Cytokinins: Metabolism and Biological Activity of N6-(Δ2-Isopentenyl)adenosine and N6-(Δ2-Isopentenyl)adenine in Tobacco Cells and Callus

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Abstract
The cytokinin N6-(.DELTA.2-isopentenyl)adenine was at least 3.3 times as active as N6-(.DELTA.2-isopentenyl)adenosine in promoting the growth of cytokinin-requiring tobacco (Nicotiana tabacum) callus. Absorption rates of N6-(.DELTA.2-isopentenyl)adenine and N6-(.DELTA.2-isopentenyl)adenosine by tobacco cells in liquid suspension do not differ significantly. In these cells N6-(.DELTA.2-isopentenyl)adenosine-5''-monophosphate, di- and tri-phosphate are synthesized in both cases, but 7-glucosylation occurs significantly only with N6-(.DELTA.2-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. It is suggested that N6-(.DELTA.2-isopentenyl)adenine-7-glucoside is a protected or storage form of the cytokinin which could account for the higher biological activity of N6-(.DELTA.2-isopentenyl)adenine than of N6-(.DELTA.2-isopentenyl)adenosine.