristem identity genes APETALA 1
(AP1) directly and LEAFY (LFY) gene indirectly via activation of SUPPRESSOR OF
OVEREXPRESSION OF CO 1 (SOC1) gene.
✓ The floral meristem identity genes (AP1 and SOC1) then activates the group of genes called
Flower Organ Identity genes (Class A (AP1; AP2), Class B (AP3; PI) and Class C genes (AG)
causing flowering.
Gene Classes Genes involved Floral organ formation
Class A Genes APETALA1 (AP1); APETALA2 (AP2) Sepal and Petal
Class B Genes APETALA (A3); PISTILLATA (PI) Petal and Stamen
Class C Genes AGAMOUS (AG) Stamen and Carpel

FLOWER ORGAN IDENTITY GENES forming flower

GENES INVOLVED
CO (Constans)
FT (Flowering Lotus T)
FD (Flowering D)
CO gene
Co Protein
Solar radiation & Internal
Clock
(Circadian Clock)
(Stable in
light)
FT gene (In the companion
cells of the phloem
of leaves and
stems.)
FT Protein (Moves from leaves
to SAM and enter
nucleus & forms
complex with FD
protein)
FD/FT Complex
Expressed
in Phloem
of leaf/
stem
SOC1 gene
LFY gene
AP1 gene
Both LFY & AP1
gene together
activates
Floral Organ
Identity Genes
[+] [+]
[+]
[+]
[+]
[+]
PHOTOPERIODIC
PATHWAY
Promotes

Previous year Questions

Vernalization
✓ Vernalization refers to flowering after exposer to cold (0-5oC). It is reversible
and can be lost as a result of exposer to condition of high temp. It can be
maintained through tissue culture.
✓ It causes molecular changes in stem apical meristem making it competent to
flower.
✓ Initial experiment proof about vernalization was provided by Melcher who
grafted a vernalized henbane plant to a non-vernalized plant that has
never experienced low temperature, it flowered. The substance which
diffused from the scion to stock was referred to as Vernalin.
✓ Successive attempts to isolate & identity vernalin failed. Whether vernalin is
same as florigen or its precursor in not known.
✓ Anton Lang found that GAs application to biennials induces flowering without
low temp. treatment. He concluded that GAs can substitute vernalization.
Vernalization
(Cold treatment)

MOLECULAR Mechanism of Vernalization
✓ Vernalization or cold (0-5oC) treatment activates the expression of Vernalization insensitive 3
(VIN3); Vernilization1 (VRN1) and Vernilization2 (VRN2) genes.
✓ VIN3 protein causes chromatin remodeling (epigenetic changes) (H3K4 me3 to H3K27 me3/
H3K49 me3) in the Flowering locus C (FLC) gene, causing the amount of FLC mRNA to
decrease inactivating it, thus promoting flowering. VRN1 and VRN2 also repress FLC genes
but in different manner.
✓ Under normal conditions FLC encodes a MADS-BOX transcription factor that represses
flowering by directly repressing FT gene in leaves . This will result in inactivation of floral
meristem identity genes and floral organ identity genes.
Remember: VIN3, VRN1, VRN2, inactivates FLC, while FLC inactivates FT gene
✓ In SAM FT protein, thus can not forms a complex with Flowering D (FD) gene. The [ ]
gene complex then fails to activates floral meristem identity genes LEAFY (LFY) and AP1 gene
inactivating flowering.

