Register      Login
Functional Plant Biology Functional Plant Biology Society
Plant function and evolutionary biology
EVOLUTIONARY REVIEW

Cytokinins – recent news and views of evolutionally old molecules

Lukáš Spíchal
+ Author Affiliations
- Author Affiliations

A Centre of the Region Haná for Biotechnological and Agricultural Research, Faculty of Science, Palacký University, Šlechtitelů 11, CZ-78371 Olomouc, Czech Republic. Email: lukas.spichal@upol.cz

This paper is part of an ongoing series: ‘The Evolution of Plant Functions’.

Functional Plant Biology 39(4) 267-284 https://doi.org/10.1071/FP11276
Submitted: 14 December 2011  Accepted: 6 March 2012   Published: 24 April 2012

Abstract

Cytokinins (CKs) are evolutionally old and highly conserved low-mass molecules that have been identified in almost all known organisms. In plants, they evolved into an important group of plant hormones controlling many physiological and developmental processes throughout the whole lifespan of the plant. CKs and their functions are, however, not unique to plants. In this review, the strategies and mechanisms of plants – and phylogenetically distinct plant-interacting organisms such as bacteria, fungi, nematodes and insects employing CKs or regulation of CK status in plants – are described and put into their evolutionary context. The major breakthroughs made in the last decade in the fields of CK biosynthesis, degradation and signalling are also summarised.

Additional keywords: biosynthesis, degradation, metabolism, signaling.


References

Abe I, Tanaka H, Abe T, Noguchi H (2007) Enzymatic formation of unnatural cytokinin analogs by adenylate isopentenyltransferase from mulberry. Biochemical and Biophysical Research Communications 355, 795–800.
Enzymatic formation of unnatural cytokinin analogs by adenylate isopentenyltransferase from mulberry.Crossref | GoogleScholarGoogle Scholar |

Aguinaldo AMA, Turbeville JM, Linford LS, Rivera MC, Garey JR, Raff RA, Lake JA (1997) Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387, 489–493.
Evidence for a clade of nematodes, arthropods and other moulting animals.Crossref | GoogleScholarGoogle Scholar |

Akiyoshi DE, Klee H, Amasino RM, Nester EW, Gordon MP (1984) T-DNA of Agrobacterium tumefaciens encodes an enzyme of cytokinin biosynthesis. Proceedings of the National Academy of Sciences of the United States of America 81, 5994–5998.
T-DNA of Agrobacterium tumefaciens encodes an enzyme of cytokinin biosynthesis.Crossref | GoogleScholarGoogle Scholar |

Anantharaman V, Iyer LM, Aravind L (2007) Comparative genomics of protists: new insights into the evolution of eukaryotic signal transduction and gene regulation. Annual Review of Microbiology 61, 453–475.
Comparative genomics of protists: new insights into the evolution of eukaryotic signal transduction and gene regulation.Crossref | GoogleScholarGoogle Scholar |

Anjard C, Loomis WF (2008) Cytokinins induce sporulation in Dictyostelium. Development 135, 819–827.
Cytokinins induce sporulation in Dictyostelium.Crossref | GoogleScholarGoogle Scholar |

Arata Y, Nagasawa-Iida A, Uneme H, Nakajima H, Kakimoto T, Sato R (2010) The phenylquinazoline compound S-4893 is a non-competitive cytokinin antagonist that targets Arabidopsis cytokinin receptor CRE1 and promotes root growth in Arabidopsis and rice. Plant & Cell Physiology 51, 2047–2059.
The phenylquinazoline compound S-4893 is a non-competitive cytokinin antagonist that targets Arabidopsis cytokinin receptor CRE1 and promotes root growth in Arabidopsis and rice.Crossref | GoogleScholarGoogle Scholar |

Armstrong DJ (1994) Cytokinin oxidase and the regulation of cytokinin degradation. In ‘Cytokinins. Chemistry, activity and function’. (Ed. DWS Mok, MC Mok) pp. 139–154. (CRC Press: Boca Raton)

Ashikari M, Sakakibara H, Lin S, Yamamoto T, Takashi T, Nishimura A, Angeles ER, Qian Q, Kitano H, Matsuoka M (2005) Cytokinin oxidase regulates rice grain production. Science 309, 741–745.
Cytokinin oxidase regulates rice grain production.Crossref | GoogleScholarGoogle Scholar |

Åstot C, Doležal K, Nordström A, Wang Q, Kunkel T, Moritz T, Chua NH, Sandberg G (2000) An alternative cytokinin biosynthesis pathway. Proceedings of the National Academy of Sciences of the United States of America 97, 14778–14783.
An alternative cytokinin biosynthesis pathway.Crossref | GoogleScholarGoogle Scholar |

Bae E, Bingman CA, Bitto E, Aceti DJ, Phillips GN (2008) Crystal structure of Arabidopsis thaliana cytokinin dehydrogenase. Proteins 70, 303–306.
Crystal structure of Arabidopsis thaliana cytokinin dehydrogenase.Crossref | GoogleScholarGoogle Scholar |

Barnes MF, Tien CL, Gray JS (1980) Biosynthesis of cytokinins by potato cell cultures. Phytochemistry 19, 409–412.
Biosynthesis of cytokinins by potato cell cultures.Crossref | GoogleScholarGoogle Scholar |

Barry GF, Rogers SG, Fraley RT, Brand L (1984) Identification of a cloned cytokinin biosynthetic gene. Proceedings of the National Academy of Sciences of the United States of America 81, 4776–4780.
Identification of a cloned cytokinin biosynthetic gene.Crossref | GoogleScholarGoogle Scholar |

Bartrina I, Otto E, Strnad M, Werner T, Schmülling T (2011) Cytokinin regulates the activity of reproductive meristems, flower organ size, ovule formation, and thus seed yield in Arabidopsis thaliana. The Plant Cell 23, 69–80.
Cytokinin regulates the activity of reproductive meristems, flower organ size, ovule formation, and thus seed yield in Arabidopsis thaliana.Crossref | GoogleScholarGoogle Scholar |

Bassil NV, Mok D, Mok MC (1993) Partial purification of a cis-trans-isomerase of zeatin from immature seed of Phaseolus vulgaris L. Plant Physiology 102, 867–872.

Beattie GA (2007) Plant-associated bacteria: survey, molecular phylogeny, genomics and recent advances. In ‘Plant-associated bacteria’. (Ed. SS Gnanamanickam) pp. 1–56. (Springer: Berlin)

Beinsberger SEI, Valcke RLM, Deblaere RY, Clijsters HMM, De Greef JA, Van Onckelen H (1991) Effects of the introduction of Agrobacterium tumefaciens T-DNA ipt gene in Nicotiana tabaccum L. cv. Petit Havana Sr1 plant cels. Plant & Cell Physiology 32, 489–496.

Bilyeu KD, Cole JL, Laskey JG, Riekhof WR, Esparza TJ, Kramer MD, Morris RO (2001) Molecular and biochemical characterization of a cytokinin oxidase from maize. Plant Physiology 125, 378–386.
Molecular and biochemical characterization of a cytokinin oxidase from maize.Crossref | GoogleScholarGoogle Scholar |

Bird DMcK, Koltai H (2000) Plant parasitic nematodes: habitats, hormones, and horizontally-acquired genes. Journal of Plant Growth Regulation 19, 183–194.

Blackwell JR, Horgan R (1991) A novel strategy for production of a highly expressed in an active form. FEBS Letters 295, 10–12.
A novel strategy for production of a highly expressed in an active form.Crossref | GoogleScholarGoogle Scholar |

Bölker M, Basse CW, Schirawski J (2008) Ustilago maydis secondary metabolism – from genomics to biochemistry. Fungal Genetics and Biology 45, S88–S93.
Ustilago maydis secondary metabolism – from genomics to biochemistry.Crossref | GoogleScholarGoogle Scholar |

Brault M, Caiveau O, Pédron J, Maldiney R, Sotta B, Miginiac E (1999) Detection of membrane-bound cytokinin-binding proteins in Arabidopsis thaliana cells. European Journal of Biochemistry 260, 512–519.
Detection of membrane-bound cytokinin-binding proteins in Arabidopsis thaliana cells.Crossref | GoogleScholarGoogle Scholar |

Brinegar C (1994) Cytokinin-binding proteins and receptors. In ‘Cytokinins. Chemistry, activity and function’. (Ed. DWS Mok, MC Mok) pp. 217–232. (CRC Press: Boca Raton)

Brownlee BG, Hall RH, Whitty CD (1975) 3-Methyl-2-butenal: an enzymatic degradation product of the cytokinin, N-6-(delta-2 isopentenyl)adenine. Canadian Journal of Biochemistry 53, 37–41.
3-Methyl-2-butenal: an enzymatic degradation product of the cytokinin, N-6-(delta-2 isopentenyl)adenine.Crossref | GoogleScholarGoogle Scholar |

