2. In the molecular events of flowering development we basically study the role of various gene
which regulate the formation of flower.
To understand this let us first understand the basic structure of flower:
3. The floral development occur when the shoot vegetative meristem is transformed into floral
meristem and then floral meristem further undergo changes to form a flower.
Shoot vegetative meristem Floral meristem Flower
Shoot vegetative meristem is transformed into floral meristem with the help of Floral
Meristem Identity Gene.
Floral meristem identity gene
→ LEAFY(LFY)
→ APETALA1
→ CAULIFLOWER
Floral meristem is transformed into flower with the help of FLORAL ORGAN IDENTITY GENE.
4. Floral organ identity gene
→ APETALA 1
→ APETALA 2
→ APETALA 3
→ PISTILLATA
→ AGAMOUS
Genes which identify floral organ identity are MADS box genes.
MADS box gene have a conserved nucleotide sequence known as MADS box which encode a
particular protein structure known as the MADS domain. The MADS domain enables these
transcription factor to bind to DNA that has a specific nucleotide sequence.
Most of the homeotic gene belongs to MADS box gene.
5. ABC Model of flowering plant:
First discovered by George Haughn and Chris Somerville in 1998.
Arabidopsis thaliana
Three classes of homeotic gene regulates the development of flower.
The 3 classes of gene include
→ Class A
→ Class B
→ Class C
and hence its name is given as ABC Model.
This 3 classes of genes leads to the formation of 4 types of whorls .
Whorl 1- Sepal , Whorl 2- Petals , Whorl 3- Stamen , Whorl 4- Carpel
6. Class A - APETALA 1(AP1) , APETALA2(AP2)
Class B - APETALA 3 (AP3) , PISTALLATA
Class C - AGAMOUS
Let us understand this with the help of a floral diagram
7. Diagrammatic representation of how the different classes of gene is regulation the different
whorl formation.
Whorls
Activity type
Structure Sepal Petal Stamen Carpel
Genes
1 2 3 4
B
A C
AP3,PI
AP2 AG
8. MUTATION
Class A and C are mutually antagonist to each other that means in the absence of A, C is
expressed and in the absence of A, C is expressed.
Class B gene is expressed with A and C
If there is a mutation in class A
Carpel Stamen Stamen Carpel
B
C C
9. If there is a mutation in class B
Sepal Carpel
If there is a mutation in class C
Sepal Petals Petals Sepal
A C
B
A A
10. ABCD Model of flowering plant
A new class of gene is added.
Class D gene
→ SEEDSTICK(STK)
It is a ovule specific gene.
11. ABCDE Model of flowering plant
It is the most recent model of flower development
5 Classes of genes are responsible for the development of flower.
Class E genes is required for the normal functioning of A B C D.
Class E – SEPALLATA
→ sep1
→ sep2
→ sep3
→ sep4
sep1 sep2 sep3 mutant produces sepals in all four whorls
Mutation in sep4 results in the formation of all floral organs into leaves.
12. Sepals Petals Stamens carpels
Ovules
A+E= Sepals
A+B+E= Petals C+E= Carpel
B+C+E= Stamen C+D+E= Ovules
B
A C
E
D
13. References:
1) Teiz and Zeiger's(2012). Plant Physiology; 4th edition
2) Kumar and Mani's. Life Science Vol II; 8th Edition
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