Download 08 Cytokinins

Chapter 21
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Differentiated cells can resume division
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Wounding induces division at the wound site
Generally self-limiting
 Agrobacterium tumefaciens infection
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Diffusible factors control cell division
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Division stops because signal stops?
Plant tissues and organs can be cultured
Arabidopsis.info
Roots could be cultured; stems were recalcitrant
 Crown gall tissue, OTOH …..
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Roots – grow fine with no hormones
Shoots – no growth even with meristems
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Unless adventitious roots present
 difference between regulation in shoot
and root
 root derived factors
regulate shoot
Crown Gall tissue
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First cytokinin discovered -- kinetin.
Not reported in plants
 Reported in human urine.
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H
N
CH2
N
N
N
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N
H
O
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Coconut endosperm (coconut milk) –
supported continued division of mature,
differentiated cells
1940s-50s – Adenine had some effect
Aged herring sperm …..
 Kinetin
 Corn endosperm (1973) – zeatin
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Notice the double bond …..
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Zeatin isomerase
trans form generally predominates
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Zea and Oryza – cis form
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Derivatives of adenosine monophosphate (AMP).
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R determines type
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isoprenoid cytokinin.
aromatic cytokinin – least common
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Biological activities similar to trans-zeatin
Inducing cell division in the presence of auxin
 Promoting bud or root formation in appropriate
ratios to auxin
 Delaying leaf senescence
 Promoting expansion of dicot cotyledons
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Almost all are aminopurine deriviatives
Thidiazuron
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Occur both bound and free
Some plant pathogens secrete free
cytokinins!
Agrobacterium
 Fungi
 Insects
 Nematodes
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Found in actively dividing tissues
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E.g., seeds, fruits, leaves, root tips,
Found in bleeding sap (think wound
healing!)
Occur in algae, mosses, horsetails, ferns, and
conifers
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Primary site of synthesis -- root tips.
High concentrations -- immature seeds and
developing fruits
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synthesis *OR* transport?
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Evidence indicates that locally produced cytokinins
required to release buds from dormancy
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Cytokinins only stimulate cell division in the
presence of auxin!
Tobacco tissue culture – callus only
 In the absence of meristem or cambium – cell
division happened!
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Specific role -- regulate the progression of the
cell cycle.
In absence of either auxin or cytokinin -- G1 or G2
 Supply missing hormone -- 12-24 hours – division
begins
 Activate cyclin-dependent kinases (CDK).
 Activation of CDK – allows transition from G2 to
mitosis.
 Promotion of accumulation of cyclins allow
transition from G1 to S
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Removes
inhibitory
phosphate
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Promotes
accumulation
of G1 cyclins
(CycD)
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Auxin/cytokinin interaction regulates formation of
tissues in cell culture.
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Both hormones in similar concentrations -- maintain cells as
undifferentiated callus.
High auxin -- only plant roots form.
High cytokinin -- only shoots form.
Proper ratios – callus can produce an intact plant.
Cytokinins
Sugarcane callus
High IAA and High Cytokinin
Cytokinins
Development of Shoots
High IAA and Low Cytokinin
Cytokinins
Shoots Elongate
High IAA and no Cytokinin
Cytokinins
Development of Roots
High IAA and no Cytokinin
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Cytokinins and apical dominance.
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Application of cytokinin to meristem or to the
axillary bud will release the bud!
Plants overproducing auxin – increased apical dominance
 Plants overproducing cytokinin – decreased dominance
 Application of cytokinin to plants overproducing auxin
releases buds as well
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Such results illustrate that the ratio of auxin to
cytokinin
 “witch’s broom”
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Extreme lateral bud release
 overproduction of cytokinin?
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Witches broom in Pinus strobus
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Crown gall tumors -- Agrobacterium
tumifaciens.
Excised tumors can exist with no added hormones
 Contain tumor-inducing (Ti) plasmid
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3 classes of proteins
 Auxin and cytokinen production
 Opines – nutrients for the bacterium
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Plasmid inserted into nuclear DNA upon infection
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Cytokinins are involved in the formation of crown
gall tumors in plants.
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Transferred DNA (T-DNA) from the plasmid is
incorporated into the host plant genome.
The T-DNA contains three genes.
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Two of the genes are necessary for the synthesis of auxins and
cytokinins.
The third gene causes the plant to produce opines, which are a
nutrient amino acid for the bacteria.
The natural ability of A. tumifaciens to genetically
reprogram plants can been adapted to facilitate the
transfer of other genes into plants.
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Cytokinins -- inhibitors of
senescence.
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Exogenous cytokinins delay
senescence.
On detached leaves – no
senescence
 Leaves treated with auxin (for
adventious root formation) – no
senescence
 Leaves spotted with cytokinin –
spots stay green
 20 week tobacco plants …..
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Delayed senescence
appears to deal with
distribution of
nutrients
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Exogenous
application
Spots stay green
 Nutrients (and AA)
migrate to spot
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Cytokinins stimulate
metabolism?
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Meristem – delicate balance between dividing
cells and differentiating cells
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Cytokinins play a part
Reducing in vivo concentrations
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increased degradation or loss-of-function receptors
Retarded or halted shoot development
 Dwarf, late flowering plants
 Reduced size of the shoot meristem
 Slowed formation of leaf primordia
 A reduced number of leaf cells
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Cytokinin maintains
meristem by controlling
cell division
Deficient mutants –
lonely guy (LOG)
Gene activates cytokinins
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Homeotic genes – genes that control placement and
spatial organization of body parts by controlling the
developmental fate of groups of cells
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Contain 180 nucleotide sequence called a homeobox
Homeobox – specifies homeodomain in the protein
Homeodomain – part of the protein that binds to the DNA
when the protein functions as transcriptional regulator
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Can bind to any DNA segment
Other domains determine which genes the protein regulates
Plant homeobox genes include some MADS-box genes and
KNOX genes
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KNOX genes -- regulate biosynthesis of cytokinins
and gibberellins to specify meristems
KNOX genes -- normally expressed in SAM but not
leaf primordia or developing leaves.
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A dominant, gain-of-function mutation
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Loss-of-function mutants
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expression of KNOX genes in developing leaves, causing “knots” of cells or
ectopic shoots to form.
Accumulation of KNOX proteins – division only, no differentiation
A defect in STM (ShootMeristemless) prevents formation of apical
meristem
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KNOX proteins -- transcription factors that
regulate transition from indeterminate growth to
differentiation.
KNOX proteins :
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Up-regulate IPT genes for cytokinin synthesis
Suppress the GA20-oxidase gene in gibberellin
synthesis
KNOX proteins -- maintain the high cytokinin/low
gibberellin ratio needed for formation and
maintenance of shoot meristems.
Cytokinins
Totipotency
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Summary from Taiz & Zeiger
Promote shoot growth by increasing cell
proliferation in the SAM
 Inhibit root growth by promoting the exit of cells
from the RAM
 Regulate components of the Cell Cycle
 Modify apical dominance and promote lateral bud
growth
 Delay leaf senescence
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Summary from Taiz & Zeiger
Promote movement of nutrients
 Affect light signaling
 Regulate vascular development
 Involved in the formation of nitrogen-fixing
nodules
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Chemical Nature: N6-adenine derivatives, phenyl urea
compounds. Zeatin is the most common cytokinin
Sites of Biosynthesis: primarily in root tips
Transport: transported in the xylem from roots to shoots
Effects: promotion of cell division; promotion of shoot
formation in tissue culture; delay of leaf senescence;
application of cytokinin can cause release of lateral buds
from apical dominance and can increase root development
in arid conditions
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