The cytokinins. Synthetic and naturally occurring N6-substituted adenine derivatives profoundly affect plant growth

Genome-wide identification and characterization of cytokinin oxidase/dehydrogenase family genes in Medicago truncatula

Cytokinin oxidase/dehydrogenases (CKXs) play a key role in the irreversible degradation of phytohormone cytokinin that is necessary for various plant growth and development processes. However, thus far, detailed investigations of the CKX gene family in the model legume Medicago truncatula are limited. In this study, we identified 9 putative CKX homologues with conserved FAD- and cytokinin-binding domains in the M. truncatula genome. We analyzed their phylogenetic relationship, gene structure, conserved domain, expression pattern, protein subcellular locations and other properties. The tissue-specific expression profiles of the MtCKX genes are different among different members and these MtCKXs also displayed different patterns in response to synthetic cytokinin 6-benzylaminopurine (6-BA) and indole-3-acetic acid (IAA), suggesting their diverse roles in M. truncatula development. To further understand the biological function of MtCKXs, we identified and characterized mutants of each MtCKX by taking advantage of the Tnt1 mutant population in M. truncatula. Results indicated that M. truncatula plants harboring Tnt1 insertions in each single MtCKX genes showed no morphological changes in aerial parts, suggesting functional redundancy of MtCKXs in M. truncatula shoot development. However, disruption of Medtr4g126160, which is predominantly expressed in roots, leads to an obvious reduced primary root length and increased lateral root number, indicating the specific roles of cytokinin in regulating root architecture. We systematically analyzed the MtCKX gene family at the genome-wide level and revealed their possible roles in M. truncatula shoot and root development, which shed lights on understanding the biological function of CKX family genes in related legume plants.

Cytokinins: synthesis, mass spectra, and biological activity of compounds related to zeatin

Compounds related to dihydrozeatin that define the influence of the location of the hydroxyl group along the side chain have been synthesized and tested for cytokinin activity. The compounds compared are in the series: 6-(X-hydroxy-3-methylbutylamino)purines and their ribosides, where X = 2, 3, and 4.Hydroxy substitution on the 4-position of the side chain enhances, but in the 2-, 3-, or 2- and 3- positions, decreases cytokinin activity as compared with the unsubstituted isopentyl (or isopentenyl) chains. This differential influence of the position of the hydroxyl group in the N(6)-chain holds also for the similarly related 9-beta-D-ribofuranosides. The relatively higher activity of 3,4-dihydroxy as compared with 2,3-dihydroxy derivatives is consistent with this position effect.Compounds related to zeatin possessing side-chain ester moieties have also been synthesized and tested comparatively. Among these, 6-(4-acetoxy-3-methyl-trans-2-butenylamino)purine is at least as active as zeatin, the most active presently known cytokinin in the tobacco bioassay, whereas the analog, methyl 2-methyl-4-(purin-6-ylamino)-trans-crotonate, with the ester function effectively reversed, has vastly lower activity, and its riboside is practically inactive.

Endogenous cytokinins in vernalized winter wheat grains

The soybean callus assay was used to study the effect of vernalization on the endogenous cytokinin levels of the grains of winter wheat (Triticum vulgare L.), cv. Ferto and Fema. Vernalization of the grains was carried out at 3° C in the dark for 0, 9, 18, 27, 36 and 45 days. Cytokinin activities could be detected in both the aqueous and butanol extracts of vernalized grains of both cultivars. It is suggested that flowering of winter wheat may involve changes in the levels of endogenous cytokinins during vernalization.

Biosynthesis and cytokinin activity of 8-hydroxy and 2,8-dihydroxy derivatives of zeatin and N-6(increment-2-isopentenyl)adenine

8-Hydroxy and 2,8-dihydroxy derivatives of the cytokinins, 6-(4-hydroxy-3-methyl-trans-2-butenylamino)purine and N-6-(increment -2-isopentenyl)adenine, have been biosynthesized by xanthine oxidase oxidation. 8-Hydroxy derivatives have been shown to be the major intermdeiates. These compounds were tested for cytokinin activity in the tobacco bioassay. The results suggest that substitution of the 8 position with a hydroxyl group causes less decrease of cytokinin activity than substitution of both the 2 and 8 positions with hydroxyl groups.

