Discovery:

Haberlandt (1915) found soluble substance in vascular tissue that stimulated division of potato tuber tissue.

Skoog (1955) autoclaved herring sperm DNA stimulated mitosis in tobacco tissue culture - due to a degradation product of DNA: Kinetin (synthetic cytokinin).

Letham (1974) Zeatin. Naturally occurring cytokinin isolated from coconut milk (liquid endosperm) and maize endosperm.  Coconut milk been known for many years to stimulate cell division in tissue culture (in presence of auxin).

Biosynthesis: See Figs 21.2 and 21.3 for structures of kinetin and zeatin.

Cytokinins synthesized from the terpenoid isopentenyl pyrophosphate (IPP) and adenosine monophosphate (AMP).  See Fig. 21.8.  Cytokinin synthase catalyzes this first step in biosynthetic pathway.

Transport:

Occur as free molecules throughout plants and algae.  Also as modified bases in some tRNA molecules.  Cytokinin conjugates are also present e.g., glucosyl zeatin.  Cytokinin oxidase responsible for degradation of cytokinins but probably does not affect conjugates. Conjugates facilitate storage and transport.  Also cytokinin oxidase does not degrade synthetic forms (e.g., kinetin and benzylaminopurine - BAP).

Synthesized in tissues with high meristematic activity (cambium, apical meristem regions, young leaves).  Root apex main site of production in young seedlings.  Move in xylem transpiration to shoot.  Drought or damage to root influences (reduces) cytokinins in transpiration stream.  Movement of cytokinins in xylem is passive process - usually the cytokinins are moved in inactive form.

Physiological Roles:  As with most plant hormones, cytokinins work in concert with other hormones.

Cell Cycle Regulation:  G1-phase of cell cycle: increase in cell mass;  S-phase: replication of DNA.

Two major control points in plant cell cycle.  One regulates start of DNA replication, other start of mitosis.  If plant cell goes through DNA replication without mitosis then becomes polyploid (does not occur in animal cells).

• Auxin regulates events leading to DNA replication
• Cytokinin regulates events leading to mitosis.

Thus, in tobacco tissue culture auxin alone causes DNA replication but does not allow mitosis.  Cytokinin alone allows cells to divide (mitosis/cytokinesis) if they already had DNA replicated.  To cause both need correct ratio of auxin and zeatin (kinetin).

Regulation of Morphogenesis in Culture:

• High auxin: kinetin ratio - root formation
• High kinetin: auxin ratio - shoot formation

Experiments: mutations in the Ti plasmid in gall-forming bacterium Agrobacterium (this bacterium is used to transform cells).  Transforms plant tissues.  “Shooty” mutations if has iinactivated auxin synthesis genes.  “Rooty” mutation if has inactivated zeatin biosynthesis genes.

Delay Senescence: Treating detached leaves with cytokinin delays senescence: ie slows the loss of Chl, RNA, protein, lipids.  Also works on whole plants sprayed with kinetin.  How?  Cytokinin-treated plant parts have nutrients preferentially transported to them.  Seems to alter the source-sink relationships.  Postulate: senescence associated with low [cytokinin].

Maturation of chloroplasts: Dark and light-grown seedlings differ in morphology.  Etiolation in dark, and have etioplasts.  Etioplasts develop into chloroplasts if exposed to light.  But even in dark they form chloroplasts if treated with cytokinin, but need light for complete development of Chl and enzymes.

Cell Expansion: Also have role in cell enlargement in some tissues/organs, especially dicots.  Similar to auxin in that it increases plasticity of wall so turgor pressure can enlarge cell.  But unlike auxin, it is not due to increased acidification.
What hormones affect cell size?
NOTE: if use stem segments of soybean hypocotyl then cytokinin inhibits auxin-induced cell elongation.  This shows have complex interactions.

Mechanism of Action:

• Regulation of protein synthesis.  Increase rate of protein synthesis and change the specific proteins being made.  Stimulates synthesis of some, inhibits others
• Slows degradation of mRNA for certain proteins e.g., small subunit of Rubisco and Chl a/b binding protein
• Cytokinins in special tRNA molecules may play role in determing type of protein made
• Cytokinins can influence [Ca²⁺] in cell.  In moss Funaria cytokinin promotes uptake of  Ca²⁺ by cell and increase cytosolic levels.  Ca²⁺ may act as second messenger in signal transduction pathway initiated by cytokinin.  Ca²⁺ may be a biochemical switch to transform hormone signal so regulates for example bud formation
• Specific cytokinin binding proteins have been identified (CBF-1) in cereal grains.  May be ribosomal proteins which regulate protein synthesis.

Link to Brassinosteroids