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At least 19 recordsLinked to original sources
PubMed2002

Conformational preferences of anticodon 3'-adjacent hypermodified nucleic acid base cis-or trans-zeatin and its 2-methylthio derivative, cis-or trans- ms(2)zeatin.

Conformational preferences of the hypermodified nucleic acid bases N6-(Delta(2)-cis-hydroxyisopentenyl)adenine, cis-io(6)Ade also known as cis-zeatin, and N(6)-(Delta(2)-trans-hydroxyisopentenyl)adenine, trans-io(6)ade or trans-zeatin, and 2-methylthio derivatives of these cis-ms(2)io(6)Ade or cis-ms(2)zeatin, and trans-ms(2)io6Ade or trans-ms(2)zeatin have been investigated theoretically by the quantum chemical Perturbative Configuration Interaction with Localized Orbitals (PCILO) method. Automated geometry optimization using quantum chemical MNDO, AM1 and PM3 methods has also been made to compare the salient features. The predicted most stable conformation of cis-io(6)Ade, trans-io(6)Ade, cis-ms(2)io(6)Ade and trans-ms(2)io(6)Ade are such that in each of these molecules the isopentenyl substituent spreads away (has "dista" conformation) from the five membered ring imidazole moiety of the adenine. The atoms N(6), C(10) and C(11) remain coplanar with the adenine ring in the predicted preferred conformation for each of these molecules. In cis-io(6)Ade as well as cis-ms(2)io(6)Ade the hydroxyl oxygen may participate in intramolecular hydrogen bonding with the H-C(10)-H group. In trans-io(6)Ade the hydroxyl group is oriented towards the H-C(2) instead. This orientation is retained in trans-ms(2)io(6)Ade, possible O-H...S hydrogen bonding may be a stabilizing factor. In all these four modified adenines C(11)-H is favourably placed to participate in intramolecular hydrogen bonding with N(1). In cis-ms(2)io(6)Ade as well as trans-ms(2)io(6)Ade the 2-methylthio group preferentially orients on the same side as C(2)-N(3) bond, due to this non-obstrusive placing, orientation of the hydroxyisopentenyl substituent remains unaffected by 2-methylthiolation. Thus the N(1) site remains shielded irrespective of the 2-methylthiolation status in these various cis-and trans-zeatin analogs alike. Firmly held orientation of hydroxyisopentenyl substituent in zeatin isomers and derivatives, in contrast to adaptable orientation of isopentenyl substituent in i(6)Ade and ms(2)i(6)Ade, may account for the increased efficiency of suppressor tRNA and reduced codon context sensitivity accompanied with the occurrence of ms(2)-zeatin (ms(2)io(6)Ade) modification.

Anticodon↗
PubMed1976

Developmental Effects of Zeatin, Ribosyl-Zeatin, and Agrobacterium tumefaciens B(6) on Certain Mosses.

Eight species of mosses studied were divided into two groups on the basis of their developmental responses to ribosyl-trans-zeatin and Agro-bacterium tumefaciens B(6). All eight produced either gametophores or callus on the protonema in response to 6-(gamma,gamma-dimethylallylamino) purine and trans-zeatin. Three which produced normal gametophores with A. tumefaciens yielded callus or abnormal gametophores with ribosyl-trans-zeatin. Ribosyl-trans-zeatin and A. tumefaciens were relatively ineffective on five other mosses. Characteristics of protonemal growth common to each of these two groups are described.

Journal Article↗
PubMed1984

Changes in Cytokinins and Gibberellin-Like Substances in Pinus radiata Buds during Lateral Shoot Initiation and the Characterization of Ribosyl Zeatin and a Novel Ribosyl Zeatin Glycoside.

Based on detection and quantitation by bioassay, endogenous gibberellin-like substances (GAs) and cytokinins (CKs) in Pinus radiata D. Don buds during sequential shoot initiation shift from less polar to more polar forms (GAs) and from conjugated to free forms (CKs). As the terminal bud moves from the production of "short shoots" (needle fascicles) to "long shoots" (lateral branches or female conebuds), a more polar GA appears while a glucoside-conjugate of zeatin riboside is reduced, and zeatin riboside levels increase markedly.Permethyl derivatives of the two highly active CK fractions were examined by capillary gas chromatography-mass spectrometry after separation by C(18) reverse phase high performance liquid chromatography. The mass spectra indicated the presence of: 9-beta-d-ribofuranosyl-6-(4-hydroxy-3-methyl-but-2-enylamino)purine (zeatin riboside) and 9-[hexosyl(probably glucosyl)-beta-d-ribofuranosyl]-6-(4-hydroxy-3-methyl-but-2- enylamino)purine (a glycoside of zeatin riboside in which the glycosyl moiety is attached directly to the ribosyl moiety at an unknown position).

