59. Individual and overlapping expression of homoerotic genes in adjacent whorls of a flower determine the pattern of floral organ development. In an Arabidopsis
    mutant, floral organs are distributed as follows:
    Whorl 1 (outer most) — carpel
    Whorl 2 — stamens
    Whorl 3 – stamens
    Whorl 4 (inner most) — carpel
    Loss of function mutation in which one of the following genes would have caused the above pattern of floral organ development?
    (1) APETALA 2 (2) APETALA 3
    (3) PISTILLATA (4) AGAMOUS

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Introduction

The floral organ development pattern in Arabidopsis is tightly regulated by the activity of homeotic genes categorized into classes A, B, and C. Among these, AGAMOUS (AG), a C-class homeotic gene, plays a central role in controlling the identity of reproductive organs, including stamens and carpels, and in terminating floral meristem activity. Mutations in AG result in distinctive homeotic transformations that disrupt normal floral organ patterning.

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The Role of AGAMOUS in Floral Patterning

• AG is expressed in the third and fourth whorls of the flower, where it specifies stamens and carpels, respectively.

• It acts as a floral meristem termination gene, ensuring that floral organs develop in a determinate pattern and the floral meristem ceases growth after organ formation.

• Mutations in AG lead to the production of flowers with homeotic transformations, typically causing the reproductive organs to be replaced by floral organs normally found in outer whorls.

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Mutant Phenotype: Flower-within-Flower

• In agamous mutants, the flowers exhibit a homeotic conversion where the third whorl (stamens) and the fourth whorl (carpels) convert into additional whorls of outer organ types, such as sepals and petals.

• This mutation causes the flowers to appear as “flowers within flowers” because the floral meristem remains indeterminate and produces extra floral organs.

Organ Patterning in Mutants

• The mutation causes the third whorl (stamens) to develop as carpels and the fourth whorl (carpels) to develop as additional carpels or sometimes sepals, depending on mutant severity.

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Summary

The mutation in AGAMOUS leads to the transformation of stamens and carpels into floral organs of outer whorls (sepals and petals). This pattern of floral organ development underscores AG’s critical functional role in specifying reproductive organ identity and floral meristem termination in Arabidopsis.