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NCERT gives cytokinins one short story: Skoog's group saw tobacco callus grow only when the medium had auxin plus something like coconut milk, yeast extract or DNA, and Miller and colleagues then crystallised kinetin in 1955. The real story is stranger and more useful. It involves a rejected paper, a bottle that worked when new ones did not, and a pressure cooker. This post tells it with the checks you can do yourself.
Who was Folke Skoog?
Folke Skoog (1908 to 2001, per a 2001 obituary in Plant Molecular Biology Reporter) was a professor in the Botany Department at the University of Wisconsin. Richard Amasino's account of the discovery, written for its 50th anniversary, adds that Skoog had run the 1,500-metre race for Sweden at the 1932 Olympics, and that he was not someone who let a rebuff pass.
In the early 1950s his group was studying how new shoots form on pieces of tobacco stem in culture. Results varied badly, so Skoog wanted a steady supply of dividing tissue. Coconut milk sometimes helped and sometimes inhibited growth. When Skoog asked F. C. Steward at Cornell for his coconut milk purification method, Steward declined. Within minutes, Amasino writes, Skoog called the biochemist Frank Strong to start a rival project.
How did an old bottle of DNA lead to kinetin?
In 1951 Carlos Miller joined Skoog's group as a postdoctoral researcher. Adding yeast extract to a root-forming medium with auxin gave him not roots but a mass of dividing cells. In June 1953 he found the active material could be precipitated with silver nitrate, a reagent that also precipitates purines. Pure adenine and other bases did little, but he kept his hunch that a purine derivative was involved.
Then he tested bottles of nucleic acid on the lab shelf. RNA did nothing. Herring sperm DNA, at as little as 50 mg/L, gave very good growth. Skoog ordered a large batch from the same company. It was completely inactive.
Miller collected DNA samples from labs across the campus. Of seven tested, three were active, and they were the ones stored at room temperature for a long time. So something was changing in old DNA. He autoclaved fresh DNA in water for thirty minutes, and it became strongly active. On the evening of 16 December 1954, crystals formed on his purification column.
Can you check the formula yourself?
Strong's group found an equivalent weight of 215.2 and an empirical formula of C10H9N5O. You can check that these agree:
Carbon: 10 × 12.011 = 120.11
Hydrogen: 9 × 1.008 = 9.072
Nitrogen: 5 × 14.007 = 70.035
Oxygen: 1 × 15.999 = 15.999
Total: 120.11 + 9.072 + 70.035 + 15.999 = 215.216, which rounds to 215.2.
Now compare with adenine, C5H5N5. If one hydrogen of adenine is replaced by a side group, that group must be C10H9N5O minus C5H4N5, which is C5H5O. That is a furfuryl group. Miller proposed 6-furfurylaminopurine, Strong's lab synthesised it with a 60 percent yield on the first attempt, and it matched. The name kinetin was proposed in a paper submitted in January 1955 and published that March in the Journal of the American Chemical Society.
Is kinetin a natural plant hormone?
No, and this is a classic exam trap. NCERT Class 11, Chapter 13 states that kinetin, a modified adenine obtained from autoclaved herring sperm DNA, does not occur naturally in plants. Amasino notes that Miller and Skoog themselves questioned in 1956 whether kinetin exists in nature, because heating adenine with deoxyribose can make it. The search for natural substances led to zeatin from maize, reported by Letham and Miller in 1965. The class name cytokinin was proposed in 1965 by Skoog and colleagues, to avoid confusion with animal kinins.
Why does the auxin to cytokinin ratio matter?
In 1957 Skoog and Miller showed that the balance of the two hormones decides what callus becomes. As a review in Molecular Plant summarises, a high auxin to cytokinin ratio induces roots, while a low ratio promotes shoots. This is the principle behind regenerating whole plants in the lab, covered in our post on plant tissue culture.
Common mistakes to avoid:
Writing that kinetin was extracted from coconut milk. It came from heated herring sperm DNA.
Reversing the ratio. High auxin gives roots; high cytokinin gives shoots.
Calling kinetin and zeatin the same. Zeatin is natural; kinetin is not.
Growth regulators are listed in the NEET-UG 2026 syllabus; check the current year's version.
What can a student take from Skoog's lab?
Amasino records that Miller and Skoog's first manuscript on the factor was rejected. They did not argue with the reviewers; they kept working until the compound was in their hands. The discovery also needed two departments: botanists with a bioassay and chemists who could name what the assay found. When you hit a result that does not fit, write down what changed between the run that worked and the one that did not. If the hormone interactions still feel tangled, the Fermi Concept Library covers every concept in biology, each with its own lesson and its place in the map.
Before your next revision session, recalculate the molar mass of kinetin without looking, then explain in two lines why autoclaving DNA made it active.
