CYTOKININS

Cytokinin stimulates the cell division in plants. It has following history of discovery:

  1. Earlier, it was believed that coconut milk contain cell division causing factors.
  2. The developing endosperm of maize also contains such factors.
  3. Later, herring (fish) sperm DNA was heated. It gives cell division causing factor. This substance can cause cell division in tobacco pith cells. It was named kinetin. But kinetin does not occur in any of the plants. It is only a degradation product of DNA.
  4. Letham (1963) extracted, purified and crystallized such a hormone front the developing maize grain. This substance was named zeatin. Zeatin was found in many plants.
  5. it was soon found that both kinetin and zeatin are similar in chemical structure. All such cell division causing substances are called cytokinins (cytokinais cell division)Le6. The chemists synthesized two very important cytokinins. One kinetin. The other is benzyladenine (BA).

    ROLE OF CYTOKININS IN PLANT GROWTH (PHYSIOLOGICAL ROLES)

    1. Cell Division

    Tissue culture helped to understand the role of cytokin ills in plants. Pith tissues were taken out from the stem of tobacco plant. They were supplied mineral salts, vitamins and sucrose in vitro. Pith cells do not gross on this medium. Auxin is added to the medium. But they do not divide and slightly enlarge. Then kinetin is added alone. The cells again do not divide. Then both auxin and kinetin were together put in the medium. Now the pith cells starts dividing rapidly by mitosis. B forms a I ump of undifferentiated cells. The lump of undifferentiated cells is known as callus. Therefore. cytokinin causes cell division in tobacco pith cells in the presence of auxin.

    1. Morphogenesis

    The differentiation of different tissues like root, shoot from callous is called morphogenesis. The ratio of cytokinin % to auxin controls the morphogenesis.

    (a)   The cytokinin to auxin ratio is 10:1. It produces callus.


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    (b)  This ratio was increased bv increasing the concentration of auxin or decreasing the concentration of cytokinin (100:1). Now the cells organize themselves. They differentiated into numerous roots.

    (c)   Now the auxin to cytokinin ratio is decreased ( I:1 ). A number of shoot buds are differentiated on the callus. These buds finally develop into tiny tobacco plants.

    1. Delay of Leaf Senescence

    The degradative changes which lead to death of an organ or organism are called leaf senescence. When a leaf becomes old, it becomes senescent. It dies or falls down. Proteins, chlorophyll and nucleic acids are broken down into simpler compounds during this process. They are broken into sugars, amino acids and other metabolites. These soluble substances are exported out of the leaf to the other plant parts. The leaf becomes chlorotic. It ‘dries up or falls down. The application of cytokinin delays Inc senescence. Cytokinin maintains the level of metabolites. It prevent; their degradation.

    1. Role in Apical doininipst

    Cytokinin is antagonistic to auxin. It releases the plant from apical dominance. It promotes the growth of lateral buds

 

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