Summary
Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. There are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin, and 6-benzylaminopurine, and phenylurea-type cytokinins like diphenylurea and thidiazuron (TDZ). Most adenine-type cytokinins are synthesized in roots. Cambium and other actively dividing tissues also synthesize cytokinins. No phenylurea cytokinins have been found in plants. Cytokinins participate in local and long-distance signalling, with the same transport mechanism as purines and nucleosides. Typically, cytokinins are transported in the xylem. Cytokinins act in concert with auxin, another plant growth hormone. The two are complementary, having generally opposite effects. The idea of specific substances required for cell division to occur in plants actually dates back to the German physiologist J. Wiesner, who, in 1892, proposed that initiation of cell division is evoked by endogenous factors, indeed a proper balance among endogenous factors. Somewhat later, the Austrian plant physiologist, G. Haberlandt, reported in 1913 that an unknown substance diffuses from the phloem tissue which can induce cell division in the parenchymatic tissue of potato tubers. In 1941, Johannes Van Overbeek found that the milky endosperm of immature coconut also had this factor, which stimulated cell division and differentiation in very young Datura embryos. Jablonski and Skoog (1954) extended the work of Haberlandt and reported that a substance present in the vascular tissue was responsible for causing cell division in the pith cells. Miller and his co-workers (1954) isolated and purified the cell division substance in crystallised form from autoclaved herring fish sperm DNA. This active compound was named as Kinetin because of its ability to promote cell division and was the first cytokinin to be named.
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