Biology:Cytokinin

From HandWiki
Short description: Class of plant hormones promoting cell division
The cytokinin zeatin is named after the genus of corn, Zea.

Cytokinins (CK) are a class of plant hormones 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.

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).[1] Most adenine-type cytokinins are synthesized in roots.[2] Cambium and other actively dividing tissues also synthesize cytokinins.[3] No phenylurea cytokinins have been found in plants.[4] Cytokinins participate in local and long-distance signalling, with the same transport mechanism as purines and nucleosides.[5] Typically, cytokinins are transported in the xylem.[2]

Cytokinins act in concert with auxin, another plant growth hormone. The two are complementary,[6] [7] having generally opposite effects.[2]

History

The idea of specific substances required for cell division to occur in plants actually dates back to the German physiologist J. Wiesner, who, in 1892, proposed that initiation of cell division is evoked by endogenous factors, indeed a proper balance among endogenous factors. Somewhat later, the Austrian plant physiologist, G. Haberlandt, reported in 1913 that an unknown substance diffuses from the phloem tissue which can induce cell division in the parenchymatic tissue of potato tubers.[8] 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.[9][10]

Jablonski and 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 pith cells.[11][12] Miller and his co-workers (1954) isolated and purified the cell division substance in crystallised form from autoclaved herring fish sperm DNA.[11] 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.[8]

The first naturally occurring cytokinin was isolated and crystallised simultaneously by Miller and D.S. Lethum (1963–65) from the milky endosperm of corn (Zea mays) and named Zeatin. Lethem (1963) proposed the term Cytokinins for such substances.[13]

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] 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.[2] Moreover, it has been shown that cytokinin alone has no effect on 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. A higher ratio of cytokinin induces growth of shoot buds, while a higher ratio of auxin induces root formation.[2]

Cytokinins have been shown to slow aging of plant organs by preventing protein breakdown, activating protein synthesis, and assembling nutrients from nearby tissues.[2] 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.[14]

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[15] and Nicotiana tabacum.[16] Also in context of 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..[17]

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.[18]

Mode of action

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 transcriptions factors. The phosphorylated, and thus activated, type-B RRs regulate the transcription of numerous genes, including the type-A RRs. The type-A RRs negatively regulate the pathway.[19]

Biosynthesis

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 pyrophosphate (DMAPP) or hydroxymethylbutenyl pyrophosphate (HMBPP) as prenyl donors.[20] This reaction is the rate-limiting step in cytokinin biosynthesis. DMADP and HMBDP used in cytokinin biosynthesis are produced by the methylerythritol phosphate pathway (MEP).[20]

Cytokinins can also be produced by recycled tRNAs in plants and bacteria.[20][21] 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.[20] The prenylation of these adenines is carried out by tRNA-isopentenyltransferase.[21]

Auxin is known to regulate the biosynthesis of cytokinin.[22]

Uses

Because cytokinins promote plant cell division and growth, they have been studied since the 1970s as potential agrochemicals, however they have yet to be widely adopted, probably due to the complex nature of their effects.[23] One study found that applying cytokinin to cotton seedlings led to a 5–10% increase in yield under drought conditions.[24] Some cytokinins are utilized in tissue culture of plants and can also be used to promote the germination of seeds.