Bruce SA, Saville BJ, Emery RJN (2011) Ustilago maydis produces cytokinins and abscisic acid for potential regulation of tumor formation in maize. Journal of Plant Growth Regulation 30, 51–63.
Ustilago maydis produces cytokinins and abscisic acid for potential regulation of tumor formation in maize.Crossref | GoogleScholarGoogle Scholar |

Brugière N, Humbert S, Rizzo N, Bohn J, Habben JE (2008) A member of the maize isopentenyl transferase gene family, Zea mays isopentenyl transferase 2 (ZmIPT2), encodes a cytokinin biosynthetic enzyme expressed during kernel development. Cytokinin biosynthesis in maize. Plant Molecular Biology 67, 215–229.
A member of the maize isopentenyl transferase gene family, Zea mays isopentenyl transferase 2 (ZmIPT2), encodes a cytokinin biosynthetic enzyme expressed during kernel development. Cytokinin biosynthesis in maize.Crossref | GoogleScholarGoogle Scholar |

Brzobohatý B, Moore I, Kristoffersen P, Bako L, Campos N, Schell J, Palme K (1993) Release of active cytokinin by a beta-glucosidase localized to the maize root meristem. Science 262, 1051–1054.
Release of active cytokinin by a beta-glucosidase localized to the maize root meristem.Crossref | GoogleScholarGoogle Scholar |

Bürkle L, Cedzich A, Döpke C, Stransky H, Okumoto S, Gillissen B, Kühn C, Frommer WB (2003) Transport of cytokinins mediated by purine transporters of the PUP family expressed in phloem, hydathodes, and pollen of Arabidopsis. The Plant Journal 34, 13–26.
Transport of cytokinins mediated by purine transporters of the PUP family expressed in phloem, hydathodes, and pollen of Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Burrows WJ, Armstrong DJ, Skoog F, Hecht SM, Boyle JT, Leonard NJ, Occolowitz J (1968) Cytokinin from soluble RNA of Escherichia coli: 6-(3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine. Science 161, 691–693.
Cytokinin from soluble RNA of Escherichia coli: 6-(3-methyl-2-butenylamino)-2-methylthio-9-beta-D-ribofuranosylpurine.Crossref | GoogleScholarGoogle Scholar |

Caesar K, Thamm AMK, Witthöft J, Elgass K, Huppenberger P, Grefen C, Horák J, Harter K (2011) Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum. Journal of Experimental Botany 62, 5571–5580.
Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum.Crossref | GoogleScholarGoogle Scholar |

Caillet J, Droogmans L (1988) Molecular cloning of the Escherichia coli miaA gene involved in the formation of delta 2-isopentenyl adenosine in tRNA. Journal of Bacteriology 170, 4147–4152.

Choi J, Choi D, Lee S, Ryu C-M, Hwang I (2011) Cytokinins and plant immunity: old foes or new friends? Trends in Plant Science 16, 388–394.
Cytokinins and plant immunity: old foes or new friends?Crossref | GoogleScholarGoogle Scholar |

Chu H-M, Ko T-P, Wang AH-J (2010) Crystal structure and substrate specificity of plant adenylate isopentenyltransferase from Humulus lupulus: distinctive binding affinity for purine and pyrimidine nucleotides. Nucleic Acids Research 38, 1738–1748.
Crystal structure and substrate specificity of plant adenylate isopentenyltransferase from Humulus lupulus: distinctive binding affinity for purine and pyrimidine nucleotides.Crossref | GoogleScholarGoogle Scholar |

Cooper JB, Long SR (1994) Morphogenetic rescue of Rhizobium meliloti nodulation mutants by trans-zeatin secretion. The Plant Cell 6, 215–225.

D’Agostino IB, Kieber JJ (1999) Phosphorelay signal transduction: the emerging family of plant response regulators. Trends in Biochemical Sciences 24, 452–456.
Phosphorelay signal transduction: the emerging family of plant response regulators.Crossref | GoogleScholarGoogle Scholar |

D’Agostino IB, Deruere J, Kieber JJ (2000) Characterization of the response of the Arabidopsis response regulator gene family to cytokinin. Plant Physiology 124, 1706–1717.
Characterization of the response of the Arabidopsis response regulator gene family to cytokinin.Crossref | GoogleScholarGoogle Scholar |

De Meutter J, Tytgat T, Witters E, Gheysen G, van Onckelen H, Gheysen G (2003) Identification of cytokinins produced by the plant parasitic nematodes Heterodera schachtii and Meloidogyne incognita. Molecular Plant Pathology 4, 271–277.
Identification of cytokinins produced by the plant parasitic nematodes Heterodera schachtii and Meloidogyne incognita.Crossref | GoogleScholarGoogle Scholar |

Delwiche CF, Palmer JD (1996) Rampant horizontal transfer and duplication of Rubisco genes in eubacteria and plastids. Molecular Biology and Evolution 13, 873–882.

Dihanich ME, Najarian D, Clark R, Gillman EC, Martin NC, Hopper AK (1987) Isolation and characterization of MOD5, a gene required for isopentenylation of cytoplasmic and mitochondrial tRNAs of Saccharomyces cerevisiae. Molecular and Cellular Biology 7, 177–184.

Dixon SC, Martin RC, Mok MC, Shaw G, Mok DWS (1989) Zeatin glycosylation enzymes in Phaseolus. Isolation of O-glucosyltransferase from P. lunatus and comparison to O-xylosyltransferase from P. vulgaris. Plant Physiology 90, 1316–1321.
Zeatin glycosylation enzymes in Phaseolus. Isolation of O-glucosyltransferase from P. lunatus and comparison to O-xylosyltransferase from P. vulgaris.Crossref | GoogleScholarGoogle Scholar |

Dorchin N, Hoffman JH, Stirk WA, Novák O, Strnad M, van Staden J (2009) Sexually dimorphic gall structures correspond to differential phytohormone contents in male and female wasp larvae. Physiological Entomology 34, 359–369.
Sexually dimorphic gall structures correspond to differential phytohormone contents in male and female wasp larvae.Crossref | GoogleScholarGoogle Scholar |

Duke CC, MacLeod JK, Summons RE, Letham DS, Parker CW (1978) The structure and synthesis of cytokinin metabolites II. Lupinic acid and O-β-D-glucopyranosyl zeatin from Lupinus augustifolius. Australian Journal of Chemistry 31, 1291–1301.
The structure and synthesis of cytokinin metabolites II. Lupinic acid and O-β-D-glucopyranosyl zeatin from Lupinus augustifolius.Crossref | GoogleScholarGoogle Scholar |

Faiss M, Zalubilová J, Strnad M, Schmülling T (1997) Conditional transgenic expression of the ipt gene indicates a function for cytokinins in paracrine signaling in whole tobacco plants. The Plant Journal 12, 401–415.
Conditional transgenic expression of the ipt gene indicates a function for cytokinins in paracrine signaling in whole tobacco plants.Crossref | GoogleScholarGoogle Scholar |

Frébort I, Šebela M, Galuszka P, Werner T, Schmülling T, Peč P (2002) Cytokinin oxidase/cytokinin dehydrogenase assay: optimized procedures and applications. Analytical Biochemistry 306, 1–7.
Cytokinin oxidase/cytokinin dehydrogenase assay: optimized procedures and applications.Crossref | GoogleScholarGoogle Scholar |

Frébort I, Kowalska M, Hluska T, Frébortová J, Galuszka P (2011) Evolution of cytokinin biosynthesis and degradation. Journal of Experimental Botany 62, 2431–2452.
Evolution of cytokinin biosynthesis and degradation.Crossref | GoogleScholarGoogle Scholar |

Frébortová J, Fraaije MW, Galuszka P, Šebela M, Peč P, Hrbáč J, Novák O, Bilyeu KD, English JT, Frébort I (2004) Catalytic reaction of cytokinin dehydrogenase: preference for quinones as electron acceptors. Biochemical Journal 380, 121–130.
Catalytic reaction of cytokinin dehydrogenase: preference for quinones as electron acceptors.Crossref | GoogleScholarGoogle Scholar |

Frébortová J, Novák O, Frébort I, Jorda R (2010) Degradation of cytokinins by maize cytokinin dehydrogenase is mediated by free radicals generated by enzymatic oxidation of natural benzoxazinones. The Plant Journal 61, 467–481.
Degradation of cytokinins by maize cytokinin dehydrogenase is mediated by free radicals generated by enzymatic oxidation of natural benzoxazinones.Crossref | GoogleScholarGoogle Scholar |

Frugier F, Kosuta S, Murray JD, Crespi M, Szczyglowski K (2008) Cytokinin: secret agent of symbiosis. Trends in Plant Science 13, 115–120.
Cytokinin: secret agent of symbiosis.Crossref | GoogleScholarGoogle Scholar |

Fusseder A, Ziegler P (1988) Metabolism and compartmentation of dihydrozeatin exogenously supplied to photoautotrophic suspension cultures of Chenopodium rubrum. Planta 173, 104–109.
Metabolism and compartmentation of dihydrozeatin exogenously supplied to photoautotrophic suspension cultures of Chenopodium rubrum.Crossref | GoogleScholarGoogle Scholar |

