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PP 503: Physiological and molecular responses of plant to abiotic stresses (2+1)

Cytokinin Biosynthesis and transduction

Presented by: Ekatpure Sachin Chandrakant (PhD Research Scholar)

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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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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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Cytokinins

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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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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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Basic chemistry

All cytokinins have basic Adenine ring structure.

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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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Produced Cytokinins are produced mainly in the :

1. Roots2. Fruits3. Young leaves4. Developing tissues

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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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Biosynthesis of Cytokinins

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1. Biosynthesis 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

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2. Crown gall cytokinin biosynthesis 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

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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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Cytokinin Signaling Transduction

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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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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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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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Thank You