Transcript - University of Arid Agriculture Rawalpindi

Cytokinins Biosynthesis, Signaling
Pathways, Functions and Cross talk
with Auxins
ARSHAD MAHMOOD KHAN
[email protected]
PhD Scholar (13-arid-3581)
BOTANY DEPARTMENT
UAAR RAWALPINDI
LECTURE OUTLINE
CYTOKININS
● Cytokinins
○
○
○
○
○
○
Discovery
Chemical Structure
Biosynthesis
Transport
Signaling
Functions
DISCOVERY
CYTOKININS
• Haberlandt (1913) – compound in phloem
stimulates cell division
• Van Overbeek (1941) – coconut milk
(endosperm) also has the ability to stimulate cell
division
• Jablonski & Skoog (1954) – compounds in
vascular tissues promote cell division
• Miller (1955) – first cytokinin isolated from
herring (Fish) sperm, named kinetin
DISCOVERY
CYTOKININS
●Miller (1955) – first cytokinin isolated from herring sperm, named kinetin
●Miller (1961) – first naturally occurring cytokinin found in plants (Zea mays
L.), later called Zeatin
●Lathum & Guillfoyle (1974)
Zeatin
CHEMICAL STRUCTURE
CYTOKININS
●Adenine structure (amino purine ring)
●N6 Side chain
○Degree of unsaturation
○Number of carbons in side chain
CHEMICAL STRUCTURE
CYTOKININS
Free base forms are most hormonally
active (Yamada et al. 2001)
May have sugars attached to make
molecule more inactive and useful for
transport
Ribosides
Ribotides
Glycosides
BIOSYNTHESIS
CYTOKININS
Biosynthesis started by the IPT
(isopentenyl tranferase) enzyme
Major pathway
Figure on slide no. 9
Other pathway
tRNA biosynthesis
Figure on slide no. 10
Plant physiology, 4th Edi. Fig. 21.6, 2006 Sinauer Associate. Inc
BIOSYNTHESIS
Plant physiology, 4th Edi. Fig. 21.7, 2006 Sinauer Associate. Inc
BIOSYNTHESIS
tRNA breakdown not
the major pathway for
cytokinin synthesis
BIOSYNTHESIS
CYTOKININS
●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
TRANSPORT
CYTOKININS
●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).
SIGNALING PATHWAY
Cytokinins perception and pathway
Cytokinin Signaling Pathway in Arabidopsis consists of four steps:
1.
Initiation of a Phosphorelay Cascade by distinct Plasma
membrane His (Histidine) Protein Kinases upon Cytokinin
perception
2.
Convergence of the signals initiated at His Protein Kinases, on
AHP proteins, that serve as Phospho-relay carriers between the
Cytokinin receptors and the downstream nuclear responses
3.
Activation of B-type ARR proteins by nuclear AHP
4.
Translocation and transcriptional activation of A-type ARRs
Mechanism:
•
First of all Cytokinins binds with the CHASE (Cyclases/Histidine
kinases–Associated Sensory Extracellular) domain of receptors
e.g. (CRE1, AHK2, AHK3) present in PM
•
The binding of Cytokinin activates the Transmembrane Domain,
which Autophosphorylates on a His domain.
•
The phosphate then transferred to Receiver Domain and an AHP
protein, which translocates to the nucleus, where it activates
ARRs.
Cytokinins perception and pathway
• ARR form a large gene family composed of 22 genes that includes
two major classes (Type-A and Type-B)
• Type-A ARRs (ARR3,4, 5, 6, 7,8,9,15,16,17,18,19)
• Type-B ARRs (ARR1, ARR2, and ARR10,11,12,13,14)
AHP activates Type-B ARRs increases the transcription of the
Type-A ARRs, which feed back to inhibit their own transcription.
This transcription and translation of specific ARRs produces
specific cytokinins responses in plants
SIGNALING SUMMARY
CYTOKININS
●Cytokinin changes the structure of the His Kinase
domain
○Phosphate moves from the kinase domain to the receiver
domain
○Phosphate moves onto AHP
○AHP moves from cytoplasm into nucleus
○Phosphate on AHP moves onto receiver domain of the
response regulator
○Change in structure of the receiver domain affects the output
domain
○Output domain then signals transcription to cytokinin responses
FUNCTIONS
CYTOKININS
●FUNCTIONS
○Cell division
○Root and shoot meristems
○Cell differentiation
○Leaf senescence
○Inflorescence growth
○Nutrient mobilization
○Cotyledon expansion
○Apical dominance
○ Seedling morphology
CELL DIVISION
CYTOKININ FUNCTION
● Auxin and cytokinins influence the
activity of:
○Cyclin-dependant protein
kinases (CDKs) and cyclins
Both are proteins that regulate
transitions between G1 to S
and G2 to mitosis stages in
the cell cycle
○Auxin stimulates the production
of CDKs and cyclins
Cytokinins activate CDKs and cyclins
through phosphrorylation and allow
transition between stages
cyclin
cyclin
amino
synthesized acids
cyclin
degraded
cyclin
CDK
CDK
(inactive)
phosphorylation
ATP?