Journal Article↗
PubMed1990

Influence of Endogenous Cytokinins on Reverse Mobilization in Cotyledons of Cicer arietinum L: Reproduction of Endogenous Levels of Total Cytokinins, Zeatin, Zeatin Riboside, and Their Corresponding Glucosides.

The embryonic axis plays an essential role in the mobilization of the main reserves of the cotyledons of seeds of Cicer arietinum L. cv Castellana. This control by the axis of the metabolism of the storage products of the cotyledons largely takes place through the cytokinins, which are transported from the embryonic axis to the cotyledons where the mobilization of reserves begins. The principal regulatory role of the endogenous cytokinins concerns the metabolism of carbohydrates and proteins; there is less influence on lipid metabolism. However, each cytokinin seems to have a different role in the mobilization processes. The glucosides, glucosyl zeatin riboside, and glucosyl zeatin act only as storage forms of the hormones. Zeatin riboside affects mainly the mobilization of carbohydrates and has less effect on protein mobilization. Zeatin regulates both the mobilization of carbohydrates and that of proteins and is more marked in the latter case.

Journal Article↗
PubMed1975

Comparative Activity of Isomers of Zeatin and Ribosyl-Zeatin on Funaria hygrometrica.

The activities of isomers of zeatin, ribosyl-zeatin, and 6-(gamma,gamma-dimethylallylamino) purine (i(6)Ade) on the moss Funaria hygrometrica are compared by measuring the ability of the cytokinins to induce callus or gametophores. The cis- and trans-ribosyl-zeatins were inactive, and therefore this kind of bioassay cannot be used as evidence for the presence or absence of a cytokinin in tests on natural products.

Journal Article↗
PubMedDate not supplied

[A new micromethod for differential quantitative assay of zeatin and zeatin riboside].

A new method is proposed for differential quantitative assay of two major endogenous cytokinin forms. It is based on determination of two effective parameters-concentrations of zeatin and zeatin riboside--with the use of appropriate antigens as standards. The method can be used for determining cytokinins in small samples of plant tissues without extract fractionation. This study pioneers in quantitation of changes in the hormonal status of ovules and ovaries of Triticum aestivum L. at early stages of embryogeny. A gradual increase in the content of the active and storage forms of the hormones from the ovary to the ovule was revealed.

Adenosine↗
PubMed1972

Comparison of zeatin indoleacetate with zeatin and indoleacetic Acid in the tobacco bioassay.

Zeatin indole-3-acetate, 6-[4-(indole-3-acetoxy)-3-methyl-trans-2-butenylamino]purine, is at least as effective as zeatin on a molar basis in satisfying the cytokinin requirement for growth and bud formation in tobacco bioassays. It is less effective than indole-3-acetic acid and is needed as a variable function of the cytokinin concentration for satisfying the optimal requirement of an auxin. Comparisons of the types of growth and yield of tissue obtained with serial concentration of the ester and with equimolar mixtures of its free base and acid indicate that the relative requirement for auxin changes with the concentration of cytokinin and is related to the types of callus growth and differentiation which occur. The results also suggest that the ester serves as a source of auxin only after modification, presumably by hydrolysis to indoleacetic acid.

Journal Article↗
PubMed1984

Concentration Gradients of trans-Zeatin Riboside and trans-Zeatin in the Maize Stem: Measurement by a Specific Enzyme Immunoassay.

