54. Which of the floral whorls is affected in apetala 3/pistillata (ap3/pi) mutants?
    (1) Sepals and petals. (2) Petals and stamens.
    (3) Stamens and carpels. (4) Sepals and stamens.

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Introduction

Floral organ identity in Arabidopsis is governed by a combination of homeotic gene classes A, B, and C. The B-class genes, particularly APETALA3 (AP3) and PISTILLATA (PI), specify the identities of petals and stamens during flower development. Mutations in these genes lead to disruptions in flower organ formation, specifically impacting the second and third floral whorls.

Role of APETALA3 and PISTILLATA in Floral Development

• APETALA3 and PISTILLATA encode MADS-box transcription factors essential for petal (2nd whorl) and stamen (3rd whorl) development.

• These genes function as obligate heterodimers to regulate the expression of downstream targets required for organ differentiation.

• Mutations in either gene disrupt normal petal and stamen formation, causing homeotic conversions where petals develop as sepals, and stamens develop as carpels.

Mutant Phenotype Description

• Flowers of ap3 or pi mutants show sepals in place of petals (2nd whorl) and carpels in place of stamens (3rd whorl).

• The outer and inner whorls (sepals and carpels) remain unaffected, regulated by A and C class genes respectively.

Summary Table: Floral Organ Identity in AP3/PI Mutants

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Conclusion

Mutations in the B-class genes APETALA3 and PISTILLATA specifically disrupt the identity of petals and stamens, leading to flowers with sepals and carpels in the second and third whorls, consistent with option (2).