The cytokinins were discovered in the search for factors that stimulate plant cells to divide.
The first cytokinin discovered was Kinetin- Synthetic cytokinin, discovered from herring sperm DNA. During investigation of Miller and Skoog they observed stimulation of cell division when cultured pith tissue was treated with autoclaved Herring sperm DNA. This indicated that DNA degradation product caused stimulation of cell division in tobacco pith culture. This compound was identified as kinetin since it caused cytokinesis (Fig. 16.1). It is now characterized as 6-furfurylaminopurine.
As kinetin is not synthesized by plants so called as Synthetic Cytokinin.
The first natural cytokinin discovered was Zeatin from corn endosperm. The immature endosperm from corn (Zea mays) was found to contain a substance with biological activity similar to kinetin. This substance stimulates mature plant cells to divide when added to a culture medium along with auxin. The active ingredient was later identified as zeatin [trans-6-(4-hydroxy-3-methyl-2-butenylamino) purine]
Zeatin can exist in cis as well as trans-form, the trans-form is more active although cis form has been found in high levels in a number of plant species.
Cytokinins can be present in plants as ribosides (in which ribose sugar is attached to the 9 nitrogen of the purine ring), ribotides (in which the ribose sugar moiety contains a phosphate group), or a glycosides (in which a sugar molecule is attached to 3, 7, or 9 nitrogen of the purine ring).
Cytokinins are also known to occur abundantly in coconut milk. The most abundant cytokinin in coconut water is trans-zeatin riboside.
Cytokinin are synthesized from adenine derivative compounds.
Site Of Biosynthesis: Plastid and Endoplasmic Reticulum
There are two types of pathways for biosynthesis of Cytokinins
1.Direct pathway
2. Indirect pathway
DIRECT PATHWAY:
In this pathway the first step in condensation of ATP/ADP with DMAPP.
ATP is AdenosineTriPhosphate and DMAPP-Is IPP derivative DiMethyAllylPyroPhosphate.
This results in formation of iPTP Or iPDP.
This involves transfer of the isopentenyl group of dimethylallyl diphosphate(DMAPP) to Adenosine moeity-ATP/ADP.
If IPP group of DMAPP is transfered to ATP it will forms-iPTP on other side if IPP group of DMAPP is transferred to AMP it will form iPDP
The direct pathway involves formation of N6 -isopentyladenosine monophosphate (iPMP) from 50 -AMP and dimethylallyl pyrophosphate (DMAPP), followed by hydroxylation of the side chain to form zeatin-type compounds.
Conversion of DMAPP and AMP to iPMP occurs in plastids because DMAPP is synthesized in plastids. This reaction is the rate limiting step in cytokinin biosynthesis.
The next step is the hydroxylation of the isopentenyl side chain by a member of the cytochrome P450 monooxygenase (CYP) family of enzymes.
Hydroxylation reaction occurs largely on ER membranes and is regulated by several hormones. iPMP is then converted to zeatin by unidentified hydroxylases. Various phosphorylated forms can be interconverted, and free zeatin can be formed from the riboside by the action of enzymes of general purine metabolism. Also, reduction of the double bond in isopentenyl side chain of zeatin gives rise to dihydrozeatin (diHZ).
The indirect pathway of cytokinin biosynthesis involves turnover of tRNA containing cis-zeatin.
The tRNA Cytokinin are synthesized by modification of specific adenine residue within fully transcribed tRNA.
The isopentyl group are transferred to the adenine molecule of tRNA from DMAPP by an enzyme tRNA-IPT
The tRNA dervide cytokinin act as hormonol signals for plants if the tRNA is broken or degraded and goes back to the cells
tRNAs with anticodons that start with a uridine and carrying an already-prenylated adenosine adjacent to the anticodon release the adenosine as a cytokinin upon degradation. Prenylation of these adenines is carried out by tRNA isopentenyl transferase. (Prenylation refers to addition of hydrophobic molecule to a chemical compound.)
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