The correct option is Gibberellins.
Explanation
The discovery of major Plant Growth Regulators (PGRs) was largely accidental. The identification of Gibberellins is historically linked to a specific pathological condition observed in rice plants in Japan.
Detailed Analysis
- Gibberellins (Correct): The 'bakanae' (foolish seedling) disease in rice seedlings was caused by a fungal pathogen Gibberella fujikuroi. Infected plants grew excessively tall and were sterile. In 1926, E. Kurosawa demonstrated that the symptoms could be induced in healthy seedlings by treating them with sterile filtrates of the fungus. The active substance causing this elongation was later identified as Gibberellic Acid (GA).
- Auxins (Incorrect): The discovery of Auxins is associated with observations of phototropism (bending towards light) in the coleoptiles of canary grass, initially studied by Charles Darwin and his son Francis Darwin. F.W. Went later isolated Auxin from the tips of oat coleoptiles.
- Cytokinins (Incorrect): These were discovered during tissue culture experiments. F. Skoog and his co-workers observed that callus proliferation occurred only when the medium contained extracts like yeast extract, coconut milk, or DNA. The active substance, kinetin, was crystallized from autoclaved herring sperm DNA.
- Abscisic Acid (Incorrect): This regulator was discovered independently by three different researchers who isolated inhibitors named inhibitor-B, abscission II, and dormin. These were later found to be chemically identical and named Abscisic Acid.
Key Takeaway: The 'bakanae' disease, caused by the fungus Gibberella fujikuroi, led to the discovery of Gibberellins, a class of hormones primarily responsible for stem elongation.