A, B, D only is correct. Gibberellins are terpenoids from isoprene units (A true), GA12 is the key early precursor (B true), and GA applications enlarge seedless grapes (D true), but C and E are false.

Question Breakdown

This multi-statement question tests gibberellin (GA) biochemistry, biosynthesis, and applications in plant physiology. GAs are diterpenoid hormones (>130 types) promoting growth via DELLA protein degradation. Evaluate each statement against established facts.

Statement Analysis

A: Gibberellins are terpenoid compounds, made up of isoprene units

True. GAs are C20/C19 diterpenoids synthesized from 4 isoprene (C5) units via geranylgeranyl diphosphate (GGPP) in plastids. Tetracyclic ent-gibberellane skeleton confirms terpenoid nature.

B: GA12 is the precursor of all the other gibberellins

True. GA12 (kaurenoic acid derivative) is the first committed C20-GA precursor after ent-kaurene oxidation. All subsequent GAs (early 13-hydroxylation or non-13-hydroxy pathways) derive from it.

C: GA1 and GA3 are directly responsible for the various effects in plants

False. Bioactive GAs are GA1, GA3, GA4, and GA7, but GA4/GA7 often predominate (e.g., Arabidopsis). No single pair is “directly responsible”; activity depends on species/context. GA3 (gibberellic acid) is commercial but not universal.​

D: Size of seedless grapes can be increased by GA applications

True. GA3 sprays on Thompson Seedless grapes induce parthenocarpy and cell expansion in berries, doubling size commercially. Standard horticultural practice.​

E: Gibberellin biosynthesis can be inhibited by application of Auxins

False. Auxins (e.g., IAA) often synergize with GAs or have complex interactions, but do not inhibit GA biosynthesis. Paclobutrazol or uniconazole inhibit via ent-kaurene oxidase; auxins promote hypocotyl GA levels.​

Correct Option: A, B, D only

True about gibberellins includes their terpenoid nature from isoprene units, GA12 as universal precursor, and GA applications for larger seedless grapes. Understanding these separates bioactive roles from myths in plant hormone studies.

Gibberellin Biosynthesis Pathway

• Acetate → IPP (isoprene) → GGPP (C20) → ent-kaurene → GA12 → GA53/GA4 → Bioactive GAs (GA1/3/4/7).

• Early (GA12 → GA53) vs. non-early pathways converge at GA4.

Practical Applications

GA3 enlarges fruits (grapes, citrus), breaks seed dormancy, induces bolting – but GA1/GA3 aren’t sole effectors; GA4 key in many species.​