Cytokinins are the phytohormones which stimulate cell division. They prevent the onset of senescence in tissues.
Miller, Skoog (1955) observed that 2 substances were effective in enhancing the growth of mature plant cells and tissues in culture. The two substances were coconut milk and degraded samples of yeast DNA. The active ingredient in both was found to be - furfuryl amino purine a modified form of adenine. They named this compound kinetin
because of its property to activate cell division. About 18 compounds have so far been discovered which showed similar activity. They are now called as cytokinins
Structure of Kinetin, Zeatin and Adenine
The first naturally occurring cytokinin was isolated from the milky endosperm of Zea mays (corn) and named Zeatin.
Cytokinins promote cell division. The stimulate cell division even in non-meristematic cells.
In tissue cultures of parenchyma, mitosis is accelerated when both auxin and cytokinin are present, but no response is seen with auxin or cytokinin present alone.
Cytokinin - auxin ratio and their interaction controls morphogenetic differentiation in shoot and root meristems.
High cytokinin to auxin ratio causes differentiation of shoots.
A low ratio of cytokinin to auxin causes root formation.
Intermediate cytokinin to auxin ratio causes formation of roots as well as shoots.
Intermediate cytokinin to low auxin causes growth of large amount of callus.
Cytokinins break the dormancy of many seeds and promote their germination.
Cytokinins stimulate the growth of lateral buds and thus suppress apical dominance.
Cytokinins delay ageing of plant organs by controlling protein synthesis and mobilisation of resources.
Cytokinins promote femaleness during sex expression in the flowering of plants.
Cytokinins induce parthenocarpy in many plants.
Cytokinins are known to stimulate several enzymes especially those concerned with photosynthesis.
Cytokinins are put to some use in agriculture.
Cytokinins are used to prevent premature senescence in crop plants.
Cytokinins are applied to the marketed vegetables, cut shoots and flowers to keep them fresh for several days.
Cytokinins are used in tissue culture because of their role in cell division and morphogenesis.