cytokinins biosynthesis, signaling pathways, functions and cross talk with auxins arshad mahmood...
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![Page 1: Cytokinins Biosynthesis, Signaling Pathways, Functions and Cross talk with Auxins ARSHAD MAHMOOD KHAN arshadbotanist@gmail.com PhD Scholar (13-arid-3581)](https://reader035.vdocument.in/reader035/viewer/2022062217/56649ec55503460f94bd0431/html5/thumbnails/1.jpg)
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Cytokinins Biosynthesis, Signaling Pathways, Functions and Cross talk with Auxins
ARSHAD MAHMOOD KHAN
PhD Scholar (13-arid-3581)
BOTANY DEPARTMENT
UAAR RAWALPINDI
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LECTURE OUTLINE
CYTOKININS
● Cytokinins ○ Discovery ○ Chemical Structure ○ Biosynthesis ○ Transport ○ Signaling ○ Functions
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DISCOVERYCYTOKININS
• 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
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●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)Lathum & Guillfoyle (1974)
ZeatinZeatin
DISCOVERYCYTOKININS
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CHEMICAL STRUCTURECYTOKININS
●Adenine structure (amino purine ring) ●N6 Side chain
○Degree of unsaturation ○Number of carbons in side chain
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CHEMICAL STRUCTURECYTOKININS
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
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BIOSYNTHESISCYTOKININS
Biosynthesis started by the IPT (isopentenyl tranferase) enzyme
Major pathway Figure on slide no. 9
Other pathway
tRNA biosynthesis Figure on slide no. 10
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BIOSYNTHESIS
Plant physiology, 4th E
di. Fig. 21.6, 2006 Sinauer A
ssociate. Inc
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BIOSYNTHESIStRNA breakdown not the major pathway for
cytokinin synthesis
Plant physiology, 4th E
di. Fig. 21.7, 2006 Sinauer A
ssociate. Inc
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BIOSYNTHESISCYTOKININS
●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
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TRANSPORTCYTOKININS
●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).
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SIGNALING PATHWAY
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Cytokinins perception and pathwayCytokinin 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.
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• 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
Cytokinins perception and pathway
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●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
SIGNALING SUMMARY CYTOKININS
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FUNCTIONSCYTOKININS
●FUNCTIONS ○Cell division ○Root and shoot meristems ○Cell differentiation ○Leaf senescence ○Inflorescence growth ○Nutrient mobilization ○Cotyledon expansion ○Apical dominance ○ Seedling morphology
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● 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
CELL DIVISIONCYTOKININ FUNCTION
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Fig. 15-7, p. 241
cyclin
cyclinsynthesized
aminoacids
cyclin
(active)phosphorylation
ATP?GTP?
(inactive)
cyclindegraded
dephosphorylationtriggers mitosis
or DNA synthesis
P
cyclin
C-PK
CDK CDK CDK
26S Proteosome
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● 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 MERISTEMSCYTOKININ FUNCTION
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ROOT AND SHOOT MERISTEMS
CYTOKININ FUNCTION
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Fig. 15-10, p. 244
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CELL DIFFERENTIATIONCYTOKININ FUNCTION
Bacteria: Agrobacterium tumefaciens
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●Auxin:cytokinin affects cell differentiation in callus tissue ●More auxin leads to roots development●More cytokinin leads to shoots development
●Skoog and Miller (1965)
CELL DIFFERENTIATIONCYTOKININ FUNCTION
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LEAF SENESCENCECYTOKININ FUNCTION
Cytokinin delays leaf senescence Delay the degradation of chloroplasts May increase the growing season for agricultural purposes.
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LEAF SENESCENCECYTOKININ FUNCTION
• 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
Control Sprayed
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●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.
INFLORESCENCE GROWTHCYTOKININ FUNCTION
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●Promotes expansion in cotyledons ●Extends the cell wall ●Differs from auxin expansion
○Unlike auxin, no proton extrusion through the cell wall happens
COTYLEDON EXPANSIONCYTOKININ FUNCTION
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●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.
APICAL DOMINANCECYTOKININ FUNCTION
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●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 MORPHOLOGYCYTOKININ FUNCTION
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SEEDLING MORPHOLOGYCYTOKININ 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
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●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
REFERENCESCYTOKININS
Thanks for listening!
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