The cytokinin zeatin is named after the genus of corn, Zea, in which it was first discovered.
Cytokinins (CK) are a class of plant growth substances (plant hormones) active in promoting cell division, and are also 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.
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.[1] 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.[2] 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.[3]
Cytokinins are involved in many plant processes, including cell division, shoot and root morphogenesis, chloroplast maturation, cell enlargement, auxiliary bud release and senescence.[4] 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.[5]
Biosynthesis
Adenosine phosphate-isopentenyltransferase (IPT) is the enzyme that catalyses the first reaction in the biosynthesis of isoprene cytokinins. It may use any of the phosphorylated versions of the nucleotide base adenine (ATP, ADP or AMP) as a substrate and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl diphosphate (HMBDP) as prenyl donors.[6] This is also the rate limiting step of cytokinin biosynthesis [13] DMAPP and HMBDP are produced from the methylerythritol phosphate pathway (MEP).[6]
Cytokinins can also be produced by recycled tRNAs in plants and bacteria.[6][7] Some tRNAs with anticodons that start with a uridine carry an already prenylated adenosine adjacent to the anticodon.[6] When the tRNA is degraded the adenosine is released as a cytokinin. The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.[7]
References
- ^ Mok, DWS and Mok, MC. 2001. Cytokinin metabolism and action. Annual Review of Plant Physiology and Plant Molecular Biology 52: 89-118
- ^ Chen, C. et al. 1985. Localization of Cytokinin Biosynthetic Sites in Pea Plants and Carrot Roots. Plant Physiology 78:510–513.
- ^ Sakakibara, H. 2006. Cytokinins: Activity, Biosynthesis, and Translocation. Annual Review of Plant Biology 57: 431-449
- ^ 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
- ^ 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
- ^ a b c d Ildoo Hwang, Hitoshi Sakakibara (2006) Cytokinin biosynthesis and perception Physiologia Plantarum 126 (4), 528–538
- ^ 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
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