LECTURE OUTLINE
                            CYTOKININS



● Cytokinins
   ○   Discovery
   ○   Chemical Structure
   ○   B...
DISCOVERY
        CYTOKININS




● Haberlandt (1913) – compound in
  phloem stimulates cell division
● Van Overbeek (1941)...
DISCOVERY
                          CYTOKININS


● Miller (1955) – first cytokinin isolated from herring sperm,
  named ki...
CHEMICAL STRUCTURE
                        CYTOKININS


● Adenine structure (amino purine ring)
● N6 Side chain
   ○ Degre...
CHEMICAL STRUCTURE
       CYTOKININS




  ● Free base forms are most
    hormonally active (Yamada et al.
    2001)
  ● M...
BIOSYNTHESIS
                         CYTOKININS


● Biosynthesis started by the IPT (isopentenyl tranferase)
  enzyme
● M...
BIOSYNTHESIS
BIOSYNTHESIS
   CYTOKININS




      ● tRNA breakdown not the
        major pathway for cytokinin
        synthesis
BIOSYNTHESIS
                              CYTOKININS


● Generated mostly in the root apical meristems but also
  found i...
BIOSYNTHESIS
   CYTOKININS



     ● Other organisms make
       cytokinins to influence the
       plant for their own be...
TRANSPORT
                        CYTOKININS




● Cytokinins move up the plant through the xylem
● By contrast, auxin mov...
SIGNALING
SIGNALING
                            CYTOKININS


● Cytokinin changes the structure of the His Kinase
  domain
   ○ Phosp...
FUNCTIONS
                          CYTOKININS




● FUNCTIONS
  ○   Cell division
  ○   Root and shoot meristems
  ○   Ce...
CELL DIVISION
CYTOKININ FUNCTION



        ● Auxin and cytokinins influence
          the activity of:
           ○ Cycli...
ROOT AND SHOOT MERISTEMS
      CYTOKININ FUNCTION


                 ● Optimal levels of
                   cytokinins are...
ROOT AND SHOOT MERISTEMS
      CYTOKININ FUNCTION
CELL DIFFERENTIATION
            CYTOKININ FUNCTION



Bacteria: Agrobacterium tumefaciens
CELL DIFFERENTIATION
                        CYTOKININ FUNCTION


●   Auxin:cytokinin affects cell differentiation in call...
LEAF SENESCENCE
  CYTOKININ FUNCTION




     ● Cytokinin delays leaf
       senescence
         ○ Delay the degradation o...
LEAF SENESCENCE
  CYTOKININ FUNCTION




          ● Delayed leaf senescence
            help plants recover from
        ...
INFLORESCENCE GROWTH
     CYTOKININ FUNCTION




       ● Cytokinins induce division in
         inflorescence tips
      ...
CYTOKININ-INDUCED NUTRIENT MOBILIZATION
                          CYTOKININ FUNCTION


 ● Causes a new source-sink relatio...
COTYLEDON EXPANSION
    CYTOKININ FUNCTION


            ● Promotes expansion in
              cotyledons
            ● Ex...
APICAL DOMINANCE
  CYTOKININ FUNCTION



          ● Antagonistic hormone
            interaction between
            cyto...
SEEDLING MORPHOLOGY
    CYTOKININ FUNCTION




     ● Etiolated leaves treated with
       cytokinins produce more
       ...
SEEDLING MORPHOLOGY
                           CYTOKININ FUNCTION




            1 (control)    2 (most          3       ...
REFERENCES
                                CYTOKININS


● Ashikari, Motoyuki. “Cytokinin Oxidase Regulates Rice Grain Prod...
Cytokinins
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Cytokinins

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Cytokinins

  1. 1. LECTURE OUTLINE CYTOKININS ● Cytokinins ○ Discovery ○ Chemical Structure ○ Biosynthesis ○ Transport ○ Signaling ○ Functions
  2. 2. 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
  3. 3. DISCOVERY CYTOKININS ● Miller (1955) – first cytokinin isolated from herring sperm, named kinetin ● Miller (1961) – first naturally occurring cytokinin found in plants, later called zeatin
  4. 4. CHEMICAL STRUCTURE CYTOKININS ● Adenine structure (amino purine ring) ● N6 Side chain ○ Degree of unsaturation ○ Number of carbons in side chain
  5. 5. 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
  6. 6. BIOSYNTHESIS CYTOKININS ● Biosynthesis started by the IPT (isopentenyl tranferase) enzyme ● Major pathway ○ Figure 21.6 ● Other pathway ○ tRNA biosynthesis
  7. 7. BIOSYNTHESIS
  8. 8. BIOSYNTHESIS CYTOKININS ● tRNA breakdown not the major pathway for cytokinin synthesis
  9. 9. 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
  10. 10. BIOSYNTHESIS CYTOKININS ● Other organisms make cytokinins to influence the plant for their own benefit ○ Bacteria ○ Fungi ○ Insects ○ Nematodes
  11. 11. 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).
  12. 12. SIGNALING
  13. 13. SIGNALING 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  cytokinin responses
  14. 14. FUNCTIONS CYTOKININS ● FUNCTIONS ○ Cell division ○ Root and shoot meristems ○ Cell differentiation ○ Leaf senescence ○ Inflorescence growth ○ Nutrient mobilization ○ Cotyledon expansion ○ Apical dominance ○ Seedling morphology
  15. 15. 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
  16. 16. 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
  17. 17. ROOT AND SHOOT MERISTEMS CYTOKININ FUNCTION
  18. 18. CELL DIFFERENTIATION CYTOKININ FUNCTION Bacteria: Agrobacterium tumefaciens
  19. 19. CELL DIFFERENTIATION CYTOKININ FUNCTION ● Auxin:cytokinin affects cell differentiation in callus tissue ● More auxin  roots ● More cytokinin  shoots ● Skoog and Miller (1965)
  20. 20. LEAF SENESCENCE CYTOKININ FUNCTION ● Cytokinin delays leaf senescence ○ Delay the degradation of chloroplasts ○ May increase the growing season for agricultural purposes.
  21. 21. LEAF SENESCENCE CYTOKININ FUNCTION ● Delayed leaf senescence help plants recover from environmental stresses ○ Drought (Rivero et al. 2007) ○ Flooding (Zhang et al. 2000) (Hyunh et al. 2005)
  22. 22. 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.
  23. 23. CYTOKININ-INDUCED NUTRIENT MOBILIZATION CYTOKININ FUNCTION ● Causes a new source-sink relationship in the growing parts in the plant ● If a plant needs more nutrients to grow, cytokinins accumulate in the root zone to stimulate growth. ○ Nitrogen is often the limiting nutrient most sought after
  24. 24. 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
  25. 25. 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. ● This relationship was also shown in our experiment.
  26. 26. 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
  27. 27. SEEDLING MORPHOLOGY CYTOKININ FUNCTION 1 (control) 2 (most 3 4 (most dilute) concentrated) Average 8.8 2.9 5.4 8.1 (mm) Standard 1.398 0.567 0.699 0.875 Deviation (mm) Molarity none 4.651 x 10-4 4.651 x 10-5 4.651 x 10-6 (mol/L) • These effects are positively correlated with cytokinin solution strength.
  28. 28. 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 Thanks for listening!
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