Question

Which plant bio regulator blocks the synthesis of gibberellin?

• A. Abscisic acid
• B. Ethylene
• C. Benzothiadiazole
• D. Prohexadione-calcium

Hint:

Plant growth regulators can inhibit gibberellin (GA) synthesis to control plant height and growth.
Prohexadione-calcium is a specific inhibitor of late-stage GA biosynthesis.
Others include paclobutrazol, chlormequat (CCC).
ABA and Ethylene have complex interactions with GAs but are not primary "blockers" of GA synthesis in the same way as these PGRs.

Answer 1

The Correct Option is D

Gibberellins (GAs) are plant hormones that promote cell elongation, stem growth, and other developmental processes. Plant growth regulators (PGRs) that inhibit GA synthesis are used to reduce plant height, control lodging, and manage vegetative growth.
(1) Abscisic acid (ABA): A plant hormone primarily involved in stress responses and dormancy. It often acts antagonistically to GAs but doesn't primarily block GA synthesis as its main mode of action in this context; it affects GA signaling or response.
(2) Ethylene: A plant hormone involved in ripening, senescence, and stress responses. It can interact with GA pathways but is not primarily a direct GA synthesis blocker.
(3) Benzothiadiazole (BTH): A plant defense activator, known for inducing systemic acquired resistance (SAR). Not primarily a GA synthesis inhibitor.
(4) Prohexadione-calcium: This is a well-known plant growth regulator that specifically inhibits the late stages of gibberellin biosynthesis. It blocks the action of certain dioxygenase enzymes involved in converting inactive GAs to active GAs (e.g., GA\(_1\)). This leads to reduced levels of active GAs, resulting in controlled vegetative growth, shorter internodes, and often improved fruit set or quality in some crops. Other GA synthesis inhibitors include paclobutrazol, uniconazole, chlormequat chloride (CCC), and trinexapac-ethyl. Prohexadione-calcium is a prominent example. Prohexadione-calcium