Q.54 The semi-dwarf trait of corn, wheat and rice plants used in breeding program during 1960s resulted
in green revolution. Later this ‘green-revolution gene’ has been identified to be involved in either
signal transduction pathway or biosynthesis of
(A) Auxin (B) Gibberellin (C) Cytokinin (D) Ethylene
Correct Answer: (B) Gibberellin
The semi-dwarf trait driving the Green Revolution in corn, wheat, and rice resulted from mutations disrupting gibberellin biosynthesis or signal transduction. Genes like Rht-B1/Rht-D1 (wheat), sd1 (rice), and dwarf8 (maize) reduce bioactive GA levels or receptor sensitivity, shortening stems while maintaining fertility for higher yields.
Option Analysis
(A) Auxin: Incorrect. Auxin regulates apical dominance and tropisms; no Green Revolution genes target IAA pathways. Dwarfing via auxin would stunt roots/overall growth.
(B) Gibberellin: Correct. Wheat Rht1 encodes DELLA proteins (GA signaling repressors); rice sd1 blocks GA20-oxidase biosynthesis; maize d8 affects GA response. All limit internode elongation.
(C) Cytokinin: Incorrect. Cytokinins promote cell division/shoot growth; blocking them causes bushy dwarfs unsuitable for mechanized harvest.
(D) Ethylene: Incorrect. Ethylene triggers senescence/fruit ripening; no documented Green Revolution genes target ethylene pathways for semi-dwarfism.
The semi-dwarf trait of corn, wheat and rice plants used in 1960s breeding programs resulted in green revolution through gibberellin pathway disruption, doubling global cereal yields via Norman Borlaug’s wheat varieties and IR8 rice.
Key Green Revolution Genes
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Wheat Rht-B1b/Rht-D1b: DELLA proteins insensitive to GA degradation; stems shorten 20-30 cm.
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Rice sd1: GA20-oxidase mutation blocks GA4/GA1 production (isolated 2002).
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Maize dwarf8: Transcription factor reducing GA response.
Mechanism of Action
Gibberellin promotes internode elongation via DELLA degradation:
GA → GID1 receptor → DELLA ubiquitination → stem growth
Mutant: DELLA accumulates → growth repression
Impact & Legacy
Yields increased 200-300%; fertilizer response improved 3x. Modern breeding stacks Rht alleles with disease resistance for climate resilience.
