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cytokinin

 
Dictionary: cy·to·ki·nin   (sī'tə-kī'nĭn) pronunciation
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n.
Any of a class of plant hormones that promote cell division and growth and delay the senescence of leaves.


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Wikipedia: Cytokinin
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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 a scientist 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.[citation needed] 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] The correlation of auxins and cytokinins in the plants is a constant (A/C = const.).

Contents

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

  1. ^ J.J. Kieber (2002): Tribute to Folke Skoog: Recent advances in our understanding of cytokinin biology. Journal of Plant Growth Regulation 21, 1-2. [1]
  2. ^ Chen, C. et al. 1985. Localization of Cytokinin Biosynthetic Sites in Pea Plants and Carrot Roots. Plant Physiology 78:510–513.
  3. ^ Mok, DWS and Mok, MC. 2001. Cytokinin metabolism and action. Annual Review of Plant Physiology and Plant Molecular Biology 52: 89-118
  4. ^ Sakakibara, H. 2006. Cytokinins: Activity, Biosynthesis, and Translocation. Annual Review of Plant Biology 57: 431-449
  5. ^ 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
  6. ^ Eva L. Decker, Wolfgang Frank, Eric Sarnighausen, Ralf Reski (2006): Moss systems biology en route: Phytohormones in Physcomitrella development. Plant Biology 8, 397-406 [2]
  7. ^ a b c d Ildoo Hwang, Hitoshi Sakakibara (2006) Cytokinin biosynthesis and perception Physiologia Plantarum 126 (4), 528–538
  8. ^ a b 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
  9. ^ 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

External links

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Plant hormones

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Cytokinin".  Read more