Aromatic cytokinins

Aromatic cytokinins are found in poplar,379-381 anise,382 tomato,383 Cocos nucifera m and Arabidopsis,381 In addition to BA, aromatic cytokinins are found as ortho-topolins, wc/ -topolins, hydroxylated BAs, ortho-meth-oxytopolins, and rwfto-methoxytopolins (Figure 1 1).380,381 Other derivatives have been isolated as N3-, N7-, and /VP-glucosides and A P-alanine on the adenine ring,385 and nucleosides and a glucoside linked to the ribosyl moiety.386 In spite of the several reports of their occurrence, it is not clear whether the aromatic species are common in plants. 

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). 

Origin of aromatic cytokinins and metabolic pathways participating in their biosynthesis has not yet been identified and a possible implication of adenylate isopentenyltransferase has not been examined. 

All native cytokinins are derivatives of adenine with at least one substituent (at position). According to this substituent, these compounds may be classed into (1) isoprenoid (zeatin, A -A -isopentenyladenine and their derivatives), (2) isoprenoid-derived (dihydrozeatin and its derivatives) and (3) aromatic cytokinins. Native cytokinins and their derivatives are summarised in Fig. 1 together with abbreviations used here. 

To date there is no report about the biosynthesis of aromatic cytokinins. In view of the dissimilarity between the aromatic and the isoprenoid(-derived) A side chains it is likely that their biosynthetic pathways are quite different. Phenylalanine may be considered as a starting compound and benzaldehyde and/or hydroxylated benzaldehydes as immediate side chain precursors. However, the existence of some crossing-points between aromatic and isoprenoid side chain formation cannot be completely excluded. There is also the possibility that the enzymes of adenine and/or purine metabolism, which are not strictly specific, may catalyse some mutual conversions among BA-bases, nucleosides and nucleotides [81]. 

However, many naturally occurring and synthetic cytokinins that are not recognised as substrates by cytokinin oxidase (e.g. DHZ-type and aromatic cytokinins) are degraded in vivo by the side chain cleavage in a number of plant tissues (reviewed in [81,92,93,104]). Degradation of such cytokinin-oxidase-non-substrate cytokinins may be attributed to an as yet unknown separate enzyme system which makes the control of cytokinin catabolism even more complex. 

The structures of two novel Pt(IV) complexes with aromatic cytokinins, 9-[6-(benzylamino)purine] and 9-[6-(furfurylamino)purine] penta-chloroplatinates have been studied by Vicha et al by the use of NMR spectroscopy including a variety of spin-spin couplings. Among others, /cpt of 18 Hz has indicated that Pt in these complexes is coordinated at the N9 atom, which is in accordance with the solid-state structure characterized by X-ray diffraction. 

Natural cytokinins are. / /-substituted adenine derivatives. The N6 side chain is either isoprenoid or aromatic (Figure 10), with the former occurring in greater abundance than the latter. The prenyl side chains vary in the presence or absence of a hydroxyl group and the stereoisomeric position. Common active isoprenoid cytokinins are isopentenyladenine (iP /V6-(A2-isopentenyl)adenine), trans-zeatin (tZ / / -(4-hydro.xyisopentenyl)adenine (A)-2-methyl-4-(1 //-purin-6-ylamino)but-2-en-l -ol), ax-zeatin (cZ (Z)-2-methyl-4-(l//-purin-6-ylamino)but-... 

Active cytokinins possess an aromatic ring system which can interact with the purine... 

Fig. 3A-E. Schematic substrate-receptor complex of cytokinin-agonistic and antagonistic compounds. Models are for -substituted adenines and N,N -diphenylureas (A) 4-substituted-2-methylpyrrolo[2,3-d]pyrimidines (B) 4-substituted-2-methylthiopyrido[2,3-d]pyrimidines (C) N--substituted 2-amino-4-chloro-6-ethylamino-5-triazines (D) and N-phenylcarbamates (E). In A. the ends of the bars of the structures represent hydrogen atoms, and the double bonds of the aromatic rings are omitted. C and D were reproduced from previous papers [12,18] with permission from the American Chemical Society...

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