Editor’s Choice
1955: Kinetin Arrives. The 50th Anniversary of a New
Plant Hormone
Richard Amasino*
Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706
Fifty years ago, scientists from the Botany and Biochemistry
departments at the University of Wisconsin
announced the isolation, crystallization, characterization,
and synthesis of 6-furfurylaminopurine (Miller
et al., 1955a, 1955b, 1956), a plant hormone in a class
now referred to as cytokinins (Skoog et al., 1965). They
proposed the trivial name kinetin for this substance,
and described its ability to promote cell division in test
tissues from tobacco (Nicotiana tabacum). The discovery
of kinetin marked the culmination of several years of
research on a variety of projects conducted by many
people in the two departments.
A description of the little victories, the interesting
twists, and the roles of persistence, patience, and good
luck in this discovery has never been presented in
print. Hopefully, this account will serve as an illustration
of how scientific advancement can arise from
fruitful collaboration and the utilization of many odd
bits of information often obtained rather fortuitously.
In the early 1950s, Professor Folke Skoog and his
group in the Botany Department were continuing their
work on the chemical control of new shoot formation in
excised pieces of tobacco stems grown in culture (Skoog
fondly referred to these de novo formed shoots as
‘‘buddies’’). This research effort was hindered by the
fact that the stem pieces, which were obtained from
plants grown in the greenhouse, were difficult to obtain
in a sterile and completely healthy condition. The
results obtained with the stem segments were quite
variable, probably because of the harsh sterilization
protocol (mercuric chloride was employed for sterilization),
varying light intensities and temperatures at
different times of year, as well as greenhouse management
procedures. To avoid the difficulties and variability
of using greenhouse-grown stem pieces, Skoog
sought an alternative tissue to use in these studies. By
the 1950s, there had been advances in the art of plant
cell culture; in particular, several laboratories had
reported that certain complex materials added to an
otherwise chemically defined culture medium (consisting
of minerals, a carbon source such as Suc, and
often the plant hormone auxin) could result in the
sustained growth of plant tissues or cells. These materials
included coconut (Cocos nucifera) milk, which had
first been used by van Overbeek to stimulate the growth
of young Datura stramonium embryos in culture (van
Overbeek et al., 1941). Skoog explored whether his
group could continuously culture tobacco tissue on
coconut milk-containing medium, and thereby produce
‘‘standardized’’ samples of tissue for their studies
of shoot formation. Skoog’s group found that although
coconut milk sometimes promoted the proliferation of
tobacco tissue, the effect was quite variable; in fact,
some batches of coconut milk inhibited growth. They
were using coconuts purchased at a local grocery store,
and recognized that some of this variability might be
due to inhibitors present in certain batches of coconuts.
Skoog therefore sought advice from Professor F.C.
Steward at Cornell University. Steward’s laboratory
was attempting to purify and identify materials in
coconut milk that promote the growth of carrot (Daucus
carota) cells in culture. Skoog hoped to obtain some
details of Steward’s coconut milk purification procedure,
not with the intent to compete with Steward on
the identification of the active factor(s) but rather to
partially purify the coconut milk factors to an extent
that would enable his group to prepare consistently
healthy cell cultures for their studies of new shoot
formation.
Steward replied to this request by sending a letter to
Skoog stating that it would be best to leave such matters
to the Cornell investigators, and Steward also included
a few newspaper clippings that praised his own work
but provided no useful details of the purification methods.
Skoog, who competed in the 1,500-meter race for
Sweden in the 1932 Olympics, was not the type of
person who would let such a rebuff go unchallenged.
Within minutes of receiving Steward’s reply, he contacted
Professor Frank Strong, a natural products
expert in the Department of Biochemistry at the University
of Wisconsin, to initiate a collaboration to
compete with Steward’s group on the identification of
the coconut milk factor(s).
The early stages of the project moved rather quickly.
