ISSN 10214437, Russian Journal of Plant Physiology, 2011, Vol. 58, No. 4, pp. 743–747. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © G.A. Romanov, 2011, published in Fiziologiya Rastenii, 2011, Vol. 58, No. 4, pp. 631–635.
In 2011, plant biologists commemorate an impor
tant event: ten years ago new research methodologies
enabled plant biologists to raise the curtain of mystery
concealing such elusive phytohormones as cytokinins.
It was in 2001 that the nature of the cytokinin recep
tors was clarified and the pathway for intracellular sig
nal transduction to primary response genes was eluci
dated. Moreover, in the same 2001, cytokinin synthe
sis in plants was ultimately substantiated, based on the
discovery of the enzymes (and corresponding genes)
providing for biosynthesis of these phytohormones.
Thus, in history of plant biology, 2001 will remain the
triumphal year of cytokinins.
THE DISCOVERY OF CYTOKININ
Early in 2001, two groups of Japanese researchers
almost simultaneously reported the discovery of cyto
kinin receptors in Arabidopsis. The paper from the
Kakimoto laboratory (Osaka University) appeared in
the February issue of
, whereas the Mizuno
group (Nagoya University) presented their data as a
rapid paper in
Plant Cell Physiology
, the journal pub
lished in Japan . By the date of submission, the
Kakimoto group was the first one: their paper was
received on October 16, 2000, while the Mizuno team
sent their manuscript on December 11.
Both research groups presented sound evidence
that one of the sensor histidine kinases in Arabidopsis,
a 121kD multidomain transmembrane protein was a
specific cytokinin receptor. Kakimoto and his collab
orators dubbed this protein CRE1 (for Cytokinin
Response 1), whereas Mizuno and his coauthors,
AHK4 (for Arabidopsis Histidine Kinase 4). There
fore the receptor is usually referred to by its double
name CRE1/AHK4, although in their subsequent
publications, each group kept using the initial name.
In addition to
encoding the receptor in
question, both groups of researchers discovered in
Arabidopsis two more closely related genes (
); these were later shown to code also for cytoki
Earlier the sensor histidine kinases in microorgan
isms, such as bacteria and yeast, were shown to trans
duce signals in the mode of socalled twocomponent
system comprising both the sensor kinase and a tran
scription factor called a response regulator. In micro
organisms, signal transduction from a protein sensor
to a response regulator is implemented by transferring
highenergy phosphate from receptor phosphohisti
dine to aspartate residue in the response regulator .
Such phosphorylation usually results in the activation
of the response regulator and affects transcription of
the regulatorsensitive genes. No wonder that both
reports on discovery of cytokinin receptor also sug
gested the presumable pathway for transducing the
cytokinin signal to response genes (promoters) via the
consecutive phosphorylation of proteins in the two
component system; such hypothesis was soon corrob
orated in the most brilliant way [4, 5].
The event was enthusiastically appraised by plant
biologists: the newly discovered receptors were
described and mentioned in hundreds of reviews and
experimental papers. It is notable that the rapid
The Discovery of Cytokinin Receptors and Biosynthesis
of Cytokinins: A True Story
G. A. Romanov
Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (499) 9778018; email: firstname.lastname@example.org
Received November 29, 2010
—The year 2001 saw an amazing progress in cytokinin studies. Ten years ago, cytokinin receptor
genes and genes encoding cytokinin biosynthetic enzymes together with the corresponding proteins were
identified in plants. These studies elucidated the molecular mechanism of cytokinin effects on the expression
of cytokinin responsive genes and ultimately established the endogenous synthesis of cytokinins in plant cells,
justifying their membership among plant hormones. The paper describes in short the edifying and sometimes
paradoxical story of these fundamental and captivating discoveries.
: plant, cytokinin, receptor, histidine kinase, signal transduction, primary response gene, control of
gene expression, cytokinin biosynthesis.