ISSN 1021-4437, Russian Journal of Plant Physiology, 2009, Vol. 56, No. 2, pp. 268–290. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © G.A. Romanov, 2009, published in Fiziologiya Rastenii, 2009, Vol. 56, No. 2, pp. 295–319.
HISTORY OF CYTOKININ DISCOVERY
Cytokinins (initially termed kinins
) were discov-
ered in the laboratory of F. Skoog (United States) as
early as half-century ago . However, only now, in the
21th century, the basics of their molecular action start
to be disclosed. The ﬁrst artiﬁcially obtained cytokinin
(kinetin) initially displayed its biological activity in the
axenic plant tissue culture as a compound promoting
cell division and callus growth. Considering the fact
that G. Haberlandt (Germany) supposed the occurrence
of such regulators in plants as early as in the beginning
of the 20th century (see ), cytokinins were hastily
added to the list of the basic plant hormones. It seems
now evident that this was made somewhat prematurely
because, at that time, the presence of cytokinin-like
compounds in plants and their intracellular biosynthe-
sis were not strictly proven.
In fact, the ﬁrst natural cytokinin zeatin was isolated
from maize (
) embryos by D. Letham et al.
(Australia) [4, 5] almost ten years after the ﬁrst syn-
thetic cytokinin kinetin was obtained by Skoog et al.,
whereas cytokinin biosynthesis in plants was proven by
Japanese researchers only in 2001 [6, 7], i.e., already in
the 21th century.
After the materials of Chailakhyan Lecture delivered in 2006.
It would be more correctly to designate them as phytokinins, thus
indicating both their functions and their belonging to the plant
kingdom, as was suggested by Köhler and Conrad in 1966 ).
Figure 1 presents the structures of the basic cytoki-
nins. Naturally occurring cytokinins are relatively sim-
ple adenine derivatives modiﬁed on the nitrogen atom
at the position 6 of the six-member heterocycle. In most
cytokinins, a short aliphatic chain of the isopentenyl
residue is attached to adenine at this position. Hor-
mones, in which this chain is not modiﬁed, belong to
the group of isopentenyl-type (iP-type) cytokinins.
However, there are enzymes in plants hydroxylating the
aliphatic chain on its terminal carbon . Cytokinins
with hydroxylated aliphatic chain were termed zeatins
(Z-type cytokinins). Two zeatin stereoisomers are pos-
hydroxyl group of the isopentenyl side chain is oriented
away from the adenine heterocycle.
believed to be the most widespread and active plant
-zeatin, in contrast, the end hydroxyl
group is oriented toward the adenine ring, so that the
hydrogen bond can be formed between the hydrogen of
the OH-group and the nitrogen atom at position 1 of the
adenine heterocycle (Fig. 1) . Among other natural
cytokinins, there are such compounds as dihydrozeatin
(with a reduced double bond in the side chain), 6-ben-
zyladenine (BA), and topolin (an aromatic cytokinin
ﬁrstly found in poplar plants with the 3-hydroxybenzyl
residue instead of aliphatic chain ).
Cytokinins and their analogs can be produced by
relatively simple chemical synthesis. Therefore,
numerous synthetic cytokinins differing in their activity
have been produced. In practice, most easily synthe-
sized and stable cytokinins, such as kinetin, BA, and/or
isopentenyladenine, are usually applied.
Along with derivatives of adenine, cytokinin activ-
ity was detected in synthetic derivatives of phenylurea
How Do Cytokinins Affect the Cell?
G. A. Romanov
Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (495) 977-8018; e-mail: firstname.lastname@example.org
Received June 30, 2008
—The lecture presents modern knowledge of the mechanisms of cytokinin perception and signal
transduction to the genes of primary and secondary responses. It also demonstrates the relations between the
rapid cytokinin-induced processes and cytokinin-induced physiological effects. The characteristics of the cyto-
kinin regulatory system and its role in the control of plant growth and development are discussed.
Key words: cytokinin - receptor - signal transduction - primary response genes - control of gene expression -
plant cell - long-distance communication
: ARR—Arabidopsis response regulator; BA—ben-
zyladenine; DZ—dihydrozeatin; GUS—
isopentenyladenine; PLD—phospholipase D; Z—zeatin.