Graduate students Jack Mauney (Botany), Rod Clayton
(Biochemistry), and Bill Hillman (Botany) did the
tough laboratory work of developing methods to purify
the active factor(s) and developing a bioassay to test
each step of the purification. At first, their assay employed
serially cultured tobacco tissue, but later they
* E-mail amasino@biochem.wisc.edu; fax 608–262–3453.
www.plantphysiol.org/cgi/doi/10.1104/pp.104.900160.
Plant Physiology, July 2005, Vol. 138, pp. 1177–1184, www.plantphysiol.org Ó 2005 American Society of Plant Biologists 1177
used callus tissue derived from carrot root pieces. As
a starting material for purification of the active factor(s),
they shifted to an extract of ground coconut meat
rather than the milk because their assays indicated the
meat was a richer source of activity. In late 1952, their
work was published in Physiologia Plantarum (Mauney
et al., 1952). They described the preparation of afraction
that was over 4,000 times as active on a fresh weight
basis as the crude coconut meat extract. The active substance(s)
in this fraction was heat stable, acid and alkali
labile, nonvolatile, water soluble, and organic. It was
present in a Neuberg mercuric precipitate and was
bound quite strongly by activated charcoal. The factor
moved toward the cathode during electrodialysis and
appeared to be amphoteric. The paper also reported
that ion-exchangers such as Dowex 50 ‘‘held the activity
so tenaciously that successful elution of the factor
was never accomplished’’ (p. 495). Progress slowed as
graduate students completed their theses or shifted
emphasis, and this paper was the only publication from
Wisconsin on the activity from coconut.
In 1951, Carlos Miller, who had recently received
a Ph.D. from Ohio State University, began postdoctoral
work in Skoog’s group. Two of the projects that Miller
initiated were to converge, prompting a new line of
investigation in the search for growth promoting activity.
One project continued the investigation of ways
to manipulate organ formation from tobacco stem
pieces in culture. As discussed above, results with
greenhouse-grown stem segments were quite variable.
Miller thought that enriching the culture medium
might mitigate this variability, for example, stem segments
from less vigorous plants might respond better
if a range of metabolites (that might otherwise be
limiting) were supplied to the culture medium. To test
this, he added yeast (Saccharomyces cerevisiae) extract to
a medium that was known to promote root formation
because it contained a certain level of the auxin indoleacetic
acid. To his surprise, the yeast extract did
not enhance root formation but instead caused a massive
amount of undifferentiated growth and cell division!
In a separate project, Miller was searching for
a source of growth-promoting material that was more
reliable than coconut extract in the carrot tissue assay.
He found that yeast extract was quite active in promoting
growth of the carrot tissue when used directly
in the culture medium. In the Physiologia Plantarum
paper (Mauney et al., 1952), which primarily reported
on the coconut project, it was noted that a sample of
yeast extract was inactive when used directly in the
carrot tissue assay (p. 490), but it was also noted that
some activity could be obtained from yeast extract
after partial purification (p. 496). However, Miller had
fortuitously chosen a particular bottle containing
a sample of yeast extract that had relatively high
activity when used directly in the culture medium.
Miller set out to purify the active material(s) from
yeast extract. There was a dwindling supply of yeast
extract in the particular bottle that had relatively
high activity, so several large containers of different
Anheuser-Busch brewer yeasts were ordered in the
hope of finding a plentiful source of factor(s). Unfortunately,
none of these batches exhibited activity. Yet
Miller made progress working with the limited supply
from the small bottle of yeast extract that had activity.
The chemical techniques available at that time for
compound identification from a crude mixture were
limited; one common technique was precipitation with
metal ions. As previously found in the coconut work,
considerable activity was present in a mercuric precipitate,
and some activity was adsorbed onto activated
charcoal. Every step of the purification had to be
evaluated using the bioassay of cell proliferation in
culture. The carrot culture assay was giving variable
results and so, in May of 1953, Miller started using
callus tissue from tobacco stem pieces. When tobacco
stem pieces are placed on a simple medium without
yeast extract, there is typically an initial burst of
growth and cell division from the basal end of the
stem piece (this burst of growth formed what was sometimes
called ‘‘wound callus’’). However, in Miller’s
assays there was no further proliferation (even if the
tissue was transferred to fresh medium) unless the
activity from yeast extract was present in the medium.
