Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. Zeatin is an adenine-type cytokinin . The synthesis of cis-type cytokinins is carried out in the cytosol through prenylation of tRNA molecules and dimethylallyl diphosphate precursors provided through the mevalonate pathway. Hormonal homeostasis is thought to be maintained by glucosylation that inactivates cytokinins. O-glucosylation is reversible and is hypothesized to be involved with transport, storage, and protection against zeatin oxidases. Only the enzymes that catalyze the first and last steps of this pathway have been elucidated (all predicted enzymes are coloured orange in the image). First, tRNA dimethylallyltransferase transfers a dimethylallyl group onto the adenine at position 37 in tRNAs that have uridine-starting codons. It requires a magnesium ion as a cofactor. Second, the predicted enzyme cis-Hydroxy-prenyl-tRNA catalyzes a reaction whereby N6-dimethylallyladenosine37 in tRNA is converted into cis-hydroxy-prenyl-tRNA. Third, the predicted enzyme cZRMP synthase catalyzes a reaction whereby cis-hydroxy-prenyl-tRNA is converted into cis-zeatin riboside monophosphate (cZRMP). Fourth, the predicted enzyme cis-Zeatin riboside synthase catalyzes a reaction whereby cis-zeatin riboside monophosphate (cZRMP) is converted into cis-zeatin riboside. Fifth, the predicted enzyme cis-Zeatin synthase catalyzes whereby cis-zeatin riboside is converted into cis-zeatin. Sixth, UDP glucose:cytokinin glycosyltransferase catalyzes a reaction whereby cis-zeatin is converted into cis-zeatin-O-glucoside.

cis-Zeatin-O-Glucoside Biosynthesis References