MOLECULAR Mechanism of Vernalization
✓ Vernalization or cold (0-5oC) treatment activates VIN3, VRN1 & VRN2 genes inactivating FLC,
thus activating FT expression, causing interaction activating LFY & SOC1 ultimately
. activating floral organ identity genes.
Remember: FLC inactivates FT
FLC encodes a MADS box transcription factor:
MADS - DNA binding
✓ MCM1 from budding yeast, S. cereviciae.
✓ AGAMOUS from the thale cress, A. thaliana
✓ DEFICIENS from the snapdragon, A. majus
✓ SRF from the human H. sapiens

Vernalization
(Cold/Chilling Period)
[+]
VRN1, VRN2 & VIN3 genes
These genes represses
FLC gene epigenetically Inhibit
FLC gene (Represses Flowering)
[+]
LFY gene
AP1 gene
[+]
[+]
SOC1 gene
Floral Organ
Identity Genes
VERNALIZATION
PATHWAY
[-]
[-]
GENES INVOLVED
VRN1 (Vernalization 1)
VRN2 (Vernalization 2)
VIN3 (Ventralization
insensitive 3)
FLC (Flowering Locus C)
Inhibit
Promotes
FT Protein
[+]

33
GA based MOLECULAR Mechanism of FLOWERING
✓ Since LFY gene is not expressed after stage V, of flower development, other factors presumably
act to maintain their expression, these includes the hormone Gibberellin (GA).
✓ GA deficient mutant produces flower which have growth arrest and immature. The GA appears
to be an important regulator of homeotic gene flower development.
✓ Signaling through the GA pathway involves inactivation of a family of “DELLA proteins” that act
as negative regulators of GA responses.
✓ Mutations in these DELLA genes can repress the floral defects of GA mutant flowers, suggesting
that GA responses are essential for completion of floral organ development.
✓ GA promotes [+] expression of the class B and C genes and the continued development of floral
organs by negatively [-] regulating DELLA proteins.

33
GA based MOLECULAR Mechanism of FLOWERING

Previous year Questions
• Predict the results of the following Mutations:
Q1. GA [-] Mutants: The flower will be immature and sterile.
Q2. Della [-] Mutant: The flower will be normal (constituting express GA).
Q3. GA [-]/ Della [-] Mutant: The flower will be normal i.e., a mutation in Della will cause .
. upregulation [+] of GA-response genes.
Q4. GA [+]/ Della [+]: Inducible w.r.t GA presence [+] or absence [-].
Q5. GA [+]/ Della [-]: The flower will be normal.
Q6. GA [-]/ Della [+]: The flower will be immature and sterile.

Sensing of Plant Age
[+] FCA, FLK, FLD & LD genes
Inhibit
[-]
FLC gene
[+]
LFY gene
AP1 gene
[+]
SOC1 gene
Inhibit
[+]
Floral Organ
Identity Genes
Both LFY & AP1 gene
together activates
AUTONOMOUS
PATHWAY
GENES INVOLVED
FCA (Flowering control locus A)
FLK (Flowering locus K)
FLD (Flowering locus D)
LD (Luminidependens)
FLC (Flowering Locus C)
SOC1 (Suppressor of
Overexpression of Constant 1)
LFY (Leafy)
AP1 (Apetala-1)
[-]
Promotes

Summary of Genetic interactions in
• Arabidopsis
• Thaliana
• Flower development

VERNALIZATION PATHWAY AUTONOMOUS PATHWAY
Photoperiodic
Pathway
Gibberellic Acid
Pathway
“Cold Treatment” “Its sense Plant age”
[+]
VRN1, VRN2
& VIN3 genes
Overview
FRI gene
[+] [+]
FLC gene
[-]
[-]
FCA, FLK, FLD & LD genes
[-]
SOC1
LFY ⇋ AP1
Floral Organ Identify Genes
Flowering
[+] [+]
[+]
[+]
FT/FD complex
(Formed)
(moves to SAM &
enters nucleus)
FT Protein
FT gene
CO gene
[+] [+]
[+]
It inhibits FLC
Expression

Photoperiodism & Vernalization L1-3.pdf

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Photoperiodism & Vernalization L1-3.pdf

Similar to Photoperiodism & Vernalization L1-3.pdf (20)

Recently uploaded

Recently uploaded (20)

Photoperiodism & Vernalization L1-3.pdf