The ABC Model Explains the Determination of Floral Organ Identity

In 1991 the ABC model was proposed to explain how homeotic genes control organ identity. The ABC model postulates that organ identity in each whorl is determined by a unique combination of the three organ identity gene activities (see Figure 24.6):

• Activity of type A alone specifies sepals.

• Activities of both A and B are required for the formation of petals.

• Activities of B and C form stamens.

• Activity of C alone specifies carpels.

The model further proposes that activities A and C mutually repress each other (see Figure 24.6); that is, both A- and C-type genes have cadastral function in addition to their function in determining organ identity.

The patterns of organ formation in the wild type and most of the mutant phenotypes are predicted and explained by this model (Figure 24.8). The challenge now is to understand how the expression pattern of these organ identity genes is controlled by cadastral genes; how organ identity genes, which encode transcription factors, alter the pattern of other genes expressed in the developing organ; and finally how this altered pattern of gene expression results in the development of a specific floral organ.

Whorl

Genes

Whorl

1

2

3

4

Structure Sepal Petal Stamen Carpel

Genes

B

A

C

Structure Sepal Petal Stamen Carpel

FIGURE 24.8 Interpretation of the phe-notypes of floral homeotic mutants based on the ABC model. (A) Wild type. (B) Loss of C function results in expansion of the A function throughout the floral meristem. (C) Loss of A function results in the spread of C function throughout the meristem. (D) Loss of B function results in the expression of only A and C functions.

(B) Loss of C function

(B) Loss of C function

(C) Loss of A function

Whorl

Genes

Whorl

1

2

3

4

Structure Sepal Petal Petal Sepal

Genes

B

A

Structure Sepal Petal Petal Sepal

(C) Loss of A function

(D) Loss of B function

Whorl

1

2

3

4

(D) Loss of B function

Genes

Whorl

B

Genes

C

Structure

Carpel

Stamen

Stamen

Carpel

Whorl

1

2

3

4

Structure Sepal Sepal Carpel Carpel

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