1. Cytokinins are plant hormones that promote cell division. The first was discovered in 1955 and is called kinetin, derived from degraded herring sperm DNA. 2. Cytokinins are synthesized primarily in root apical meristems but also other growing tissues. They move up through the xylem while auxin moves down. 3. In Arabidopsis, cytokinin signaling involves receptors that phosphorylate histidine phosphotransfer proteins, leading to phosphorylation of ARR transcription factors and expression of genes causing cytokinin responses.

1. ntroduction
• Cytokinin (CK) are plant growth hormones which are basic
in nature, either amino purine or phenyl urea derivatives,
that promote cytokinesis (= cell division) either alone or in
conjunction with auxin.
• The first cytokinin was discovered from degraded
autoclaved Herring sperm DNA by Miller 1955.It is called
kinetin (6-furfuryl amino-purine).
• Kinetin does not occur naturally. It is a synthetic hormone.
• The first natural cytokinin was obtained from unripe maize
grain known as zeatin (6-hydroxy 3-methyl trans 2-butenyl
amino-purine). It also occurs in coconut milk.
(6-hydroxy 3-methyl
trans 2-butenyl
amino-purine)

2. In 1931, an Austrian plant physiologist. G. Haberlandt,
discovered that soluble substances present in phloem tissues
could cause cell division in parenchyma cells of wounded potato.
In 1954, C. O. Miller found that the aged DNA from sperms
stimulates cell division in pith cells from tobacco tissue
cultures, and the active part of DNA was found to be kinetin.

3. • Jablonski & Skoog (1954) - compounds in
vascular tissues promote cell division
Naturally occurring Cytokinin Sir Skoog

4. ➢ Cytokinin extracted from coconut milk, Tomato juice
➢ flowers and fruits of pear, plum
➢ Cambium tissues of Eucalyptus, Nicotina
➢ Immature fruits of Zea Mays, Musa sp.
➢ Root exudates of Sunflower
➢ Ribosylzeatin,
➢ Zeatin,
➢ Dihydrozeatin.
Naturally occuring Cytokinin
Cytokinin found in plants
Dihydrozeatin
Zeatin

5. CIS-ZEATIN TRANS-ZEATIN

6. Kinetin
Zeatin
Benzyl aminopurine
Cytokinin
Structure

7. The major forms of cytokinin differ
in different plant species. For
example in rice the isoprenoid
cytokinin cis zeatin is the major
form, whereas in Arabidopsis it is
the trans-zeatin and iP forms that
predominate.
Common Type of
Cytokinin

8. Biosynthesis location
• Roots seem to be the major source of
cytokinin synthesis. From roots the cytokinin
pass upwardly through xylem.
• Some cytokinin synthesis also takes place in
other areas where cell divisions are occurring
like endosperm region of seeds, growing
embryos and developing seeds, young fruits,
developing shoots buds, etc.
•Coconut milk is a rich source of cytokinin.

9. • Generated mostly in the root apical meristems but also found in:
➢ Root cap cells
➢ Ovules
➢ Phloem cells
➢ Leaf axils
➢ Tips of young inflorescences
➢ Fruit
➢ Seeds
Biosynthesis

10. Biosynthesis
• Other organisms make cytokinin to
influence the plant for their own
benefit
➢ Bacteria
➢ Fungi
➢ Insects.
➢ Nematodes

11. • Cytokinins move up the plant
through the xylem
• By contrast, auxin moves from
top down.
• Some signal in the shoot can
also induce cytokinin transport
from the root (Beveridge 2000).
TRANSPORT

12. ATP/ADP
DMAPP(Dimethyl Allyl
Phosphate)
iPDP, or iPTP
(isopentenyladenosine-5’-
monophosphate)
Zeatin
The first committed step in cytokinin biosynthesis is the
addition of the isopentenyl side chain from DMAPP to
an adenosine moiety. The plant and bacterial IPT
enzymes differ in the adenosine substrate used; the
plant enzyme appears to utilize both ADP and ATP, and
the bacterial enzyme utilizes AMP. The products of
these reactions (iPMP, IPDP, or IPTP) are converted to
zeatin by an unidentified hydroxylase. The various
phosphorylated forms can be interconverted and free
trans-Zeatin can be formed from the riboside by
enzymes of general purine metabolism. Trans-Zeatin
can be metabolized in various ways
Biosynthesis

13. Cytokinin Synthesis

14. Cytokinin are rapidly metabolized by cytokinin oxidase, thereby
inactivating cytokinin's
Activity of Cytokinin oxidase induced by high cytokinin concentrations.
Metabolism
Of
Cytokinin

15. ➢ The cytokinin receptors are encoded by a multigene
family.
➢ Receptor- CRE1(CYTOKININ RESPONSE 1),contain a
conserved extra-cellular Cytokinin-binding domain called
the CHASE domain.
➢ Receptor in Arabidopsis for CK signaling- AHKs
(AHK2,AHK3 and AHK4) /Arabidopsis hybrid kinase
➢ CHASE domain bind to Cytokinin in pH dependent
manner.
➢ (AHP) Histidine phosphotransfer protein Hpt proteins are
predicted to mediate the phosphotransfer between
sensor kinases and response regulators.
CYTOKININS SIGNALING PATHWAY

17. 1. Cytokinin binds to CRE1, which is likely to occur as a
dimer. Cytokinin binds to an extracellular portion of CRE1
called the CHASE domain.Two other hybrid sensor
kinases (AHK2 and AHK3) containing a CHASE domain are
also likely to act as cytokinin receptors in Arabidopsis.
2. Cytokinin binding to these receptors activates their
histidine kinase activity. The phosphate is transferred to
an asparate residue (D) on the fused receiver domains.
3. The phosphate is then transferred to a conserved
histidine present in an AHP protein
STEPS -

18. 4. Phosphorylation causes the AHP protein to move into
the nucleus, where it transfers the phosphate to an
asparate residue located within the receiver domain of a
type-B ARR
5. The phosphorylation of the type-B ARR activates the
output domain to induce transcription of genes encoding
type-A ARRs.
6. The type-A ARRs are likely also to be phosphorylated
by the AHP proteins.
7. The phosphorylated type-A ARRS interact with various
effectors to mediate the changes in cell function
appropriate to cytokinin (indicated in the model as
"cytokinin responses").
STEPS -