Comparative Analysis of the Biosynthesis of Isoprenoid and Aromatic Cytokinins

To compare the biosynthesis pathways of aromatic and isoprenoid cytokinins, a series of nucleoside derivatives of natural cytokinins was synthesized and their cytokinin activity was determined in a test system based on the model plant Arabidopsis thaliana. Cytokinin nucleosides are known to lack the hormonal activity until cleaving the ribose moiety at the position 9. Our experiments have shown that both ribo- and 5'-deoxyribo derivatives of N⁶-isopentenyladenine were able to turn into active cytokinins in planta exhibiting cytokinin activity. By contrast, 5'-deoxy nucleosides of aromatic cytokinins did not show similar activity. Since 5'-deoxy nucleosides cannot phosphorylate in vivo, the direct pathway of active cytokinin formation by cleavage of nucleotides is blocked here. The detected activity in 5'-deoxy nucleosides of isoprenoid cytokinins and the lack of the activity in 5'-deoxy nucleosides of aromatic cytokinins indicates the difference in the biosynthesis of these compounds.