Q58. The diagram describes the ABC model of flower patterning in Arabidopsis
where the A, B and C functions are operational in the whorls (1+2), (2+3)
and (3+4), respectively, in the wild–type flower. Removal of A or C function
results in the floral organ arrangements as (carpel; stamen; stamen; carpel)
or (sepal; petal; petal; sepal), respectively. Based on these observations,
which ONE of the following molecular pathways is CORRECT for floral
organ pattern generation? Arrow indicates activation and bar indicates
inhibition.
ABC Model of Flower Patterning in Arabidopsis: Correct Molecular Pathway for Whorl 2
The ABC model explains floral organ identity in Arabidopsis through combinatorial gene functions across four whorls, with arrows indicating activation and bars showing inhibition. Based on the diagram, option (C) correctly identifies the B function molecular pathway active in whorl 2 (petals).
Wild-Type Whorls
Arabidopsis flowers develop sepals (whorl 1), petals (whorl 2), stamens (whorl 3), and carpels (whorl 4). A genes (APETALA1, APETALA2) alone specify sepals; A + B genes (APETALA3, PISTILLATA) specify petals; B + C genes (AGAMOUS) specify stamens; C genes alone specify carpels. A and C functions mutually repress each other.
Option Analysis
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Option A: Incorrectly assigns A function to whorls 1 and 2 without B involvement for petals; petals require A + B activation.
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Option B: Wrongly places B function solely in whorls 2 and 3; ignores A + B for petals and B + C for stamens.
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Option C (Correct): Accurately shows A in whorl 1 (sepals), A + B in whorl 2 (petals), B + C in whorl 3 (stamens), C in whorl 4 (carpels), with mutual A-C inhibition.
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Option D: Falsely limits C to whorls 3 and 4 without B + C for stamens.
Key Genes
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A: AP1, AP2 (whorls 1-2).
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B: AP3, PI (whorls 2-3).
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C: AG (whorls 3-4).
Mutations confirm: B mutants convert petals to sepals, stamens to carpels.
The ABC model of flower patterning in Arabidopsis thaliana revolutionized understanding of floral organ development, specifying whorl functions through A, B, and C gene classes. This framework, essential for CSIR NET Life Sciences, details how molecular pathways control sepals in whorl 1, petals in whorl 2, stamens in whorl 3, and carpels in whorl 4. Dive into activations, inhibitions, and mutants for exam mastery.
ABC Model Basics
A genes activate sepals (whorl 1) and contribute to petals (whorl 2); B genes join A for petals and C for stamens (whorls 2-3); C genes drive stamens and carpels (whorls 3-4). Mutual antagonism between A and C ensures precise boundaries, with arrows showing activation and bars inhibition in diagrams.
Key genes include:
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A: APETALA1 (AP1), APETALA2 (AP2).
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B: APETALA3 (AP3), PISTILLATA (PI).
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C: AGAMOUS (AG).
Whorl-Specific Functions
| Whorl | Organs | Gene Combination | Mutant Effect |
|---|---|---|---|
| 1 | Sepals | A | Carpels in ap2 |
| 2 | Petals | A + B | Sepals in ap3/pi |
| 3 | Stamens | B + C | Petals in ag |
| 4 | Carpels | C | Sepals in ag |
This table highlights combinatorial control in ABC model flower patterning Arabidopsis whorl functions.
Mutant Phenotypes
Loss-of-function reveals roles: B-class mutants (ap3, pi) yield sepals-petals-carpels-carpels, confirming B for whorl 2 petals and whorl 3 stamens. C mutants (ag) produce sepals-petals-petals-sepals with indeterminacy. Triple ABC mutants form leaf-like organs, underscoring necessity.
CSIR NET Relevance
For competitive exams, focus on pathway accuracy: Correct identification of B activation in whorl 2 (A+B for petals) distinguishes options. E-class (SEPALLATA) genes refine the ABCE model, required across whorls.