Gajdošová S, Spíchal L, Kamínek M, Hoyerová K, Novák O, Dobrev PI, Galuszka P, Klíma P, Gaudinová A, Žižková E, Hanuš J, Dančák M, Trávníček B, Pešek B, Krupička M, Vaňková R, Strnad M, Motyka V (2011) Distribution, biological activities, metabolism, and the conceivable function of cis-zeatin-type cytokinins in plants. Journal of Experimental Botany 62, 2827–2840.
Distribution, biological activities, metabolism, and the conceivable function of cis-zeatin-type cytokinins in plants.Crossref | GoogleScholarGoogle Scholar |

Galichet A, Hoyerová K, Kamínek M, Gruissem W (2008) Farnesylation directs AtIPT3 subcellular localization and modulates cytokinin biosynthesis in Arabidopsis. Plant Physiology 146, 1155–1164.
Farnesylation directs AtIPT3 subcellular localization and modulates cytokinin biosynthesis in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Galuszka P, Frébort I, Šebela M, Sauer P, Jacobsen S, Peč P (2001) Cytokinin oxidase or dehydrogenase? Mechanism of cytokinin degradation in cereals. European Journal of Biochemistry 268, 450–461.
Cytokinin oxidase or dehydrogenase? Mechanism of cytokinin degradation in cereals.Crossref | GoogleScholarGoogle Scholar |

Galuszka P, Frébortová J, Luhová L, Bilyeu KD, English JT, Frébort I (2005) Tissue localization of cytokinin dehydrogenase in maize: possible involvement of quinone species generated from plant phenolics by other enzymatic systems in the catalytic reaction. Plant & Cell Physiology 46, 716–728.
Tissue localization of cytokinin dehydrogenase in maize: possible involvement of quinone species generated from plant phenolics by other enzymatic systems in the catalytic reaction.Crossref | GoogleScholarGoogle Scholar |

Galuszka P, Popelková H, Werner T, Frébortová J, Pospíšilová H, Mik V, Köllmer I, Schmülling T, Frébort I (2007) Biochemical characterization of cytokinin oxidases/dehydrogenases from Arabidopsis thaliana expressed in Nicotiana tabacum L. Journal of Plant Growth Regulation 26, 255–267.
Biochemical characterization of cytokinin oxidases/dehydrogenases from Arabidopsis thaliana expressed in Nicotiana tabacum L.Crossref | GoogleScholarGoogle Scholar |

Galuszka P, Spíchal L, Kopečný D, Tarkowski P, Frébortová J, Šebela M, Frébort I (2008) Metabolism of plant hormones cytokinins and their function in signaling, cell differentiation and plant development. In ‘Studies in natural products chemistry, Vol. 34.’ (Ed. Atta-ur-Rahman) pp. 203–264. (Elsevier: Amsterdam)

Gan S, Amasino RM (1995) Inhibition of leaf senescence by autoregulated production of cytokinin. Science 270, 1986–1988.
Inhibition of leaf senescence by autoregulated production of cytokinin.Crossref | GoogleScholarGoogle Scholar |

Giraud E, Moulin L, Vallenet D, Barbe V, Cytryn E, Avarre JC, Jaubert M, Simon D, Cartieaux F, Prin Y, Bena G, Hannibal L, Fardoux J, Kojadinovic M, Vuillet L, Lajus A, Cruveiller S, Rouy Z, Mangenot S, Segurens B, Dossat C, Franck WL, Chang WS, Saunders E, Bruce D, Richardson P, Normand P, Dreyfus B, Pignol D, Stacey G, Emerich D, Verméglio A, Médigue C, Sadowsky M (2007) Legumes symbioses: absence of Nod genes in photosynthetic bradyrhizobia. Science 316, 1307–1312.
Legumes symbioses: absence of Nod genes in photosynthetic bradyrhizobia.Crossref | GoogleScholarGoogle Scholar |

Golovko A, Hjälm G, Sitbon F, Nicander B (2000) Cloning of a human tRNA isopentenyl transferase. Gene 258, 85–93.
Cloning of a human tRNA isopentenyl transferase.Crossref | GoogleScholarGoogle Scholar |

Gonzalez-Rizzo S, Crespi M, Frugier F (2006) The Medicago truncatula CRE1 CK receptor regulates lateral root development and early symbiotic interaction with Sinorhizobium meliloti. The Plant Cell 18, 2680–2693.
The Medicago truncatula CRE1 CK receptor regulates lateral root development and early symbiotic interaction with Sinorhizobium meliloti.Crossref | GoogleScholarGoogle Scholar |

Gu R, Fu J, Guo S, Duan F, Wang Z, Mi G, Yuan L (2010) Comparative expression and phylogenetic analysis of maize cytokinin dehydrogenase/oxidase (CKX) gene family. Journal of Plant Growth Regulation 29, 428–440.
Comparative expression and phylogenetic analysis of maize cytokinin dehydrogenase/oxidase (CKX) gene family.Crossref | GoogleScholarGoogle Scholar |

Haegeman A, Jones JT, Danchin EG (2011) Horizontal gene transfer in nematodes: a catalyst for plant parasitism? Molecular Plant—Microbe Interactions 24, 879–887.
Horizontal gene transfer in nematodes: a catalyst for plant parasitism?Crossref | GoogleScholarGoogle Scholar |

Hare PD, Van Staden J (1994) Cytokinin oxidase: biochemical features and physiological significance. Physiologia Plantarum 91, 128–136.
Cytokinin oxidase: biochemical features and physiological significance.Crossref | GoogleScholarGoogle Scholar |

Hejátko J, Pernišová M, Eneva T, Palme K, Brzobohatý B (2003) The putative sensor histidine kinase CKI1 is involved in female gametophyte development in Arabidopsis. Molecular Genetics and Genomics 269, 443–453.
The putative sensor histidine kinase CKI1 is involved in female gametophyte development in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Hejátko J, Ryu H, Kim G-T, Dobešová R, Choi S, Choi SM, Souček P, Horák J, Pekárová B, Palme K, Brzobohaty B, Hwang I (2009) The histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS HISTIDINE KINASE2 and 3 regulate vascular tissue development in Arabidopsis shoots. The Plant Cell 21, 2008–2021.
The histidine kinases CYTOKININ-INDEPENDENT1 and ARABIDOPSIS HISTIDINE KINASE2 and 3 regulate vascular tissue development in Arabidopsis shoots.Crossref | GoogleScholarGoogle Scholar |

Hewelt A, Prinsen E, Schell J, Van Onckelen H, Schmülling T (1994) Promoter tagging with a promoterless ipt gene leads to cytokinin-induced phenotypic variability in transgenic tobacco plants: implications of gene dosage effects. The Plant Journal 6, 879–891.
Promoter tagging with a promoterless ipt gene leads to cytokinin-induced phenotypic variability in transgenic tobacco plants: implications of gene dosage effects.Crossref | GoogleScholarGoogle Scholar |

Heyl A, Ramireddy E, Brenner WG, Riefler M, Allemeersch J, Schmülling T (2008) The transcriptional repressor ARR1-SRDX suppresses pleiotropic cytokinin activities in Arabidopsis. Plant Physiology 147, 1380–1395.
The transcriptional repressor ARR1-SRDX suppresses pleiotropic cytokinin activities in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Heyl A, Riefler M, Romanov GA, Schmülling T (2012) Properties, functions and evolution of cytokinin receptors. European Journal of Cell Biology 91, 246–256.
Properties, functions and evolution of cytokinin receptors.Crossref | GoogleScholarGoogle Scholar |

Higuchi M, Pischke MS, Mähönen AP, Miyawaki K, Hashimoto Y, Seki M, Kobayashi M, Shinozaki K, Kato T, Tabata S, Helariutta Y, Sussman MR, Kakimoto T (2004) In planta functions of the Arabidopsis cytokinin receptor family. Proceedings of the National Academy of Sciences of the United States of America 101, 8821–8826.
In planta functions of the Arabidopsis cytokinin receptor family.Crossref | GoogleScholarGoogle Scholar |

Hirose N, Makita N, Yamaya T, Sakakibara H (2005) Functional characterization and expression analysis of a gene, OsENT2, encoding an equilibrative nucleoside transporter in rice suggest a function in cytokinin transport. Plant Physiology 138, 196–206.
Functional characterization and expression analysis of a gene, OsENT2, encoding an equilibrative nucleoside transporter in rice suggest a function in cytokinin transport.Crossref | GoogleScholarGoogle Scholar |

Hothorn M, Dabi T, Chory J (2011) Structural basis for cytokinin recognition by Arabidopsis thaliana histidine kinase 4. Nature Chemical Biology 7, 766–768.
Structural basis for cytokinin recognition by Arabidopsis thaliana histidine kinase 4.Crossref | GoogleScholarGoogle Scholar |

