Zeatin metabolism

Genetic Differences in the Enzymatic Regulation of Zeatin Metabolism in Phaseobts Embryos... 

Since no specific enzymes interconverting cytokinin bases to ribosides and nucleotides and vice versa have been found, it is generally thought that these conversions are mediated by the enzymes of purine metabolism. However, a low affinity of these enzymes toward cytokinins may be insufficient due to physiological concentrations of cytokinins in plant tissues that are up to six orders lower than those of adenine derivatives. The only cytokinin-specific enzymes have been found for glycosylation of cytokinins at N and atoms of the adenine ring or at the hydroxyl group of zeatin. The main supposed cytokinin-specific metabolic pathways are displayed in Fig. (3). 

Fig. (3). Basic metabolic pathways of isoprenoid cytokinins. Enzymes participating in individual steps 1 -adenylate isopcntcnyltransfcrasc, EC 2.5.1.27 2 - cytochrome P-450 tnotiooxygcnasc, HC 1.14.13.- 3 - zeatin reductase, EC 1.3.1.69 4 - zeatin isomcrasc 5 - zcatin-O-glucosyltransferase, EC 2.4.1.203 and EC 2.4.1.215 6 - zealin-O-xylosyltamsferase, EC 2.4.2.40 7 - / -gtucosiduse, EC 3.2.1.21 8 — cytokinin-JV-glucosyltransl erose, EC 2.4.1.118 9 cytokinin dehydrogenase, EC 1.5.99.12. Dashed line indicates only predicted metabolic pathway, which have not been proved yet.

An unsettling feature of the system is the instability of its characteristics other than cytokinin-dependency. For example, the time taken following transfer to cytokinin-free medium (at a fixed cell density) before mitotic arrest is liable to shift between experiments. A simple explanation for the shifts might be the spontaneous occurrence in the culture of variant types with altered cytokinin metabolism. The spontaneous occurrence of an unstable line with enhanced activity of zeatin side chain cleavage has been described [32]. It seems that the renowned phenotypic instability of callus tissue [33] is accentuated by suspension culture, in which each small clump of cells is a separate entity with its own selective advantages in the fierce competition to contribute cells to the inoculum for the next batch. 

Gordon ME, Letham DS, Parker CW (1974 a) Regulators of cell division in plant tissues XVIII. The metabolism and translocation of zeatin in intact radish seedlings. Ann Bot38 809-825... 

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