Cytokinins are plant growth hormones which are basic in nature, either aminopurine or phenyl urea derivatives that promote cytokinesis or cell division. Cytokinin have little or no effect on extension growth.
Carlos Miller and his associates (1955),while working in F. Skoog’s laboratory, cultured tobacco pith tissue in a medium containing auxins. They observed enormous increase in cell size. There was no wall formation hence, the cells were multinucleate.
Later, Miller and his colleagues were able to induce normal cell division by adding extracts from yeast. This showed that yeast extract contained some growth regulator. Later, this growth regulator was isolated from yeast cell and was named kinetin. The name cytokinin came from cytokinesis, meaning cell division.
Degraded yeast DNA and coconut milk both have a common active substance – deoxy-adenosine. It is commonly called kinetin and is chemically known as 6-furfurylaminopurine. It has the ability to induce cell division.
The first naturally occurring cytokinin to be chemically identified was from Zea mays grains in 1963. It was named zeatin. Coconut milk, is another common kinetin and is called coconut-milk factor.
Mode of action of cytokinin:
It has definite role in nucleic acid metabolism and protein synthesis. It has been found that cytokinin application in particular site in plant results in accumulation of amino acid, in organic phosphate, sugar and other solutes. These prevent yellowing of the leaves during senescence. It is also a potent component of certain t-RNA. This probably influences the synthesis of specific enzymes which help in synthesis of certain amino acid and prevent the breakdown of protein during senescence.
Synthesis and distribution:
All cytokinins are derivatives of purine based. They are synthesized in meristematic tissues. They occur abundantly in young roots, young leaves and developing fruits. Cytokinins are found in the tissue where rapid cell division takes place. Embryos, developing fruits and roots are particularly rich in cytokinins. In mature plants, they are frequently synthesized in the root and subsequently move to the shoot along the transpiration stream through xylem. Apple trees have high concentration of cytokinins in the xylem sap during spring season.
Bioassay of cytokinin:
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.
Retardation of leave 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.
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.
Physiological effects of cytokinin:
- Cell division: Cytokinins promote cell division, however, only in the presence of auxins. The balances of these two hormones determine the growth of root and shoot systems. With high concentrations of both cytokinin and auxins, the tissue continues to grow as a callus (lump of undifferentiated tissue). If the concentration of auxins is higher as compared to the cytokinin concentration, roots develop in the cultured tissue.On the other hand, if the concentration of cytokinin is higher, the tissue forms shoot with leaves. Thus, the relative amounts of these two hormones control the differentiation of tissues.
- Counteraction of apical dominance: Presence of cytokinin in an area causes preferential movement of nutrients towards it. Auxins and cytokinins act opposite to one another in relation to apical dominance. Auxins allow dominance of apical bud while cytokinins stimulate the growth of the lateral buds. Application of cytokinins to lateral buds relieves them from apical inhibition.
- Delay in senescence: One of the important properties of cytokinins is their ability to delay the normal process of senescence in leaves. Disappearance of chlorophyll and degradation of protein are two important symptoms of senescence. They are useful in delaying the process of ageing and senescence of leaves by reducing the breakdown of chlorophyll, protein and nucleic acids. This effect of cytokinin is known as Richmond–Lang effect.
- Shelf life: Application of cytokinins keeps harvested plant production fresh. Lettuce, cabbage and mushrooms and many other fruits and vegetables are treated with cytokinin to prolong their shelf life. Florists use cytokinin to keep cut flowers fresh.
- Breaking of dormancy: Cytokinin application breaks dormancy of many seeds and promotes their germination.
- Initiation of interfascicular cambium: Cytokinin can induce the formation of interfascicular cambium in plant which helps in secondary growth.
- Resistance: Cytokinins increase resistance to high or low temperature and disease.
- Accumulation of salts: It can induce accumulation of salts inside the cells.
- Flowering: Cytokinins can replace photoperiodic requirement of flowering in certain cases.
- Sex expression: Cytokinins promote femaleness in flowers.