How does the ABC Model of Plant Development work? What are A genes, B genes and C genes?

The ABC Model of plant development is a conceptual framework that explains how specific genes regulate the development of floral organs in plants. It was first proposed by Coen and Meyerowitz in 1991 and has since become a foundational model for understanding how flowers are formed in angiosperms (flowering plants).
The ABC model proposes that the formation of the four types of floral organs — sepals, petals, stamens, and carpels — is controlled by three classes of genes: A genes, B genes, and C genes. These genes are expressed in specific patterns in different regions of the developing flower and work in combination to determine which organ develops in each floral whorl.
The ABC Model Breakdown:
The flower is typically organized into four concentric whorls:
1. Whorl 1: Sepals (outermost)
2. Whorl 2: Petals
3. Whorl 3: Stamens
4. Whorl 4: Carpels (innermost)
Gene Function:
The A, B, and C genes regulate the formation of these organs through their interactions.
A Genes:
• Function: A genes are responsible for the development of the sepals and petals.
• In the ABC model, A genes are expressed in Whorls 1 and 2.
• Key A gene example: APETALA1 (AP1) and APETALA2 (AP2) are classic examples of A-class genes.
• Effect: When A genes are expressed in Whorl 1, they promote the formation of sepals. In Whorl 2, A genes promote the formation of petals. If A gene expression is disrupted, these organs may not form properly.
B Genes:
• Function: B genes are responsible for the formation of petals and stamens.
• B genes are expressed in Whorls 2 and 3.
• Key B gene example: APETALA3 (AP3) and PISTILLATA (PI) are examples of B-class genes.
• Effect: In Whorl 2, B genes work with A genes to form petals, while in Whorl 3, B genes work with C genes to form stamens. Disruption of B genes can result in the absence or alteration of petals and stamens.
C Genes:
• Function: C genes are responsible for the development of stamens and carpels.
• C genes are expressed in Whorls 3 and 4.
• Key C gene example: AGAMOUS (AG) is a classic example of a C-class gene.
• Effect: In Whorl 3, C genes promote the formation of stamens, and in Whorl 4, they promote the formation of carpels. Disruption of C gene expression leads to the absence of carpels and/or stamens.
Interactions Between Genes:
The ABC model works through gene interactions, meaning the correct floral organ formation is determined not by one gene alone but by the interplay of A, B, and C genes in different whorls.
• Whorl 1 (Sepals): A genes are active and suppress B and C genes.
• Whorl 2 (Petals): Both A and B genes are active, leading to the formation of petals.
• Whorl 3 (Stamens): Both B and C genes are active, leading to the formation of stamens.
• Whorl 4 (Carpels): C genes are active, leading to the formation of carpels.
Key Points:
• A, B, and C genes work in specific spatial patterns to ensure that the right organs form in the correct whorls.
• The ABC model assumes that these genes act in combination, not in isolation. For example, in the second whorl, both A and B genes must be active together to form petals.
• The model also suggests epistasis, meaning the expression of one set of genes can influence or mask the expression of others.
Mutations and their Effects:
• Mutation in A genes: Leads to the absence of sepals and petals, and the flower may form only stamens and carpels.
• Mutation in B genes: Leads to a loss of petals and stamens, resulting in only sepals and carpels.
• Mutation in C genes: Leads to a loss of stamens and carpels, resulting in only sepals and petals.
The ABC model provides a clear understanding of how floral organs are determined by a combination of A, B, and C genes, each contributing to the formation of different organs at specific whorls. The spatial and temporal expression of these genes in the flower helps create the variety of floral forms seen in angiosperms. The interactions of these genes allow for precise control over the flower's structure, which is crucial for pollination and reproduction.