57. Based on ABC model during flower development, loss of class A activity results in the formation of only stamen and carpel. Which of the following floral organ
    identity genes controls the class A activity?
    (1) APETALA 1 and APETALA 2
    (2) APETALA 3 and PISTILLATA
    (3) Only PISTILLATA
    (4) Only AGAMOUS

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Introduction

In Arabidopsis thaliana, floral organ development is regulated by the ABC model, where Class A, B, and C genes specify organ identity in the four floral whorls. Class A genes control the identity of sepals and petals. Among these, APETALA1 (AP1) and APETALA2 (AP2) play essential roles in Class A activity. Loss of Class A function leads to flowers lacking sepals and petals, instead forming stamens and carpels.

Functions of APETALA1 and APETALA2

• APETALA1 (AP1) is a MADS-box transcription factor that specifies floral meristem identity and sepal and petal development, primarily expressed in the first two whorls.

• APETALA2 (AP2) encodes a transcription factor affecting the development of sepals and petals, with broader expression but crucial for early organ identity specification.

• Both genes synergistically maintain Class A function, repressing Class C gene (AGAMOUS) expression in outer whorls to prevent reproductive organs from forming there.

Consequences of Loss of Class A Function

• Mutations in AP1 or AP2 disrupt sepal and petal development leading to homeotic replacement by reproductive organs (stamens and carpels) due to expanded Class C activity.

• Flowers from Class A mutants show whorl composition of stamens and carpels only, due to the loss of sepals and petals.

Summary Table: Class A Gene Functions and Flower Phenotypes

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Conclusion

APETALA1 and APETALA2 are the floral organ identity genes responsible for Class A activity controlling sepals and petals in Arabidopsis. Their loss results in flowers with only stamens and carpels, confirming option (1).