The ABC model of flower development describes the genetic mechanisms that establish floral organ identity. Studies in Arabidopsis thaliana and Antirrhinum majus in the early 1990s led to the isolation of floral organ-identity genes, known as MADS-box genes, and the establishment of the ABC model. The ABC model proposes that different combinations of the A, B, and C class floral organ-identity genes specify the four types of floral organs.

1. Topic
Floral identity gene and
ABC model

2. The ABC model of flower development is a
scientific model of the process by which flowering
plants produce a pattern of gene expression in
meristems that leads to the appearance of an organ
oriented towards sexual reproduction, a flower
ABC MODEL

4. In the early 1990s, genetic studies on
Arabidopsis thaliana and Antirrhinum majus
led to the isolation and characterization of
floral organ-identity genes (also called floral
homeotic genes) and the establishment of the
seminal ABC model for flower development.
This model proposed that different organ-
identity genes act alone and in various
combinations to specify each of the four types
of floral organs.

5. The ABC model of flower development was
first formulated by George Haughn and Chris
Somerville in 1988. It was first used as a
model to describe the collection of genetic
mechanisms that establish floral organ
identity in the Rosids, as exemplified by
Arabidopsis thaliana, and the Asterids, as
demonstrated by Antirrhinum majus.
HISTORY

6. Antirrhinum Arabidopsis

8. Most of the genes of ABCDE model are MADS-box
genes
The MADS box is a conserved sequence found in
genes which comprise the MADS-box gene family
The floral homeotic MADS box gene in
• Arabidopsis is AGAMOUS
• Antirrhinum is DEFCIENS
The length of the MADS-box are in the range of 168
to 180 base pairs
MADS –BOX GENES

9. In plants, MADS-box genes are involved in controlling
all major aspects of development, including male &
female gametophyte development, embryo and seed
development, as well as root, flower and fruit
development, floral organ identity and flowering
time determination

10. The identities of different organs are
specified by the actions of floral organ-
identity genes in different regions of a
developing flower.

11. A class genes
Act to specify sepal and petal
development
B class genes
Act to specify petal and stamen
development
C class gene
Acts to specify stamen and carpel
development
BASIC ABC MODEL OF FLORAL ORGAN
DEVELOPMENT

13. In A. thaliana, function A is mainly represented
by two genes APETALA1 (AP1) and APETALA2
(AP2)
Mutations in the class A gene AP2 result in C
activity spreading into whorls one and two and
consequently homeotic transformations in
organ identity with carpels replacing sepals in
whorl one and stamens replacing petals in
whorl two
A CLASS GENE

15. In A. thaliana the type-B function mainly
arises from two genes, APETALA3 (AP3) and
PISTILLATA (PI), both of which are MADS-box
genes. A mutation of either of these genes
causes the homeotic conversion of petals into
sepals and of stamens into carpels.
B CLASS GENES

17. In A. thaliana, the C function is derived from
one MADS-box type gene called AGAMOUS
(AG)
Mutations in the class C gene AG result in
class A activity in all four whorls. The ag
mutants produced indeterminate flowers
repeating the pattern of organs (Se Pe Pe)n
CCLASS GENES

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