18. Which plant growth regulator is involved in activating alpha-amylase activity in germinating seeds?

(1) Auxin                            (2) Cytokinin

(3) Gibberlic acid           (4) Brassinosteroids

Correct answer: (3) Gibberlic acid

Gibberellins produced by the embryo during seed germination diffuse to the aleurone layer and induce de novo synthesis and secretion of α‑amylase, which then hydrolyses starch to sugars that feed the growing seedling.
Therefore, the plant growth regulator directly involved in activating α‑amylase in germinating seeds is gibberellic acid (GA).

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Option-wise explanation

(1) Auxin

• Auxin mainly controls cell elongation, apical dominance, root formation, and tropic responses, not the classic induction of α‑amylase in cereal aleurone cells.

• It may modulate some aspects of germination indirectly, but it is not the hormone known for activating α‑amylase.

(2) Cytokinin

• Cytokinins promote cell division and delay senescence and can influence sink strength and nutrient mobilization, but they are not the principal trigger of α‑amylase synthesis in germinating cereal grains.

(3) Gibberlic acid – correct

• In barley, wheat and rice, GA synthesized in the embryo moves to the aleurone where it up‑regulates transcription of α‑amylase genes (and other hydrolases) via GA–GID1–DELLA signaling and GAMYB‑type transcription factors.

• The resulting α‑amylase converts stored starch in the endosperm to maltose and glucose, providing energy for radicle and plumule growth.

(4) Brassinosteroids

• Brassinosteroids regulate cell expansion, vascular differentiation, and stress responses, and crosstalk with GA, but are not the textbook hormone for α‑amylase activation in germinating seeds.