(kinin) Any of a group of growth substances whose primary effect is to stimulate cell division. Cytokinins were discovered during work on tissue culture media when it was found that cells of tobacco pith explants could be stimulated to divide by adding the purine adenine to the medium. Subsequently various adenine derivatives, e.g. kinetin, were found to have even greater effects on cell division. However these effects are not seen in the absence of auxin. Moreover, by changing the proportion of cytokinin to auxin, different types of meristematic activity may be induced (see auxin ). Cytokinins are probably active in most aspects of plant growth and development. Their most obvious effects include the delay of senescence, the induction of flowering in certain species, and the breaking of dormancy in axillary buds and some seeds. All natural cytokinins are derivatives of the base adenine. Some synthetic cytokinins are substituted phenylureas. It has been suggested they may act by regulating nucleic acid activity, particularly that of transfer RNA (see IPA). Endogenous cytokinins are found in the greatest concentrations in embryos and developing fruits, e.g. in the 'milk' of the coconut. Before fruit set, it is probable that most cytokinin synthesis is in the root. Abnormally high levels of cytokinin are associated with certain plant diseases, e.g. witches broom and crown gall. See kinetin, zeatin. |
