Editing Cytokinin

{{Short description|Class of plant hormones promoting cell division}}
{{Distinguish|text=[[Cytokine]], a class of small proteins important in cell signaling}}
[[Image:Zeatin.png|thumb|right|upright=0.65|The cytokinin [[zeatin]] is named after the genus of corn, ''[[Teosinte|Zea]]''.]]

'''Cytokinins''' ('''CK''') are a class of [[plant hormone]]s that promote [[cell division]], or [[cytokinesis]], in plant roots and shoots. They are involved primarily in [[Cell (biology)|cell]] growth and [[cellular differentiation|differentiation]], but also affect [[apical dominance]], [[axillary bud]] growth, and leaf [[plant senescence|senescence]].

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).<ref>{{cite journal | vauthors = Aina O, Quesenberry K, Gallo M | year = 2012 | title = Thidiazuron-Induced Tissue Culture Regeneration from Quartered-Seed Explants of Arachis paraguariensis | url = https://acsess.onlinelibrary.wiley.com/doi/10.2135/cropsci2011.07.0367| journal = Crop Science | volume = 52 | issue = 3| page = 555 | doi = 10.2135/cropsci2011.07.0367 | s2cid = 82510749 | url-access = subscription }}</ref>  Most adenine-type cytokinins are synthesized in roots.<ref name=campbell>{{cite book | vauthors = Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Jackson RB  |title=Biology |publisher=Pearson, Benjamin Cummings |location=San Francisco |year=2008 |pages=827–30 |edition=8th | isbn = 978-0-555-03883-3 }}</ref>  [[Cambium (botany)|Cambium]] and other actively dividing tissues also synthesize cytokinins.<ref>{{cite journal | vauthors = Chen CM, Ertl JR, Leisner SM, Chang CC | title = Localization of cytokinin biosynthetic sites in pea plants and carrot roots | journal = Plant Physiology | volume = 78 | issue = 3 | pages = 510–513 | date = July 1985 | pmid = 16664274 | pmc = 1064767 | doi = 10.1104/pp.78.3.510 }}</ref> No phenylurea cytokinins have been found in plants.<ref>{{cite journal | vauthors = Mok DW, Mok MC | title = Cytokinin Metabolism and Action | journal = Annual Review of Plant Physiology and Plant Molecular Biology | volume = 52 | issue = 1 | pages = 89–118 | date = June 2001 | pmid = 11337393 | doi = 10.1146/annurev.arplant.52.1.89 }}</ref> Cytokinins participate in local and long-distance signalling, with the same transport mechanism as purines and nucleosides.<ref>{{cite journal | vauthors = Sakakibara H | title = Cytokinins: activity, biosynthesis, and translocation | journal = Annual Review of Plant Biology | volume = 57 | issue = 1 | pages = 431–449 | year = 2006 | pmid = 16669769 | doi = 10.1146/annurev.arplant.57.032905.105231 | s2cid = 25584314 }}</ref> Typically, cytokinins are transported in the [[xylem]].<ref name=campbell/>

Cytokinins act in concert with [[auxin]], another plant growth hormone. The two are complementary,<ref>{{cite journal | vauthors = Schaller GE, Bishopp A, Kieber JJ | title = The yin-yang of hormones: cytokinin and auxin interactions in plant development | journal = The Plant Cell | volume = 27 | issue = 1 | pages = 44–63 | date = January 2015 | pmid = 25604447 | pmc = 4330578 | doi = 10.1105/tpc.114.133595 }}</ref>
<ref>{{cite journal | vauthors = Großkinsky DK, Petrášek J | title = Auxins and cytokinins - the dynamic duo of growth-regulating phytohormones heading for new shores | journal = The New Phytologist | volume = 221 | issue = 3 | pages = 1187–1190 | date = February 2019 | pmid = 30644580 | doi = 10.1111/nph.15556 | doi-access = free }}</ref> having generally opposite effects.<ref name=campbell/>

