Crack the ABC Model of Flowering in 5 Minutes!

The ABC Model of Flowering

A high-yield masterclass for CSIR NET Life Sciences. Understand how homeotic genes dictate the fate of floral meristems to form Sepals, Petals, Stamens, and Carpels.

⚡ Quick Facts for the Exam

• Proposed by: Coen and Meyerowitz in 1991.
• Model Organisms: Mostly studied in Arabidopsis thaliana (Thale cress) and Antirrhinum majus (Snapdragon).
• The "E" Genes: Discovered later! SEPALLATA (SEP) genes are Class E. Without them, ALL whorls turn into plain leaves. They act as "glue" for A, B, and C proteins.
• Protein Complexes: These genes encode proteins that form tetramers (Quartet Model) to bind to DNA and trigger organ formation.

Core Theory & Molecular Basis

• Homeotic Genes: The ABC model relies on floral homeotic genes. If these mutate, one floral organ is replaced by another (homeosis).
• Transcription Factors: Almost all of these genes encode transcription factors containing a highly conserved MADS-box domain (except AP2). They bind to DNA and control the transcription of downstream genes required for organ development.
• Class A Genes: Include APETALA1 (AP1) and APETALA2 (AP2). Determine whorls 1 & 2.
• Class B Genes: Include APETALA3 (AP3) and PISTILLATA (PI). Determine whorls 2 & 3.
• Class C Genes: Represented by AGAMOUS (AG). Determines whorls 3 & 4 and stops the flower from growing infinitely (meristem determinacy).

The Wild-Type (Normal) Blueprint

A typical flower consists of four concentric rings called whorls. Here is how the genes translate into actual physical structures:

Whorl 1
Sepals
(Gene A only)

Whorl 2
Petals
(Genes A + B)

Whorl 3
Stamens
(Genes B + C)

Whorl 4
Carpels
(Gene C only)

Cross-Section of Floral Whorls

The Gene Overlap Model

THE GOLDEN RULE OF MUTATION:
Class A and Class C genes are mutually antagonistic!
If A mutates, C expands into whorls 1 & 2.
If C mutates, A expands into whorls 3 & 4.

🎯 Cracking the CSIR NET PYQ

Let's apply the logic to the exact question.

Q. 29. Based on a typical ABC model of flowering, which of the following statements are correct regarding the phenotype of the resultant flower from outer to inner whorl?

A. Loss of A function results into Carpel, Stamen, Stamen and Carpel.
B. Loss of A and B functions result into Carpel, Carpel, Carpel and Carpel.
C. Loss of C function results into Sepal, Sepal, Petal and Petal.
D. Loss of B function results into Sepal, Stamen, Carpel and Carpel.

• ✔️ (1) A and B only [Correct Answer]
• (2) A and C only
• (3) B and C only
• (4) B and D only

🔥 The "Hot" Explanation (Step-by-Step Logic)

Statement A (Loss of A function):
Because of the Golden Rule, if Gene A is missing, Gene C spreads to Whorls 1 and 2.
• Whorl 1: C alone → Carpel
• Whorl 2: C + B → Stamen
• Whorl 3: C + B → Stamen
• Whorl 4: C alone → Carpel
Verdict: Correct!

Statement B (Loss of A and B functions):
Gene A is missing, so Gene C spreads everywhere. Gene B is also dead.
Therefore, ONLY Gene C is active across all 4 whorls.
• Whorl 1 to 4: C alone → Carpel, Carpel, Carpel, Carpel
Verdict: Correct!

Statement C (Loss of C function):
If Gene C is missing, Gene A spreads to Whorls 3 and 4.
• Whorl 1: A alone → Sepal
• Whorl 2: A + B → Petal
• Whorl 3: A + B → Petal
• Whorl 4: A alone → Sepal
Verdict: Result is Sepal-Petal-Petal-Sepal. The statement claims Sepal-Sepal-Petal-Petal. Incorrect!

Statement D (Loss of B function):
Gene B is missing. Gene A and C stay in their normal lanes.
• Whorl 1: A alone → Sepal
• Whorl 2: A alone → Sepal
• Whorl 3: C alone → Carpel
• Whorl 4: C alone → Carpel
Verdict: Result is Sepal-Sepal-Carpel-Carpel. The statement claims Sepal-Stamen-Carpel-Carpel. Incorrect!