Houba-Hérin N, Pethe C, d’Alayer J, Laloue M (1999) Cytokinin oxidase from Zea mays: purification, cDNA cloning and expression in moss protoplasts. The Plant Journal 17, 615–626.
Cytokinin oxidase from Zea mays: purification, cDNA cloning and expression in moss protoplasts.Crossref | GoogleScholarGoogle Scholar |

Hussain A, Krischke M, Roitsch T, Hasnain S (2010) Rapid determination of cytokinins and auxin in cyanobacteria. Current Microbiology 61, 361–369.
Rapid determination of cytokinins and auxin in cyanobacteria.Crossref | GoogleScholarGoogle Scholar |

Hutchison CE, Li J, Argueso C, Gonzalez M, Lee E, Lewis MW, Maxwell BB, Perdue TD, Schaller GE, Alonso JM, Ecker JR, Kieber JJ (2006) The Arabidopsis histidine phosphotransfer proteins are redundant positive regulators of cytokinin signaling. The Plant Cell 18, 3073–3087.
The Arabidopsis histidine phosphotransfer proteins are redundant positive regulators of cytokinin signaling.Crossref | GoogleScholarGoogle Scholar |

Hwang I, Sheen J (2001) Two-component circuitry in Arabidopsis cytokinin signal transduction. Nature 413, 383–389.
Two-component circuitry in Arabidopsis cytokinin signal transduction.Crossref | GoogleScholarGoogle Scholar |

Imamura A, Hanaki N, Nakamura A, Suzuki T, Taniguchi M, Kiba T, Ueguchi C, Sugiyama T, Mizuno T (1999) Compilation and characterization of Arabidopsis thaliana response regulators implicated in His–Asp phosphorelay signal transduction. Plant & Cell Physiology 40, 733–742.

Inoue T, Higuchi M, Hashimoto Y, Seki M, Kobayashi M, Kato T, Tabata S, Shinozaki K, Kakimoto T (2001) Identification of CRE1 as a cytokinin receptor from Arabidopsis. Nature 409, 1060–1063.
Identification of CRE1 as a cytokinin receptor from Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Iwamura H (1994) Cytokinin antagonists: synthesis and biological activity. In ‘Cytokinins. Chemistry, activity and function’. (Ed. DWS Mok, MC Mok) pp. 43–55. (CRC Press: Boca Raton)

Joshi MV, Loria R (2007) Streptomyces turgidiscabies possesses a functional cytokinin biosynthetic pathway and produces leafy galls. Molecular Plant—Microbe Interactions 20, 751–758.
Streptomyces turgidiscabies possesses a functional cytokinin biosynthetic pathway and produces leafy galls.Crossref | GoogleScholarGoogle Scholar |

Kaiser W, Huguet E, Casas J, Commin C, Giron D (2010) Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts. Proceedings. Biological Sciences 277, 2311–2319.
Plant green-island phenotype induced by leaf-miners is mediated by bacterial symbionts.Crossref | GoogleScholarGoogle Scholar |

Kakimoto T (1996) CKI1, a histidine kinase homolog implicated in cytokinin signal transduction. Science 274, 982–985.
CKI1, a histidine kinase homolog implicated in cytokinin signal transduction.Crossref | GoogleScholarGoogle Scholar |

Kakimoto T (2001) Identification of plant cytokinin biosynthetic enzymes as dimethylallyl diphosphate: ATP/ADP isopentenyltransferases. Plant & Cell Physiology 42, 677–685.
Identification of plant cytokinin biosynthetic enzymes as dimethylallyl diphosphate: ATP/ADP isopentenyltransferases.Crossref | GoogleScholarGoogle Scholar |

Kamínek M, Armstrong DJ (1990) Genotypic variation in cytokinin oxidase from Phaseolus callus cultures. Plant Physiology 93, 1530–1538.
Genotypic variation in cytokinin oxidase from Phaseolus callus cultures.Crossref | GoogleScholarGoogle Scholar |

Kamínek M, Trčková M, Fox JE, Gaudinová A (2003) Comparison of cytokinin-binding proteins from wheat and oat grains. Physiologia Plantarum 117, 453–458.
Comparison of cytokinin-binding proteins from wheat and oat grains.Crossref | GoogleScholarGoogle Scholar |

Kasahara H, Takei K, Ueda N, Hishiyama S, Yamaya T, Kamiya Y, Yamaguchi S, Sakakibara H (2004) Distinct isoprenoid origins of cis- and trans-zeatin biosyntheses in Arabidopsis. The Journal of Biological Chemistry 279, 14049–14054.
Distinct isoprenoid origins of cis- and trans-zeatin biosyntheses in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Kieber JJ, Schaller GE (2010) The perception of cytokinin: a story 50 years in the making. Plant Physiology 154, 487–492.
The perception of cytokinin: a story 50 years in the making.Crossref | GoogleScholarGoogle Scholar |

Kim HJ, Ryu H, Hong SH, Woo HR, Lim PO, Lee IC, Sheen J, Nam HG, Hwang I (2006) Cytokinin-mediated control of leaf longevity by AHK3 through phosphorylation of ARR2 in Arabidopsis. Proceedings of the National Academy of Sciences of the United States of America 103, 814–819.
Cytokinin-mediated control of leaf longevity by AHK3 through phosphorylation of ARR2 in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Kowalska M, Galuszka P, Frébortová J, Šebela M, Béres T, Hluska T, Šmehilová M, Bilyeu KD, Frébort I (2010) Vacuolar and cytosolic cytokinin dehydrogenases of Arabidopsis thaliana: heterologous expression, purification and properties. Phytochemistry 71, 1970–1978.
Vacuolar and cytosolic cytokinin dehydrogenases of Arabidopsis thaliana: heterologous expression, purification and properties.Crossref | GoogleScholarGoogle Scholar |

Krall L, Raschke M, Zenk MH, Baron C (2002) The Tzs protein from Agrobacterium tumefaciens C58 produces zeatin riboside 5′-phosphate from 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate and AMP. FEBS Letters 527, 315–318.
The Tzs protein from Agrobacterium tumefaciens C58 produces zeatin riboside 5′-phosphate from 4-hydroxy-3-methyl-2-(E)-butenyl diphosphate and AMP.Crossref | GoogleScholarGoogle Scholar |

Kurakawa T, Ueda N, Maekawa M, Kobayashi K, Kojima M, Nagato Y, Sakakibara H, Kyozuka J (2007) Direct control of shoot meristem activity by a cytokinin-activating enzyme. Nature 445, 652–655.
Direct control of shoot meristem activity by a cytokinin-activating enzyme.Crossref | GoogleScholarGoogle Scholar |

Kuroha T, Tokunaga H, Kojima M, Ueda N, Ishida T, Nagawa S, Fukuda H, Sugimoto K, Sakakibara H (2009) Functional analyses of LONELY GUY cytokinin-activating enzymes reveal the importance of the direct activation pathway in Arabidopsis. The Plant Cell 21, 3152–3169.
Functional analyses of LONELY GUY cytokinin-activating enzymes reveal the importance of the direct activation pathway in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Kwade Z, Swiatek A, Azmi A, Goossens A, Inzé D, Van Onckelen H, Roef L (2005) Identification of four adenosine kinase isoforms in tobacco By-2 cells and their putative role in the cell cycle-regulated cytokinin metabolism. The Journal of Biological Chemistry 280, 17512–17519.
Identification of four adenosine kinase isoforms in tobacco By-2 cells and their putative role in the cell cycle-regulated cytokinin metabolism.Crossref | GoogleScholarGoogle Scholar |

Laloue M, Fox JE (1985) Characterization of an imine intermediate in the degradation of isopentenylated cytokinins by a cytokinin oxidase from wheat. In ‘Abstracts of the 12th International Conference on Plant Growth Substances’. (Ed. M Bopp) pp. 23. (Springer: Berlin)

Laukens K, Lenobel R, Strnad M, Van Onckelen H, Witters E (2003) Cytokinin affinity purification and identification of a tobacco BY-2 adenosine kinase. FEBS Letters 533, 63–66.
Cytokinin affinity purification and identification of a tobacco BY-2 adenosine kinase.Crossref | GoogleScholarGoogle Scholar |

Letham DS (1973) Cytokinins from Zea mays. Phytochemistry 12, 2445–2455.
Cytokinins from Zea mays.Crossref | GoogleScholarGoogle Scholar |

Lohrmann J, Buchholz G, Keitel C, Sweere U, Kircher S, Bäurle I, Kudla J, Schäfer E, Harter K (1999) Differential expression and nuclear localization of response regulator-like proteins from Arabidopsis thaliana. Plant Biology 1, 495–505.
Differential expression and nuclear localization of response regulator-like proteins from Arabidopsis thaliana.Crossref | GoogleScholarGoogle Scholar |

Lomin SN, Yonekura-Sakakibara K, Romanov GA, Sakakibara H (2011) Ligand-binding properties and subcellular localization of maize cytokinin receptors. Journal of Experimental Botany 62, 5149–5159.
Ligand-binding properties and subcellular localization of maize cytokinin receptors.Crossref | GoogleScholarGoogle Scholar |