== History ==
The idea of specific substances required for cell division to occur in plants actually dates back to the Swiss physiologist [[Julius Wiesner|J. Wiesner]], who, in 1892, proposed that initiation of cell division is evoked by endogenous factors, specifically a proper balance among them. Austrian plant physiologist, [[Gottlieb Haberlandt|G. Haberlandt]], reported in 1913 that an unknown substance diffuses from the phloem tissue which can induce cell division in the [[Parenchyma (botany)|parenchymatic]] tissue of [[Potato|potato tubers]].<ref name="Moore-1979">{{Citation | vauthors = Moore TC |title=Cytokinins |date=1979 |url=https://doi.org/10.1007/978-1-4684-0079-3_4 |work=Biochemistry and Physiology of Plant Hormones |pages=147–180 | veditors = Moore TC |place=New York, NY |publisher=Springer US |language=en |doi=10.1007/978-1-4684-0079-3_4 |isbn=978-1-4684-0079-3 |access-date=2022-06-17|url-access=subscription }}</ref> In 1941, [[Johannes Van Overbeek]] found that the milky [[endosperm]] of immature [[coconut]] also had this factor, which stimulated cell division and differentiation in very young ''[[Datura]]'' embryos.<ref>{{cite journal | vauthors = VAN Overbeek J, Conklin ME, Blakeslee AF | title = Factors in Coconut Milk Essential for Growth and Development of Very Young Datura Embryos | journal = Science | volume = 94 | issue = 2441 | pages = 350–351 | date = October 1941 | pmid = 17729950 | doi = 10.1126/science.94.2441.350 | bibcode = 1941Sci....94..350V }}</ref><ref>{{Cite journal | vauthors = Collins S |date=1964-08-14 |title=Plant Physiology: The Lore of Living Plants . By Johannes van Overbeek and Harry K. Wong. National Science Teachers Association, Washington, D.C., 1964. 160 pp. 50g. |url=https://www.science.org/doi/10.1126/science.145.3633.698.c |journal=Science |language=en |volume=145 |issue=3633 |pages=698–699 |doi=10.1126/science.145.3633.698.c |s2cid=239878163 |issn=0036-8075|url-access=subscription }}</ref>

[[Aleksander Jabłoński|Jablonski]] and [[Folke K. Skoog|Skoog]] (1954) extended the work of Haberlandt and reported that a substance present in the vascular tissue was responsible for causing cell division in the sith cells.<ref name="Amasino-2005">{{cite journal | vauthors = Amasino R | title = 1955: kinetin arrives: the 50th anniversary of a new plant hormone | journal = Plant Physiology | volume = 138 | issue = 3 | pages = 1177–1184 | date = July 2005 | pmid = 16009993 | pmc = 1176392 | doi = 10.1104/pp.104.900160 }}</ref><ref>{{Cite journal |vauthors = Eckardt NA |date=2003-11-01 |title=A New Classic of Cytokinin Research: Cytokinin-Deficient Arabidopsis Plants Provide New Insights into Cytokinin Biology |journal=The Plant Cell |language=en |volume=15 |issue=11 |pages=2489–2492 |doi=10.1105/tpc.151110 |issn=1040-4651 |pmc=540265}}</ref> [[Carlos O. Miller|Miller]] and his co-workers (1954) isolated and purified the cell division substance in crystallised form from [[autoclave]]d [[Herring|herring fish]] sperm DNA.<ref name="Amasino-2005" /> This active compound was named as [[kinetin]] because of its ability to promote cell division and was the first cytokinin to be named. Kinetin was later identified to be 6-furfuryl-amino purine. Later on, the generic name kinin was suggested to include kinetin and other substances having similar properties.<ref name="Moore-1979" />

The first naturally occurring cytokinin was isolated and crystallised simultaneously by Miller and D.S. Lethum (1963–65) from the milky endosperm of [[Maize|corn]] (''Zea mays'') and named zeatin. Lethem (1963) proposed the term cytokinins for such substances.<ref>{{Citation | vauthors = Chen CM |chapter=The Discovery of Cytokinins |date=1998-02-01 |title=Discoveries in Plant Biology |volume=1 |pages=1–15 |publisher=WORLD SCIENTIFIC |doi=10.1142/9789812817563_0001 |isbn=978-981-02-1313-8}}</ref>

== Function ==

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. According to the "direct inhibition hypothesis", these effects result from the ratio of cytokinin to auxin.{{Citation needed|date=June 2021}} 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.<ref name=campbell/> Moreover, it has been shown that cytokinin alone has no effect on [[Ground tissue#Parenchyma|parenchyma]] cells. When cultured with auxin but no cytokinin, they grow large but do not divide.  When cytokinin and auxin are both added together, the cells expand and differentiate. When cytokinin and auxin are present in equal levels, the parenchyma cells form an undifferentiated [[callus (cell biology)|callus]].  A higher ratio of cytokinin induces growth of [[Shoot (botany)|shoot]] buds, while a higher ratio of auxin induces [[root]] formation.<ref name=campbell/>

