Cytokinins

Michael Riefler, Institute for Biology/Applied Genetics, Berlin, Germany
Tomáš Werner, Institute for Biology/Applied Genetics, Berlin, Germany
Alexander Heyl, Institute for Biology/Applied Genetics, Berlin, Germany

Published online: January 2007

DOI: 10.1002/9780470015902.a0020095

Cytokinins are a class of plant growth regulators that play an important role in many physiological and developmental processes in the plant, such as regulation of cell division, leaf senescence, nutrient uptake and pathogen resistance. Recent years have witnessed rapid progress in our understanding of several aspects of cytokinin biology, especially cytokinin signalling and metabolism.

Keywords: cytokinin; two-component system; CKX; IPT; Zeatin; cell cycle

Figure 1. Structure of several cytokinins.
Figure 2. Model of cytokinin biosynthesis. tZ can be produced via the iPMP-dependent pathway or the alternative iPMP-independent pathway. However, tZ might also be produced via an isomerization of cZ. Abbreviations used: cZ, cis-zeatin; DMAPP, dimethylallyl diphosphate; HMBPP, hydroxymethylbutenyl diphosphate; iP, isopentenyladenine; iPA, isopentenyladenosine; iPMP, isopentenyladenosine-5¢-monophosphate; iPDP, isopentenyladenosine-5¢-diphosphate; iPTP, isopentenyladenosine-5¢-triphosphate; IPT, isopentenyltransferase; tZ, trans-zeatin; tZR, trans-zeatin riboside; tZMP, tZR-5¢-monophosphate; tZDP, tZR-5¢-diphosphate; tZTP, tZR-5’-triphosphate.
Figure 3. Model of cytokinin signalling via the complex TCS. Cytokinin binds to the cytokinin receptor, which might act as a dimer, and induces autophosphorylation. The phosphoryl group is then transferred to the histidine residue of a phospho-transmitter protein (AHP). The phosphorylated AHP moves into the nucleus and transfers the phosphoryl group to type-B and type-A ARRs. The activated type-B ARRs can induce transcription of type-A ARRs rapidly. Type-A ARRs modulate several cytokinin responses. Additionally they negatively regulate on the signal transduction cascade via the AHPs. C, cytokinin; D, aspartate residue, H, histidine residue; P, phosphoryl group.
Figure 4. Arabidopsis hypocotyl tissue cultures. Hypocotyls were cultured on growth media supplemented with different hormones for 4 weeks. (a) Hypocotyls grown on hormone free media remain unchanged; (b) 100 ng/mL–1 naphtyl acetic acid (=NAA) and 1000 ng/mL–1 isopentenyladenine (=iP) proliferate and differentiate shoots; (c) hypocotyls grown on media supplemented with 1000 ng/mL–1 NAA and 100 ng/mL–1 iP proliferate and differentiate roots.
Figure 5. Phenotype of Arabidopsis thaliana wild type and cytokinin receptor mutant ahk2 ahk3 cre1/ahk4. (a) Three-week old Arabidopsis plants, wild type (upper plant) and ahk2 ahk3 cre1/ahk4 (lower plant); (b) wild-type seeds; (c) seeds of ahk2 ahk3 cre1/ahk4.
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 References
    Ashikari M, Sakakibara H, Lin S et al. (2005) Cytokinin oxidase regulates rice grain production. Science 309: 741–745.
    book Heyl A, Werner T and Schmülling T (2006) "Cytokinin metabolism and signal transduction". In: Hedden P and Thomas S (eds) Plant Hormone Signaling, Annual Plant Reviews, pp. 93–123. Oxford: Blackwell Publishing Ltd
    Hwang I and Sheen J (2001) Two-component circuitry in Arabidopsis cytokinin signal transduction. Nature 413: 383–389.
    Inoue T, Higuchi M, Hashimoto Y et al. (2001) Identification of CRE1 as a cytokinin receptor from Arabidopsis. Nature 409: 1060–1063.
    Mähönen AP, Higuchi M, Tormakangas K et al. (2006) Cytokinins regulate a bidirectional phosphorelay network in Arabidopsis. Current Biology 16: 1116–1122.
    Riefler M, Novak O, Strnad M and Schmülling T (2006) Arabidopsis cytokinin receptor mutants reveal functions in shoot growth, leaf senescence, seed size, germination, root development and cytokinin metabolism. Plant Cell 18: 40–54.
    book Taiz L and Zeiger E (2002) Plant Physiology, 3rd edition. Sunderland, MA: Sinauer Associates, Inc.
    To JP, Haberer G, Ferreira FJ et al. (2004) Type-A Arabidopsis response regulators are partially redundant negative regulators of cytokinin signaling. Plant Cell 16: 658–671.
    Werner T, Motyka V, Laucou V et al. (2003) Cytokinin-deficient transgenic Arabidopsis plants show multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristem activity. Plant Cell 15: 532–550.
 Further Reading
    Asakura Y, Hagino T, Ohta Y et al. (2003) Molecular characterization of His-Asp phosphorelay signaling factors in maize leaves: implications of the signal divergence by cytokinin-inducible response regulators in the cytosol and the nuclei. Plant Molecular Biology 52: 331–341.
    Hass C, Lohrmann J, Albrecht V et al. (2004) The response regulator 2 mediates ethylene signalling and hormone signal integration in Arabidopsis. EMBO Journal 3: 3290–3302.
    Higuchi M, Pischke MS, Mahonen AP et al. (2004) In planta functions of the Arabidopsis cytokinin receptor family. Proceedings of the National Academy of Sciences of the USA 101: 8821–8826.
    Kim HJ, Ryu H, Hong SH et al. (2006) Cytokinin-mediated control of leaf longevity by AHK3 through phosphorylation of ARR2 in Arabidopsis. Proceedings of the National Academy of Sciences of the USA 103: 814–819.
    Leibfried A, To JP, Busch W et al. (2005) WUSCHEL controls meristem function by direct regulation of cytokinin-inducible response regulators. Nature 438: 1172–1175.
    Mähönen AP, Bishopp A, Higuchi M et al. (2006) Cytokinin signaling and its inhibitor AHP6 regulate cell fate during vascular development. Science 311: 94–98.
    Mason MG, Li J, Mathews DE et al. (2004) Type-B response regulators display overlapping expression patterns in Arabidopsis. Plant Physiology 135: 927–937.
    Mok DW and Mok MC (2001) Cytokinin metabolism and action. Annual Review of Plant Physiology and Plant Molecular Biology 52: 89–118.
    Nishimura C, Ohashi Y, Sato S et al. (2004) Histidine kinase homologs that act as cytokinin receptors possess overlapping functions in the regulation of shoot and root growth in Arabidopsis. Plant Cell 16: 1365–1377.
    Tajima Y, Imamura A, Kiba T et al. (2004) Comparative studies on the type-B response regulators revealing their distinctive properties in the His-to-Asp phosphorelay signal transduction of Arabidopsis thaliana. Plant & Cell Physiology 45: 28–39.
    Takei K, Sakakibara H and Sugiyama T (2001) Identification of genes encoding adenylate isopentenyltransferase, a cytokinin biosynthesis enzyme, in Arabidopsis thaliana. The Journal of Biological Chemistry 276: 26405–26410.

Related Sub-Topics:

Intracellular and Extracellular Signalling | Plant Secondary Metabolism | Plant Development
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Article Title: Cytokinins
 
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Riefler, Michael, Werner, Tomáš, and Heyl, Alexander(Jan 2007) Cytokinins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020095]