References

  1. ↑ "Thidiazuron-Induced Tissue Culture Regeneration from Quartered-Seed Explants of Arachis paraguariensis". Crop Science 52 (3): 555. 2012. doi:10.2135/cropsci2011.07.0367. https://acsess.onlinelibrary.wiley.com/doi/10.2135/cropsci2011.07.0367. 
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Biology (8th ed.). San Francisco: Pearson, Benjamin Cummings. 2008. pp. 827–30. ISBN 978-0-555-03883-3. 
  3. ↑ "Localization of cytokinin biosynthetic sites in pea plants and carrot roots". Plant Physiology 78 (3): 510–513. July 1985. doi:10.1104/pp.78.3.510. PMID 16664274. 
  4. ↑ "Cytokinin Metabolism and Action". Annual Review of Plant Physiology and Plant Molecular Biology 52 (1): 89–118. June 2001. doi:10.1146/annurev.arplant.52.1.89. PMID 11337393. 
  5. ↑ "Cytokinins: activity, biosynthesis, and translocation". Annual Review of Plant Biology 57 (1): 431–449. 2006. doi:10.1146/annurev.arplant.57.032905.105231. PMID 16669769. 
  6. ↑ "The yin-yang of hormones: cytokinin and auxin interactions in plant development". The Plant Cell 27 (1): 44–63. January 2015. doi:10.1105/tpc.114.133595. PMID 25604447. 
  7. ↑ "Auxins and cytokinins - the dynamic duo of growth-regulating phytohormones heading for new shores". The New Phytologist 221 (3): 1187–1190. February 2019. doi:10.1111/nph.15556. PMID 30644580. 
  8. ↑ 8.0 8.1 "Cytokinins" (in en), Biochemistry and Physiology of Plant Hormones (New York, NY: Springer US): pp. 147–180, 1979, doi:10.1007/978-1-4684-0079-3_4, ISBN 978-1-4684-0079-3, https://doi.org/10.1007/978-1-4684-0079-3_4, retrieved 2022-06-17 
  9. ↑ "Factors in Coconut Milk Essential for Growth and Development of Very Young Datura Embryos". Science 94 (2441): 350–351. October 1941. doi:10.1126/science.94.2441.350. PMID 17729950. Bibcode: 1941Sci....94..350V. 
  10. ↑ "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." (in en). Science 145 (3633): 698–699. 1964-08-14. doi:10.1126/science.145.3633.698.c. ISSN 0036-8075. https://www.science.org/doi/10.1126/science.145.3633.698.c. 
  11. ↑ 11.0 11.1 "1955: kinetin arrives: the 50th anniversary of a new plant hormone". Plant Physiology 138 (3): 1177–1184. July 2005. doi:10.1104/pp.104.900160. PMID 16009993. 
  12. ↑ "A New Classic of Cytokinin Research: Cytokinin-Deficient Arabidopsis Plants Provide New Insights into Cytokinin Biology" (in en). The Plant Cell 15 (11): 2489–2492. 2003-11-01. doi:10.1105/tpc.151110. ISSN 1040-4651. 
  13. ↑ "The Discovery of Cytokinins", Discoveries in Plant Biology, 1, WORLD SCIENTIFIC, 1998-02-01, pp. 1–15, doi:10.1142/9789812817563_0001, ISBN 978-981-02-1313-8 
  14. ↑ "Enzymes of nitrogen mobilization in detached leaves of Lolium temulentum during senescence". Plant Physiology 142 (2): 161–169. January 1978. doi:10.1104/pp.116.1.329. PMID 24408097. 
  15. ↑ "The cytokinin-activated transcription factor ARR2 promotes plant immunity via TGA3/NPR1-dependent salicylic acid signaling in Arabidopsis". Developmental Cell 19 (2): 284–295. August 2010. doi:10.1016/j.devcel.2010.07.011. PMID 20708590. 
  16. ↑ "Cytokinins mediate resistance against Pseudomonas syringae in tobacco through increased antimicrobial phytoalexin synthesis independent of salicylic acid signaling". Plant Physiology 157 (2): 815–830. October 2011. doi:10.1104/pp.111.182931. PMID 21813654. 
  17. ↑ "Cytokinin production by Pseudomonas fluorescens G20-18 determines biocontrol activity against Pseudomonas syringae in Arabidopsis". Scientific Reports 6: 23310. March 2016. doi:10.1038/srep23310. PMID 26984671. Bibcode: 2016NatSR...623310G. 
  18. ↑ "Moss systems biology en route: phytohormones in Physcomitrella development". Plant Biology 8 (3): 397–405. May 2006. doi:10.1055/s-2006-923952. PMID 16807833. 
  19. ↑ "Cytokinin signaling in Arabidopsis". The Plant Cell 14 (Suppl): S47–S59. 2002-01-01. doi:10.1105/tpc.010444. PMID 12045269. 
  20. ↑ 20.0 20.1 20.2 20.3 "Cytokinin biosynthesis and perception". Physiologia Plantarum 126 (4): 528–538. 2006. doi:10.1111/j.1399-3054.2006.00665.x. 
  21. ↑ 21.0 21.1 "Expression of cytokinin biosynthetic isopentenyltransferase genes in Arabidopsis: tissue specificity and regulation by auxin, cytokinin, and nitrate". The Plant Journal 37 (1): 128–138. January 2004. doi:10.1046/j.1365-313x.2003.01945.x. PMID 14675438. 
  22. ↑ "Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: a factor of potential importance for auxin-cytokinin-regulated development". Proceedings of the National Academy of Sciences of the United States of America 101 (21): 8039–8044. May 2004. doi:10.1073/pnas.0402504101. PMID 15146070. Bibcode: 2004PNAS..101.8039N. 
  23. ↑ "Use of cytokinins as agrochemicals". Bioorganic & Medicinal Chemistry. Recent Developments in Agrochemistry 24 (3): 484–492. February 2016. doi:10.1016/j.bmc.2015.12.022. PMID 26719210. 
  24. ↑ Yao S (March 2010). "Plant Hormone Increases Cotton Yields in Drought Conditions". News & Events. Agricultural Research Service (ARS), U.S. Department of Agriculture. http://www.ars.usda.gov/is/pr/2010/100310.htm. 

External links



Retrieved from "https://handwiki.org/wiki/index.php?title=Biology:Cytokinin&oldid=3622060"