Cytokinins were described as compounds that regulate cell division in plants in experiments in which compounds were screened for their effects on tissue cultures. From these experiments came two compounds: the first, kinetin, was isolated as the active ingredient in herring sperm DNA that caused massive cell proliferation in plant cell culture. Zeatin was the first natural plant cytokinin, isolated from the liquid endosperm of the coconut. The cytokinins constitute a small number of compounds, which are derivatives of adenine or amino purine.
Cytokinin effects are generally associated with promotion of growth and development and delay of senescence. Cytokinins applied to leaves will delay senescence; they speed up chloroplast maturation in etiolated (dark-grown)
tissue and promote cell expansion in young leaves. Cytokinins applied to lateral buds in plants showing strong apical dominance will overcome growth inhibition by auxin, causing the bud to grow out. Cytokinins, with auxins, are involved in plant tumor formation and in morphogenesis, the development of roots and shoots. Figure 2 illustrates the effects of varying the ratio of auxin to cytokinin on the growth and morphogenesis of plant material in tissue culture.
Like gibberellins, cytokinins contain isoprene subunits. The first stage involves the reaction of isopentenyl pyrophosphate with adenosine monophosphate (AMP), catalyzed by the enzyme cytokinin synthase to yield isopentenyl adenine ribotide. From this compound, cytokinin ribotides, ribosides and cytokinins are formed. Cytokinins are also synthesized by gene products resulting from the insertion of bacterial genes from Agrobacterium tumifaciens (Topic P3) The TI (tumor- inducing) plasmid from A. tumifaciens introduces the gene for isopentenyltransferase. This enzyme generates isopentenyl adenine, which is converted to trans-zeatin and dihydrozeatin in the plant. These hormones, together with auxin produced by another TI-plasmid gene product, cause a tumor or crown gall to form at the site of infection.
Cytokinins are synthesized in various tissues and organs, though the root apical meristem (Topic C2) is a major site of its production. They have been identified in the xylem flow from cut roots, suggesting that this may be a route for longdistance transport of cytokinins through the plant. Cytokinins in the root xylem are predominantly zeatin ribosides, which are rapidly converted to freecytokinin in leaves. Cytokinin inactivation occurs when they are oxidized to adenine by the enzyme cytokinin oxidase.