Which of the following floral whorls are absent in agamous (ag) mutant of Arabidopsis?
1. petals and stamens
2. sepals and petals
3. stamens and carpels
4. sepals and carpels

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Introduction

In plant developmental biology, Arabidopsis thaliana serves as a powerful model organism. One of its well-studied floral mutations is the agamous (ag) mutant, which disrupts normal floral organ formation. This article explains the function of the AGAMOUS gene, what happens when it’s mutated, and why stamens and carpels are missing in the agamous mutant.

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Floral Whorls in Arabidopsis

A typical Arabidopsis flower contains four concentric whorls of organs:

1. Sepals (outermost)

2. Petals

3. Stamens (male reproductive organs)

4. Carpels (female reproductive organs)

The development of each whorl is controlled by specific floral homeotic genes, categorized under the ABC model of flower development.

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What is the AGAMOUS Gene?

The AGAMOUS (AG) gene is a C-class gene in the ABC model. It plays two crucial roles:

• Specifies the identity of stamens (3rd whorl) and carpels (4th whorl)

• Promotes the termination of floral meristem, preventing indefinite floral growth

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agamous (ag) Mutant Phenotype

In the agamous mutant, the AG gene is nonfunctional or deleted. As a result:

• Stamens and carpels are absent

• The 3rd and 4th whorls are replaced by petals and sepals, respectively

• Flowers display indeterminate growth, leading to repeated development of sepals and petals

Summary of Floral Whorls in ag Mutant:

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Why Are Stamens and Carpels Absent?

Because AG gene expression is required for stamen and carpel identity, its mutation leads to their homeotic transformation—that is, one organ type being replaced by another. This highlights the importance of transcription factors in developmental genetics.

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Conclusion

The agamous mutant of Arabidopsis is a classic example of how genetic mutations affect organ identity and development. By lacking stamens and carpels, ag mutants provide critical insights into floral patterning and gene function. Understanding this mutation not only enriches our knowledge of plant biology but also aids in genetic engineering and plant breeding research.