Cytokinins are plant hormones that promote cell division. They were first discovered in 1913 and isolated in the 1950s. There are two main types - adenine-type and phenylurea-type. Cytokinins are produced in roots, fruits, young leaves and developing tissues, and move through xylem and phloem. They stimulate cell division, promote chloroplast maturation, stimulate bud growth, and delay senescence.

1. CYTOKININS

2. Definition
• Cytokinins (CK) are a class of plant growth
substances (phyto hormones) that promote
cell division, in plant roots and shoots.

3. History
1. Discovery:
• 1913, Gottlieb Haberlandt.
• 1941, Johannes van Overbeek
• 1954, Jablonski and Skoog.

4. History
2. Isolation:
• The first cytokinin (kinetin) was isolated in
1955 by Miller.
• The first naturally occurring cytokinin
(zeatin) was isolated in 1961 by Miller.

5. Basic chemistry
• All cytokinins have
basic Adenine ring
structure.

7. Types
• There are two types of cytokinins:
1. adenine-type cytokinins ; represented by
kinetin, zeatin, and 6-benzylaminopurine.
2. phenylurea-type cytokinins like
diphenylurea and thidiazuron (TDZ) .

8. Presence
• Found in almost all higher plants as well
as:
1. Mosses.
2. Fungi.
3. Bacteria.
4. Algae.
5. Diatoms.

9. Produced
• Cytokinins are produced mainly in the :
1. Roots.
2. Fruits.
3. Young leaves.
4. Developing tissues.

10. Occurrence
• Cytokinins occur in free form or
conjugated form.
• tRNA-cyto are formed in every living cell in
cytoplasm, chloroplast and mitochondria.

11. Biosynthesis Process
1. isopentyl pyrophosphate is isomerized.
2. isomer then react with adenosine
monophosphate with the aid of an
enzyme.
3. isopentenyl adenosine-5'-phosphate
(isopentenyl AMP) is obtained.
4. isopentenyl adenosine is produced by
the removal of phosphate.
5. further converted to isopentenyl adenine.

12. Transport
• Cytokinins move from xylem, phloem, and
parenchyma cells.
• Mainly xylem from root to shoot.

13. Degradation
• Cytokinins are degraded by an enzyme
called cytokinin oxidase.

14. Physiological roles/mode of action
• Cytokinins are 0.01 to 1.0 in plants.
• Cytokinins effect on translation but not on
transcription.
• cytokinins are involved in the regulation of
the cell cycle.

15. Functions
• Stimulates cell division.
• Promote cell expansion.
• Promote Chloroplast maturation.
• Stimulates morphogenesis.
• Stimulates the growth of lateral buds.
• Stimulates leaf expansion.

16. Functions
• Delay of senescence.
• Involved in releasing seed dormancy.