28. In which stage of Arabidopsis embryogenesis is hypophysis first observed?
    (1) Octant (2) Dermatogen
    (3) Globular (4) Transition

Introduction

Arabidopsis embryogenesis progresses through well-defined stages, including octant, dermatogen, globular, heart, and torpedo stages. The hypophysis, an important cell contributing to root development, is first identified at the globular stage, marking a critical point for root apical meristem initiation and early embryonic patterning.

Hypophysis Formation at the Globular Stage

• The hypophysis is the uppermost cell of the suspensor, which undergoes an asymmetric division during the globular stage.​

• This division produces a lens-shaped apical daughter that forms the quiescent center and a basal daughter that helps form the root cap, key components of the root apical meristem (RAM).​

• The formation and division of the hypophysis are regulated by auxin signaling critical for embryonic root initiation.​

• Before the globular stage, the suspensor primarily acts as a conduit for nutrients; the globular stage marks its functional specialization to contribute to RAM.​

Developmental Context

• The globular stage is characterized by rapid cell division and the establishment of the apical-basal axis, providing spatial cues for root and shoot meristem formation.​

• This stage links early embryo polarity with the tissue differentiation required for proper root and shoot formation.​

Contrasts with Other Stages

• The octant and dermatogen stages involve earlier cell divisions but do not yet show distinct hypophysis development.​

• The transition stage follows the globular stage, leading into heart and torpedo stages with further differentiation and organ development.​

Summary Table: Appearance of Hypophysis during Embryogenesis

Conclusion

The hypophysis first appears and divides asymmetrically during the globular stage of Arabidopsis embryogenesis, playing a fundamental role in establishing the root apical meristem.