Competitive inhibition of beef heart cyclic AMP phosphodiesterase by cytokinins and related compounds

Two cytokinins and four related analogs, none of which is a cyclic ribonucleotide, have been shown to act as competitive inhibitors of the high K(m) cyclic-AMP phosphodiesterase (3':5'-cyclic-AMP 5'-nucleotidohydrolase, EC 3.1.4.17) activity from beef heart. Weak inhibition of the low K(m) cyclic AMP phosphodiesterase activity was also observed, suggesting a possible mechanism for regulation of intracellular cyclic AMP levels by the exogenously added compounds. In addition to the kinetic data, obtained on the six inhibitors in four different heterocyclic series, 15 other cytokinins and related compounds have been shown to inhibit the high K(m) cyclic AMP phosphodiesterase activity at single concentrations of substrate and inhibitor. Heterocycles such as adenosine and 7-amino-3-methylpyrazolo[4,3-d]pyrimidine, which lack the N-substituent, were inactive as cyclic AMP phosphodiesterase inhibitors. The observed inhibition of cyclic AMP phophodiesterase supports prior observations which implicate exogenously added cytokinins in cyclic AMP metabolism.

Protein, nucleic acid and starch metabolism in the duckweed, Spirodela oligorrhiza, treated with cytokinins

Bacteria-free cultures of Spirodela oligorrhiza continue to increase in frond number for 2 to 3 days after transfer to darkness. There is then no further increase in frond number for 3 to 4 weeks, although DNA, RNA and protein synthesis continue at decreased rates and starch accumulates in the plants. We refer to such ;non-growing' plants in darkness as dormant. Adding kinetin to dormant Spirodela initiated increased DNA, RNA and protein synthesis within 1h, although new fronds were not detected until 24h after the addition of kinetin. The frond number then continued to increase. Starch accumulated in dormant plants. Accumulation of starch appeared to be a consequence of inhibition of growth rather than the converse. No evidence was obtained for a block in [(14)C]glucose metabolism that might explain the lack of growth in darkness in the absence of kinetin. In darkness, more ribosomes were membrane-bound in dormant Spirodela than in Spirodela growing with kinetin. Similarities between the response of Spirodela to darkness, stringent control in bacteria and pleiotypic controls in animal cells are discussed. It is suggested that all three processes are ultimately controlled by specific protein kinases that are individually sensitive to different effectors.

Cell division and prophage induction in Escherichia coli: effects of pantoyl lactone and various furan derivatives

Strain T-44 is a thermosensitive mutant of Escherichia coli in which both cell division and prophage repression are altered at elevated temperatures. The effects of various ribosides, pantoyl lactone, and the furfural derivatives nitrofurazone and 5-methyl furfural suggest that some low-molecular-weight compound is important in the control of cell division and prophage repression in this strain. This low-molecular-weight compound may have a five-membered oxygen-containing ring as part of its structure.

Cytokinins: development of a potent antagonist

A systematic search has resulted in the synthesis of a class of cytokinin antimetabolites. The development and biological properties of the anticytokinins are discussed in terms of one member of the class, 3- methyl - 7 - (3 - methylbutylamino)pyrazolo[4,3 - d]- pyrimidine.