A sensitive, specific enzyme immunoassay (EIA) for trans-zeatin riboside (ZR) and trans-zeatin (Z) in the 0.3 to 30 picomole range has been described. The reliability of the method for measuring ZR + Z in partially purified extracts of Zea mays L. tissues was verified by highperformance liquid chromatography. EIA measurements showed that there was a concentration gradient of ZR + Z along the length of the Zea stem. The topmost internodes, internodes 7 and 8 counting from the coleoptilar node, had the highest concentration ( approximately 130 picomoles per gram fresh weight). Moving basipetally, the concentration dropped approximately 10-fold to a minimum at internode 4, and then increased slightly in internodes 2 and 3. There were also gradients within each internode. The five lowest internodes contained the highest concentrations toward their apical end, the region which included the node; this asymmetry was less pronounced near the top of the plant.

Journal Article↗
PubMed1967

Zeatin and zeatin riboside from a mycorrhizal fungus.

The puffball fungus Rhizopogon roseolus produces and releases three cytokinins when it is cultured in liquid media. Two of these compounds have been isolated in crystalline form from such media and were found to have properties identical to those of synthetic zeatin and zeatin riboside.

Basidiomycota↗
PubMed2003

O-glucosylation of cis-zeatin in maize. Characterization of genes, enzymes, and endogenous cytokinins.

trans-Zeatin is a major and ubiquitous cytokinin in higher plants. cis-Zeatin has traditionally been viewed as an adjunct with low activity and rare occurrence. Recent reports of cis-zeatin and its derivatives as the predominant cytokinin components in some plant tissues may call for a different perspective on cis-isomers. The existence of a maize (Zea mays) gene (cisZOG1) encoding an O-glucosyltransferase specific to cis-zeatin (R.C. Martin, M.C. Mok, J.E. Habben, D.W.S. Mok [2001] Proc Natl Acad Sci USA 98: 5922-5926) lends further support to this view. Results described here include the isolation of a second maize cisZOG gene, differential expression of cisZOG1 and cisZOG2, and identification of substantial amounts of cis-isomers in maize tissues. The open reading frame of cisZOG2 has 98.3% identity to cisZOG1 at the nucleotide level and 97.8% at the amino acid level. The upstream regions contain common and unique segments. The recombinant enzymes have similar properties, K(m) values of 46 and 96 microM, respectively, for cis-zeatin and a pH optimum of 7.5. Other cytokinins, including N(6)-(delta(2)-isopentenyl)adenine, trans-zeatin, benzyladenine, kinetin, and thidiazuron inhibited the reaction. Expression of cisZOG1 was high in maize roots and kernels, whereas cisZOG2 expression was high in roots but low in kernels. cis-Zeatin, cis-zeatin riboside, and their O-glucosides were detected in all maize tissues, with immature kernels containing very high levels of the O-glucoside of cis-zeatin riboside. The results are a clear indication that O-glucosylation of cis-zeatin is a natural metabolic process in maize. Whether cis-zeatin serves as a precursor to the active trans-isomer or has any other unique function remains to be demonstrated.

Amino Acid Sequence↗
PubMed1973

The Metabolism of Hormones during Seed Germination and Release from Dormancy: III. The Effects and Metabolism of Zeatin in Dormant and Nondormant Ash Embryos.

Zeatin and zeatin-9, beta-ribonucleoside enhance the germination of dormant ash embryos. While the first macroscopic signs of germination appear only after about 72 hours, 12 hours of exposure to 50 mum zeatin is as effective as continuous incubation. There must be barriers against transport out of the embryos since 8-(14)C-zeatin and its metabolites, zeatin-9, beta-ribonucleoside, the 5'-mono and the suspected di- and triphosphates, accumulate against a concentration gradient. Zeatin ribonucleoside is about as effective as zeatin in enhancing embryo germination, yet the internal 8-(14)C-zeatin level is lower by a factor of about 50 when the ribonucleoside is fed. The physiological effects of zeatin and abscisic acid on the germination of ash embryos are antagonistic. There is, however, no evidence that abscisic acid has a significant effect on 8-(14)C-zeatin uptake or conversions.

Journal Article↗
PubMed1975

A kinetic analysis of the effects of gibberellic Acid, zeatin, and abscisic Acid on leaf tissue senescence in rumex.

