On the biogenesis of cytokinins in roots of Phaseolus vulgaris.

Roots of intact bean plants were supplied with [(14)C]adenine by pulse-chase experiments. The rate of incorporation of radioactivity into tRNA and oligonucleotides of roots as well as the content of radioactive labeled cytokinin nucleotides in these RNA fractions were determined. On the average, 1/70 of the radioactivity incorporated into tRNA was localized in N(6)(Δ(2)isopentenyl)adenosine. The half life of tRNA was estimated to be 65-70 h. Shortly after the pulse period, oligonucleotides contained zeatin riboside at a ratio of 1:800, on the basis of radioactivity. The half life of these oligonucleotides was determined to be about 8 h. The main free radioactive cytokinin of roots and leaves was zeatin. Comparing the rate of degradation of (14)C-labeled tRNA and the oligonucleotides of roots and the rate of appearance of radioactive cytokinins in roots and leaves, we found strong indications for their dependency. The results contradict the hypothesis of de novo synthesis of cytokinins in roots of intact bean plants.