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PubMed1994

Cell-autonomous cytokinin-independent growth of tobacco cells transformed by Agrobacterium tumefaciens strains lacking the cytokinin biosynthesis gene.

Mutations at the cytokinin biosynthesis locus (tmr) of Agrobacterium tumefaciens usually result in strains that induce tumors exhibiting the rooty phenotype associated with high auxin-to-cytokinin ratios. However, tobacco (Nicotiana tabacum cv Havana 425) leaf disc explants responded to tmr- mutant strain A356 by producing rapidly growing, unorganized tumors, indicating that these lines can grow in a cytokinin-independent fashion despite the absence of a functional tmr gene. Several methods have been used to characterize the physiological and cellular basis of this phenotype. The results indicate that tmr- tumors have a physiologically distinct mechanism for cytokinin-independent growth in comparison to tumors induced by wild-type bacteria. The cytokinin-independent phenotype of the tmr- transformants appears to be cell autonomous in nature: only the transformed cells and their progeny were capable of cytokinin-independent growth. Specifically, the tmr- tumors did not accumulate cytokinin, and clonal analysis indicated the tmr- transformed cells were not capable of stimulating the growth of neighboring nontransformed cells. Finally, the cytokinin-independent phenotype of the tmr- transformants was shown to be cold sensitive, whereas the wild-type tumors exhibited a cold-resistant cytokinin-independent phenotype. Potential mechanisms for this novel form of cytokinin-independent growth, including the role of the dehydrodiconiferyl alcohol glucosides found in both tumor types, are discussed.

Agrobacterium tumefaciens↗

PubMed1999

Increased steady state mRNA levels of the STM and KNAT1 homeobox genes in cytokinin overproducing Arabidopsis thaliana indicate a role for cytokinins in the shoot apical meristem.

This study investigates the consequences of endogenously enhanced biosynthesis of the plant hormone cytokinin in Arabidopsis thaliana (L.) Heynh. Transcriptional control of the bacterial ipt gene by the Drosophila melanogaster hsp70 promoter enabled temperature-dependent increased cytokinin production in transgenic plants. Heat-treated plants accumulated higher levels of unbound and bound zeatin-type cyto-kinins, the latter being preferentially N-conjugated glucosides. Cytokinin overproduction significantly increased the biomass of seedlings. Ipt transgenics had higher steady state mRNA levels of the shoot meristem specifying homeobox genes KNAT1 and STM, similar to the cytokinin-overproducing shoot meristem mutant amp1 (hpt, cop2, pt) This finding, together with previously described phenotypic similarities between transgenic cytokinin-overproducing plants and plants overexpressing the KNAT1 or KN1 genes, suggests that these factors act on the same pathway. We hypothesize that cytokinins act upstream of KNAT1 and STM. The influence of cytokinins on homeobox genes provides a link between the hormone and the developmental genes and indicates a role for cytokinins in the shoot apical meristem.

Alkyl and Aryl Transferases↗

PubMed2000

The Cytokinin-hypersensitive genes of Arabidopsis negatively regulate the cytokinin-signaling pathway for cell division and chloroplast development.

We isolated Arabidopsis thaliana mutants that respond more sensitively than the wild type to cytokinins. The calli produced from the mutants exhibit typical cytokinin responses, including rapid proliferation and chloroplast development in response to lower levels of cytokinins than in the wild type. The mutations are recessive and belong to two complementation groups designated ckh1 and ckh2 for cytokinin-hypersensitive. CKH1 and CKH2 were mapped to the top of chromosome I and the middle of chromosome II, respectively. The cytokinin levels in these mutants were not increased. We speculate that the CKH1 and CKH2 gene products negatively regulate the signaling pathway leading from cytokinin perception to cell proliferation and chloroplast development.

Arabidopsis↗

PubMed2001

Nitrogen-dependent accumulation of cytokinins in root and the translocation to leaf: implication of cytokinin species that induces gene expression of maize response regulator.

We have described the spatial and temporal accumulation pattern of various cytokinin species in roots, xylem sap and leaves during the resupply of nitrogen in maize. Upon addition of nitrate to nitrogen-depleted maize plants, isopentenyladenosine-5'-monophosphate (iPMP) started to accumulate in roots within 1 h preceding accumulation of trans-zeatin riboside-5'-monophosphate (ZMP), trans-zeatin riboside (ZR) and trans-zeatin (Z). In the xylem flow, both exudation rate of xylem sap and the concentration of the cytokinins increased, and ZR was the dominant species in the sap. In leaf tissue, the accumulation level of Z, which was the dominant form, started to increase 4 h after nitrate resupply to plants and the level was maintained for at least 24 h. Administration of a near physiological concentration of Z, ZR or ZMP (Z-type cytokinins) to detached leaves induced the accumulation of ZmRR1 transcript, that encode maize response regulators, but administration of isopentenyladenine, isopentenyladenosine or iPMP did not. These results strongly suggest that cytokinins are transported across the roots to shoots in response to nitrogen availability, and that, most probably, Z-type cytokinin(s), trigger the induction of ZmRR1.