In June of 1953, Miller found that the active material
present in yeast extract was precipitated by silver
nitrate under slightly acid conditions (Fig. 1). This
finding was extremely important to what ensued. The
silver nitrate method was more selective than the
mercuric precipitation procedure. Miller recognized
that purine and pyrimidines are one class of compounds
precipitated by silver nitrate, so he tested
individual purine or pyrimidines, such as adenine,
cytosine, guanine, thymine, uracil, etc., in the tobacco
assay. Although none of the tested compounds had
any substantial growth promotion activity, the notion
that a purine derivative might be involved remained
very appealing to him. Skoog’s group had previously
shown (beginning with the work of Cheng Tsui) that
addition of the purine adenine to the culture medium
of tobacco stem segments would enhance shoot formation
(Skoog and Tsui, 1948, 1951; Miller and Skoog,
1953) and, in the presence of auxin, would cause some
cell division (Skoog and Tsui, 1951). Some weak cell
division activity from adenine had also been noted in
the Physiologia Plantarum paper on coconut milk
(Mauney et al., 1952).
Miller continued to pursue his hunch that a purine
derivative might be involved. He sampled bottles of
nucleic acids from the laboratory shelf for activity in
the bioassay because he thought they might contain
additional purine derivatives. RNA did not promote
growth, but, almost unbelievably, very good growth
occurred when as little as 50 mg/L of herring sperm
DNA was added to the culture medium. Moreover, as
had been found with yeast extract, the active material
could be precipitated from a solution of herring sperm
DNA with silver nitrate. That a concentrated source of
activity was commercially available was cause for
great celebration! To ensure a large supply of active
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1178 Plant Physiol. Vol. 138, 2005
material for further studies, Skoog ordered a keg of
herring sperm DNA that had been prepared by the
same company using the same procedures by which
the small active sample that Miller had first tested had
been prepared. However, the new batch of herring
sperm DNA did not possess any trace of growthpromoting
activity. The excitement abated. Miller had
serendipitously chosen samples of both yeast extract
and herring sperm DNA with relatively high activity,
but no activity was present in any newly purchased
batches.
For the next several months, work continued with
the dwindling supply of active samples of yeast extract
and herring sperm DNA. Miller achieved a
partial purification of the activity by preparing ethanolic
extracts of both sources, followed by silver
precipitation, partition into diethyl ether or n-butanol,
and paper chromatography. This yielded preparations
that were active in the tobacco stem callus test at less
than 1 mg/L. Clearly, the active materials were quite
potent. The absorption spectra of the ether-soluble
factors from both yeast extract and herring sperm
DNA were very similar in several solvents and exhibited
a similar shift in alkaline solution. In addition,
both activities migrated almost identically on paper
chromatograms. Thus, each of the two sources seemed
to have yielded the same or closely related active
substances. Moreover, if the ether-soluble fraction was
subjected to chromatography on filter paper using
only water as the developing solvent, the activity
could be visualized as a dark quenching spot under
shortwave UV light at about Rf 0.5. Miller and Skoog
submitted a manuscript describing these findings to
the Proceedings of the Society for Experimental Biology and
Medicine. It was rejected. Part of the reviewers’ concerns
had to do with the rather poor quantitative
nature of the assay for growth-promoting activity, but
Miller and Skoog considered many other points raised
by the reviewers to lack validity. They knew the project
was gaining momentum, and they decided not to
address the reviewers’ comments or to make any
additional effort to publish their progress. The rejection
increased their resolve to identify the active
compound(s) and caused Skoog to terminate his
membership in the Society for Experimental Biology
and Medicine (this latter point is from Skoog, 1994).
