Cytokinins

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Lecture 28
Cytokinins Regulators of Cell Division and
Ethylene The Gaseous Hormone
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Differentiated plant cells can resume division

• Examples
• secondary meristems (vascular cambium or cork
  cambium)
• wounding of plant cells initiates cell divisions
  at the wound
• site

Infection with Agrobacterium tumefaciens at the
wound site causes tumor-forming disease
crown-gall (unorganized mass of tumor-like
tissue) ? natural evidence of the mitotic
potential of mature plant cells Agrobacterium
changes the character of cells, makes them
tumor-like
Tomato stem
Haberlandt 1913 vascular tissue contains a
water-soluble substance that will stimulate
division of wounded potato tuber tissue ?
discovery of cytokinins in 1950s
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Discovery, identification, properties
Philip White nutrient medium, supplemented with
auxin and 10-20 coconut milk supports cell
division of mature, differentiated cells from a
variety of tissues and species, leading to
formation of callus tissue. ? Coconut milk
contains substance/s that stimulate/s mature
cells to enter and remain in cell division
cycle. Coconut milk was shown to contain the
cytokinin zeatin. The very first cytokinin
discovered was the synthetic analog kinetin.
Kinetin A breakdown product of DNA
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Zeatin is the most abundant natural cytokinin
Extracts of immature endosperm of corn (Zea mays)
contain a substance with the same biological
effect as kinetin. This substance stimulates
mature plant cells to divide when added to
culture medium along with auxin. Letham (1973)
isolated that substance, called zeatin.
Kinetin
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Some plant pathogenic bacteria, insects, and
nematodes secrete free cytokinins
Infection of plant tissue by these organisms
causes tissue to divide and form special
structures in some cases (insect galls used as
feeding sites)
Witches broom on balsam fir caused by
Corynebacterium fascians Lateral buds (normally
dormant) are stimulated to grow by bacterial
cytokinin
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Biosynthesis, transport, and metabolism
Root apical meristem major site of free
cytokinins in whole plant.
Transport of cytokinins from roots to shoot via
xylem.
Cytokinins are rapidly metabolized by cytokinin
oxidase, thereby inactivating cytokinins.
(isopentenyladenine)
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Biological roles of cytokinins
Cytokinins regulate cell division in vivo
Tobacco plant overexpressing the gene for
cytokinin oxidase ? reduction in endogenous
cytokinin levels ? Inhibition of shoot
growth due to reduction in rate of cell
proliferation in SAM
AtCKX1-OX
AtCKX1 Arabidopsis cytokinin oxidase
WT
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Cytokinin is required for normal growth of SAM
SAM of wild-type tobacco SAM transgenic tobacco
overexpressing AtCKX1 ? cytokinin
deficiency ? reduced size of SAM
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Cytokinin suppresses growth of roots
Cytokinins suppress size and cell division
activity of roots. Cytokinins have opposite
roles in regulating cell proliferation in RAM
and SAM.
WT tobacco
AtCKX1-OX Cytokinin-deficient
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The auxincytokinin ratio regulates morphogenesis
in cultured tissues
Shoots
No growth
Callus
Plant tissue
Roots
Nutrient agar
Auxin Kinetin
Ratio 15 150 0.003
High auxincytokinin ratio root Low
auxin/cytokinin ratio shoot Intermediate
auxin/cytokinin ratio callus
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Cytokinins delay leaf senescence
Senescence programmed aging process leading
to death
Leaf senescence is retarded in transgenic tobacco
containing high levels of cytokinin.
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Ethylene
H2C-CH2
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Discovery of ethylene
19th century coal gas used for street
illumination ? defoliation ethylene
is active component
1901 Dimitry Neljubov discovery of the triple
response pea-seedlings exposed to illuminating
(coal) gas reduced stem/hypocotyl
elongation reduced root elongation
(swelling) exaggeration of curvature of apical
hook by growing plants in fresh air, they
regained their normal morphology and rate of
growth
Ethylene
3-day-old Arabidopsis seedlings
Air
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Biosynthesis of ethylene

• May be synthesized in all organs
• Mostly in meristematic and nodal regions
• Increased during leaf abscission, flower
  senescence,
• fruit ripening, wounding, stress

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Factors affecting ethylene biosynthesis
Stimulation of ethylene biosynthesis
by developmental state (fruit
ripening) environmental conditions
(stress) other plant hormones (auxin,
cytokinin) physical and chemical injury
(wounding)
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Ethylene promotes ripening of some fruits
Fruit ripening tissue softening enzymatic
breakdown of cell walls starch hydrolysis sugar
accumulation disappearance of organic acids and
phenolic compounds Plant seeds are ready for
dispersal
Banana Climacteric rise in CO2 and Ethylene,
which triggers ripening process
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Some physiological effects of ethylene
Triple response
Epinasty (downward bending of leaves)
Air
C2H4
Air
C2H4
Air
C2H4
Tomato
Arabidopsis
Pea
Promotion of root hair formation
Inhibition of flower senescence
Lettuce
Air
C2H4
STS Silver thiosulfate
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Roles of ethylene and auxin during leaf abscission
Abscission the shedding of leaves, fruits,
flowers
Jewelweed (Impatiens)

• (A)
• 2 or 3 layers in abscission zone undergo cell
  wall breakdown
• (B)
• Resulting protoplasts round up and increase
• in volume, pushing apart the xylem cells,
• facilitating separation of leaf from stem

Abscission zone
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Roles of ethylene and auxin during leaf abscission
Model of hormonal control of leaf abscission
Leaf maintenance Shedding induction
Shedding phase
High auxin from leaf reduces ethylene sensitivity
of abscission zone and prevents leaf shedding.
Reduction in leaf auxin increases ethylene
production and ethylene sensitivity in abscission
zone, which triggers the shedding phase.
Synthesis of enzymes that hydrolyze the cell wall
polysaccharides, resulting in cell separation and
leaf abscission.