29. In which of the following stages of Arabidopsis embryogenesis do the visible distinctions between the adaxial and abaxial tissues of the cotyledons become initially apparent?
    (1) Globular stage (2) Zygotic stage
    (3) Torpedo stage (4) Mature stage

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Introduction

Embryogenesis in Arabidopsis thaliana progresses through defined morphological stages including zygotic, globular, heart, torpedo, and mature stages. A crucial developmental event is the establishment of leaf polarity, which begins with the visible differentiation of adaxial (upper) and abaxial (lower) sides of the cotyledons. This polarity first becomes visually apparent during the torpedo stage, key for proper organ formation and function.

Key Features of the Torpedo Stage

• The cotyledons elongate and differentiate distinctly, exhibiting visible differences in cell morphology between their adaxial and abaxial sides.

• Adaxial tissues face toward the shoot apical meristem and typically develop palisade-like cells; abaxial tissues face away and have more spongy cell morphology and stomatal development.

• This differentiation is guided by spatially regulated gene expression such as HD-ZIP III genes on the adaxial surface and KANADI and YABBY genes on the abaxial.

Comparison with Other Stages

• In the zygotic stage, the embryo is a single cell without tissue differentiation.

• In the globular stage, early polarity and organ primordia form but distinct adaxial-abaxial differences are not yet visible.

• The mature stage represents full differentiation but polarity is established earlier during the torpedo stage.

Biological Significance

• Establishment of adaxial-abaxial polarity is essential for leaf blade expansion, vascular patterning, and photosynthetic efficiency later in plant development.

• Failure to establish this polarity can lead to malformed leaves and impaired plant function.

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Summary Table: Arabidopsis Embryogenesis and Cotyledon Polarity

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Conclusion

The torpedo stage of Arabidopsis embryogenesis marks the first visible signs of differentiation between the adaxial and abaxial tissues of cotyledons, a fundamental step in establishing leaf polarity and proper development.