The biosynthesis of cytokinin
Since their discovery in 1955 much work has been carried out on cytokinins. They are known to be formed from adenine, and there are two main classes, one containing aromatic side chains has members such as meta-topolin; the other group has isoprene side chains and includes cis-zeatin. Cytokinins have many roles to play in plants; they are involved in cell proliferation, differentiation, and many processes that are under signal transduction control. The following section takes a look into how cytokinin is biosynthesized.
Isoprenoid cytokinin biosynthesis
The first step of the biosynthesis of isoprenoid cytokinins involves the enzyme adenosine phosphate-isopentenyltransferase. This enzyme catalyses the N-prenylation of adenosine 5'-phosphates such as ATP, ADP and AMP with either hydroxymethylbutenyl diphosphate or dimethylallyl diphosphate.
There are two main pathways involved in cytokinin biosynthesis, these are the MEP (methylerythritol phosphate) and the MVA (mevalonate) pathway. The majority of iP and trans-zeatin cyttokinins are derived from the MEP pathway, whereas the MVA pathway is largely responsible for the creation of cis-zeatins. For further information on the biosynthesis of isoprenoid cytokinins it is recommended to read the papers by Takei and Sakakibara (see references).
Aromatic cytokinin biosynthesis
Little is know about how aromatic cytokinins are biosynthesized in plants. It is thought that the pathways involved in the creation of aromatic biosynthesis may be shared with those of isoprenoid cytokinin biosynthesis. This was concluded because many of the receptors and enzymes involved in isoprenoid cytokinin synthesis are able to recognise aromatic cytokinins too. In addition to this it is thought that P450 enzymes may be involved in the biosynthesis of the topolins that are found in many aromatic cytokinin side chains.
Kasahara et al. (2004). Distinct isoprenoid origins of cis- and trans-zeatin biosyntheses in Arabidopsis. J. Biol. Chem. 279:14049–54
Sakakibara (2006) Cytokinins: Activity, Biosynthesis, and translocation. Annu. Rev. Plant Bio. 57: 431 to 449
Takei et al. (2001). Identification of genes encoding adenylate isopentenyltransferase, a cytokinin biosynthesis enzyme, in Arabidopsis thaliana. J. Biol. Chem. 276: 26405–10
Takei et al. (2004). Arabidopsis CYP735A1 and CYP735A2 encode cytokinin hydroxylases that catalyze the biosynthesis of trans-zeatin. J. Biol. Chem. 279:41866–72