By this time, Miller was becoming increasingly
confident that the active substance was an adenine
derivative. Some of this confidence came from a chemical
test that he had performed earlier for the presence
of indoleacetic acid and other indole compounds in the
active fraction. At that time, many growth phenomena
were regarded as being influenced or even caused by
auxin. Therefore, Miller had tested the active material
for the presence of indoles by performing paper
chromatography of the material with water as the
solvent and then spraying with Salkowski reagent (a
mixture of sulfuric acid and ferric chloride). No color
appeared after several minutes, i.e. the test for indoles
was negative. However, to evaluate whether the sample
might contain bound or conjugated indoles that
could not react with the Salkowski reagent, Miller held
the chromatogram over a hot plate. As the paper began
to char, a pink spot appeared at the position of the
active material. Miller then found that a spray of
sulfuric acid alone was even more effective at producing
the color, indicating that the color was not due
to the reaction of a bound indole compound. His
search of the literature revealed that a mixture of
sulfuric acid with Cys (known as the Dische reagent)
would form a pink product when reacted with deoxyribonucleosides.
He found that the Dische reagent
was quite effective at reacting with the active material
to form a pink compound. As discussed below, this
would turn out to provide an important clue for the
determination of the structure of kinetin. In addition,
when some of the purified active factor was treated
with the Dische reagent (or with sulfuric acid alone)
and then subjected to chromatography, a new quenching
compound (a compound with a different Rf value)
was detected. The new compound appeared to be
adenine because of its position on the chromatogram
as well as other chemical properties, i.e. acid treatment
appeared to liberate adenine from the active material;
thus, there was growing evidence that the factor might
be a substituted adenine!
Many further attempts were made to obtain active
substances from the newer but inactive batch of
herring sperm DNA, such as hydrolysis of the nucleic
acid, but these attempts were not successful. However,
the new batch of DNA, which had been stored at room
temperature for many months, was repeatedly assayed
and very gradually began to show activity. Some
change was occurring in the DNA sample during
storage! In fact, this gradual acquisition of activity had
been noted in the rejected manuscript. Miller collected
samples of DNA from laboratories across the
Wisconsin campus with the idea to test the samples for
Figure 1. Growth of tobacco cells on control medium containing auxin
(left) and on the same medium with addition of the silver nitrate
precipitable material. This image is from the original work of Miller in
1953.
Editor’s Choice
Plant Physiol. Vol. 138, 2005 1179
activity and, if any additional samples tested positive,
to explore whether the active samples had anything in
common. Seven DNA samples were tested for activity
in the tobacco stem callus assay. Three of them had
activity. Samples that had been stored at room temperature
for relatively long times had activity, whereas
newly purchased samples or old bottles of DNA kept
desiccated and refrigerated were inactive.
With these observations in mind, Miller tried a simple
method to ‘‘age’’ a new sample of inactive DNA:
he scooped some DNA into water in a 1-L flask and
autoclaved it for thirty minutes. After cooling, he extracted
the darkened, autoclaved slurry of DNA with
n-butanol and examined the butanol fraction for a
compound with the same chromatographic properties
(UV quenching and Dische reagent reactivity at the
same Rf value) and spectroscopic characteristics of the
active substance from the original samples. A compound
with the right properties was present! Bioassays
confirmed that autoclaving the DNA produced
an enormous amount of a cell division-promoting activity.
A procedure for activating the DNA had been
found! Further tests, conducted in early November of
1954, indicated that an acidic condition during autoclaving
was necessary for the activation; in the initial
test, the DNA sample itself had provided sufficient
acidity for activity to be generated. Miller then prepared
a large batch of the active material. The autoclaved
slurry was extracted with n-butanol, and the
butanol was driven off by distillation, leaving an
aqueous preparation from which insolubles were removed
by filtration. The remaining solution was
adjusted to pH 6.8 and then passed through a large
column of the cation-exchanger Dowex 50 (H1
). The
exchanger bound the activity very strongly; recall
that activity from the coconut extract had been held
by, but not recovered from, Dowex 50. Therefore,
Miller tried more severe elution conditions than
those previously used in the coconut work. He found
that the active substance could be gradually eluted
from the Dowex resin by strong acid (1.5 N HCl) or
more rapidly eluted by 1 N ammonium hydroxide.