Hormones which inhibit senescence in Rumex leaf tissue in the dark include gibberellic acid and the cytokinin zeatin. Abscisic acid accelerates senescence in this tissue. Other workers have proposed that cytokinins, but not gibberellins, interact with abscisic acid in senescing Rumex leaf tissue. The present study reinvestigates the question of interaction using measurements of chlorophyll degradation kinetics as parameters of senescence rate and draws the conclusion that neither zeatin nor gibberellic acid interact with abscisic acid in this system. In support of this conclusion are these results. Zeatin clearly cannot overcome the effects of abscisic acid when hormone solutions are replaced every other day. The kinetics of chlorophyll breakdown for tissue treated with unreplaced saturating zeatin solutions is different from that of tissue exposed to saturating zeatin plus abscisic acid. The observed rates of chlorophyll breakdown for tissue treated with abscisic acid and zeatin agree closely with predicted rates using a multiplicative model for independent action of the two hormones.Zeatin solutions, when replaced every other day, show up to a 550-fold increase in effective concentration in the retardation of senescence. Less than a 10-fold increase could be accounted for by the addition of more zeatin molecules to the tissue. A nonbiological inactivation of zeatin or the production of an inhibitor of zeatin action by the tissue could not be demonstrated. It seems that zeatin is metabolically inactivated or sequestered in this tissue. The possible physiological significance of the inactivation of cytokinins in leaf tissue is discussed.

Journal Article↗
PubMed2002

tRNA is the source of low-level trans-zeatin production in Methylobacterium spp.

Pink-pigmented facultatively methylotrophic bacteria (PPFMs), classified as Methylobacterium spp., are persistent colonizers of plant leaf surfaces. Reports of PPFM-plant dialogue led us to examine cytokinin production by PPFMs. Using immunoaffinity and high-performance liquid chromatography (HPLC) purification, we obtained 22 to 111 ng of trans-zeatin per liter from culture filtrates of four PPFM leaf isolates (from Arabidopsis, barley, maize, and soybean) and of a Methylobacterium extorquens type culture originally recovered as a soil isolate. We identified the zeatin isolated as the trans isomer by HPLC and by a radioimmunoassay in which monoclonal antibodies specific for trans-hydroxylated cytokinins were used. Smaller and variable amounts of trans-zeatin riboside were also recovered. trans-Zeatin was recovered from tRNA hydrolysates in addition to the culture filtrates, suggesting that secreted trans-zeatin resulted from tRNA turnover rather than from de novo synthesis. The product of the miaA gene is responsible for isopentenylation of a specific adenine in some tRNAs. To confirm that the secreted zeatin originated from tRNA, we mutated the miaA gene of M. extorquens by single exchange of an internal miaA fragment into the chromosomal gene. Mutant exconjugants, confirmed by PCR, did not contain zeatin in their tRNAs and did not secrete zeatin into the medium, findings which are consistent with the hypothesis that all zeatin is tRNA derived rather than synthesized de novo. In germination studies performed with heat-treated soybean seeds, cytokinin-null (miaA) mutants stimulated germination as well as wild-type bacteria. While cytokinin production may play a role in the plant-PPFM interaction, it is not responsible for stimulation of germination by PPFMs.

Adenine↗
PubMed1972

Cytokinins: synthesis and biological activity of geometric and position isomers of zeatin.

Geometric and position isomers of zeatin and of ribosylzeatin and other compounds closely related to zeatin have been tested in the tobacco (Nicotiana tabacum var. Wisconsin No. 38) bioassay. None was more active than zeatin itself. There was a much greater difference in activity (> 50-fold) between trans- and cis-zeatin than between trans-isozeatin [6-(4-hydroxy-2-methyl-trans-2-butenylamino) purine] and cis-isozeatin [6-(4-hydroxy-2-methyl-cis-2-butenylamino) purine], the latter being less active than cis-zeatin and trans-isozeatin. Higher concentrations were required for equivalent callus growth stimulated by the 9-ribosyl derivatives, which followed an order of decreasing activity: ribosyl-trans-zeatin > ribosyl-cis-zeatin > ribosyl-trans-isozeatin > ribosyl-cis-isozeatin, corresponding roughly to that of the bases. The effect of side chain, double bond saturation was to diminish the activity, and in the dihydro series the shift of the methyl group from the 3- to the 2-position in going from dihydrozeatin to dihydroisozeatin [6-(4-hydroxy-2-methylbutylamino) purine] resulted in a 70-fold decrease in activity. cis-Norzeatin [6-(4-hydroxy-cis-2-butenylamino) purine], which was less than one-fifth as active as cis-zeatin, showed the effect of complete removal of the side chain methyl group, and cyclic-norzeatin [6-(3,6-dihydro-1,2-oxazin-2-yl) purine] was about 1/100 as active as cis-norzeatin. These findings delineate completely the effect on the cytokinin activity of zeatin of variation in side chain geometry, presence and position of the methyl substituent, presence and geometry of hydroxyl substitution, presence of the double bond, and of side chain cyclization.

