HORMONE PHYSIOLOGY AND
SIGNAL TRANSDUCTION
Huseyin Tombuloglu, Phd.
GBE 304 Spring 2015
CYTOKININS
Regulators of Cell Division
Cytokinins
promote cell division….cytokinesis.
Discovered in the 1950s during the “golden
age” of tissue culture of plants.
Were discovered in the search for factors
that stimulate plant cells to divide (i.e.,
undergo cytokinesis)
Most are purines…refers to organic
chemical structure.
Kinetin as an example.
There are many forms of this
hormone
Figure 21.2 Structures of other aminopurines that are active as cytokinins
Functions of Cytokinins
Close relationship w/ auxins in plants.
Promotes mitosis…cell divisions.
Cell expansion of leaves, stems, roots.
Formation of bud tissues.
Involved in light responses.
Regulate protein synthesis.
Cytokinins and Auxins
Concentration of cytokinins relative to auxins are
very important
In tissue culture…auxin : cytokinin ratios affect
growth of tissues.
 auxin :  cytokinins = Increased ROOT growth.
 auxin :  cytokinins = Increased SHOOT growth.
Some Plant Pathogenic Bacteria, Insects, and
Nematodes Secrete Free Cytokinins
Many of these microorganisms produce and secrete substantial
amounts of cytokinins and/or cause the plant cells to synthesize
plant hormones, including cytokinins
Infection of plant tissues with these microorganisms can induce the
tissues to divide and, in some cases, to form special structures, such
as mycorrhizae, in which the microorganism can reside in a
mutualistic relationship with the plant.
Mycorrhizae
Witches’broom on balsam fir (Abies balsamea) Balsam köknarı
Figure 21.3 Witches’ broom on a fir tree
Agrobacterium rhizogenes infection
Tobacco plants overexpressing the gene for
cytokinin oxidase. The plant on the left is wild type.
The two plants on the right are overexpressing two
different constructs of the Arabidopsis gene for
cytokinin oxidase: AtCKX1and AtCKX2.
Shoot growth is strongly inhibited in
the transgenic plants
This is evident that without cytokinin plant growth
is strongly inhibited.
Cytokinins can also control branching in some
species….
Cytokinins promote lateral buds.
Auxins suppress lateral buds (apical dominance).
Root
development
Buds development
The regulation of growth and organ formation in cultured tobacco callus at different
concentrations of auxin and kinetin.
No differentiation
Figure 21.13 Cytokinin suppresses the growth of roots
The cytokinin-deficient
AtCKX1 tobacco-right
WT- left
Figure 21.14 Cytokinin suppresses the size and cell division activity of roots
Figure 21.23 Cytokinin regulates grain yield in rice (A)
Figure 21.23 Cytokinin regulates grain yield in rice (B)
Figure 21.7 Phenotypes of Arabidopsis plants harboring mutations in the cytokinin receptors