Arabidopsis thaliana is a model plant species that is used to study flowering genes. There are four classes of genes that control flowering: 1) floral meristem identity genes that convert vegetative shoot meristems into floral meristems, 2) shoot meristem identity genes that maintain vegetative growth, 3) floral organ identity genes that determine the identity of floral organs, and 4) boundary setting genes that regulate the expression of organ identity genes. Key floral organ identity genes include AP1, AP2, AP3, PI, and AG.

1. Flowering Model
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2. FLOWERING GENES IN Arabidopsis thaliana
❑Arabidopsis thaliana is long day plant (LDP) and a
member of family Brassicaceae (Mustard family) widely
used as genetic model in plants.
❑Flower is modified shoot of limited growth.
❑Flowering requires reprogramming of Shoot Apical
Meristem (SAM).
❑Biological function of a flower is to help in reproduction.
❑Genetic analysis have identified 4 Classes of genes
which control flowering in Arabidopsis thaliana.
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3. Class 1
Floral meristem
identity gene
Class 2
Shoot meristem
identity gene
Class 4
Boundary setting Gene/
Cadastral gene
Class 3
Floral organ
identity gene
Flowering gene
in Arabidopsis
thaliana
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4. FLORAL MERISTEM IDENTITY GENES
There Are 6 Floral meristem identity gene identified in
Arabidopsis thaliana
These genes Are
expressed in
early floral stage.
LFY (leafy) Gene
AP1 (Apetala 1) Gene
CAL (Cauliflower) Gene
AP2 (Apetala 2) Gene
FUL (Fruitful) Gene
UFO Gene
FUNCTION
CONVERTS VEGETATIVE
SHOOT MERISTEM INTO
FLORAL MERISTEM i.e.
CONVERT VEGETATIVE
SHOOT INTO FLOWER
MERISTEM.
Class 1
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5. SHOOT MERISTEM IDENTITY GENES
Class-2
Shoot Meristem Identity Genes identified in Arabidopsis thaliana are:
1. TFL 1 (Terminal Flower 1)
2. TFL 2 (Terminal Flower 2)
FUNCTION:
❑ IT HAS OPPOSITE EFFECT OF CLASS 1
GENES (FLORAL MERISTEM IDENTITY GENES).
❑ ACTS ON SAM TO REPRESS THE FLORAL TRANSITION AND MAINTAINS VEGETATIVE GROWTH
i.e. IT PREVENTS TRANSITION OF INFLORESCENCE SHOOT INTO FLORAL SHOOT.
Vegetative
meristem
Inflorescence
meristem
Flower
meristem
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6. Class-3 FLORAL ORGAN IDENTITY GENES
There are 5 Floral Organ Identity Genes identified in
Arabidopsis thaliana
These genes are organ specific so their expression leads to development
of different organs of flower like sepal, petal, stamen & carpel.
1. AP1 (APETALA 1) GENE
2. AP2 (APETALA 2) GENE
3. AP3 (APETALA 3) GENE
4. PI (PISTILLATA) GENE
5. AG (AGAMOUS) GENE
Class-A
Class-B
Class-C
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7. Class-4 BOUNDARY SETTING GENES/ CADASTRAL GENES
They regulate Floral Organ Identity gene by setting boundaries for their
expression and thus controls the correct distribution of the floral organs.
EXAMPLE:
1. SUP (SUPERMAN) GENE
FUNCTION:
It acts to maintain boundary between 3rd and 4th whorl. It define boundary
between male reproductive organ (stamen) and female reproductive organ
(carpel) in the flower.
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8. Flowering Pathways in Arabidopsis thaliana
Vernalization Pathway
Gibberellic Acid Pathway
Autonomous Pathway
Photoperiodic Pathway
In Arabidopsis, flowering is controlled by:
All of these Pathways
converge to activate the
Floral Meristem Identity
genes such as LFY and
AP1 genes.
