Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Folke Skoog discovered their effects using coconut milk in the 1940s at the University of Wisconsin–Madison. 
There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin, and 6-benzylaminopurine, and phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ). Most adenine-type cytokinins are synthesized in roots. Cambium and other actively dividing tissues also synthesize cytokinins. No phenylurea cytokinins have been found in plants. Cytokinins participate in local and long-distance signalling, with the same transport mechanism as purines and nucleosides. Typically, cytokinins are transported in the xylem.
 Mode of action
The ratio of auxin to cytokinin plays an important role in the effect of cytokinin on plant growth. Cytokinin alone has no effect on parenchyma cells. When cultured with auxin but no cytokinin, they grow large but do not divide. When cytokinin is added, the cells expand and differentiate. When cytokinin and auxin are present in equal levels, the parenchyma cells form an undifferentiated callus. More cytokinin induces growth of shoot buds, while more auxin induces root formation.
Cytokinins are involved in many plant processes, including cell division and shoot and root morphogenesis. They are known to regulate axillary bud growth and apical dominance. The "direct inhibition hypothesis" posits that these effects result from the cytokinin to auxin ratio. This theory states that auxin from apical buds travels down shoots to inhibit axillary bud growth. This promotes shoot growth, and restricts lateral branching. Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth. Simple experiments support this theory. When the apical bud is removed, the axillary buds are uninhibited, lateral growth increases, and plants become bushier. Applying auxin to the cut stem again inhibits lateral dominance.
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. 
Cytokinins have been shown to slow aging of plant organs by preventing protein breakdown, activating protein synthesis, and assembling nutrients from nearby tissues. A study that regulated leaf senescence in tobacco leaves found that wild-type leaves yellowed while transgenic leaves remained mostly green. It was hypothesized that cytokinin may affect enzymes that regulate protein synthesis and degradation.
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. 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).
Cytokinins can also be produced by recycled tRNAs in plants and bacteria. 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. The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.
Auxin is known to regulate the biosynthesis of cytokinin.
Because cytokinin promotes plant cell division and growth, produce farmers use it to increase crops. One study found that applying cytokinin to cotton seedlings led to a 5–10% yield increase under drought conditions. 
Cytokinins have recently been found to play a role in plant pathogenesis. For example, cytokinins have been described to induce resistance against Pseudomonas syringae in Arabidopsis thaliana and Nicotiana tabacum.
- Kieber JJ (March 2002). "Tribute to Folke Skoog: Recent Advances in our Understanding of Cytokinin Biology". J. Plant Growth Regul. 21 (1): 1–2. doi:10.1007/s003440010059. PMID 11981613.
- Campbell, Neil A.; Reece, Jane B.; Urry, Lisa Andrea.; Cain, Michael L.; Wasserman, Steven Alexander.; Minorsky, Peter V.; Jackson, Robert Bradley (2008). Biology (8th ed.). San Francisco: Pearson, Benjamin Cummings. pp. 827–30.
- Chen CM, Ertl JR, Leisner SM, Chang CC (July 1985). "Localization of cytokinin biosynthetic sites in pea plants and carrot roots". Plant Physiol. 78 (3): 510–3. PMC 1064767. PMID 16664274.
- Mok DW, Mok MC (June 2001). "Cytokinin Metabolism and Action". Annu. Rev. Plant Physiol. Plant Mol. Biol. 52: 89–118. doi:10.1146/annurev.arplant.52.1.89. PMID 11337393.
- Sakakibara H (2006). "Cytokinins: activity, biosynthesis, and translocation". Annu Rev Plant Biol 57: 431–49. doi:10.1146/annurev.arplant.57.032905.105231. PMID 16669769.
- Decker EL, Frank W, Sarnighausen E, Reski R (May 2006). "Moss systems biology en route: phytohormones in Physcomitrella development". Plant Biol (Stuttg) 8 (3): 397–405. doi:10.1055/s-2006-923952. PMID 16807833.
- Wingler A, Von Schaewen A, Leegood RC, Lea PJ, Quick WP (January 1998). "Regulation of Leaf Senescence by Cytokinin, Sugars, and Light". Plant Physiol 116 (1): 329–335. PMC 35173.
- Hwang I, Sakakibara H (2006). "Cytokinin biosynthesis and perception". Physiologia Plantarum 126 (4): 528–538.
- Miyawaki K, Matsumoto-Kitano M, Kakimoto T (January 2004). "Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate". Plant J. 37 (1): 128–38. PMID 14675438.
- Nordström A, Tarkowski P, Tarkowska D, et al. (May 2004). "Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: a factor of potential importance for auxin-cytokinin-regulated development". Proc. Natl. Acad. Sci. U.S.A. 101 (21): 8039–44. doi:10.1073/pnas.0402504101. PMC 419553. PMID 15146070.
- Yao S (March 2010). "Plant Hormone Increases Cotton Yields in Drought Conditions". News & Events. Agricultural Research Service (ARS), U.S. Department of Agriculture.
- Choi J, Huh SU, Kojima M, Sakakibara H, Paek K-H, Hwang I (August 2010). "The cytokinin-activated transcription factor ARR2 promotes plant immunity via TGA3/NPR1-dependent salicylic acid signaling in arabidopsis". Developmental Cell 19 (2): 284–295. doi:10.1016/j.devcel.2010.07.011. PMID 20708590.
- Großkinsky DK, Naseem M, Abdelmohsen UR, Plickert N, Engelke T, Griebel T, Zeier J, Novák O, Strnad M, Pfeifhofer H, van der Graaff E, Simon U, Roitsch T (October 2011). "Cytokinins mediate resistance against Pseudomonas syringae in tobacco through increased antimicrobial phytoalexin synthesis independent of salicylic acid signaling". Plant Physiology 157 (2): 815–830. doi:10.1104/pp.111.182931. PMC 3192561. PMID 21813654.
- Agrares Fertilizer with cytokinins
- Taiz, Lincoln; Zeiger, Eduardo (2010). "Ch. 21: Cytokinins: Regulators of Cell Division". Plant Physiology (5th ed.). Sinauer. ISBN 978-0-87893-866-7.
- Plant Physiology:Cytokinin
- Regulation of Leaf Senescence by Cytokinin, Sugar, and Light