Lorteau MA, Ferguson BJ, Guinel FC (2001) Effects of CK on ethylene production and nodulation in pea (Pisum sativum) cv. Sparkle. Physiologia Plantarum 112, 421–428.
Effects of CK on ethylene production and nodulation in pea (Pisum sativum) cv. Sparkle.Crossref | GoogleScholarGoogle Scholar |

Mähönen AP, Bonke M, Kauppinen L, Riikonen M, Benfey PN, Helariutta Y (2000) A novel two-component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root. Genes & Development 14, 2938–2943.
A novel two-component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root.Crossref | GoogleScholarGoogle Scholar |

Mähönen AP, Bishopp A, Higuchi M, Nieminen KM, Kinoshita K, Törmäkangas K, Ikeda Y, Oka A, Kakimoto T, Helariutta Y (2006) Cytokinin signaling and its inhibitor AHP6 regulate cell fate during vascular development. Science 311, 94–98.
Cytokinin signaling and its inhibitor AHP6 regulate cell fate during vascular development.Crossref | GoogleScholarGoogle Scholar |

Malito E, Coda A, Bilyeu KD, Fraaije MW, Mattevi A (2004) Structures of Michaelis and product complexes of plant cytokinin dehydrogenase: implications for flavoenzyme catalysis. Journal of Molecular Biology 341, 1237–1249.
Structures of Michaelis and product complexes of plant cytokinin dehydrogenase: implications for flavoenzyme catalysis.Crossref | GoogleScholarGoogle Scholar |

Martin RC, Mok MC, Shaw G, Mok DWS (1989) An enzyme mediating the conversion of zeatin to dihydrozeatin in Phaseolus embryos. Plant Physiology 90, 1630–1635.
An enzyme mediating the conversion of zeatin to dihydrozeatin in Phaseolus embryos.Crossref | GoogleScholarGoogle Scholar |

Martin RC, Mok MC, Habben JE, Mok DW (2001) A maize cytokinin gene encoding an O-glucosyltransferase specific to cis-zeatin. Proceedings of the National Academy of Sciences of the United States of America 98, 5922–5926.
A maize cytokinin gene encoding an O-glucosyltransferase specific to cis-zeatin.Crossref | GoogleScholarGoogle Scholar |

Maruyama S, Matsuzaki M, Misawa K, Nozaki H (2009) Cyanobacterial contribution to the genomes of the plastid-lacking protists. BMC Evolutionary Biology 9, 197–220.
Cyanobacterial contribution to the genomes of the plastid-lacking protists.Crossref | GoogleScholarGoogle Scholar |

Mason MG, Li J, Mathews DE, Kieber JJ, Schaller GE (2004) Type-B response regulators display overlapping expression patterns in Arabidopsis. Plant Physiology 135, 927–937.
Type-B response regulators display overlapping expression patterns in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Mason MG, Mathews DE, Argyros DA, Maxwell BB, Kieber JJ, Alonso JM, Ecker JR, Schaller GE (2005) Multiple type-B response regulators mediate cytokinin signal transduction in Arabidopsis. The Plant Cell 17, 3007–3018.
Multiple type-B response regulators mediate cytokinin signal transduction in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Medford JI, Horgan R, El-Sawi Z, Klee HJ (1989) Alterations of endogenous cytokinins in transgenic plants using a chimeric isopentenyl transferase gene. The Plant Cell 1, 403–413.

Miller CO (1967) Zeatin and zeatin riboside from a mycorrhizal fungus. Science 157, 1055–1057.
Zeatin and zeatin riboside from a mycorrhizal fungus.Crossref | GoogleScholarGoogle Scholar |

Miller CO, Skoog F, von Saltza MH, Strong FM (1955) Kinetin, a cell division factor from deoxyribonucleic acid. Journal of the American Chemical Society 77, 1392
Kinetin, a cell division factor from deoxyribonucleic acid.Crossref | GoogleScholarGoogle Scholar |

Mitreva M, Blaxter ML, Bird DM, McCarter JP (2005) Comparative genomics of nematodes. Trends in Genetics 10, 573–581.

Miyawaki K, Matsumoto-Kitano M, Kakimoto T (2004) Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate. The Plant Journal 37, 128–138.
Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate.Crossref | GoogleScholarGoogle Scholar |

Miyawaki K, Tarkowski P, Matsumoto-Kitano M, Kato T, Sato S, Tarkowská D, Tabata S, Sandberg G, Kakimoto T (2006) Roles of Arabidopsis ATP/ADP isopentenyltransferases and tRNA isopentenyltransferases in cytokinin biosynthesis. Proceedings of the National Academy of Sciences of the United States of America 103, 16598–16603.
Roles of Arabidopsis ATP/ADP isopentenyltransferases and tRNA isopentenyltransferases in cytokinin biosynthesis.Crossref | GoogleScholarGoogle Scholar |

Mizuno T (1998) His–Asp phosphotransfer signal transduction. Journal of Biochemistry 123, 555–563.

Mok MC (1994) Cytokinins and plant development – an overview. In ‘Cytokinins. Chemistry, activity and function’. (Ed. DWS Mok, MC Mok) pp. 155–166. (CRC Press: Boca Raton)

Mok DW, Mok MC (2001) Cytokinin metabolism and action. Annual Review of Plant Physiology and Plant Molecular Biology 52, 89–118.
Cytokinin metabolism and action.Crossref | GoogleScholarGoogle Scholar |

Mok MC, Martin RC, Dobrev PI, Vanková R, Ho PS, Yonekura-Sakakibara K, Sakakibara H, Mok DW (2005) Topolins and hydroxylated thidiazuron derivatives are substrates of cytokinin O-glucosyltransferase with position specificity related to receptor recognition. Plant Physiology 137, 1057–1066.
Topolins and hydroxylated thidiazuron derivatives are substrates of cytokinin O-glucosyltransferase with position specificity related to receptor recognition.Crossref | GoogleScholarGoogle Scholar |

Morris RO, Bilyeu KD, Laskey JG, Cheikh NN (1999) Isolation of a gene encoding a glycosylated cytokinin oxidase from maize. Biochemical and Biophysical Research Communications 255, 328–333.
Isolation of a gene encoding a glycosylated cytokinin oxidase from maize.Crossref | GoogleScholarGoogle Scholar |

Motyka V, Vaňková R, Čapková V, Petrášek J, Kamínek M, Schmülling T (2003) Cytokinin-induced upregulation of cytokinin oxidase activity in tobacco includes changes in enzyme glycosylation and secretion. Physiologia Plantarum 117, 11–21.
Cytokinin-induced upregulation of cytokinin oxidase activity in tobacco includes changes in enzyme glycosylation and secretion.Crossref | GoogleScholarGoogle Scholar |

Mougel C, Zhulin IB (2001) CHASE: an extracellular sensing domain common to transmembrane receptors from prokaryotes, lower eukaryotes and plants. Trends in Biochemical Sciences 26, 582–584.
CHASE: an extracellular sensing domain common to transmembrane receptors from prokaryotes, lower eukaryotes and plants.Crossref | GoogleScholarGoogle Scholar |

Murray JD, Karas BJ, Sato S, Tabata S, Amyly L, Szczyglowski K (2007) A CK perception mutant colonized by Rhizobium in the absence of nodule organogenesis. Science 315, 101–104.
A CK perception mutant colonized by Rhizobium in the absence of nodule organogenesis.Crossref | GoogleScholarGoogle Scholar |

Nishimura C, Ohashi Y, Sato S, Kato T, Tabata S, Ueguchi C (2004) Histidine kinase homologs that act as cytokinin receptors possess overlapping functions in the regulation of shoot and root growth in Arabidopsis. The Plant Cell 16, 1365–1377.
Histidine kinase homologs that act as cytokinin receptors possess overlapping functions in the regulation of shoot and root growth in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Nisler J, Zatloukal M, Popa I, Doležal K, Strnad M, Spíchal L (2010) Cytokinin receptor antagonists derived from 6-benzylaminopurine. Phytochemistry 71, 823–830.
Cytokinin receptor antagonists derived from 6-benzylaminopurine.Crossref | GoogleScholarGoogle Scholar |

Nomura T, Tanaka Y, Abe H, Uchiyama M (1977) Cytokinin activity of discadenine: a spore germination inhibitor of Dictyostelium discoideum. Phytochemistry 16, 1819–1820.
Cytokinin activity of discadenine: a spore germination inhibitor of Dictyostelium discoideum.Crossref | GoogleScholarGoogle Scholar |

Pačes V, Werstiuk E, Hall RH (1971) Conversion of N 6-(Δ2-isopentenyl)adenosine to adenosine by enzyme activity in tobacco tissue. Plant Physiology 48, 775–778.
Conversion of N 6-(Δ2-isopentenyl)adenosine to adenosine by enzyme activity in tobacco tissue.Crossref | GoogleScholarGoogle Scholar |

Parkinson JS, Kofoid EC (1992) Communication modules in bacterial signaling proteins. Annual Review of Genetics 26, 71–112.
Communication modules in bacterial signaling proteins.Crossref | GoogleScholarGoogle Scholar |

