Q.55 An Arabidopsis thaliana mutant plant developed defective flowers with altered
floral organ identity and patterning. In this mutant, the four floral whorls contain
Sepal-Sepal-Carpel-Carpel, from the periphery to the center of the flower.
Based on the typical ABC model of floral organ patterning, which among the
following are mutated in this plant?
(A) Class A gene(s)
(B) Class B gene(s)
(C) Class C gene(s)
(D) Double mutant for Class A and Class C genes
Class B genes are mutated in this Arabidopsis thaliana mutant. The phenotype shows sepals in whorls 1 and 2 (instead of sepals and petals) and carpels in whorls 3 and 4 (instead of stamens and carpels), which matches the classic class B mutant under the ABC model of floral organ identity.
ABC Model Basics
The ABC model explains floral organ patterning in Arabidopsis thaliana through three classes of homeotic genes acting combinatorially across four whorls. Class A genes (e.g., APETALA1, APETALA2) specify sepals in whorl 1 and contribute to petals in whorl 2; class B genes (e.g., APETALA3, PISTILLATA) act with A for petals (whorl 2) and with C for stamens (whorl 3); class C genes (e.g., AGAMOUS) specify stamens (whorl 3) and carpels (whorl 4). A and C functions mutually repress each other to restrict expression domains.
Option Analysis
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(A) Class A gene(s): A mutants show carpels in whorl 1, stamens in whorl 2, and normal stamens/carpels in whorls 3/4 due to ectopic C expression in outer whorls. This does not match sepal-sepal-carpel-carpel.
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(B) Class B gene(s): B mutants lack B activity, yielding A alone in whorls 1-2 (sepals) and C alone in whorls 3-4 (carpels), exactly matching the phenotype. Examples include apetala3 or pistillata mutants.
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(C) Class C gene(s): C mutants show petals (A+B) in whorl 3 and sepals (A) in whorl 4, with indeterminate flowers, not carpels in inner whorls.
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(D) Double mutant for Class A and C genes: A+C double mutants produce leaf-like organs or sepals across whorls due to loss of both A and C, with B activity possibly forming modified petals/stamens in middle whorls—not sepal-sepal-carpel-carpel.
Correct Answer
(B) Class B gene(s)
The Arabidopsis thaliana mutant sepal-sepal-carpel-carpel phenotype reveals key insights into the ABC model of flower development, a cornerstone of plant developmental biology for CSIR NET aspirants. This homeotic mutation disrupts normal whorl patterning—sepals (whorl 1), petals (whorl 2), stamens (whorl 3), carpels (whorl 4)—and directly implicates class B genes like APETALA3 (AP3) and PISTILLATA (PI).
ABC Model in Arabidopsis Thaliana Flower Development
In wild-type Arabidopsis thaliana flowers, MADS-box transcription factors orchestrate organ identity via combinatorial gene activity. Class A (AP1, AP2) alone forms sepals; A+B forms petals; B+C forms stamens; C alone forms carpels. Mutual antagonism between A and C confines B to whorls 2-3. Mutations cause predictable homeotic shifts, as seen in classic studies from the 1990s.
Phenotype of Sepal-Sepal-Carpel-Carpel Mutant
This mutant replaces petals with sepals (whorl 2) and stamens with carpels (whorl 3), yielding uniform outer sepals and inner carpels. Loss of B function defaults whorl 2 to A-only (sepals) and whorl 3 to C-only (carpels), confirming class B gene mutation. No indeterminacy or ectopic organs rules out C defects.
Why Class B Genes? Detailed Mutant Comparison
| Mutant Type | Whorl 1 | Whorl 2 | Whorl 3 | Whorl 4 | Key Genes Affected |
|---|---|---|---|---|---|
| Wild Type | Sepals (A) | Petals (A+B) | Stamens (B+C) | Carpels (C) | – |
| Class A | Carpels (C) | Stamens (B+C) | Stamens (B+C) | Carpels (C) | AP1, AP2 |
| Class B | Sepals (A) | Sepals (A) | Carpels (C) | Carpels (C) | AP3, PI |
| Class C | Sepals (A) | Petals (A+B) | Petals (A+B) | Sepals (A) | AG |
| A+C Double | Leaves/Sepals | Petals/Stamens (B) | Petals/Stamens (B) | Leaves/Sepals | AP1/AP2 + AG |
This table highlights how only class B loss matches the query phenotype perfectly.
CSIR NET Exam Relevance
For competitive exams like CSIR NET Life Sciences, master ABC model mutants: B mutants (ap3, pi) show sepal-sepal-carpel-carpel; A mutants carpel-stamen-stamen-carpel; C mutants sepal-petal-petal-sepal. Double mutants add complexity, but single B fits here. Practice with Arabidopsis thaliana diagrams for quick recall.
