The cytokinin zeatin is named after the genus of corn, Zea, in which it was discovered.

Cytokinins (CK) are a class of plant growth substances (plant hormones) that promote cell division. They are primarily involved in cell growth, differentiation, and other physiological processes. Their effects were first discovered through the use of coconut milk in the 1940s by Adicott at the University of Wisconsin-Madison named Folke Skoog. ^[1]

There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin and 6-benzylaminopurine, as well as phenylurea-type cytokinins like diphenylurea or thidiazuron (TDZ). The adenine-type cytokinins are synthesised in stems, leaves and roots, which is the major site. Cambium and possibly other actively dividing tissues are also sites of cytokinin biosynthesis.^[2] There is no evidence that the phenylurea cytokinins occur naturally in plant tissues.^[3] Cytokinins are involved in both local and long distance signalling, the latter of which involves the same in planta transport mechanism as used for transport of purines and nucleosides.^[4]

Mode of Action

Cytokinins are involved in many plant processes, including cell division, shoot and root morphogenesis, chloroplast maturation, cell enlargement, auxiliary bud release and senescence.^[5] The ratio of auxin to cytokinin is crucial during cell division and the differentiation of plant tissues.

While cytokinin action in vascular plants is described as pleiotropic, this class of plant hormones specifically induces the transition from apical growth to growth via a three-faced apical cell in moss protonema. This bud induction can be pinpointed to differentiation of a specific single cell, and thus is a very specific effect of cytokinin. ^[6]

Biosynthesis

Adenosine phosphate-isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins. It may use ATP, ADP or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl diphosphate (HMBDP) as prenyl donors.^[7] This reaction is the rate limiting step in cytokinin biosynthesis. DMAPP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).^[7]

Cytokinins can also be produced by recycled tRNAs in plants and bacteria.^[7][8] tRNAs with anticodons that start with a uridine and carrying an already prenylated adenosine adjacent to the anticodon release on degradation the adenosine as a cytokinin.^[7] The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.^[8]

Auxin is known to regulate the biosynthesis of cytokinin.^[9]

References

External links

• Chapter 21 of Plant Physiology Book: Cytokinins: Regulators of Cell Division
• Plant Physiology:Cytokinin

cs:Cytokinin de:Cytokinine es:Citoquinina fr:Cytokinine ko:???? he:????????? nl:Cytokinine ja:??????? pl:Cytokininy fi:Sytokiniini tr:Sitokinin