The elution by strong acid provided a significant
purification: several inactive compounds eluted in
the early fractions as the column was washed with
acid, whereas the activity came off in much later
fractions. Miller combined the later-eluting fractions,
and to remove the 1.5 N acid without risking destruction
of the active factor, he simply diluted the
combined fractions to about 0.5 N HCl and ran them
through Dowex 50 again (the activity bound to the
Dowex at this lower level of acid). He washed the
Dowex column thoroughly with water and then
added 1 N ammonium hydroxide to elute the activity.
On the evening of December 16, 1954, Miller saw a
swirling white band of crystals moving down the
column with the ammonium hydroxide front against
the dark brown background of the exchange resin!
He collected the crystals as they exited the column,
decanted the excess ammonium hydroxide solution,
and washed the crystals with water before drying
them.
To determine if the crystals might represent the
purified factor, Miller dissolved a small portion in
ethanol and performed paper chromatography as described
above. All of the properties of the crystalline
material were identical to those of the active substance
obtained from the old bottle of DNA: quenching
material moved to the expected position on a paper
chromatogram, the region of the chromatogram with
the active substance turned pink when sprayed with
the Dische reagent, and the material had the expected
absorption peak. The next morning, Miller showed the
crystals to Skoog, confidently declaring them to be the
cell-division factor. On December 21, 1954, after recrystallization
to provide a further purification, Miller
set up a series of tobacco stem callus bioassays using
several concentrations of the factor. He then went to
Ohio to spend the holiday break with his family.
During the break, Skoog sent a letter to Ohio excitedly
reporting that the factor was giving positive results.
The next phase of the research was an intense effort
to learn the structure of the compound. Frank Strong
and his group in the Biochemistry Department, which
included Malcom von Saltza, generated much critical
data in an incredibly short time. Strong’s group found
that the crystalline compound had an equivalent
weight of 215.2 and, by elemental analysis, fit an empirical
formula of C10H9N5O. They also determined the
infrared spectrum, melting point, and sublimation
temperature, and noted two pKa values (indicating
that the compound was amphoteric). Another key
finding was that the compound could not be acetylated.
In January of 1955, approximately 1 month after
the crystals first appeared in the Dowex column,
a paper was submitted to the Journal of the American
Chemical Society reporting their progress on defining
the chemical properties of the active compound and
suggesting that it be named kinetin. This paper appeared
in the March 5 issue (Miller et al., 1955a).
A very important question remained unanswered:
What was the structure of kinetin? Assuming that
kinetin was a substituted adenine and that the equivalent
and molecular weights were the same, the
substituting group (which presumably replaced a hydrogen)
must consist of C5H5O. That kinetin was
precipitated by silver nitrate indicated the substituting
group was not on the 9-position of adenine. Acetylated
derivatives are typically produced on free amino
groups, and the failure to obtain acetylated derivatives
raised the possibility that the substituting group was
on the amino group of adenine. But what was the
configuration of the substituting group?
One hint came from an absorption band at 8.01 m in
the infrared spectrum. Miller’s after-dinner perusals
of chemical journals revealed that compounds having
C–O–C ether bonds absorbed sharply at this wavelength.
Furthermore, his prior tinkering with the
Salkowski test for the presence of indoles had led to
the discovery of the reactivity of the compound with
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1180 Plant Physiol. Vol. 138, 2005
the Dische reagent. Reading the chemical literature,
Miller learned that the route of deoxyribonucleosides
to a pink product in the Dische reagent was likely to
involve the conversion of deoxy-ribose into a furancontaining
compound. He thought that a related reaction
might have contributed to production of the
active factor from DNA. Miller thus proposed that the
active material was 6-furfurylaminopurine (Fig. 2),
a compound with a furan derivative attached to the
amino group of adenine at the 6 position of the purine
ring.
Acting on Miller’s proposal, Frank Strong and
Francis Okumura rapidly synthesized 6-furfurylaminopurine
by refluxing 6-methylmercaptopurine in furfurylamine
overnight. As was typical for work from
Strong’s lab, the initial attempt was successful—the
protocol gave a 60% yield of kinetin. All of the
chemical and biological properties of the synthesized
compound were identical to those of the factor obtained
from DNA, confirming the proposed structure!
