Spring.wmf (18300 bytes) Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Plant Hormones - Cytokinins

I. General

II. Chemistry

A. General

B. Synthetic cytokinins

C. Cytokinins and nucleic acids

III. Synthesis

IV. Transport

V. Bioassays/Analysis

A. Bioassay

  1. Callus culture cell proliferation - not used too much because it takes too long
  2. Expansion of radish or cocklebur cotyledons
  3. Inhibition of chlorophyll loss by detached oat leaves during senescence

B. Methods of Analysis – liquid chromatography, mass spectroscopy; radioimmunoassays

VI. Disposal

VII. Actions

A. Control morphogenesis

callus + auxin + no cytokinin little growth of callus
callus + auxin + cytokinin  callus grows well, undifferentiated

B. Crown Gall

C. Regulates the cell cycle/cell division (hence, the name "cytokinins) – especially by controlling the transition from G2 mitosis. This effect is moderated by cyclin-dependent protein kinases (CDK's) and their subunits, cyclins.

D. Delay senescence

The exact mechanism by which this occurs is unclear but likely involves the ability of cytokinins to mobilize nutrients. Application of cytokinin to a leaf will cause it to act as a sink and nutrients will be directed towards it.

E. Greening
    Promotes the light-induced formation of chlorophyll and conversion of etioplasts to chloroplasts (greening process).

F. Promote lateral bud development
    Cytokinin application to dormant buds will cause them to develop. A witches’ broom is caused by a pathogen such as the bacterium Corynebacterium fascians (or A. tumefaciens) that produces cytokinin which, in turn, causes stimulates lateral bud development (branching). These results suggest that apical dominance may be related to cytokinin, too.

    For example, when tobacco cells are infected with the Ti-plasmid that has been modified to possess the heat shock promoter, a heat treatment stimulates the cells to produce increased amounts of cytokinin. These plants exhibit less apical dominance and remain green longer than non-heat treated controls. Thus, these results support the conclusion that senescence and apical dominance are related to cytokinin levels.

G. Promote cell expansion
     Cytokinins stimulate the expansion of cotyledons. This is the basis for the classical bioassay. The mechanism is associated with increased plasticity of the cell wall, not associated with acidification.

VIIII. Mechanism of action - Specific binding sites (receptor) for cytokinin are known. These may be ribosomal proteins. Thus, it is not too surprising that cytokinins have been shown to regulate protein synthesis. 

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Last updated:  01/07/2009     Copyright  by SG Saupe