Cytokinin

1. Cytokinins are substituted adenine compounds that
promote cell divisions in specific plant tissues.
The ratio of auxin to cytokinin plays an important role
in the effect of cytokinin on plant growth.

2. History
• 1892: Weibner thought that cell division is regulated by
endospermic compounds.
• 1941: Discovery by Johannes van Overbeek – coconut milk
could sustain cell division and prolonged growth of stem
explants (excised pieces of stem) He also showed that various
other plant species had compounds which stimulated cell
division (van Overbeek, 1941).
• 1950s: Folke Skoog and coworkers – identified a modified
purine (nucleotide) and called Kinetin
• In 1954:, Jablonski and Skoog extended the work of
Haberlandt showing that vascular tissues contained
compounds which promote cell division (Jablonski and Skoog,
1954).

3. • 1955: The first cytokinin was isolated from herring
sperm in 1955 by Miller and his associates (Miller et
al., 1955). This compound was named kinetin because
of its ability to promote cytokinesis.
• Hall and deRopp reported that kinetin could be formed
from DNA degradation products in 1955 (Hall and
deRopp, 1955).
• 1961: Miller isolated the first naturally occurring
cytokinin from corn (Miller, 1961). It was later called
zeatin. Almost simultaneous with Miller, Letham
published a report on zeatin as a factor inducing cell
division and later described its chemical properties
(Letham, 1963).
• It is Miller and Letham that are credited with the
simultaneous discovery of zeatin. Since that time,
many more naturally occurring cytokinins have been
isolated and the compound is ubiquitous to all plant
species in one form or another (Arteca, 1996;
Salisbury and Ross, 1992).

4. All cytokinins have basic Adenine ring structure

5. Natural Cytokinins
• 6-furfuryl-aminopurine,
• Ribosyl zeatin,
• Zeatin,
• Isopentinyladenine
• Dihydrozeatin.

7. There are synthetic cytokinins derived from diphenylurea (DPU) that are structurally
unrelated to the adeninetype cytokinins.

8. Occurrence
• Cytokinins have been found in almost all higher plants as
well as mosses, fungi, bacteria,
• Cytokinins are found in actively growing tissues where cell
division takes place (root tip, shoot tip, expanding leaf,
developing endosperm – e.g. Liquid endosperm of coconut,
immature maize endosperm.
• Today there are more than 200 natural and synthetic
cytokinins.
• Root tips are the probable sites of cytokinin synthesis.
• The balance of cytokinins and auxins acting together causes
development of organs like shoots and roots

9. • Most important function of cytokinins is
cytokinesis. Application of cytokinins promote
cell division by increasing the change of cell
from G2 to mitosis
• This is done by enhancing protein synthesis,
since specific enzymes are required for
mitosis.
• Cytokinins effect on translation but not on
transcription.
Physiological Roles/ Mode of Action

10. Cytokinin Functions
• Stimulates cell division.
• When cytokinin is provided to a liquid culture
medium containing plant cells, the protein
synthetic activity of the cells is greatly
stimulated. Moreover, some of the proteins
thus synthesized are new ones. These
observations suggest that cytokinins control
cytokinesis by regulating the synthesis of
some specific protein factors that are required
for cytokinesis.

11. Promote cell expansion
• Cytokinins are equally good in causing cell
expansion like auxins.
• only in dicot seedlings but in stem and roots
they inhibit inhibit cell expansion probably
due to production of ethylene in stem and
roots

12. Stimulates morphogenesis
• Shoot initiation/bud formation in tissue
culture.
• Involved in differentiation of tracheids
through activation of lignin biosynthesis.
• Involved in induction of flowering

13. Release of apical dominance
• Stimulates the growth of lateral buds-release
of apical dominance.

14. • Delay of senescence. Yellowing (loss of chlorophyll) of
leaf can be delayed with the addition of cytokinins
• This effect of cytokinin has been explained as due to
the prevention of degradative catabolic processes by
the way of repression activity of few hydrolysing
enzymes like protease, RNAse, DNAse etc.
• Furthermore, cytokinin facilitates the chlorophyll
synthesis.
• It also sustains the activity of carbon fixation, RNA
synthesis and protein synthesis.

15. Richmond - Lang effect
• The delay of senescence of leaves and other
organs of the plants by cytokinins is called
Richmond - Lang effect. It was discovered by
Richmond and Lang in 1967.

16. • Involved in releasing seed dormancy. They
substitute the red light requirement for
breaking the dormancy of seeds. The site of
action of cytokinin on seed germination is the
cotyledon, which expands to break the seed
coat and facilitates the germination of seeds.

17. • Stimulates leaf expansion resulting from cell
enlargement. During cytokinin induced cell
enlargement, respiratory activity increases
significantly and greater amounts of K+ ions
are accumulated in the cells. At the same
time, cells in response to the hormones
induce the synthesis of few minor species of
RNAs and some proteins.
• May enhance stomatal opening in some
species.

18. Promote Chloroplast maturation
• :Promotes the conversion of etioplasts into
chloroplasts via stimulation of chlorophyll
synthesis.
• Etiolated leaves treated with cytokinins
develop chloroplast

19. Role of Cytokinins in Apical Dominance
• Measurements of cytokinin levels in axillary buds of Douglas fir
(Pseudotsuga menziesii) show a very good correlation between
endogenous cytokinin levels and bud growth (Pilate et al. 1989).
• Much of the cytokinin of the plant is synthesized in the root and
transported to the shoot.
• Studies with the 14C-labeled cytokinin benzyladenine (BA), have shown
that when the labeled compound is applied to roots, more [14C]BA is
transported to the shoot apex than to the axillary bud.
– Decapitation increases the accumulation of [14C]BA by the axillary bud, and
application of auxin to the apical stump reduces this accumulation.
– Thus auxin makes the shoot apex a sink for cytokinin from the root, and this
may be one of the factors involved in apical dominance.

20. cytokinins and auxin are
antagonistic-
• Auxin from terminal bud causes shoot to
lengthen.
• Cytokinins from roots stimulate axillary bud
• Auxin stimulates lateral root formation while
cytokinins restrain it.