The luck of testing old bottles of DNA and yeast
extract, and the blending of the particular talents,
knowledge, and insights of the groups in the Biochemistry
and Botany Departments, had led to the
discovery of the first highly active member of the
cytokinin class of plant hormones. In March of 1955,
the paper describing the structure and properties of
kinetin was submitted to the Journal of the American
Chemical Society, and it appeared in the May 5 issue of
the journal (Miller et al., 1955b; a more complete
description with substantiating data appeared later:
Miller et al., 1956).
EPILOGUE
This discovery received much attention. There were
press conferences and photography sessions (Fig. 3).
The research had been supported in part by the
American Cancer Society, and, based on the ability of
kinetin to control cell proliferation, speculation that
the discovery would lead to strategies to combat
cancer appeared in the popular press. Cancer patients
sought advice from the discoverers. Representatives of
drug and chemical companies visited the Wisconsin
laboratories to obtain additional information. The
Wisconsin Alumni Research Foundation patented the
discovery.
Despite all the attention, laboratory work continued.
In Biochemistry, new purine derivatives related to
kinetin were synthesized, and their effects on plant
cells were tested in Botany. 6-Benzyladenine, now
a commonly used cytokinin, was the first of several
highly active cytokinins to be discovered by Skoog and
Strong and their co-workers quite soon after the
discovery of kinetin. The effect of kinetin on developmental
processes that were also affected by red light,
such as leaf expansion (Miller, 1956) and seed germination
(Miller, 1958), was discovered. Other groups
quickly discovered other developmental processes
that could be influenced by cytokinin, such as leaf
senescence (Richmond and Lang, 1957).
In the same year that the identification of kinetin was
published, Steward’s group published a paper reporting
that diphenylurea was a compound from coconut
milk active in promoting cell division (Shantz and
Steward, 1955). In their studies, Steward and his group
had collected a large batch of coconuts that had become
available after a storm in Florida (Jacobs, 1979). To
prepare this large batch, commercial equipment from
a DuPont plant was used (Jacobs, 1979; P. Davies,
personal communication). Steward’s group did not
know that the equipment had been previously used
for preparing phenylurea herbicides. It was unfortunate
that they diluted residual diphenylurea with coconut
milk only to repurify it as an active factor, but
fortunately this mishap led to the discovery that disubstituted
ureas also have cytokinin activity. Recently,
it has been shown that a di-substituted urea (thidiazuron)
can activate a putative cytokinin receptor similar
to adenine-type cytokinins (Inoue et al., 2001;
Yamada et al., 2001). Therefore, active di-substituted
ureas should be considered as true cytokinins. It should
be noted, however, that there is also evidence that
Figure 2. The structures of kinetin, adenine, and a furan ring.
Editor’s Choice
Plant Physiol. Vol. 138, 2005 1181
thidiazuron can act to prevent the metabolism of
adenine-type cytokinins by inhibiting cytokinin oxidase
(Laloue and Fox, 1989).
In 1956, the term kinin was proposed for substances
that promote cytokinesis (i.e. the partition of a cell into
new cells) and therefore ‘‘permit continuous growth of
various plant tissues in vitro’’ (Miller et al., 1956;
p. 1376). By this time, the ability of diphenylurea to
promote cell division had been published, and, thus,
kinetin and diphenylurea were the first kinins to be
reported. In their 1956 paper, Miller and Skoog and coworkers
question ‘‘whether kinetin exists as such and
functions as a kinin in nature’’ (p. 1377)—i.e. perhaps it
was not naturally occurring (Miller et al., 1956). Indeed,
kinetin can be formed by simply autoclaving
adenine and deoxy-ribose at a moderately low pH
(Hall and deRopp, 1955). The deoxy-ribose apparently
can rearrange to a furfuryl group and become joined to
the 6 amino group of adenine. However, as noted by
Skoog (1994), it cannot be ruled out that some kinetin
could be formed in plants under certain conditions,
such as those in wounded tissue.