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9. FRI Gene
FLC Gene
SOC1
LFY AP1
Floral organ identity Gene
FCA, FLK, FLD & LD gene
FT protein
FT gene
CO gene
FT/FD Complex
(formed)
VIN 3, VRN 1 &
VRN 2 GENE
Gibberellic
Acid
pathway
(Move to SAM &
enters nucleus)
Autonomous Pathway
Its sense plant age
It promote FLC
expression
Vernalization Pathway
Cold
Treatment
Photoperiodic
pathway
Flowering
Internal clock
+
Light
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10. FLC Gene
SOC1
LFY AP1
VIN 3, VRN 1 &
VRN 2 GENE
Vernalization Pathway
Floral organ identity Gene Flowering
Activate
Activate
Activate
Activate
Inhibit
Inhibit
Represses Flowering
These gene are represses
FLC gene epigenetically
FLC (flowering Locus C ) gene
VIN3 ( vernalization Incentive 3)
Gene
VRN 1 (Vernalization 1) Gene
VRN 2 (Vernalization 2) Gene
Chilled/Cold
Climate
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11. SOC 1
LFY gene
Floral organ
identity Gene
Gibberellic
Acid
pathway
Flowering
AP1
Activate
Activate
Activate
Activate
Activate
AP1 (APETALA 1) gene
LFY (LEAFY) gene
SOC1 (SUPPRESSOR OF
OVEREXPRESION OF
CONSTANTS 1) gene
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12. FLC Gene
SOC1
Floral organ identity Gene
FCA, FLK, FLD & LD gene
Flowering
LFY gene
AP1
Activate
Inhibit
Activate
Activate
PROMOTE
Inhibit
Sensing
of Plant
Age
FLC (FLOWERING LOCUS C)
LFY [LEAFY]
AP1[APETALA 1}
SOC1 (SUPPRESSOR OF
OVERREXPRESSION OF CONSTANTS 1)
FCA (FLOWER|NG CONTROL LOCUS A)
FLK (FLOWERING LOCUS K)
FLD (FLOWERING LOCUS D)
LD (LUMINIDEPENDENS)
PROMOTE
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13. SOC1
LFY
Floral organ identity Gene
FT protein
FT gene
CO gene
FT/FD Complex
(formed)
(Move from leaves SAM & enters
nucleus & forms complex with FD
protein
Photoperiodic
pathway
Flowering
AP1
(In the Companion cells of the
phloem of leaves and stem)
Activate
PROMOTE Activate
Activate
Activate
Activate
Light & Internal
clock
(circadian Clock)
FD (FLOWERING D)
CO (CONSTANS)
FT (FLOWERING LOCUS T)
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14. C St P S
S- Sepal (also known as Calyx, it protects the developing flower).
P- Petal (also known as Corolla, It attracts pollinators).
St-Stamen (It is male reproductive organ also know as Androecium).
C- Carpel (It is female reproductive organ also know as Gynoecium).
Anatomy of flower
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15. Arabidopsis thaliana is long day
plant (LDP) and a member of family
Brassicaceae (Mustard family)
widely used as genetic model in
plants.
ARABIDOPSIS THALIANA
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16. Class-3 FLORAL ORGAN IDENTITY GENES
There are 5 Floral Organ Identity Genes identified in
Arabidopsis thaliana
1. AP1 (APETALA 1) GENE
2. AP2 (APETALA 2) GENE
3. AP3 (APETALA 3) GENE
4. PI (PISTILLATA) GENE
5. AG (AGAMOUS) GENE
Class-A
Class-B
Class-C
➢ Floral Organ Identity Genes are also called Floral Homeotic Genes
➢ All accept AP2 Gene contains the same DNA binding domain MADS box.
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17. Class A gene is mutually antagonist to Class C gene I.e. A and C are
antagonist to each other
➢ If class A gene gets mutated then at that place class C gene will get expressed
and if class C Gene gets mutated then at that place Class A gene will get
expressed
➢ Class B Genes Does not express individually
A C
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18. Generalized
Gene
Expression
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19. MADS BOX GENES
MADS box is a conserve sequence motif, the genes which contains this motif are
called MADS box genes. The MADS box genes encodes DNA binding MADS
domain. MADS box genes encodes transcription factors. They are both present in
animals and plants and comes under MADS box genes family.
In plants they are involved in controlling all major aspects of
development including
1- Male and Female Gametophyte Development.
2- Embryo and Seed Development.
3- Root, Flower & Fruit Development
e.g. AP1, PI, AG, SOC1 Genes etc.
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20. A
A+B
B+C
C
CARPEL (C)
Stamen
(C+B)or (B+C)
Wild Type
(Normal)
A gene
mutated
So, if gene A gets Mutated then there will be no
sepals and petal formation
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21. C St P S C St St C
Wild Type (Normal) A gene mutated
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22. A
A+B
B+C
C Wild Type
(Normal)
B gene
mutated
CARPEL (C)
Sepals (A)
So, if gene B gets Mutated then there will be
no petals and Stamen formation
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23. C St P S
Wild Type (Normal) B gene mutated
C C S S
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24. A
A+B
B+C
C
Sepals (A)
Petals (A+B)
C gene
mutated
Wild Type
(Normal)
So, if gene C gets Mutated then there will be
no Carpel and Stamen formation
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25. A
A+B
B+C
C Wild Type
(Normal)
B & A gene
mutated
CARPEL (C)
So, if gene B&C gets Mutated then only carpel
form in all whorl
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