Cytokinins are plant hormones that promote cell division and play several important roles in plant growth and development. They are produced primarily in roots and transported throughout the plant via xylem where they stimulate cell division, inhibit senescence, promote bud development, and interact with auxins to influence tissue differentiation. The document discusses the natural and synthetic cytokinins, their sites of production, transport, and various physiological effects including cell division, morphogenesis, bud development, delaying senescence, greening, and cell expansion.
2. Cytokinins
The word for cytokinins is a generic name for all
naturally occurring substances that are known
to promote cell division..
These hormones may also be involved in
controlling leaf senescence and the growth of
lateral branches.
The most active, naturally-occurring cytokinin is
zeatin.
Cytokinins occur in most plants including
mosses, ferns, conifers, algae and diatoms.
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3. Cytokinins
• The most widely distributed cytokinins are the
synthetic benzyladenine and kinetin. Kinetin
was discovered from the tobacco pith callus
and the chemical substance was identified as 6-
furfuryl aminopurine. The natural cytokinin
appears to be made principally in apical root
meristem, inflorescences and developing fruits.
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4. Cytokinins
Cytokinins have been found to be the constituent of
certain transfer RNA molecules in a number of
different organisms.
They are also involved in stimulation of organ
formation e.g. formation of leave, fruit, buds, and
branches.
They tend to contract or overcome apical dominance
and break dormancy.
They also enhance seed germination and uniform
flowering.
• Cytokinins regulate the transportation of metabolites
in the phloem.3/9/2020 Galal Eliwa 4
5. Cytokinins
Where it produced?
Cytokinins are synthesized primarily in actively
growing tissues, particular in roots, embryos
(seeds), geminating seeds, and young
developing fruits and transported to other
organs.
This is known in part because roots can be
cultured (grown in Artificial medium in flask)
without added cytokinin, but stem cells cannot.
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6. Cytokinins
• Transport:
Cytokinins produced in the root reach to
their target tissues by moving up the plant in
the xylem sap (transpiration stream).
Zeatin ribosides are the main transport form;
converted to the free base or glucosides in
the leaves.
Some cytokinins also moves in the phloem.
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8. Cytokinins
Interact with auxins to influence differentiation of tissues
(may be used to stimulate bud formation).
Liquid endosperm of coconut called Coconut Water / Milk
contain cell division causing factors (Kinetine).
Similarly the developing endosperm of maize contain such
factors (Zeatin).
As roots begin to grow actively in the spring, they produce
large amounts of cytokinins that are transported to the
shoot, where they cause the dormant buds to become active
and expand.
Tissue cultures use cytokinins to induce shoot development.
Cytokinins may slow or prevent leaf senescence (leaf ageing
or leaf fall).3/9/2020 Galal Eliwa 8
9. Cytokinins Actions
1- Regulates the cell cycle/cell division:
Cytokinin will stimulate cell division and
influenced the pathway of differentiation when
acting with auxin hormone.
In some experment, a piece of paranchyma tissue
from stem is cultured. This is some observation of
the effect on cytokinin in cell tissue:
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Absent of cytokinin Added cytokinin &
auxin
Added cytokinin
only
The cell grow very large
but do not divided
The cell is grow and
divided
No effect
10. How Cytokinins Control Cell Division?
The ratio of cytokinins to auxin controls
the cell differentiate.
When the concentration of these two
hormone are at certain level, the mass
of cell continues to grow, but remains
undifferentiated called a callus.
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Cytokinin level increase Auxin level increase
Shoot buds develop from
callus
Root form
11. Cytokinins Actions
• 2- Control morphogenesis
• In plant tissue cultures, cytokinin is required for the
growth of callus (an undifferentiated, tumor-like mass of
cells):
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The Medium The Callus differentiation
Callus + Auxin + No Cytokinin Little growth of callus
Callus + Auxin + Cytokinin Callus grows well, undifferentiated
The concentration THE Callus differntiation
Callus + Low { cytokinin/ auxin} Little growth of callus
Callus + high { cytokinin / auxin} Callus grows well, forms meristem &
shoots
• Ratio of cytokinin and auxin are important in determining
the fate of the callus:
13. Cytokinins Actions
• 3- Bud development
Direct application of cytokinin promotes the
growth of axillary buds.
Exogenous cytokinin and auxin are thus
antagonistic in their effects on axillary bud
growth
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15. Cytokinins Actions
• 4- Delay senescence
Senescence is the
programmed aging process
that occurs in plants.
Loss of chlorophyll, RNA,
protein and lipids.
Cytokinin application to an
intact leaf markedly reduces
the extent and rate of
chlorophyll and protein
degradation and leaf drop.
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16. Cytokinins Actions
• 5- Greening
Cytokinins promotes the light-induced
formation of chlorophyll and conversion of
etioplasts to chloroplasts (greening process).
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17. Cytokinins Actions
• 6- Promote cell expansion
Cytokinins stimulate the expansion of
cotyledons.
The mechanism is associated with increased
plasticity of the cell wall, not associated with
acidification.
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