Although kinetin was the first highly active member
of the cytokinin class of plant hormones to be discovered,
whether kinetin was the first example of a chemically
defined cytokinin is arguable. As noted in Miller
(1968), Skoog and Tsui had earlier reported, in their
studies of growth and bud formation, an ‘‘array of
cytokinin effects’’ (quote from Miller, 1968; p. 44) when
adenine was added to the culture medium of tobacco
stem segments (Skoog and Tsui, 1948, 1951). Adenine
promoted new shoot formation without auxin or in the
presence of low concentrations of auxin, whereas with
higher concentrations of auxin, adenine promoted
some cell proliferation. The weak cell divisionpromoting
effects of adenine had been mentioned
again (without presenting supporting data) in the
Physiologia Plantarum paper on coconut milk (Mauney
et al., 1952). In a later study, the ability of adenine to
promote the growth of both soybean (Glycine max) and
tobacco tissues through repeated subcultures was carefully
examined (Miller, 1968). Adenine was effective in
a very narrow range of concentrations, and the promotion
required relatively high concentrations of
adenine (greater than 0.1 mM), whereas kinetin was
effective at much lower concentrations (less than 0.0001
mM; Miller, 1968). That the effect was truly due to
adenine and not a contaminant was shown by testing
several different commercial samples, recrystallizing
a sample of adenine, and following the activity in paper
chromatography (Miller, 1968). Further support that
adenine was a weak cytokinin came from the study
of the cell division-promoting activity of a series of
6-alkyl-aminopurines (e.g. n-butyl, n-propyl, ethyl, and
methyl). As the length of the alkyl substituting group
decreased, so did the activity, and adenine fit nicely into
this series as the weakest cytokinin (Miller, 1968). As
noted in the paper, however, other explanations cannot
be ruled out, for example, that adding a relatively high
level of adenine to the culture medium may permit
some cytokinins to be formed.
As discussed above, several years before the advent
of kinetin, Skoog had recognized and first published
with Tsui (Skoog and Tsui, 1948, 1951) the existence of
a relationship between adenine and auxin in relation
to organ formation in culture. The quantitative nature
Figure 3. The authors of Miller et al.
(1955b). Carlos Miller is on the left.
On the right, front to back, are Frank
Strong, Folke Skoog, Francis Okumura,
and Malcom von Saltza.
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1182 Plant Physiol. Vol. 138, 2005
of this relationship, however, was not easily detected
because the biologically effective range of adenine
concentrations was rather narrow. After the discovery
of kinetin, Miller performed experiments that showed
a similar relationship between kinetin and auxin in
relation to organ formation; in these studies, auxin and
kinetin had interacting effects over a broad concentration
range of both hormones (Fig. 4). Skoog realized
and emphasized the importance of the concentration
ratio of auxin and kinetin in the classic 1957 paper on
this topic (Skoog and Miller, 1957).
After moving to Indiana University, Miller pursued
the identity of naturally occurring cytokinins. In 1961,
he reported the partial purification from maize (Zea
mays) kernels of a cytokinin that was a purine derivative
distinct from kinetin (Miller, 1961). From
Miller’s work and that of David Letham and his group
came zeatin—the first example of a naturally occurring
adenine-based cytokinin (Letham and Miller,
1965). The currently used term, cytokinin, for this
class of plant hormones was proposed in 1965 to avoid
confusion with the kinins referred to in studies of
animal physiology (Skoog et al., 1965).
ACKNOWLEDGMENTS
This account of the discovery of kinetin was constructed from published
papers, the laboratory notebooks of Carlos Miller, and on his recollections of
this exciting time. I think this is an interesting story, and one that nicely
illustrates the varied paths to scientific discovery. On many occasions over
many years, I have asked Carlos Miller about the details of the discovery of
kinetin. I thank him for patiently discussing this history with me.
Received April 25, 2005; revised April 28, 2005; accepted April 28, 2005;
published July 11, 2005.
LITERATURE CITED
Hall RH, deRopp RS (1955) Formation of 6-furfurylaminopurine from
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