Cytokinins adenine-type

Fig. (1). Straccures of representative adenine-type and phenylurea-type cytokinins...

Structurally, cytokinins are classified into two groups, namely, adenine-type cytokinins represented by zeatin, kinetin, and 6-benzylaminopurine, and phenylurea-type cytokinins which include diphenylurea and thidiazuron. The naturally occurring cytokinins A -(A -isopentenyl)adenine 13, frans-zeatin 14, and c s-zeatin 15 differ in the stereoisomeric hydroxylation at the isoprenoid side chain. frans-Zeatin and A -(A -isopentenyl)adenine are physiologically the most important cytokinins in plants [27]. 

Ubiquinones (UQ), often called coenzyme Qio, are electron carriers in oxidative phosphorylation and photosynthesis, respectively. Ubiquinones consist of quinoid nucleus (derived from the shikimate pathway), 4-hydroxybenzoate (derived from chorismate or tyrosine), and terpenoid moiety. Zeatin, a phytohormone, is a member of the cytokinin family involved in various processes of growth and development in plants. Most cytokinins are adenine-type, where the hydrogen of amino group at Ce position of adenine is replaced with an isoprenoid. 

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). 

A recent review has considered structure-activity data on some 400 compounds [33]. Quantitative structure-activity regression analysis was carried out on several types of cytokinin and their competitive inhibitors, and the results were used to develop a map of the recognition site of the cytokinin receptor [34-36]. The map is shown in Fig. 6a, with the site being overlain by isopentenyl adenine (4-3). In Fig. 6b, the site is divided into a purine area, with the remaining area of the site being referred to as side chain domain. The pyiidyl urea (4-5) is fitted to the site. 

These results of [ Cj-adenine incubation studies directly demonstrate cytokinin synthesis by germinating seeds, and further indicate that possibly only the embryonic axes have the capacity to synthesize cytokinins, which are then translocated to the cotyledons, apparently accumulating therein to evoke physiological response. This is reflected in experiments with the intact seed where [ Cj incorporation into cytokinins (per g fw) was considerably higher in embryos than in the cotyledons (Table 2), while incorporation per organ is greater for the cotyledons [18]. Translocation experiments carried out with selective application of pH]-(diH) [9R]Z to embryos or cotyledons also indicate a polar movement of cytokinins from the embryonic axes to the cotyledons. The incorporation of [ C]-adenine into (diH) [9R]Z is particularly interesting because (diH)Z-type cytokinins predominate in lupin seed. 

Kinetin, the first cytokinin, was first found in degraded DNA. It was shown to be an artefact. For example, kinetin is also obtained when adenine, a purine base of the nucleic acids, and furfuryl alcohol, which can arise in the acid hydrolysis of sugars, are autoclaved together. Nonetheless, it was subsequently shown that there is a close relationship between cytokinins and nucleic acids. The only point is that the nucleic acids concerned are certain types of tRNA, not DNA. Substances with cytokinin activity can be detected in hydrolysates of tRNA from quite different sources (bacteria, yeast, calf liver, and higher plants). The first cytokinin was identified in yeast tRNA in 1966. It was shown to be IPA. It was then shown to be present in the tRNA of a number of other organisms. 

These findings might lead one to suspect that the mechanism of action of the cytokinins lies in their being incorporated into particular types of tRNA molecules and thus in an effect on translation. If this supposition were correct then exogenously added cytokinin ought to be incorporated into particular types of tRNA. Unfortunately, the few pieces of evidence relevant to this question, which are presently available, are contradictory. In some cases an incorporation of cytokinins into tRNA was demonstrated, in others not, although the cytokinins, which were supplied, were definitely active. What is more, there are findings which indicate that the complete cytokinins are not incorporated into tRNA. In the case of IPA residues present in tRNA it has been demonstrated several times that the isopentenyl residue is attached subsequently to an adenine moiety already present in the tRNA. 

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