A group of plant hormones (phytohormones) that, together with other plant hormones, induces plant growth and development. Since the isolation of the first naturally occurring cytokinin, zeatin, from corn seeds in 1961, more than 25 different cytokinins have been isolated from plants. In addition, more than 150 different analogs have been synthesized from kinetin, the first nonplant substance found (in 1954) to stimulate plant cell division.
Oversynthesis of cytokinins in plant tissue causes abnormal growth: crown gall tumor disease caused by the bacterium Agrobacterium tumefaciens is an example of excessive production of cytokinins in local tumor tissue. Tissue from crown gall tumors can grow on a simple medium lacking plant hormones because the tumor tissue overproduces both cytokinin and auxin. This is due to the insertion of a piece of bacterial plasmid DNA into the plant nuclear genomes causing activation of the gene responsible for the regulation of cytokinin production. Roots have been shown to be the major site of cytokinin biosynthesis, but stems and leaves are also capable of synthesizing cytokinins. It is possible that all actively dividing cells are capable of cytokinin biosynthesis. See also Crown gall.
Cytokinins exhibit a wide range of physiological effects when applied externally to plant tissues, organs, and whole plants. Exogenous applications of this hormone induce cell division in tissue culture in the presence of auxin. The formation of roots or shoots depends on the relative concentrations of auxin and cytokinin added to the culture medium. High auxin and low cytokinin concentrations lead to root formation, while low auxin and high cytokinin concentrations give shoots. Tissue culture techniques have been employed by plant biotechnologists to grow genetically engineered plant cells into whole plants. Cytokinins appear to be necessary for the correlated phenomena of mitosis and nucleic acid synthesis. Cytokinins delay the aging process in detached leaves by slowing the loss of chlorophyll. Cytokinin effects also include breaking of dormancy, promotion of seed germination, stimulation and nutrient mobilization, enhanced anthocyanin and flavanoid synthesis, increased resistance to disease, and stimulation of the opening of stomates. See also Auxin; Dormancy; Plant physiology; Tissue culture.
The mechanism of action of cytokinin on plant growth and development is poorly understood. It has been demonstrated that specific proteins are induced, enhanced, reduced, or suppressed by the hormone. Some of the enzymes or proteins regulated by cytokinins have been identified. Initial evidence suggests that cytokinins regulate, at least in part, the transcriptional process of gene expression by turning on or off specific genes and stimulating or suppressing the synthesis of specific mRNAs. Another possible action of cytokinins is the regulation of a posttranscriptional process such as stabilization of mRNA. Scientists also demonstrated that cytokinins specifically increase the rate of protein synthesis and the effect seems to be on the translation of mRNA into proteins. See also Plant growth; Plant hormones.
