ABC model of flower development

The ABC model of flower development is a scientific model that describes the process by which the floral organs of a plant are formed. This model explains how the identity of the sepals, petals, stamens, and carpels is determined by the interaction of three classes of genes, known as A, B, and C.

Diagram of the ABC model of flower development

Overview

The ABC model posits that the identity of the floral organs is determined by the combination of three classes of homeotic genes:

• A genes: These genes are responsible for the development of sepals and, in combination with B genes, petals.
• B genes: These genes, when expressed with A genes, lead to the formation of petals, and with C genes, they lead to the formation of stamens.
• C genes: These genes are responsible for the development of stamens and, in combination with B genes, carpels.

The model is based on the observation that mutations in these genes lead to predictable changes in the floral organ identity, which can be seen in various mutant phenotypes.

Mechanism

The ABC model is based on the spatial expression of the A, B, and C genes in the four concentric whorls of a flower:

• Whorl 1: A genes alone are expressed, leading to the formation of sepals.
• Whorl 2: A and B genes are expressed together, resulting in the formation of petals.
• Whorl 3: B and C genes are expressed together, leading to the formation of stamens.
• Whorl 4: C genes alone are expressed, resulting in the formation of carpels.

The model also includes the concept of "antagonism" between A and C genes, where the presence of one class inhibits the expression of the other.

Examples

The ABC model has been extensively studied in model organisms such as Arabidopsis thaliana and Antirrhinum majus.

Diagram of a mature flower showing the arrangement of floral organs

Arabidopsis thaliana

In Arabidopsis thaliana, mutations in the A, B, or C genes lead to distinct floral phenotypes:

• A gene mutants: Sepals are transformed into carpels, and petals are transformed into stamens.
• B gene mutants: Petals are transformed into sepals, and stamens are transformed into carpels.
• C gene mutants: Stamens are transformed into petals, and carpels are transformed into sepals.

File:Arabidopsis thaliana-flower.jpg

Arabidopsis thaliana flower

Antirrhinum majus

In Antirrhinum majus, similar mutations lead to changes in floral organ identity, supporting the universality of the ABC model across different species.

File:Antirrhinum majus6.jpg

Antirrhinum majus flower

Extensions of the Model

The ABC model has been expanded to include additional classes of genes, such as D and E, which are involved in the development of ovules and the specification of floral organ identity, respectively. These extensions have led to the development of the "ABCDE model" of flower development.

Related Pages

• Floral organ identity genes
• Homeotic gene
• Plant development
• Arabidopsis thaliana

Gallery

• Petunia x hybrida flower
  Petunia x hybrida flower
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  Arabidopsis mutants showing altered floral organ identity