Enzymatic modification of transfer RNA

The molecular events leading to the synthesis of mature tRNA are only now becoming amenable to experimental study. In bacterial and mammalian cells tRNA genes are transcribed into precursor tRNA. These molecules, when isolated, contain additional nucleotides at both ends (20) of the mature tRNA and lack most modified nucleosides. Presumably, specific nucleases ("trimming" enzymes) cut the precursor to proper tRNA size. The C-C-A nucleotide sequence of the amino acid acceptor end common to all tRNA's does not seem to be coded by tRNA genes (30), and may be added to the trimmed molecules by the tRNA-CMP-AMP-pyrophosphorylase (71). Modifications at the polynucleotide level of the heterocyclic bases or the sugar residues give rise to the modified nucleosides in tRNA. Although newly available substrates have allowed the detection of more of the enzymes involved in these reactions, there is still no knowledge about the sequence of modification or trimming events leading to the synthesis of active tRNA. Progress in these studies may not be easy because enzyme preparations free of nucleases or other tRNA modifying enzymes are required. The role of the modified nucleosides in the biological functions of tRNA is still unknown. Possibly pseudouridine is required for ribosome mediated protein synthesis; some other modified nucleosides in tRNA are not required for this reaction, but may enhance its rate. What might be the role of the large variety of modified nucleosides in tRNA? One is tempted to speculate that such nucleosides are important in other cellular processes in which tRNA is thought to participate such as virus infection, cell differentiation, and hormone action (2, 3). Mutants in a number of tRNA-modifying enzymes are needed in order to extend our knowledge of their purpose and of tRNA involvement in other biological processes. But unless tRNA-modifying enzymes specific for a particular tRNA species exist, no simple selection procedure can be devised. Possibly some of the regulatory mutants of amino acid biosynthesis may prove to affect tRNA-modifying enzymes (72). Transfer RNA's are macromolecules well suited for the study of nucleic acid-protein interactions. The tRNA molecules are structurally very similar, and they interact with a large number of enzymes or protein factors (2, 3). Each aminoacyl-tRNA synthetase, for instance, very precisely recognizes a set of cognate isoacceptor tRNA's (2, 73). The availability of the tRNA- modifying enzymes adds another dimension to the problem of the nature of specific recognition of tRNA by proteins. There are some tRNA-modifying enzymes, such as the uracil-tRNA methylase, which may recognize all tRNA species, while others, such as the isopentenyl-tRNA transferase, probably recognize only a selected set of tRNA molecules, even with different amino acid accepting capacities. With well-characterized RNA precursor and tRNA molecules we can hope to delineate those features of primary, secondary, and tertiary structure involved in the specific interactions of tRNA with these enzymes.

Cytokinin-induced chlorophyll formation in cucumber cotyledons

Cotyledons of cucumber plants grown in the dark for 7 days were treated with various concentrations of cytokinins for 14 h and then moved into light. After 3 h the treated cotyledons had up to 450% more chlorophyll than the water controls. This suggests that cytokinins have an important role in the formation of chlorophyll. The increase in chlorophyll level was proportional to cytokinin concentration and was apparent at concentrations as low as 0.001 mg/l. Sensitivity to cytokinins depended on the age of the cotyledons and the time of exposure to light. Gibberellic acid, indoleacetic acid, adenine and sucrose did not cause a similar increase in chlorophyll levels. This effect of cytokinins on chlorophyll formation is valuable as a simple, rapid bioassay for cytokinins.

Revised identification of the chromophore of a cell division factor from crown gall tumor cells of Vinca rosea L

Low-resolution mass spectrometry, UV spectra in acid, neutral, and alkaline solution, and nuclear magnetic resonance indicated that the chromophore of one member of a new class of cell division-promoting factors isolated from crown gall tumor tissues consisted of a 3,7-alkyl-2-alkylthio-6-purinone with one free proton in the ring. Glucose constitutes the sugar moiety.

Cholesterol inhibition of isopentenyl pyrophosphate delta3, delta2-isomerase in Mycoplasma laidlawii

Cholesterol inhibits isopentenyl Delta(3),Delta(2)-isomerase of Mycoplasma laidlawii in an apparently competitive fashion. The conversion of mevalonic acid to isopentenyl pyrophosphate is slightly stimulated. Organisms grown in the presence of mevalonic-2-(14)C acid contain small amounts of radio-label in nucleic acid and protein fractions. Most of the label is found in the lipids and is reduced dramatically in organisms grown with cholesterol. No significant accumulation of phosphorylated intermediates of polyterpene biosynthesis was observed in cells or culture supernatant fluid. All of the radioactivity appearing in the nucleic acid fraction occurs in the minor nucleoside, isopentenyl adenosine, of the transfer ribonucleic acid. The necessity for synthesis by the organisms of this minor nucleoside from mevalonic acid may explain the site of enzyme inhibition by cholesterol of polyterpene biosynthesis.

Isolation of zeatin riboside from the chicory root

From 250 kg of fresh chicory roots about 2 mg of a crystalline cytokinin were obtained. This substance was identified as ribosylzeatin (trans isomer). From the procedure employed it seems unlikely that the isolated cytokinin comes from the degradation of tRNAs; rather, it may constitute a separate pool of cytokinins.