Journal Article↗
PubMed1982

Estimation of osmotic parameters accompanying zeatin-induced growth of detached cucumber cotyledons.

Water potential (psi), the osmotic potential (psi(pi)), and the pressure potential (psi(p)) of detached cotyledons isolated from Cucumis sativus L. cv Marketer seedlings after 0, 1.5, and 3 days growth with and without zeatin were determined. From zero time to 3 days, cotyledons incubated without exogenous zeatin exhibited a slight decrease in psi (from -0.4 to -1.0 bars), while those grown with zeatin developed even more negative values (about -4 bars). Both groups showed rising psi(pi) values (decreases in solutes per unit volume), but this rise was more dramatic in those treated with zeatin. These data indicate that the capacity of zeatin-treated cotyledons to take up water more rapidly than controls and thus expand faster must be due to wall loosening, as reflected in psi(p) values which declined during 3 days from about +11 bars to about +1.4 bars.It was also found that freshly detached cotyledons or those grown without exogenous zeatin exhibited osmoregulation in polyethylene glycol (PEG) solutions. That is, while cotyledons initially lost H(2)O into certain PEG solutions, their psi values decreased over time and they began absorbing water after 1 to 4 hours. After 3 days growth, zeatin-treated cotyledons had lost most of this capacity of osmoregulate. It seems likely that osmoregulation in cotyledons not treated with zeatin is due to wall loosening rather than changes in psi(pi). Zeatin-treated cotyledons with already loosened walls may not have this option to deal with water stress and thus simply come to equilibrium with external PEG solutions.

Journal Article↗
PubMed2001

A maize cytokinin gene encoding an O-glucosyltransferase specific to cis-zeatin.

Zeatin is a naturally occurring cytokinin. Biosynthesis and metabolism studies of zeatin have been directed mostly at the trans isomer, although cis-zeatin and its riboside occur as major components in some plant species. It is not known whether parallel regulatory pathways exist for the two isomers. Based on the sequence of the gene ZOG1 encoding a trans-zeatin O-glucosyltransferase from Phaseolus (EC ), a cis-zeatin-specific O-glucosyltransferase was isolated from maize. This gene, cisZOG1, contains an ORF of 1,401 nucleotides encoding a protein of 51.1 kDa with 41% identity to the Phaseolus ZOG1 protein. Unexpectedly, the maize enzyme recognizes as substrates cis-zeatin and UDP-glucose but not cis-ribosylzeatin, trans-zeatin, or trans-ribosylzeatin. This finding indicates the existence of cis-specific regulatory elements in plants and suggests that cis-zeatin and derivatives may be more important in cytokinin homeostasis than currently recognized.

Amino Acid Sequence↗
PubMed1971

The Metabolism of Hormones during Seed Germination and Dormancy: II. The Metabolism of 8-C-Zeatin in Bean Axes.

8-(14)C-Zeatin is taken up rapidly and is extensively metabolized by excised bean axes during a 12-hour incubation at 26 C. Most of the radioactivity is found in the 80% ethanol soluble fraction and consists of zeatin, zeatin riboside, zeatin-5'-ribotide, as well as corresponding dihydrozeatin derivatives. The characterization of (14)C-dihydrozeatin included crystallization to constant specific radioactivity. No cleavage of the zeatin side chain to adenine, hypoxanthine, their ribosides, or glycylpurine was detected. Dihydrozeatin has been previously isolated from yellow lupin seeds, and our experiments indicate that it can be derived through reduction of the side chain from preexisting cytokinin. While the total amount of zeatin metabolized is not affected by growth-inhibiting concentrations of abscisic acid or cycloheximide, the conversion to dihydrozeatin derivatives is curtailed. Although somewhat less effective than zeatin and zeatin riboside, dihydrozeatin and dihydrozeatin riboside also counteract the abscisic acid-induced growth inhibition.

Journal Article↗