61. According to the ABC model of floral development in Arabidopsis as shown below

    Several genes/transcription factors e.g. AP 1, AP2, AP3, AG etc., are involved. Which one of the following statements is correct?
    (1) Apetala 2 (AP2) transcripts expressed during sepal and petal development.
    (2) Agamous AG is considered as class A gene.
    (3) AP1 expressed during carpel development.
    (4) AP3 expressed during sepal development.

Introduction

The ABC model of floral organ development in Arabidopsis thaliana explains that the overlapping expression of homeotic genes controls the identity of sepals, petals, stamens, and carpels. Among these, APETALA2 (AP2) plays a critical role in Class A gene activity, influencing sepal and petal development.

Expression of Floral Homeotic Genes

• APETALA2 transcripts are expressed prominently in the first two whorls—corresponding to developing sepals and petals.​

• APETALA1 (AP1) is also involved in early floral meristem and organ development but is a MADS-box gene distinct from AP2.​

• AGAMOUS (AG), a Class C gene, is expressed in the reproductive whorls (stamens and carpels), but not during sepal and petal formation.​

• APETALA3 (AP3) is a B-class gene expressed specifically in the petal and stamen whorls and is not associated with sepals.​

Clarification of Statements

1. APETALA2 (AP2) transcripts expressed during sepal and petal development — This is correct, aligning with its role as a Class A gene.​

2. AGAMOUS (AG) is considered as Class A gene — Incorrect. AG is a Class C gene responsible for reproductive organs.​

3. APETALA1 (AP1) expressed during carpel development — Incorrect. AP1 is a Class A gene expressed in sepals and petals, not carpels.​

4. APETALA3 (AP3) expressed during sepal development — Incorrect. AP3 is a Class B gene expressed in petals and stamens.​

Summary Table: Expression Domains of Floral Homeotic Genes

Conclusion

APETALA2 transcripts are expressed during sepal and petal development, consistent with its Class A role in the ABC model.