Figure 3.

Biosynthesis and activation of cytokinins in plants. Precursors for CK biosynthesis, HMBDP and DMAPP, come either from the methylerythritol (MEP) or mevalonate (MVA) pathways. Plant adenylate IPTs (1) utilise mostly ADP or ATP whereas tRNA IPTs (2) use the adenine in position 37 of certain tRNAs as acceptor substrate. cis-Zeatin is known to originate from tRNA, but its synthesis is unclear, as neither a cis-hydroxylated precursor nor cis-hydroxylase (3) have yet been identified in plants. Nucleotides of iP may be hydroxylated by cytochrome P450 (4) to form tZ. Upon hydrolysis of γ- and β-phosphates (5) or tRNA hydrolysis (6), the resulting monophosphates may be activated in one step by CK-specific phosphoribohydrolase named Lonely guy (7). Alternatively, the nucleotides, nucleosides, and nucleobases are probably interconverted by enzymes of purine metabolism. The free base tZ may be reduced to DHZ by zeatin reductase (8). Whether there is any de novo biosynthesis of DHZ is currently unknown. Zeatin cis-trans isomerase does not exist.