Evidence in support of biosynthesis de novo of free cytokinins

The four cytokinins in the tRNA from Lupinus luteus L. seeds have been purified and identified as ribosyl-cis-zeatin, 2-methylthio-ribosylzeatin, (Δ (2)-isopentenyl)adenosine and 2-methylthio-N(6)-(Δ (2)-isopentenyl)adenosine. These structures have been assigned on the basis of their chromatographic mobilities and the spectroscopic data of the parent materials and their silylated derivatives. The tRNA isolated from Populus x robusta Schneid. leaves contained four cytokinins with identical chromatographic properties to those identified in Lupinus luteus seed tRNA. No evidence was obtained for the presence, in tRNA, of the naturally occurring free cytokinins identified in these plant species, dihydrozeatin (Lupinus luteus) and N(6)-(2-hydroxybenzyl)adenosine (Populus x robusta). This is evidence in support of the possibility that free cytokinins can arise by biosynthesis de novo and are not exclusively by-products released intact during tRNA turnover.