20. Gibberellic acid stimulates seed germination in monocots by activation of digestive enzymes by acting on

(1) Endosperm            (2) Aleurone layer

(3) Embryo                    (4) Cotyledons

Correct answer: (2) Aleurone layer

Gibberellic acid (GA₃) from the embryo diffuses into the aleurone layer of monocot endospermic seeds (e.g., barley, wheat, rice). There it induces transcription and secretion of digestive enzymes, especially α‑amylase and various proteases, which act on the starchy endosperm to mobilize reserves for the growing embryo.

Why option (2) is correct – role of the aleurone layer

• In monocots, the aleurone is a living, protein‑rich tissue that surrounds the dead, starchy endosperm.

• During germination, GA produced by the embryo is perceived by aleurone cells, triggering expression and secretion of α‑amylase and other hydrolases.

• These enzymes diffuse into the endosperm, hydrolyse starch and storage proteins, and release soluble sugars and amino acids that are transported to the embryo.

• Therefore, GA “acts on” the aleurone, not directly on the endosperm, to activate digestive enzymes.

Why the other options are wrong

(1) Endosperm

• The endosperm is mainly a storage tissue filled with starch granules and storage proteins; its cells are typically non‑living at full maturity.

• It is the target on which the enzymes act, not the tissue where GA primarily triggers enzyme synthesis. So GA does not directly activate enzyme production in the endosperm.

(3) Embryo

• The embryo synthesizes and secretes GA into surrounding tissues and certainly responds to GA for its own growth.

• However, the classic GA‑mediated induction of α‑amylase is localized to the aleurone, not the embryo itself. The embryo is the hormone source, not the main site of digestive enzyme production.

(4) Cotyledons

• Cotyledons are the seed leaves. In most dicots, cotyledons store and mobilize reserves directly, but the question is specifically about monocots, where the scutellum (cotyledon) and aleurone/endosperm system differ from dicots.

• In cereal monocots, it is the aleurone that is textbook‑linked to GA‑induced α‑amylase secretion, not the cotyledon.