The ABC Model of Flower Development



The ABC model of flower development in angiosperms was formulated by E. Coen and E. Meyerowitz in 1991. This model is built on the observation of mutants with defects in floral organ development. The ABC model summarizes how the presence or absence of different classes of transcription factors in the different parts of the flower regulates the development of floral organs.

Two key observations have led to the ABC model. First, the discovery of homeotic mutations in which one organ develops in a location normally occupied by a different organ. Wild roses, for example, have only five petals but many stamens. Garden roses have a hometic gene that causes some of the potential stamens to develop into petals instead. Second, each of the genes that affect the identity of organs in flowers has an effect on two groups of flower organs, affecting petals and sepals or affecting petals and stamens.

Floral organ identity genes are therefore divided into three classes, depending on which organs they affect. Mutations in class A genes affect sepals and petals. Mutations in class B genes affect petals and stamens, while those in class C affect stamens and carpels. All three classes of genes are homeotic genes, which are translated into proteins. Each protein coded by these genes contains a MADS-box region that allows the protein to bind to DNA and to function as a regulator in DNA transcription. It is believe that these genes are master controlling genes, regulating the action of other genes that will control organ development.

The ABC model proposes that class A genes alone are responsible for the development of sepals, but act together with class B genes to effect petal development. Class C genes alone are responsible for initiating the development of carpels, but act together with class B genes to determine the development of stamens. Support for a dual gene interaction with class B genes comes from the nature of class B mutants. A defective B gene leads to the absence of petals and stamens; in their places develop additional sepals and carpels. Similar organ replacement occurs when other classes of genes undergo mutation.

Summary:
 * The expression of A genes induces the development of sepals.
 * The expression of B genes together with A genes induces the development of petals.
 * The expression of B genes together with C genes induces the development of stamens.
 * The expression of C genes induces the development of carpels.

In 2004 the characterization of sepallata1,2,3 triple mutant in Arabidopsis has led to the formulation of the ABCE model, which consider the importance of class E genes for the development of the floral organs.