Cytokinins are a class of plant hormones that promote cell division. This presentation discusses the history, chemistry, biosynthesis, functions, and signaling of cytokinins. It explains that cytokinins are synthesized through the transfer of an isopentenyl group from isopentenyl pyrophosphate to AMP by the enzyme cytokinin synthase. Cytokinin signaling involves histidine kinases that phosphorylate response regulators, including type A and B ARR genes, to activate cytokinin responses and form a negative feedback loop.

1. PP 503: Physiological and molecular responses of plant to abiotic
stresses (2+1)
Cytokinin Biosynthesis and
transduction
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Presented by: Ekatpure Sachin Chandrakant (PhD Research Scholar)

2. What is a hormone?
 Biochemical which regulates growth based on biological and
environmental influences
 Synonyms: Plant hormones, plant growth regulators (PGRs),
Phytohormone
 Regulate growth and development
 Mobile throughout plant
 Environment and stress responsive
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3. Major plant hormones
 Auxin – Greek: auxein; to grow or increase
 Cytokinin – cytokinesis (cell division)
 Abscisic acid – abscission
 Jasmonic acid – found in jasmine oil
 Gibberellic acid – pathogen Gibberella
 Ethylene – chemical brother to ethanol
 Brassinosteroids – derived from Brassica spp.
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4. Cytokinins
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5. Definition
Cytokinins (CK) are a class of plant growth substances (phyto hormones)
that promote cell division, in plant roots and shoots.
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6. History
1. Discovery:
 1913, Gottlieb Haberlandt.
 1941, Johannes van Overbeek
 1954, Jablonski and Skoog.
2. Isolation:
 The first cytokinin (kinetin) was isolated in 1955 by Miller
 The first naturally occurring cytokinin (zeatin) was isolated in 1961 by
Miller
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7. Basic chemistry
 All cytokinins have basic
Adenine ring structure.
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8. Types
 There are two types of cytokinins:
1. Adenine-type cytokinins ; represented by kinetin, zeatin, and 6-
benzylaminopurine (6BAP)
2. Phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ)
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9. Produced
 Cytokinins are produced mainly in the :
1. Roots
2. Fruits
3. Young leaves
4. Developing tissues
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10. Functions
 Induce cell division in callus cells in the presence of an Auxin
 Promote bud or root formation from callus cultures when in the appropriate molar ratios to
Auxin
 Delay senescence of leaves
 Promote expansion of dicot cotyledons
 Promote movement of nutrients
 Promote chloroplast development
 Promote cell expansion in leaves and cotyledons
 Regulate growth of stems and roots
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11. Biosynthesis of Cytokinins
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12. 1. Biosynthesis
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 An enzyme that synthesis cytokinin called ∆2-isopentenyl-
pyrophosphate: AMP transferase or cytokinin synthase
 Isopentenyl group of ∆2-IPP transferred to the AMP with the help of
Prenyl transferase
 The product of this reaction is isopentenyl adenine ribotide
 This compound is active as a cytokinin in bioassays
 And is readily converted to Zeatin and other cytokinins

13. 2. Crown gall cytokinin biosynthesis
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 During Agrobacterium infection:
 A small portion of Ti plasmid known as T-DNA is incorporated in to host DNA
 T DNA carries genes necessary for the auxin and cytokinins synthesis known as
Opines
 These opines are not synthesize by plant
 The T-DNA genes involved in cytokinin biosynthesis known as ipt genes
 That encodes isopentenyl transferase enzyme
 That transfers the isopentyl group from isopentenyl pyrophosphate to AMP to form
isopentenyl adenine ribotide
 Isopentenyl adenine ribotide can be converted to the active cytokinins such as trans
zeatin and dihydro zeatin by endogenous enzymes in plants

14. 3. Cytokinin Biosynthesis
 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
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15. Cytokinin Signaling Transduction
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16. Histidine Kinases
 HKs are prevailing sensors in prokaryotes
 That initiate a signaling system in which phosphoryl groups are
transferred between histidines and aspartates (phosphorelay signaling
system)
 To activate or inhibit similar downstream partners called response
regulators (RRs)
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17. Response regulators
 The first set of genes to be upregulated in response to cytokinin are the ARR
(Arabidopsis Response Regulator) genes
 In Arabidopsis, response regulators are encoded by a multigene family. They
fall into two basic classes:
1. The type A ARR genes, which are made up solely of a receiver domain, (The
rate of transcription of the type-A gene is increased within 10 minutes in
response to applied cytokinin)
2. The type B ARR genes, which contain a transcription factor domain in
addition to the receiver domain
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18. Steps to cytokinin signaling
 AHK (Arabidopsis Histidine Kinase) sensing and signaling,
 AHP (Arabidopsis Histidine Phosphotasferase) nuclear translocation,
 ARR (Arabidopsis Response Regulators) transcription activation, and
 Negative feedback loop through cytokinin-inducible ARR gene products
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21. Thank You
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