Cytokinins have been shown to slow aging of plant organs by preventing [[protein]] breakdown, activating protein synthesis, and assembling [[nutrients]] from nearby tissues.<ref name=campbell/> 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.<ref>{{cite journal | vauthors = Thomas H | title = Enzymes of nitrogen mobilization in detached leaves of Lolium temulentum during senescence | journal = Plant Physiology | volume = 142 | issue = 2 | pages = 161–169 | date = January 1978 | pmc = 35173 | doi = 10.1104/pp.116.1.329 | pmid = 24408097 }}</ref>

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]]''<ref>{{cite journal | vauthors = Choi J, Huh SU, Kojima M, Sakakibara H, Paek KH, Hwang I | title = The cytokinin-activated transcription factor ARR2 promotes plant immunity via TGA3/NPR1-dependent salicylic acid signaling in Arabidopsis | journal = Developmental Cell | volume = 19 | issue = 2 | pages = 284–295 | date = August 2010 | pmid = 20708590 | doi = 10.1016/j.devcel.2010.07.011 | doi-access = free }}</ref> and ''[[Nicotiana tabacum]]''.<ref>{{cite journal | vauthors = 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 | display-authors = 6 | title = Cytokinins mediate resistance against Pseudomonas syringae in tobacco through increased antimicrobial phytoalexin synthesis independent of salicylic acid signaling | journal = Plant Physiology | volume = 157 | issue = 2 | pages = 815–830 | date = October 2011 | pmid = 21813654 | pmc = 3192561 | doi = 10.1104/pp.111.182931 }}</ref> Also in context of [[biological pest control|biological control]] of plant diseases cytokinins seem to have potential functions. Production of cytokinins by ''[[Pseudomonas fluorescens]]'' G20-18 has been identified as a key determinant to efficiently control the infection of ''A. thaliana'' with ''P. syringae.''.<ref>{{cite journal | vauthors = Großkinsky DK, Tafner R, Moreno MV, Stenglein SA, García de Salamone IE, Nelson LM, Novák O, Strnad M, van der Graaff E, Roitsch T | display-authors = 6 | title = Cytokinin production by Pseudomonas fluorescens G20-18 determines biocontrol activity against Pseudomonas syringae in Arabidopsis | journal = Scientific Reports | volume = 6 | pages = 23310 | date = March 2016 | pmid = 26984671 | pmc = 4794740 | doi = 10.1038/srep23310 | bibcode = 2016NatSR...623310G }}</ref>

While cytokinin action in [[vascular plant]]s 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 [[Cellular differentiation|differentiation]] of a specific single cell, and thus is a very specific effect of cytokinin.<ref>{{cite journal | vauthors = Decker EL, Frank W, Sarnighausen E, Reski R | title = Moss systems biology en route: phytohormones in Physcomitrella development | journal = Plant Biology | volume = 8 | issue = 3 | pages = 397–405 | date = May 2006 | pmid = 16807833 | doi = 10.1055/s-2006-923952 | bibcode = 2006PlBio...8..397D | citeseerx = 10.1.1.319.9790 }}</ref>

==Mode of action==

[[Cytokinin signaling and response regulator protein|Cytokinin signaling]] in plants is mediated by a two-component phosphorelay. This pathway is initiated by cytokinin binding to a [[histidine kinase]] receptor in the [[endoplasmic reticulum]] membrane. This results in the [[autophosphorylation]] of the receptor, with the phosphate then being transferred to a phosphotransfer protein. The phosphotransfer proteins can then phosphorylate the type-B response regulators (RR) which are a family of [[Transcription factor|transcriptions factors]]. The phosphorylated, and thus activated, type-B RRs regulate the transcription of numerous genes, including the type-A ''RR''s. The type-A RRs negatively regulate the pathway.<ref>{{cite journal | vauthors = Hutchison CE, Kieber JJ | title = Cytokinin signaling in Arabidopsis | journal = The Plant Cell | volume = 14 | issue = Suppl | pages = S47–S59 | date = 2002-01-01 | pmid = 12045269 | pmc = 151247 | doi = 10.1105/tpc.010444 }}</ref>

