27. During which one of the following stages of Arabidopsis embryogenesis, cell elongation throughout the embryonic axis and further development of the cotyledons occur?
    (1) Globular stage. (2) Torpedo stage.
    (3) Heart stage. (4) Mature stage.

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Introduction

Arabidopsis embryogenesis is a well-studied model illustrating critical developmental stages from a single-cell zygote to a mature embryo. This process passes through distinct stages — globular, heart, torpedo, and mature. The torpedo stage is notable for coordinated cell elongation and prominent cotyledon development, marking significant embryo morphogenesis.

Features of the Torpedo Stage

• Occurs after the heart stage when the embryo develops bilateral symmetry with two cotyledon primordia.

• Characterized by elongation of cells along the embryonic axis establishing the longitudinal body plan.

• The cotyledons elongate and grow rapidly, forming leaf-like structures that will emerge post-germination.

• Vascular tissues begin to differentiate more extensively, supporting the developing embryo.

Comparison with Other Stages

• Globular stage (1): Early stage focused on cell division and basic polarity establishment, cotyledons not yet formed.

• Heart stage (3): Initiation of cotyledon primordia; embryo takes heart shape but elongation is minimal.

• Mature stage (4): Final differentiated form of embryo, completion of cotyledon growth and maturation.

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Summary Table: Key Arabidopsis Embryonic Stages

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Conclusion

The torpedo stage in Arabidopsis embryogenesis is distinguished by extensive cell elongation along the embryonic axis and the active development of cotyledons, making option (2) the correct answer.