Mechanisms of cytokinin action

Abstract

Twenty-two synthetic analogues of adenine were tested for their ability to promote the growth of Chinese cabbage leaf discs. In general an adenine residue substituted in the N6 position with a group of moderate size is required for maximum growth activity. Kinetin increases the turnover of ribosomes in floated Chinese cabbage leaf tissue. The synthetic cytokinins kinetin and 6-benzylaminopurine bind reversibly to purifies Chinese cabbage leaf ribosomes. At 4°C, 0.8 molecules of kinetin and 1.08 molecules of 6BA are bound per ribosome. Adenine and adenine derivatives that are inactive as cytokinins showed much lower affinity for ribosomes. A positive correlation between the extent of ribosome binding of synthetic adenine analogues and their biological cytokinin activity was demonstrated. ‘In vitro’ aminoacylating systems from Chinese cabbage and ‘in vitro’ protein synthesizing systems from Chinese cabbage and etiolated pea shoots were developed and characterized. Cytokinins had no significant effect on these systems. Kinetin increased gross starch degradation and sugar utilization in leaf discs floated for 3 days in the light or dark whether or not sucrose was present in the floating medium. Since kinetin induced a change in neutral sugars prior to a change in starch content of the tissue, reduced sugar pools in kinetin treated tissue are suggested as the cause of induced starch degradation. 14Co2 labelling of leaf tissue pretreated with kinetin for 24hr showed increased radioactivity in chloroform-soluble material and most sugar phosphates and a 35-40% decrease in radioactivity in the neutral sugars, glucose, sucrose and fructose. Incorporation into ATP was increase by 40% by kinetin. Isolated pea nuclei bound 6BA, kinetin and vvDMAAP, but showed low affinity for 3BA. DNase treatment of the nuclei enhanced 6BA binding by 100% while pronase treatment reduced binding. Binding sites liberated by DNase treatment showed reduced affinity for 3BA. It is suggested that nuclei contain cytokinin receptor site of a protein nature.