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Every NEET aspirant learns one line about gibberellins: "E. Kurosawa (1926) reported the appearance of symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus." It is easy to memorise and easy to forget. This post explains what Kurosawa actually tested, why a germ-free liquid was such a clever idea, how his result grew into a whole class of plant hormones, and what you can borrow from his way of thinking.
What was the foolish seedling disease?
Rice farmers in Japan knew a disease in which seedlings grew excessively tall and the plants were infertile. According to Hedden and Sponsel's history of gibberellin research, a study in 1898 showed the disease was caused by a fungal infection, and the most familiar of its local names, "bakanae", translates as silly seedling. NCERT's Class 11 chapter on plant growth calls it the foolish seedling disease and names the fungus Gibberella fujikuroi, now reclassified as Fusarium fujikuroi.
In 1912 Kenkichi Sawada, at the Imperial Research Institute in Taipei, suggested that the fungus itself supplied the stimulus for the overgrowth. That was a guess. Someone had to test it.
What did Kurosawa test in 1926?
Kurosawa was Sawada's colleague. He grew the fungus in culture, removed the living fungus, and applied the sterile liquid to healthy rice seedlings. The seedlings developed the same overgrowth symptoms. Hedden and Sponsel add that the secreted "toxin" also stimulated the growth of seedlings of several species besides rice. His paper appeared in 1926 in the Transactions of the Natural History Society of Formosa, under the title "Experimental studies on the nature of the substance excreted by the 'bakanae' fungus".
Why was a sterile filtrate such a clever test?
Think of the two possible explanations. Either the living fungus does something to the plant while it grows inside it, or the fungus releases a substance that acts on its own. An infected seedling cannot tell these apart, because both the fungus and its products are present.
Filtering the culture separates them. The fungus stays behind and the liquid passes through. If seedlings treated with the liquid alone grow too tall, explanation two wins. If they stay normal, the living fungus is needed. One clean experiment removes a whole branch of possibilities.
Students often miss this logic and remember only the date. In an exam, an assertion-reason question may hinge on exactly this point: the symptoms appeared without infection, which is why the active substance could later be purified.
How did a fungal toxin become a plant hormone?
Hedden and Sponsel trace the next steps:
1935. Teijiro Yabuta, Professor of Agricultural Chemistry at the University of Tokyo, purified a highly active sample and called it gibberellin, after the fungus.
1938. Yabuta and Sumiki obtained two crystalline substances, gibberellin A and gibberellin B, though these were later found to be impure.
1950s. Nobutaka Takahashi, Saburo Tamura and colleagues in Tokyo showed gibberellin A was a mixture and named its parts GA1, GA2 and GA3. The numbering system you use today began here.
Britain and the USA. Work outside Japan began only in the 1950s, after the war. A group at ICI in Britain obtained a pure compound they called gibberellic acid, and a USDA team in Peoria, Illinois, crystallised "gibberellin-X". The two turned out to be identical.
The review notes that gibberellins restored growth in dwarf mutants and triggered bolting and flowering in some rosette plants, which suggested plants make these compounds themselves. Chemical work confirmed that in the late 1950s. So a substance first studied as a fungal poison turned out to be a normal plant hormone that the fungus happened to make in excess.
The story even reaches your plate. The same review describes the Reduced height (Rht) genes from the Japanese wheat Norin-10, which Norman Borlaug bred into the high-yielding wheat of the Green Revolution to stabilise the stem. These semi-dwarf wheats are part of the gibberellin story too.
What does your syllabus expect you to know?
The NCERT Class 11 chapter (Reprint 2026-27) says more than 100 gibberellins are known, denoted GA1, GA2, GA3 and so on, and that GA3 was one of the first discovered and remains the most studied. It lists uses you should know:
Lengthening grape stalks and making apples elongate and improve in shape
Delaying senescence, so fruit can stay on the tree longer
Speeding up malting in the brewing industry
Raising sugarcane yield by as much as 20 tonnes per acre through longer stems
Making juvenile conifers mature early for seed production
Promoting bolting in beet, cabbage and other rosette plants
Growth regulators, including gibberellin, appear in the NMC's NEET-UG 2026 syllabus. Check the current year's syllabus before you plan revision. For the full set of five regulators side by side, read our guide to plant growth regulators.
Which discoverer goes with which hormone?
Mixing up names is the most common mistake in this topic. Keep them in one list:
Auxin: Charles and Francis Darwin's coleoptile experiments, then F. W. Went, who isolated it. See our post on Frits Went and auxin.
Gibberellin: E. Kurosawa, foolish seedling disease of rice.
Cytokinin: F. Skoog and co-workers; kinetin identified by Miller and others in 1955.
Ethylene: H. H. Cousins in 1910 confirmed that ripened oranges release a volatile substance that hastened the ripening of stored bananas; it was later identified as ethylene.
What can a student take from Kurosawa?
Kurosawa did not discover a hormone in one leap. He asked a narrow question, "is it the organism or something it releases?", and designed a test that answered only that. Others built on his answer for about thirty years. When a biology idea feels like a list of names and dates, rebuild the logic of the key experiment yourself. The Fermi Concept Library covers every concept in biology, with its own lesson, which helps when you want the reasoning behind a fact rather than the fact alone.
Tonight, write the four hormone discoveries above from memory, and for each one say in a single sentence what the key experiment ruled out.
