Cytokinins: Chemical nature, Biosynthesis, Bioassay, Physiological effects and Mode of action
Cytokinin:-
> It is mostly found in the roots of plants.
> Zeatin is the first natural cytokinin.
> Examples:-
i. Kinetin (6 – Furfuryl Amino Purine)
ii. Zeatin
iii. 2 – iP (2 – Iso Pentenyladenine)
iv. BA or BAP (Benzyl Adenine or 6 – Benzyl Amino Purine)
v. Adenine (Aminopurine)
1. Discovery of Cytokinins:-
- Skoog, Miller and co-workers in 1955 isolate and identify kinetin, a highly-active cell division factor, from autoclaved herring sperm DNA.
- In searching for naturally occurring cytokinins in plant tissues, Letham in 1963 isolated a cytokinin, zeatin, from immature corn kernels.
- A “kinetin-like” factor isolated by Miller in 1961 was later also identified as zeatin.
2. Chemical nature of Cytokinins:- All the naturally-occurring cytokinins are substituted purines. The usual way of naming a cytokinin is to express it as a substituted 6-amino purine or as N6-substituted adenine.
3. Biosynthesis of Cytokinins:-
> Cytokinin is synthesized in the roots from where they are transported to shoots by xylem tissues.
> Zeatin can be synthesized in two different pathways: the tRNA pathway and the AMP pathway.
> In the tRNA pathway zeatin is a recycled product of isopentenylated tRNAs.
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> In the AMP pathway zeatin is synthesized from an isopentenyl donor, dimethylallyl diphosphate (DMAPP), and AMP, ADP, or ATP by isopentenyltransferases.
4. Bioassay of Cytokinins:-
a. Tobacco Pith Culture:- Out of two tobacco pith cultures, one is supplied with cytokinin while the other is not. Increase in fresh weight of the tissue over the control is a measure of stimulation of cell divisions and hence cytokinin activity. The test can measure cytokinin concentration between 0.001-10 mg/litre. It takes 3-5 weeks.
b. Retardation of Leaf Senescence:- It is a rapid bioassay technique. Leaf discs are taken in two lots. In one lot cytokinin is provided. After 48-72 hours, the leaf discs are compared for chlorophyll content. Cytokinin retards the process of chlorophyll degradation. The test is sensitive in concentration of 1 pg/litre.
c. Excised Radish Cotyledon Expansion:- The test was developed by Letham. Excised Radish cotyledons are measured and placed in test solution as well as ordinary water (as control). Enlargement of cotyledons is an indication of cytokinin activity.
i. Cell Division:- Cytokinins are essential for cytokinesis though chromosome doubling can occur in their absence. In the presence of auxin, cytokinins bring about division even in permanent cells. Cell division in callus is found to require both the hormones.
ii. Morphogenesis:- Both auxin and cytokinins are essential for morphogenesis or differentiation of tissues and organs. Buds develop when cytokinins are in excess while roots are formed when their ratios are reversed.
iii. Differentiation:- Cytokinins induce formation of new leaves, chloroplasts in leaves, lateral shoot formation and adventitious shoot formation. They also bring about lignification and differentiation of inter-fascicular cambium.
iv. Senescence (Richmond-Lang Effect):- Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients.
v. Apical Dominance:- Presence of cytokinin in an area causes preferential movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bud. They thus act antagonistically to auxin which promotes apical dominance.
vi. Seed Dormancy:- Like gibberellins, they overcome seed dormancy of various types, including red light requirement of Lettuce and Tobacco seeds.
vii. Resistance:- Cytokinins increase resistance to high or low temperature and disease.
viii. Phloem Transport:- They help in phloem transport.
ix. Accumulation of Salts:- Cytokinins induce accumulation of salts inside the cells.
x. Flowering:- Cytokinins can replace photoperiodic requirement of flowering in certain cases.
xi. Sex Expression:- Like auxins and ethylene, cytokinins promote femaleness in flowers.
xii. Parthenocarpy:- Crane (1965) has reported induction of parthenocarpy through cytokinin treatment.
xiii. Stomatal opening:- It has been shown that an increased cytokinin concentration in xylem sap promotes stomatal opening.
6. Applications of Cytokinins:-
i. Tissue Culture:- Cytokinins are essential for tissue culture because besides cell division they are also involved in morphogenesis. Instead of direct addition of cytokinins, the latter may be provided to tissue culture through the addition of coconut milk or yeast extract.
ii. Shelf Life:- Application of cytokinins to marketed vegetables can keep them fresh for several days. Shelf life of cut shoots and flowers is prolonged by employing the hormones.
iii. Resistance:- Cytokinin application is helpful to plants in developing resistance to pathogens and extremes of temperature.
iv. Overcoming Senescence:- Cytokinins delay senescence of intact plant parts.
7. Mode of Action of Cytokinins:-
> Cytokinin moves from the roots into the shoots, eventually signaling lateral bud growth.
> Simple experiments support this theory.
> When the apical bud is removed, the axillary buds are uninhibited, lateral growth increases, and plants become bushier.
> Applying auxin to the cut stem again inhibits lateral dominance.
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