Pasternak O, Bujacz GD, Fujimoto Y, Hashimoto Y, Jelen F, Otlewski J, Sikorski MM, Jaskolski M (2006) Crystal structure of Vigna radiata cytokinin-specific binding protein in complex with zeatin. The Plant Cell 18, 2622–2634.
Crystal structure of Vigna radiata cytokinin-specific binding protein in complex with zeatin.Crossref | GoogleScholarGoogle Scholar |

Persson BC, Esberg B, Ólafsen Ó, Björk GR (1994) Synthesis and function of isopentenyl adenosine derivatives in tRNA. Biochimie 76, 1152–1160.
Synthesis and function of isopentenyl adenosine derivatives in tRNA.Crossref | GoogleScholarGoogle Scholar |

Pertry I, Václavíková K, Depuydt S, Galuszka P, Spíchal L, Temmerman W, Stes E, Schmülling T, Kakimoto T, Van Montagu MCE, Strnad M, Holsters M, Tarkowski P, Vereecke D (2009) Identification of Rhodococcus fascians cytokinins and their modus operandi to reshape the plant. Proceedings of the National Academy of Sciences of the United States of America 106, 929–934.
Identification of Rhodococcus fascians cytokinins and their modus operandi to reshape the plant.Crossref | GoogleScholarGoogle Scholar |

Pertry I, Václavíková K, Gemrotová M, Spíchal L, Galuszka P, Depuydt S, Temmerman W, Stes E, De Keyser A, Riefler M, Biondi S, Novák O, Schmülling T, Strnad M, Tarkowski P, Holsters M, Vereecke D (2010) Rhodococcus fascians impacts plant development through the dynamic fas-mediated production of a cytokinin mix. Molecular Plant—Microbe Interactions 23, 1164–1174.
Rhodococcus fascians impacts plant development through the dynamic fas-mediated production of a cytokinin mix.Crossref | GoogleScholarGoogle Scholar |

Pils B, Heyl A (2009) Unraveling the evolution of cytokinin signaling. Plant Physiology 151, 782–791.
Unraveling the evolution of cytokinin signaling.Crossref | GoogleScholarGoogle Scholar |

Pischke MS, Jones LG, Otsuga D, Fernandez DE, Drews GN, Sussman MR (2002) An Arabidopsis histidine kinase is essential for megagametogenesis. Proceedings of the National Academy of Sciences of the United States of America 99, 15800–15805.
An Arabidopsis histidine kinase is essential for megagametogenesis.Crossref | GoogleScholarGoogle Scholar |

Popelková H, Fraaije MW, Novák O, Frébortová J, Bilyeu KD, Frébort I (2006) Kinetic and chemical analyses of the cytokinin dehydrogenase-catalysed reaction: correlations with the crystal structure. Biochemical Journal 398, 113–124.
Kinetic and chemical analyses of the cytokinin dehydrogenase-catalysed reaction: correlations with the crystal structure.Crossref | GoogleScholarGoogle Scholar |

Punwani JA, Hutchison CE, Schaller GE, Kieber JJ (2010) The subcellular distribution of the Arabidopsis histidine phosphotransfer proteins is independent of cytokinin signaling. The Plant Journal 62, 473–482.
The subcellular distribution of the Arabidopsis histidine phosphotransfer proteins is independent of cytokinin signaling.Crossref | GoogleScholarGoogle Scholar |

Rashotte AM, Mason MG, Hutchison CE, Ferreira FJ, Schaller GE, Kieber JJ (2006) A subset of Arabidopsis AP2 transcription factors mediates cytokinin responses in concert with a two-component pathway. Proceedings of the National Academy of Sciences of the United States of America 103, 11081–11085.
A subset of Arabidopsis AP2 transcription factors mediates cytokinin responses in concert with a two-component pathway.Crossref | GoogleScholarGoogle Scholar |

Redig P, Schmülling T, Van Onckelen H (1996) Analysis of cytokinin metabolism in ipt transgenic tobacco by liquid chromatography-tandem mass spectrometry. Plant Physiology 112, 141–148.

Riechmann JL, Heard J, Martin G, Reuber L, Jiang C-Z, Keddie J, Adam L, Pineda O, Ratcliff OJ, Samaha RR, Creelman R, Pilgrim M, Broun P, Zhang JZ, Ghandehari D, Sherman BK, Yu G-L (2000) Arabidopsis transcription factors: genome-wide comparative analysis among eukaryotes. Science 290, 2105–2110.
Arabidopsis transcription factors: genome-wide comparative analysis among eukaryotes.Crossref | GoogleScholarGoogle Scholar |

Riefler M, Novak O, Strnad M, Schmülling T (2006) Arabidopsis cytokinin receptor mutants reveal functions in shoot growth, leaf senescence, seed size, germination, root development, and cytokinin metabolism. The Plant Cell 18, 40–54.
Arabidopsis cytokinin receptor mutants reveal functions in shoot growth, leaf senescence, seed size, germination, root development, and cytokinin metabolism.Crossref | GoogleScholarGoogle Scholar |

Romanov GA, Spíchal L, Lomin SN, Strnad M, Schmülling T (2005) A live cell hormone-binding assay on transgenic bacteria expressing a eukaryotic receptor protein. Analytical Biochemistry 347, 129–134.
A live cell hormone-binding assay on transgenic bacteria expressing a eukaryotic receptor protein.Crossref | GoogleScholarGoogle Scholar |

Romanov GA, Lomin SN, Schmülling T (2006) Biochemical characteristics and ligand-binding properties of Arabidopsis cytokinin receptor AHK3 compared to CRE1/AHK4 as revealed by a direct binding assay. Journal of Experimental Botany 57, 4051–4058.
Biochemical characteristics and ligand-binding properties of Arabidopsis cytokinin receptor AHK3 compared to CRE1/AHK4 as revealed by a direct binding assay.Crossref | GoogleScholarGoogle Scholar |

Sa G, Mi M, He-Chun Y, Guo-Feng L (2002) Anther-specific expression of ipt gene in transgenic tobacco and its effect on plant development. Transgenic Research 11, 269–278.
Anther-specific expression of ipt gene in transgenic tobacco and its effect on plant development.Crossref | GoogleScholarGoogle Scholar |

Sakai H, Aoyama T, Oka A (2000) Arabidopsis ARR1 and ARR2 response regulators operate as transcriptional activators. The Plant Journal 24, 703–711.
Arabidopsis ARR1 and ARR2 response regulators operate as transcriptional activators.Crossref | GoogleScholarGoogle Scholar |

Sakai H, Honma T, Aoyama T, Sato S, Kato T, Tabata S, Oka A (2001) ARR1, a transcription factor for genes immediately responsive to cytokinins. Science 294, 1519–1521.
ARR1, a transcription factor for genes immediately responsive to cytokinins.Crossref | GoogleScholarGoogle Scholar |

Sakakibara H, Kasahara H, Ueda N, Kojima M, Takei K, Hishiyama S, Asami T, Okada K, Kamiya Y, Yamaya T, Yamaguchi S (2005) Agrobacterium tumefaciens increases cytokinin production in plastids by modifying the biosynthetic pathway in the host plant. Proceedings of the National Academy of Sciences of the United States of America 102, 9972–9977.
Agrobacterium tumefaciens increases cytokinin production in plastids by modifying the biosynthetic pathway in the host plant.Crossref | GoogleScholarGoogle Scholar |

Sakamoto T, Sakakibara H, Kojima M, Yamamoto Y, Nagasaki H, Inukai Y, Sato Y, Matsuoka M (2006) Ectopic expression of KNOTTED1-like homeobox protein induces expression of cytokinin biosynthesis genes in rice. Plant Physiology 142, 54–62.
Ectopic expression of KNOTTED1-like homeobox protein induces expression of cytokinin biosynthesis genes in rice.Crossref | GoogleScholarGoogle Scholar |

Sakano Y, Okada Y, Matsunaga A, Suwama T, Kaneko T, Ito K, Noguchi H, Abe I (2004) Molecular cloning, expression and characterization of adenylate isopentenyltransferase from hop (Humulus lupulus L.). Phytochemistry 65, 2439–2446.
Molecular cloning, expression and characterization of adenylate isopentenyltransferase from hop (Humulus lupulus L.).Crossref | GoogleScholarGoogle Scholar |

Schmülling T (2001) CREam of cytokinin signalling: receptor identified. Trends in Plant Science 6, 281–284.
CREam of cytokinin signalling: receptor identified.Crossref | GoogleScholarGoogle Scholar |

Schmülling T, Beinsberger S, De Greef J, Schell J, Van Onckelen H, Spena A (1989) Construction of a heat-inducible chimeric gene to increase the cytokinin content in transgenic plant tissue. FEBS Letters 249, 401–406.
Construction of a heat-inducible chimeric gene to increase the cytokinin content in transgenic plant tissue.Crossref | GoogleScholarGoogle Scholar |

