Cytokinins: Metabolism and Biological Activity of N-(Delta-Isopentenyl)adenosine and N-(Delta-Isopentenyl)adenine in Tobacco Cells and Callus

The cytokinin, N⁶-(Δ²-isopentenyl)adenine, is found to be at least 3.3 times as active as N⁶-(Δ²-isopentenyl)adenosine in promoting the growth of cytokinin-requiring tobacco (Nicotiana tabacum) callus. Absorption rates of N⁶-(Δ²-isopentenyl)adenine and N⁶-(Δ²-isopentenyl)adenosine by tobacco cells in liquid suspension do not differ significantly. In these cells, N⁶-(Δ²-isopentenyl)adenosine-5′-monophosphate, di-, and triphosphate are synthesized in both cases, but 7-glucosylation occurs significantly only with N⁶-(Δ²-isopentenyl)adenine, protecting thereby its N⁶-isopentenyl side chain from cleavage. Degradation by N⁶-side chain removal appears to be intense, leading to the formation of adenine, adenosine, and adenylic nucleotides. Thus, it is suggested that N⁶-(Δ²-isopentenyl)adenine-7-glucoside is a protected or storage form of the cytokinin which could account for the higher biological activity of N⁶-(Δ²-isopentenyl)adenine than of N⁶-(Δ²-isopentenyl)adenosine.