Adenosine↗

PubMed2001

The Arabidopsis AHK4 histidine kinase is a cytokinin-binding receptor that transduces cytokinin signals across the membrane.

Common histidine-to-aspartate (His-->Asp) phosphorelay is a paradigm of signal transduction in both prokaryotes and eukaryotes for the propagation of certain environmental stimuli, in which histidine (His)-kinases play central roles as sensors for environmental signals. For the higher plant, Arabidopsis thaliana, it was recently suggested that the His-kinase (AHK4 / CRE1 / WOL) is a sensor for cytokinins, which are a class of plant hormones important for the regulation of cell division and differentiation. Interestingly, AHK4 is capable of functioning as a cytokinin sensor in the eubacterium, Escherichia coli (Suzuki et al. 2001, Plant Cell Physiol. 42: 107). Here we further show that AHK4 is a primary receptor that directly binds a variety of natural and synthetic cytokinins (e.g. not only N(6)-substituted aminopurines such as isopentenyl-adenine, trans-zeatin, benzyl-adenine, but also diphenylurea derivatives such as thidiazuron), in a highly specific manner (K(d) = 4.55+/-0.48x10(-9) M). AHK4 has a presumed extracellular domain, within which a single amino acid substitution (Thr-301 to Ile) was shown to result in loss of its ability to bind cytokinins. This particular mutation corresponds to the previously reported wol allele (wooden leg) that causes a striking phenotype defective in vascular morphogenesis. Collectively, evidence is presented that AHK4 and its homologues (AHK3 and possibly AHK2) are receptor kinases that can transduce cytokinin signals across the plasma membrane of A. thaliana.

Adenine↗

PubMed1997

Capillary electrophoresis of cytokinins and cytokinin ribosides.

A CE separation of cytokinins and cytokinin ribosides and some other purine and pyrimidine bases has been developed. Two electrolyte systems have been tested: 150 mM phosphoric acid, pH 1.8 and 50 mM sodium dodecylsulphate + 20 mM borate, pH 9.2. The migration times were measured and the effects of the solute structures were discussed. Preliminary experiments with plant extracts have been performed to identify the cytokinins and their ribosides. Both the systems can be used, but 150 mM phosphoric acid is better suited for identification of cytokinins in plant extracts, as the electropherograms are subject to fewer interferences.

Cytokinins↗

PubMed2001

Cytokinin oxidase or dehydrogenase? Mechanism of cytokinin degradation in cereals.

An enzyme degrading cytokinins with isoprenoid side chain, previously named cytokinin oxidase, was purified to near homogeneity from wheat and barley grains. New techniques were developed for the enzyme activity assay and staining on native electrophoretic gels to identify the protein. The purified wheat enzyme is a monomer 60 kDa, its N-terminal amino-acid sequence shows similarity to hypothetical cytokinin oxidase genes from Arabidopsis thaliana, but not to the enzyme from maize. N6-isopentenyl-2-(2-hydroxyethylamino)-9-methyladenine is the best substrate from all the cytokinins tested. Interestingly, oxygen was not required and hydrogen peroxide not produced during the catalytic reaction, so the enzyme behaves as a dehydrogenase rather than an oxidase. This was confirmed by the ability of the enzyme to transfer electrons to artificial electron acceptors, such as phenazine methosulfate and 2,6-dichlorophenol-indophenol. 2,3-Dimethoxy-5-methyl-1,4-benzoquinone, a precursor of the naturally occurring electron acceptor ubiquinone, readily interacts with the enzyme in micromolar concentrations. Typical flavoenzyme inhibitors such as acriflavine and diphenyleneiodonium inhibited this enzyme activity. Presence of the flavin cofactor in the enzyme was confirmed by differential pulse polarography and by measuring the fluorescence emission spectrum. Possible existence of a second redox centre is discussed.

Amino Acid Sequence↗

PubMed1985

Fluorescent anticytokinins as a probe for binding. Isolation of cytokinin-binding proteins from the soluble fraction and identification of a cytokinin-binding site on ribosomes of tobacco callus cells.

4-Substituted 2-methylthiopyrido[2,3-d]pyrimidines, a series of recently developed anticytokinins, have been found to fluoresce strongly in water and to be useful as probes for binding studies. The binding activity of the soluble proteins and particulate fraction of tobacco callus cells to the biologically most active member of the family, 4-n-butylamino-2-methylthiopyrido[2,3-d]pyrimidine (BAMPP), was studied fluorimetrically. We found that the binding activity is better monitored in terms of saturable binding rather than in terms of the amount of bound ligand, a conventional method used in isolation studies of hormone receptor proteins. Using this technique we isolated two kinds of high-affinity cytokinin-binding proteins from the soluble fraction and identified a high-affinity binding site on ribosomes.

Arabidopsis Proteins↗

PubMed1982