Cytokinin
496669
220985308
2008-06-22T14:55:43Z
Million Moments
2907231
biosynthesis
[[Image:ZeatinII.png|thumb|right|122px|The cytokinin [[zeatin]] is named after the genus of corn, ''[[Teosinte|Zea]]'', in which it was first discovered.]]
'''Cytokinins''' (CK) are a class of [[plant growth substance]]s ([[plant]] [[hormone]]s) active in promoting [[cell division]], and are also involved in [[Cell (biology)|cell]] growth, [[cellular differentiation|differentiation]], and other [[physiology|physiological]] processes. Their effects were first discovered through the use of [[coconut milk]] in the 1940s by a scientist at the [[University of Wisconsin-Madison]] named [[Folke Skoog]].
There are two kinds of cytokinins, adenine-type cytokinins including [[kinetin]], [[zeatin]] and [[6-benzylaminopurine]] and phenylurea-type cytokinins like [[diphenylurea]] or [[thidiazuron]] (TDZ). There is no evidence that any phenylurea cytokinins occur naturally in plant tissues.<ref> Mok, DWS and Mok, MC. 2001. Cytokinin metabolism and action. ''Annual Review of Plant Physiology and Plant Molecular Biology'' 52: 89-118 </ref> Adenine-type cytokinins are synthesised in stems, leaves and roots, but the root is the major site; furthermore [[cambium]] and possibly all actively dividing tissues are responsible for the synthesis of this group of plant hormones.<ref>Chen, C. et al. 1985. Localization of Cytokinin Biosynthetic Sites in Pea Plants and Carrot Roots. ''Plant Physiology'' 78:510–513.</ref> Cytokinin is involved in both local and long distance signalling; as a long distance signal CK shares the same transport systems used by the plant for moving purines and nucleosides.<ref>Sakakibara, H. 2006. Cytokinins: Activity, Biosynthesis, and Translocation. ''Annual Review of Plant Biology'' 57: 431-449 </ref>
Cytokinins are involved in many plant processes, including cell division, shoot and root morphogenesis, [[chloroplast]] maturation, cell enlargement, auxiliary bud release and [[senescence]].<ref>Kieber JJ (2002 Cytokinins. In CR Somerville, EM Meyerowitz, eds, [www.aspb.org/publications/arabidopsis/ The Arabidopsis Book]. American Society of Plant Biologists, Rockville, MD, doi: 10.1199/tab.0009</ref> The ratio of auxin to cytokinin is crucial during cell division and the differentiation of plant tissues and auxin is known to regulate the biosynthesis of cytokinin.<ref>Nordström, A. 2004. Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: A factor of potential importance for auxin–cytokinin-regulated development. ''PNAS'' 101:8039–8044</ref>
==Biosythesis==
Adenosine phosphate-isopentenyltransferase (IPT) is the [[enzyme]] that [[catalyst|catalyses]] the first reaction in the [[biosynthesis]] of isoprene cytokinins. It may use any of the [[phosphorylation|phosphorylated]] versions of the nucleotide base [[adenine]] ([[Adenosine triphosphate|ATP]], [[Adenosine diphosphate|ADP]] or [[Adenosine monophosphate|AMP]]) as a [[substrate]] and may use [[dimethylallyl diphosphate]] (DMAPP) or [[hydroxymethylbutenyl diphosphate]] (HMBDP) as [[Prenylation|prenyl donors]].<ref name="biosynthesis">Ildoo Hwang, Hitoshi Sakakibara (2006) Cytokinin biosynthesis and perception Physiologia Plantarum 126 (4), 528–538</ref> This is also the [[Rate-determining step|rate limiting step]] of cytokinin biosynthesis [13] DMAPP and HMBDP are produced from the [[methylerythritol phosphate pathway]] (MEP).<ref name="biosynthesis" />
Cytokinins can also be produced by recycled [[tRNA]]s in plants and bacteria.<ref name="biosynthesis" /><ref name="ipts">Kaori Miyawaki, Miho Matsumoto-Kitano, Tatsuo Kakimoto (2004) Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate The Plant Journal 37 (1), 128–138</ref> Some tRNAs with [[anticodon]]s that start with a [[uridine]] carry an already prenylated adenosine adjacent to the anticodon.<ref name="biosynthesis" /> When the tRNA is degraded the adenosine is released as a cytokinin. The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.<ref name="ipts" />
==References==
{{reflist}}
==External links==
* [http://4e.plantphys.net/chapter.php?ch=21 Chapter 21 of Plant Physiology Book: Cytokinins: Regulators of Cell Division]
* [http://employees.csbsju.edu/ssaupe/biol327/Lecture/cytokinin.htm Plant Physiology:Cytokinin]
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{{Plant_hormones}}
[[Category:Plant hormones]]
[[Category:Cytokinins| ]]
[[cs:Cytokinin]]
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[[es:Citoquinina]]
[[fi:Sytokiniini]]
[[fr:Cytokinine]]
[[he:ציטוקינין]]
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[[ja:サイトカイニン]]
[[pl:Cytokininy]]
[[tr:Sitokinin]]