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Cytokinin Plant Hormones

Cytokinins are plant growth hormones that promote cell division and are synthesized primarily in the roots, with different types occurring in various plant species. The biosynthesis process involves a series of enzymatic reactions leading to different forms of cytokinins, and their action is regulated by receptors that facilitate signal transduction pathways. Additionally, certain organisms such as bacteria and fungi can produce cytokinins to influence plant growth for their benefit.

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ntroduction • Cytokinin (CK) are plant growth hormones which are basic in nature, either amino purine or phenyl urea derivatives, that promote cytokinesis (= cell division) either alone or in conjunction with auxin. • The first cytokinin was discovered from degraded autoclaved Herring sperm DNA by Miller 1955.It is called kinetin (6-furfuryl amino-purine). • Kinetin does not occur naturally. It is a synthetic hormone. • The first natural cytokinin was obtained from unripe maize grain known as zeatin (6-hydroxy 3-methyl trans 2-butenyl amino-purine). It also occurs in coconut milk. (6-hydroxy 3-methyl trans 2-butenyl amino-purine)
Disc very In 1931, an Austrian plant physiologist. G. Haberlandt, discovered that soluble substances present in phloem tissues could cause cell division in parenchyma cells of wounded potato. In 1954, C. O. Miller found that the aged DNA from sperms stimulates cell division in pith cells from tobacco tissue cultures, and the active part of DNA was found to be kinetin.
• Jablonski & Skoog (1954) - compounds in vascular tissues promote cell division Naturally occurring Cytokinin Van Overbeek (1941) - coconut milk (endosperm) also has the ability to stimulate cell division Disc very Sir Skoog
➢ Cytokinin extracted from coconut milk, Tomato juice ➢ flowers and fruits of pear, plum ➢ Cambium tissues of Eucalyptus, Nicotina ➢ Immature fruits of Zea Mays, Musa sp. ➢ Root exudates of Sunflower ➢ Ribosylzeatin, ➢ Zeatin, ➢ Dihydrozeatin. Naturally occuring Cytokinin Cytokinin found in plants Dihydrozeatin Zeatin
CIS-ZEATIN TRANS-ZEATIN
Kinetin Zeatin Benzyl aminopurine Cytokinin Structure
The major forms of cytokinin differ in different plant species. For example in rice the isoprenoid cytokinin cis zeatin is the major form, whereas in Arabidopsis it is the trans-zeatin and iP forms that predominate. Common Type of Cytokinin
Biosynthesis location • Roots seem to be the major source of cytokinin synthesis. From roots the cytokinin pass upwardly through xylem. • Some cytokinin synthesis also takes place in other areas where cell divisions are occurring like endosperm region of seeds, growing embryos and developing seeds, young fruits, developing shoots buds, etc. •Coconut milk is a rich source of cytokinin.
• Generated mostly in the root apical meristems but also found in: ➢ Root cap cells ➢ Ovules ➢ Phloem cells ➢ Leaf axils ➢ Tips of young inflorescences ➢ Fruit ➢ Seeds Biosynthesis
Biosynthesis • Other organisms make cytokinin to influence the plant for their own benefit ➢ Bacteria ➢ Fungi ➢ Insects. ➢ Nematodes
• Cytokinins move up the plant through the xylem • By contrast, auxin moves from top down. • Some signal in the shoot can also induce cytokinin transport from the root (Beveridge 2000). TRANSPORT
ATP/ADP DMAPP(Dimethyl Allyl Phosphate) iPDP, or iPTP (isopentenyladenosine-5’- monophosphate) Zeatin The first committed step in cytokinin biosynthesis is the addition of the isopentenyl side chain from DMAPP to an adenosine moiety. The plant and bacterial IPT enzymes differ in the adenosine substrate used; the plant enzyme appears to utilize both ADP and ATP, and the bacterial enzyme utilizes AMP. The products of these reactions (iPMP, IPDP, or IPTP) are converted to zeatin by an unidentified hydroxylase. The various phosphorylated forms can be interconverted and free trans-Zeatin can be formed from the riboside by enzymes of general purine metabolism. Trans-Zeatin can be metabolized in various ways Biosynthesis
Cytokinin Synthesis
Cytokinin are rapidly metabolized by cytokinin oxidase, thereby inactivating cytokinin's Activity of Cytokinin oxidase induced by high cytokinin concentrations. Metabolism Of Cytokinin
➢ The cytokinin receptors are encoded by a multigene family. ➢ Receptor- CRE1(CYTOKININ RESPONSE 1),contain a conserved extra-cellular Cytokinin-binding domain called the CHASE domain. ➢ Receptor in Arabidopsis for CK signaling- AHKs (AHK2,AHK3 and AHK4) /Arabidopsis hybrid kinase ➢ CHASE domain bind to Cytokinin in pH dependent manner. ➢ (AHP) Histidine phosphotransfer protein Hpt proteins are predicted to mediate the phosphotransfer between sensor kinases and response regulators. CYTOKININS SIGNALING PATHWAY
1. Cytokinin binds to CRE1, which is likely to occur as a dimer. Cytokinin binds to an extracellular portion of CRE1 called the CHASE domain.Two other hybrid sensor kinases (AHK2 and AHK3) containing a CHASE domain are also likely to act as cytokinin receptors in Arabidopsis. 2. Cytokinin binding to these receptors activates their histidine kinase activity. The phosphate is transferred to an asparate residue (D) on the fused receiver domains. 3. The phosphate is then transferred to a conserved histidine present in an AHP protein STEPS -
4. Phosphorylation causes the AHP protein to move into the nucleus, where it transfers the phosphate to an asparate residue located within the receiver domain of a type-B ARR 5. The phosphorylation of the type-B ARR activates the output domain to induce transcription of genes encoding type-A ARRs. 6. The type-A ARRs are likely also to be phosphorylated by the AHP proteins. 7. The phosphorylated type-A ARRS interact with various effectors to mediate the changes in cell function appropriate to cytokinin (indicated in the model as "cytokinin responses"). STEPS -
Cytokinin Plant Hormones
Cytokinin Plant Hormones