Schmülling T, Werner T, Riefler M, Krupková E, Bartrina y Manns I (2003) Structure and function of cytokinin oxidase/dehydrogenase genes of maize, rice, Arabidopsis and other species. Journal of Plant Research 116, 241–252.
Structure and function of cytokinin oxidase/dehydrogenase genes of maize, rice, Arabidopsis and other species.Crossref | GoogleScholarGoogle Scholar |

Scholl EH, Thorne JL, McCarter JP, Bird DM (2003) Horizontally transferred genes in plant-parasitic nematodes: a high-throughput genomic approach. Genome Biology 4, R39
Horizontally transferred genes in plant-parasitic nematodes: a high-throughput genomic approach.Crossref | GoogleScholarGoogle Scholar |

Schoor S, Farrow S, Blaschke H, Lee S, Perry G, von Schwartzenberg K, Emery N, Moffatt B (2011) Adenosine kinase contributes to cytokinin interconversion in Arabidopsis. Plant Physiology 157, 659–672.
Adenosine kinase contributes to cytokinin interconversion in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Schultz JC (2002) Shared signals and the potential for phylogenetic espionage between plants and animals. Integrative and Comparative Biology 42, 454–462.
Shared signals and the potential for phylogenetic espionage between plants and animals.Crossref | GoogleScholarGoogle Scholar |

Shantz EM, Steward FD (1955) The identification of compound A from coconut milk as 1,3-diphenylurea. Journal of the American Chemical Society 77, 6351–6353.
The identification of compound A from coconut milk as 1,3-diphenylurea.Crossref | GoogleScholarGoogle Scholar |

Shaw G (1994) Chemistry of adenine cytokinins. In ‘Cytokinins. Chemistry, activity and function’. (Ed. DWS Mok, MC Mok) pp. 15–34. (CRC Press: Boca Raton)

Siemens J, Keller I, Sarx J, Kunz S, Schuller A, Nagel W, Schmülling T, Parniske M, Ludwig-Müller J (2006) Transcriptome analysis of Arabidopsis clubroots indicate a key role for cytokinins in disease development. Molecular Plant-Microbe Interactions 19, 480–494.
Transcriptome analysis of Arabidopsis clubroots indicate a key role for cytokinins in disease development.Crossref | GoogleScholarGoogle Scholar |

Simpson MG (2006) ‘Plant systematic.’ (Elsevier: Amsterdam)

Šmehilová M, Galuszka P, Bilyeu KD, Jaworek P, Kowalska M, Šebela M, Sedlářová M, English JT, Frébort I (2009) Subcellular localization and biochemical comparison of cytosolic and secreted cytokinin dehydrogenase enzymes from maize. Journal of Experimental Botany 60, 2701–2712.
Subcellular localization and biochemical comparison of cytosolic and secreted cytokinin dehydrogenase enzymes from maize.Crossref | GoogleScholarGoogle Scholar |

Spíchal L, Rakova NY, Riefler M, Mizuno T, Romanov GA, Strnad M, Schmülling T (2004) Two cytokinin receptors of Arabidopsis thaliana, CRE1/AHK4 and AHK3, differ in their ligand specificity in a bacterial assay. Plant & Cell Physiology 45, 1299–1305.
Two cytokinin receptors of Arabidopsis thaliana, CRE1/AHK4 and AHK3, differ in their ligand specificity in a bacterial assay.Crossref | GoogleScholarGoogle Scholar |

Spíchal L, Werner T, Popa I, Riefler M, Schmülling T, Strnad M (2009) The purine derivative PI-55 blocks cytokinin action via receptor inhibition. The FEBS Journal 276, 244–253.
The purine derivative PI-55 blocks cytokinin action via receptor inhibition.Crossref | GoogleScholarGoogle Scholar |

Spinola M, Galvan A, Pignatiello C, Conti B, Pastorino U, Nicander B, Paroni R, Dragani TA (2005) Identification and functional characterization of the candidate tumor suppressor gene TRIT1 in human lung cancer. Oncogene 24, 5502–5509.
Identification and functional characterization of the candidate tumor suppressor gene TRIT1 in human lung cancer.Crossref | GoogleScholarGoogle Scholar |

Stirk WA, Van Staden J (2010) Flow of cytokinins through the environment. Plant Growth Regulation 62, 101–116.
Flow of cytokinins through the environment.Crossref | GoogleScholarGoogle Scholar |

Stolz A, Riefler M, Lomin SN, Achazi K, Romanov GA, Schmülling T (2011) The specificity of cytokinin signalling in Arabidopsis thaliana is mediated by differing ligand affinities and expression profiles of the receptors. The Plant Journal 67, 157–168.
The specificity of cytokinin signalling in Arabidopsis thaliana is mediated by differing ligand affinities and expression profiles of the receptors.Crossref | GoogleScholarGoogle Scholar |

Strnad M (1997) The aromatic cytokinins. Physiologia Plantarum 101, 674–688.
The aromatic cytokinins.Crossref | GoogleScholarGoogle Scholar |

Sugawara H, Ueda N, Kojima M, Makita N, Yamaya T, Sakakibara H (2008) Structural insight into the reaction mechanism and evolution of cytokinin biosynthesis. Proceedings of the National Academy of Sciences of the United States of America 105, 2734–2739.
Structural insight into the reaction mechanism and evolution of cytokinin biosynthesis.Crossref | GoogleScholarGoogle Scholar |

Suzuki T, Sakurai K, Imamura A, Nakamura A, Ueguchi C, Mizuno T (2000) Compilation and characterization of histidine-containing phosphotransmitters implicated in His-to-Asp phosphorelay in plants: AHP signal transducers of Arabidopsis thaliana. Bioscience, Biotechnology, and Biochemistry 64, 2486–2489.
Compilation and characterization of histidine-containing phosphotransmitters implicated in His-to-Asp phosphorelay in plants: AHP signal transducers of Arabidopsis thaliana.Crossref | GoogleScholarGoogle Scholar |

Suzuki T, Miwa K, Ishikawa K, Yamada H, Aiba H, Mizuno T (2001) The Arabidopsis sensor His-kinase, AHK4, can respond to cytokinins. Plant & Cell Physiology 42, 107–113.
The Arabidopsis sensor His-kinase, AHK4, can respond to cytokinins.Crossref | GoogleScholarGoogle Scholar |

Takei K, Sakakibara H, Sugiyama T (2001) Identification of genes encoding adenylate isopentenyltransferase, a cytokinin biosynthesis enzyme, in Arabidopsis thaliana. The Journal of Biological Chemistry 276, 26405–26410.
Identification of genes encoding adenylate isopentenyltransferase, a cytokinin biosynthesis enzyme, in Arabidopsis thaliana.Crossref | GoogleScholarGoogle Scholar |

Takei K, Ueda N, Aoki K, Kuromori T, Hirayama T, Shinozaki K, Yamaya T, Sakakibara H (2004a) AtIPT3 is a key determinant of nitrate-dependent cytokinin biosynthesis in Arabidopsis. Plant & Cell Physiology 45, 1053–1062.
AtIPT3 is a key determinant of nitrate-dependent cytokinin biosynthesis in Arabidopsis.Crossref | GoogleScholarGoogle Scholar |

Takei K, Yamaya T, Sakakibara H (2004b) Arabidopsis CYP735A1 and CYP735A2 encode cytokinin hydroxylases that catalyze the biosynthesis of trans-zeatin. The Journal of Biological Chemistry 279, 41866–41872.
Arabidopsis CYP735A1 and CYP735A2 encode cytokinin hydroxylases that catalyze the biosynthesis of trans-zeatin.Crossref | GoogleScholarGoogle Scholar |

Taller B (1994) Distribution, biosynthesis and function of cytokinins in tRNA. In ‘Cytokinins. Chemistry, activity and function’. (Ed. DWS Mok, MC Mok) pp. 101–112. (CRC Press: Boca Raton)

Tan E, Besant PG, Attwood PV (2002) Mammalian histidine kinases: do they REALLY exist? Biochemistry 41, 3843–3851.
Mammalian histidine kinases: do they REALLY exist?Crossref | GoogleScholarGoogle Scholar |

Tanaka M, Takei K, Kojima M, Sakakibara H, Mori H (2006) Auxin controls local cytokinin biosynthesis in the nodal stem in apical dominance. The Plant Journal 45, 1028–1036.
Auxin controls local cytokinin biosynthesis in the nodal stem in apical dominance.Crossref | GoogleScholarGoogle Scholar |

Tarkowská D, Doležal K, Tarkowski P, Åstot C, Holub J, Fuksová K, Schmülling T, Sandberg G, Strnad M (2003) Identification of new aromatic cytokinins in Arabidopsis thaliana and Populus × canadensis leaves by LC-(+)ESI-MS and capillary liquid chromatography/frit–fast atom bombardment mass spectrometry. Physiologia Plantarum 117, 579–590.
Identification of new aromatic cytokinins in Arabidopsis thaliana and Populus × canadensis leaves by LC-(+)ESI-MS and capillary liquid chromatography/frit–fast atom bombardment mass spectrometry.Crossref | GoogleScholarGoogle Scholar |

