Arvind Kumar

Kindly consult the link below.

https://www.gdcollegebegusarai.com/course_materials/hindi/Cytokinins%20biosynthesis,%20metabolism,%20inactivation%20and%20transport%20(Part%20I).pdf

Thanks!

Cytokinin biosynthesis involves complex pathways that convert adenine derivatives into biologically active forms. It is also regulated by various factors, light, temperature, developmental signals, and interactions with other hormones.

Cytokinins are adenine-derived phytohormones whose biosynthesis proceeds via three main enzymatic steps:

1. Isopentenylation of Adenine Nucleotides

• The first committed step is catalyzed by adenosine phosphate-isopentenyltransferases (IPTs). IPTs transfer an isopentenyl group from dimethylallyl diphosphate (DMAPP) onto the N^6 position of either adenosine monophosphate (AMP), ADP, or ATP, producing N^6-(Δ^2-isopentenyl)adenosine-5′-phosphates (iP–nucleotides).
• In most plants IPTs localize to plastids or the cytosol; plastidic DMAPP arises primarily from the methylerythritol phosphate (MEP) pathway.

2. Hydroxylation to Trans-Zeatin Type Cytokinins

• Some iP–nucleotides are converted to trans-zeatin (tZ) nucleotides by cytochrome P450 monooxygenases of the CYP735A family, which hydroxylate the isopentenyl side chain at C-4 of the prenyl moiety.
• This step diversifies cytokinin structure and activity, since tZ-type cytokinins generally exhibit higher receptor affinity than iP-types.

3. Liberation of Active Bases by Phosphoribohydrolase (LOG)

• Lonely Guy (LOG) enzymes cleave the ribose-5′-phosphate moiety from iP- or tZ-nucleotides, releasing the free base N^6-isopentenyladenine (iP) or trans-zeatin (tZ).
• These free adenine derivatives are the biologically active cytokinins that bind to sensor histidine-kinase receptors in the plasma membrane, initiating downstream signaling.

Additional modifications—such as O- and N-glucosylation, methylation, or side-chain cleavage—regulate cytokinin stability, transport, and degradation. Cytokinin oxidase/dehydrogenase (CKX) enzymes irreversibly degrade free bases or nucleosides by cleaving the N^6 side chain, thereby fine-tuning hormone levels.

In summary, cytokinin biosynthesis in plants proceeds via:  1. IPT-mediated transfer of a DMAPP prenyl group to adenine nucleotides → iP–nucleotides.  2. CYP735A hydroxylation of iP–nucleotides → tZ–nucleotides.  3. LOG-catalyzed phosphoribohydrolysis → release of free iP or tZ bases.