Biosynthesis of cytokinins.

Cytokinins are adenine derivatives with an isoprenoid side chain and play an essential role in plant development. Plant isopentenyltransferases that catalyze the first and rate-limiting steps of cytokinin biosynthesis have recently been identified. Unlike bacterial enzymes, which catalyze the transfer of the isopentenyl moiety from dimethylallyldiphosphate (DMAPP) to the N(6) position of adenosine 5'-monophosphate (AMP), plant enzymes catalyze the transfer of the isopentenyl moiety from DMAPP preferentially to ATP and to ADP. The isopentenylated side chain is hydroxylated to form zeatin-type cytokinins. An alternative pathway, in which a hydroxylated side chain is directly added to the N(6) position of the adenine moiety, has also been suggested.