==Biosynthesis==
Adenosine phosphate-isopentenyltransferase (IPT) [[catalyst|catalyses]] the first reaction in the [[biosynthesis]] of [[isoprene]] cytokinins. It may use [[Adenosine triphosphate|ATP]], [[Adenosine diphosphate|ADP]], or [[Adenosine monophosphate|AMP]] as [[Substrate (biochemistry)|substrate]]s and may use [[dimethylallyl pyrophosphate]] (DMAPP) or [[hydroxymethylbutenyl pyrophosphate]] (HMBPP) as [[Prenylation|prenyl donors]].<ref name="biosynthesis">{{cite journal |vauthors=Hwang I, Sakakibara H |title=Cytokinin biosynthesis and perception |journal=Physiologia Plantarum |volume=126 |issue=4 |pages=528–538 |year=2006 |doi=10.1111/j.1399-3054.2006.00665.x|doi-access=free }}</ref> This reaction is the [[Rate-determining step|rate-limiting step]] in cytokinin [[biosynthesis]]. DMADP and HMBDP used in cytokinin biosynthesis are  produced by the [[Non-mevalonate pathway|methylerythritol phosphate pathway]] (MEP).<ref name="biosynthesis" />

Cytokinins can also be produced by recycled [[tRNA]]s in plants and [[bacteria]].<ref name="biosynthesis" /><ref name="ipts">{{cite journal | vauthors = Miyawaki K, Matsumoto-Kitano M, Kakimoto T | title = Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate | journal = The Plant Journal | volume = 37 | issue = 1 | pages = 128–138 | date = January 2004 | pmid = 14675438 | doi = 10.1046/j.1365-313x.2003.01945.x | doi-access = free }}</ref> tRNAs with [[anticodon]]s that start with a [[uridine]] and carrying an already-prenylated adenosine adjacent to the anticodon release on degradation the adenosine as a cytokinin.<ref name="biosynthesis" /> The [[prenylation]] of these adenines is carried out by [[TRNA isopentenyltransferase|tRNA-isopentenyltransferase]].<ref name="ipts" />

[[Auxin]] is known to regulate the [[biosynthesis]] of cytokinin.<ref>{{cite journal | vauthors = Nordström A, Tarkowski P, Tarkowska D, Norbaek R, Astot C, Dolezal K, Sandberg G | title = Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: a factor of potential importance for auxin-cytokinin-regulated development | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 21 | pages = 8039–8044 | date = May 2004 | pmid = 15146070 | pmc = 419553 | doi = 10.1073/pnas.0402504101 | doi-access = free | bibcode = 2004PNAS..101.8039N }}</ref>

==Uses==
Because cytokinins promote plant [[cell division]] and growth, they have been studied since the 1970s as potential [[agrochemical]]s, however they have yet to be widely adopted, probably due to the complex nature of their effects.<ref>{{cite journal | vauthors = Koprna R, De Diego N, Dundálková L, Spíchal L | title = Use of cytokinins as agrochemicals | journal = Bioorganic & Medicinal Chemistry | volume = 24 | issue = 3 | pages = 484–492 | date = February 2016 | pmid = 26719210 | doi = 10.1016/j.bmc.2015.12.022 | series = Recent Developments in Agrochemistry }}</ref> One study found that applying cytokinin to [[cotton]] [[seedling]]s led to a 5–10% increase in yield under drought conditions.<ref>{{cite web |author=Yao S |title=Plant Hormone Increases Cotton Yields in Drought Conditions |date=March 2010 |work=News & Events |publisher=Agricultural Research Service (ARS), U.S. Department of Agriculture |url=http://www.ars.usda.gov/is/pr/2010/100310.htm}}</ref> Some cytokinins are utilized in [[tissue culture]] of plants and can also be used to promote the [[germination]] of [[seed]]s.

== References ==
{{reflist}}

== External links ==
* [http://www.agrares.com/en/gibberellin-gibberellins-auxins-cytokinins-plant-growth/phytohormones_gibberellins_auxins_cytokinins_fruit_growing.htm Agrares Fertilizer with cytokinins]
* {{cite book | vauthors = Lincoln T, Zeiger E |chapter=Ch. 21: Cytokinins: Regulators of Cell Division |chapter-url=http://5e.plantphys.net/chapter.php?ch=21 |title=Plant Physiology |publisher=Sinauer |year=2010  |isbn=978-0-87893-866-7 |edition=5th |url=http://www.plantphys.net/}}
* [http://employees.csbsju.edu/ssaupe/biol327/Lecture/cytokinin.htm Plant Physiology:Cytokinin]
* [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC35173/ Regulation of Leaf Senescence by Cytokinin, Sugar, and Light]

{{Plant_hormones}}
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[[Category:Plant hormones]]
[[Category:Cytokinins| ]]
[[Category:Aging-related substances in plants]]