Cytokinins biosynthesis

The biosynthetic pathway producing isoprenoid cytokinins has been identified, whereas that of aromatic cytokinins is poorly characterized.363 385 Two distinct pathways for isoprenoid cytokinin biosynthesis have been described, and each pathway employs a different type of isopentenyltransferase at the initial step. The major pathway in higher plants, which is catalyzed by IPT, is conjugation of adenine nucleotide and DMAPP or 4-hydroxy-3-methyl-2-( )-butenyl diphosphate (HMBDP) (Figure 14). In the less frequently used pathway, cytokinins are formed by degradation of prenylated tRNAs. The initial prenylation reaction of tRNA is catalyzed by tRNA-isopentenyltransferase (tRNA-IPT) (Figure 14). 

Figure 14 IPT and tRNA-IPT reaction for primary cytokinin biosynthesis. Circles and A in tRNA show anticodon and adenosine, respectively. iPRMP, iP riboside 5 -monophosphate iPRDP, iP riboside 5 -diphosphate iPRTP, iP riboside 5 -triphosphate tZRMP, tZ riboside 5 -monophosphate PP, diphosphoric acid.

DMAPP, a prenyl-donor substrate in cytokinin biosynthesis, is an intermediate of both the methylerythritol phosphate (MEP) and mevalonate (MVA) pathways, whereas HMBDP, another substrate of Agrobacterium IPTs, is synthesized via only the MEP pathway. In general, the MEP pathway is found in bacteria and plastids, and the MVA pathway is found in the cytosol of eukaryotes.413-415... 

Figure 15 Current model of isoprenoid cytokinin biosynthesis pathway in higher plants. tZRDP, tZ riboside 5 -diphosphate tZRTP, tZ riboside 5 -triphosphate DZRMP, DZ riboside 5 -monophosphate cZRMP, cZ riboside 5 -monophosphate ...

Takeshi Kuroha was born in 1977. He graduated from the School of Biological Sciences, University of Tsukuba in 1999. He obtained his Ph.D. degree from the University of Tsukuba in 2005. He studied the mechanisms of adventitious and lateral root formation in graduate school. In 2005, he joined the Biodynamics Research Team in RIKEN Plant Science Center as research associate and is involved in the study of enzymes related to cytokinin activation. His research interests concern genetic, biochemical, and molecular physiological studies on development, local and long-distance communication, and cytokinin biosynthesis in plants. 

It is known that cytokinins induce shoot bud formation and the roots are the principal sites of cytokinin biosynthesis [18, 40]. Therefore, the adverse effects of TIBA on shoot formation might be explained by the secondary effects, such as the down regulation of cytokinin biosynthesis, caused by auxin accumulation, because auxin competitive inhibitor alone did not affect the shoot formation. 

The investigation of these non-plant genes cannot directly contribute to the understanding of cytokinin biosynthesis in plants, but it may provide (and now indeed provides) a useful tool for manipulation of the plant genome and consequently of cytokinin biosynthesis in transgenic plants. 

Down-regulation of cytokinin concentration by auxin(s) in plant cells is supposed to function either directly at the level of cytokinin biosynthesis [174,175] and/or indirectly... 

Chamberlain, W.J. Polar lipid materials in cigarette smoke condensate Tob. Sci. 20 (1976) 168-170. Chedekel, M.R. 1. Aberrant biosynthetic studies in Nicotiana tabacum and Nicotiana glauca. 11. Synthetic studies of the tobacco alkaloids. 1,4-Additions to acyl carbanion equivalents Dissertation Abstr. (1973) 2519. Chen, C.M. and D.K. Meltiz Cytokinin biosynthesis in a cell-free system from cytokinin-autotrophic tobacco tissue cultures FEBS Lett. 107 (1979) 15-20. 

Figure 6.63 Isoprenoid cytokinin biosynthesis pathway in Arabidopsis. IPT, phosphate-isopentenyl transferase AK, adenosine kinase 1, phosphatase 2,5 -ribonucleotide phosphohydrolase 3, adenosine nucleosidase 4, purine nucleoside phosphorylase 5, zeatin cis-trans isomerase and 6, zeatin reductase.

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