Taya Y, Tanaka Y, Nishimura S (1978) 5′-AMP is a direct precursor of cytokinin in Dictyostelium discoideum. Nature 271, 545–547.
5′-AMP is a direct precursor of cytokinin in Dictyostelium discoideum.Crossref | GoogleScholarGoogle Scholar |

Tirichine L, Sandal N, Madsen LH, Radutoiu S, Albrektsen AS, Sato S, Adamitu E, Tabata S, Stougaard J (2007) A gain-of-function mutation in a CK receptor triggers spontaneous root nodule organogenesis. Science 315, 104–107.
A gain-of-function mutation in a CK receptor triggers spontaneous root nodule organogenesis.Crossref | GoogleScholarGoogle Scholar |

To JP, Haberer G, Ferreira FJ, Deruère J, Mason MG, Schaller GE, Alonso JM, Ecker JR, Kieber JJ (2004) Type-A Arabidopsis response regulators are partially redundant negative regulators of cytokinin signaling. The Plant Cell 16, 658–671.
Type-A Arabidopsis response regulators are partially redundant negative regulators of cytokinin signaling.Crossref | GoogleScholarGoogle Scholar |

To JP, Deruère J, Maxwell BB, Morris VF, Hutchison CE, Ferreira FJ, Schaller GE, Kieber JJ (2007) Cytokinin regulates type-A Arabidopsis response regulator activity and protein stability via two-component phosphorelay. The Plant Cell 19, 3901–3914.
Cytokinin regulates type-A Arabidopsis response regulator activity and protein stability via two-component phosphorelay.Crossref | GoogleScholarGoogle Scholar |

Tokunaga H, Kojima M, Kuroha T, Ishida T, Sugimoto K, Kiba T, Sakakibara H (2012) Arabidopsis lonely guy (LOG) multiple mutants reveal a central role of the LOG-dependent pathway in cytokinin activation. The Plant Journal 69, 355–365.
Arabidopsis lonely guy (LOG) multiple mutants reveal a central role of the LOG-dependent pathway in cytokinin activation.Crossref | GoogleScholarGoogle Scholar |

Tsoupras G, Luu B, Hoffmann JA (1983) A cytokinin (isopentenyl-adenosyl-mononucleotide) linked to ecdysone in newly laid eggs of Locusta migratoria. Science 220, 507–509.
A cytokinin (isopentenyl-adenosyl-mononucleotide) linked to ecdysone in newly laid eggs of Locusta migratoria.Crossref | GoogleScholarGoogle Scholar |

Turner JE, Mok DWS, Mok MC, Shaw G (1987) Isolation and partial purification of an enzyme catalyzing the formation of O-xylosylzeatin in Phaseolus vulgaris embryos. Proceedings of the National Academy of Sciences of the United States of America 84, 3714–3717.
Isolation and partial purification of an enzyme catalyzing the formation of O-xylosylzeatin in Phaseolus vulgaris embryos.Crossref | GoogleScholarGoogle Scholar |

Ueguchi C, Koizumi H, Suzuki T, Mizuno T (2001a) Novel family of sensor histidine kinase genes in Arabidopsis thaliana. Plant & Cell Physiology 42, 231–235.
Novel family of sensor histidine kinase genes in Arabidopsis thaliana.Crossref | GoogleScholarGoogle Scholar |

Ueguchi C, Sato S, Kato T, Tabata S (2001b) The AHK4 gene involved in the cytokinin-signaling pathway as a direct receptor molecule in Arabidopsis thaliana. Plant & Cell Physiology 42, 751–755.
The AHK4 gene involved in the cytokinin-signaling pathway as a direct receptor molecule in Arabidopsis thaliana.Crossref | GoogleScholarGoogle Scholar |

Weerasinghe RR, Bird DM, Allen NS (2005) Root-knot nematodes and bacterial Nod factors elicit common signal transduction events in Lotus japonicus root hair cells. Proceedings of the National Academy of Sciences of the United States of America 102, 3147–3152.
Root-knot nematodes and bacterial Nod factors elicit common signal transduction events in Lotus japonicus root hair cells.Crossref | GoogleScholarGoogle Scholar |

Werner T, Schmülling T (2009) Cytokinin action in plant development. Current Opinion in Plant Biology 12, 527–538.
Cytokinin action in plant development.Crossref | GoogleScholarGoogle Scholar |

Werner T, Motyka V, Strnad M, Schmülling T (2001) Regulation of plant growth by cytokinin. Proceedings of the National Academy of Sciences of the United States of America 98, 10487–10492.
Regulation of plant growth by cytokinin.Crossref | GoogleScholarGoogle Scholar |

Werner T, Motyka V, Laucou V, Smets R, Van Onckelen H, Schmülling T (2003) Cytokinin-deficient transgenic Arabidopsis plants show multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristem activity. The Plant Cell 15, 2532–2550.
Cytokinin-deficient transgenic Arabidopsis plants show multiple developmental alterations indicating opposite functions of cytokinins in the regulation of shoot and root meristem activity.Crossref | GoogleScholarGoogle Scholar |

Werner T, Holst K, Pörs Y, Guivarc’h A, Mustroph A, Chriqui D, Grimm B, Schmülling T (2008) Cytokinin deficiency causes distinct changes of sink and source parameters in tobacco shoots and roots. Journal of Experimental Botany 59, 2659–2672.
Cytokinin deficiency causes distinct changes of sink and source parameters in tobacco shoots and roots.Crossref | GoogleScholarGoogle Scholar |

West AH, Stock AM (2001) Histidine kinases and response regulator proteins in two-component signaling systems. Trends in Biochemical Sciences 26, 369–376.
Histidine kinases and response regulator proteins in two-component signaling systems.Crossref | GoogleScholarGoogle Scholar |

Whitty CD, Hall RH (1974) A cytokinin oxidase in Zea mays. Canadian Journal of Biochemistry 52, 789–799.
A cytokinin oxidase in Zea mays.Crossref | GoogleScholarGoogle Scholar |

Wulfetange K, Lomin SN, Romanov GA, Stolz A, Heyl A, Schmülling T (2011) The cytokinin receptors of Arabidopsis thaliana are locating mainly to the endoplasmic reticulum. Plant Physiology 156, 1808–1818.
The cytokinin receptors of Arabidopsis thaliana are locating mainly to the endoplasmic reticulum.Crossref | GoogleScholarGoogle Scholar |

Yamada H, Suzuki T, Terada K, Takei K, Ishikawa K, Miwa K, Yamashino T, Mizuno T (2001) The Arabidopsis AHK4 histidine kinase is a cytokinin-binding receptor that transduces cytokinin signals across the membrane. Plant & Cell Physiology 42, 1017–1023.
The Arabidopsis AHK4 histidine kinase is a cytokinin-binding receptor that transduces cytokinin signals across the membrane.Crossref | GoogleScholarGoogle Scholar |

Yang S, Yu H, Xu Y, Goh CJ (2003) Investigation of cytokinin-deficient phenotypes in Arabidopsis by ectopic expression of orchid DsCKX1. FEBS Letters 555, 291–296.
Investigation of cytokinin-deficient phenotypes in Arabidopsis by ectopic expression of orchid DsCKX1.Crossref | GoogleScholarGoogle Scholar |

Yevdakova NA, von Schwartzenberg K (2007) Characterisation of a prokaryote-type tRNA-isopentenyltransferase gene from the moss Physcomitrella patens. Planta 226, 683–695.
Characterisation of a prokaryote-type tRNA-isopentenyltransferase gene from the moss Physcomitrella patens.Crossref | GoogleScholarGoogle Scholar |

Yonekura-Sakakibara K, Kojima M, Yamaya T, Sakakibara H (2004) Molecular characterization of cytokinin-responsive histidine kinases in maize. Differential ligand preferences and response to cis-zeatin. Plant Physiology 134, 1654–1661.
Molecular characterization of cytokinin-responsive histidine kinases in maize. Differential ligand preferences and response to cis-zeatin.Crossref | GoogleScholarGoogle Scholar |

Zhang R, Zhang X, Wang J, Letham DS, McKinney SA, Higgins TJV (1995) The effect of auxin on cytokinin levels and metabolism in transgenic tobacco tissue expressing an ipt gene. Planta 196, 84–94.
The effect of auxin on cytokinin levels and metabolism in transgenic tobacco tissue expressing an ipt gene.Crossref | GoogleScholarGoogle Scholar |

Zhou C, Huang RH (2008) Crystallographic snapshots of eukaryotic dimethylallyltransferase acting on tRNA: insight into tRNA recognition and reaction mechanism. Proceedings of the National Academy of Sciences of the United States of America 105, 16142–16147.
Crystallographic snapshots of eukaryotic dimethylallyltransferase acting on tRNA: insight into tRNA recognition and reaction mechanism.Crossref | GoogleScholarGoogle Scholar |