13.4.1—
Introduction

The term cytokinin has been accepted practically universally as a generic name for substances which promote cell division and exert other growth regulatory functions in the same manner as kinetin. The synthesis and testing of compounds for cytokinin activity began with the discovery of kinetin (6-furfurylaminopurine). Today there are probably at least one hundred known synthetic and native cytokinins (Fig. 13.4).

Structural requirements for a high order of cytokinin activity generally include an adenine molecule with the purine ring intact and with an N⁶ substituent of moderate size. One exception to the requirement for a modified purine exists, notably diphenylurea and its derivatives. Certain other substances which lack a true purine ring, such as 8-azakinetin, 6-benzylamino-8-azapurine and 6-(3-methyl-2-butenylamino)-8-azapurine are active, but each of them is less than 10% as active as its corresponding purine derivative. Substitution of either O or S for N in the N⁶ position of adenine in each case results in a more than 90% loss in growth promoting activity in the tobacco bio-assay.

Cytokinins play a role in practically all phases of plant development from cell division to the formation of flowers and fruits. They affect metabolism including the activities of enzymes and the biosynthesis of growth factors. They also influence the biogenesis of organelles and the flow of assimilates and nutrients within the plant. Cytokinins defer senescence and may protect the plant against adverse environments, such as water stress. These many diverse effects presumably stem from some primary anabolic function of the cytokinins that remains to be elucidated.

Figure 13.4
Structure of some common
native and synthetic cytokinins.

Cytokinins have been found in certain transfer-RNA (t-RNA) molecules from a large number of organisms including bacteria, yeast, plants and animals (see Cherry & Anderson, 1971). In sequence analysis the native cytokinin, isopentenyl adenine, always occurs adjacent (3' side) to the anticodon (Fig. 13.5). Furthermore, the location of the isopentenyl group is required for the

Figure 13.5
Diagram of the cloverleaf shape of seryl-transfer RNA.
The IPA residue is located adjacent to the anticodon.

specific tRNA to function efficiently in protein synthesis. Results involving bacteriophage infection of bacteria indicate that cytokinin in tRNA may have an important regulatory function in protein synthesis. In soybean seedlings (Anderson & Cherry, 1969) the application of a cytokinin (6-benzyladenine) results in increased levels of two species of leucyl-tRNAs. Although the relationship of cytokinins to tRNA may not be directly associated with the primary action of cytokinins on cellular activity, the alteration of specific tRNAs could control protein synthesis. Such a regulatory mechanism would be manifested in the control of growth and morphogenesis as the ultimate expression of the hormone.