GTP?
26S Proteosome
cyclin
dephosphorylation
P
CDK
triggers mitosis
or DNA synthesis
(active)
C-PK
Fig. 15-7, p. 241
ROOT AND SHOOT MERISTEMS
CYTOKININ FUNCTION
● Optimal levels of cytokinins are
needed for normal cell division
○Root: cytokinin
overabundance inhibits cell
division
○Shoot: cytokinins promote
cell division
● Cytokinin oxidase dictates
meristemic cytokinin
concentrations
○Mutants can either
overproduce or underproduce
this enzyme
ROOT AND SHOOT MERISTEMS
CYTOKININ FUNCTION
Fig. 15-10, p. 244
CELL DIFFERENTIATION
CYTOKININ FUNCTION
Bacteria: Agrobacterium tumefaciens
CELL DIFFERENTIATION
CYTOKININ FUNCTION
●Auxin:cytokinin affects cell differentiation in callus tissue
●More auxin leads to roots development
●More cytokinin leads to shoots development
●Skoog and Miller (1965)
LEAF SENESCENCE
CYTOKININ FUNCTION
Cytokinin delays leaf
senescence
Delay the degradation of
chloroplasts
May increase the growing
season for agricultural
purposes.
LEAF SENESCENCE
CYTOKININ FUNCTION
Control
Sprayed
• Delayed leaf senescence, help
plants to recover from environmental
stresses like
•Drought (Rivero et al. 2007)
• Flooding (Zhang et al. 2000)
• Hyunh et al. 2005
INFLORESCENCE GROWTH
CYTOKININ FUNCTION
●Cytokinins induce division in
inflorescence tips
○Leads to more flowering which yields
more fruit
●“Cytokinin Oxidase Regulates
Rice Grain Production”
(Ashikari et al. 2005)
○Plants in this study have lower levels
of cytokinin oxidase. The plants then
produce more fruit.
COTYLEDON EXPANSION
CYTOKININ FUNCTION
●Promotes expansion in
cotyledons
●Extends the cell wall
●Differs from auxin expansion
○Unlike auxin, no proton
extrusion through the cell wall
happens
APICAL DOMINANCE
CYTOKININ FUNCTION
●Antagonistic hormone interaction
between cytokinin and auxin
○Cytokinin stimulates growth in
auxiliary buds
Inhibits shoot elongation
○Auxin restrains growth in
auxiliary buds
Causes shoot to lengthen.
Mutants that overproduce
cytokinins in lateral
meristems are bushy.
SEEDLING MORPHOLOGY
CYTOKININ FUNCTION
●Etiolated leaves treated with cytokinins
produce more active/productive
chloroplasts upon illumination
●Dark-germinated seedlings treated with
cytokinins
○Shortened hypocotyls
○Expanded cotyledons
Partial development of etioplasts into
chloroplasts
SEEDLING MORPHOLOGY
CYTOKININ FUNCTION
1 (control)
2 (most
concentrated)
3
4 (most
dilute)
Average
Hypocotyle
(mm)
8.8
2.9
5.4
8.1
Standard
Deviation
(mm)
1.398
0.567
0.699
0.875
Molarity
(mol/L)
none
4.651x10-4
4.651x10-5
4.651x 10-6
REFERENCES
CYTOKININS
●Ashikari, Motoyuki. “Cytokinin Oxidase Regulates Rice Grain Production.”
Science. 23 June 2005. 1 May. 2009
<http://www.sciencemag.org/cgi/content/abstract/309/5735/741>
●Davies, Peter J. ed. Plant Hormones. Boston: Kluwer Academic Publishers,
2004
●Ma, Qing-Hu. “Genetic Engineering of Cytokinins and Their Application to
Agriculture.” Critical Reviews in Biotechnology. 28.3 (2008)
InformaWorld. University of California Santa Cruz Lib., Santa Cruz, CA, 1 May.
2009 <http://www.informaworld.com/smpp/sectioncontent=a904079709&full
text=713240928>
●Mok, David W. S. , Machteld C. Mok, eds. Cytokinins Chemistry, Activity, and
Function. Boca Raton:CRC Press Inc, 1994
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