Cytokinin metabolism: implications for regulation of plant growth and development

Over-accumulation of chloroplast-nucleus located WHIRLY1 in barley leads to a decrease in growth and an enhanced stress resistance

WHIRLY1 is a chloroplast-nucleus located DNA/RNA-binding protein with functions in development and stress tolerance. By overexpression of HvWHIRLY1 in barley, one line with a 10-fold and two lines with a 50-fold accumulation of the protein were obtained. In these lines, the relative abundance of the nuclear form exceeded that of the chloroplast form. Growth of the plants was shown to be compromised in a WHIRLY1 abundance-dependent manner. Over-accumulation of WHIRLY1 in chloroplasts had neither an evident impact on nucleoid morphology nor on the composition of the photosynthetic apparatus. Nevertheless, oeW1 plants were found to be compromised in the light reactions of photosynthesis as well as in carbon fixation. The reduction in growth and photosynthesis was shown to be accompanied by a decrease in the levels of cytokinins and an increase in the level of jasmonic acid. Gene expression analyses revealed that in nonstress conditions the oeW1 plants had enhanced levels of pathogen response (PR) gene expression indicating activation of constitutive defense. During growth in continuous light of high irradiance PR gene expression increased indicating that under stress conditions oeW1 are capable to further enhance defense.

Cytokinin N-glucosides: Occurrence, Metabolism and Biological Activities in Plants

Cytokinins (CKs) are a class of phytohormones affecting many aspects of plant growth and development. In the complex process of CK homeostasis in plants, N-glucosylation represents one of the essential metabolic pathways. Its products, CK N7- and N9-glucosides, have been largely overlooked in the past as irreversible and inactive CK products lacking any relevant physiological impact. In this work, we report a widespread distribution of CK N-glucosides across the plant kingdom proceeding from evolutionary older to younger plants with different proportions between N7- and N9-glucosides in the total CK pool. We show dramatic changes in their profiles as well as in expression levels of the UGT76C1 and UGT76C2 genes during Arabidopsis ontogenesis. We also demonstrate specific physiological effects of CK N-glucosides in CK bioassays including their antisenescent activities, inhibitory effects on root development, and activation of the CK signaling pathway visualized by the CK-responsive YFP reporter line, TCSv2::3XVENUS. Last but not least, we present the considerable impact of CK N7- and N9-glucosides on the expression of CK-related genes in maize and their stimulatory effects on CK oxidase/dehydrogenase activity in oats. Our findings revise the apparent irreversibility and inactivity of CK N7- and N9-glucosides and indicate their involvement in CK evolution while suggesting their unique function(s) in plants.

Roles of proteome dynamics and cytokinin signaling in root to hypocotyl ratio changes induced by shading roots of Arabidopsis seedlings

In nature, root systems of most terrestrial plants are protected from light exposure by growing in a dark soil environment. Hence, in vitro cultivation in transparent Petri dishes leads to physiological perturbations, but the mechanisms underlying root-mediated light perception and responses have not been fully elucidated. Thus, we compared Arabidopsis thaliana seedling development in transparent and darkened Petri dishes at low light intensity (20 µmol m(-2) s(-1)), allowing us to follow (inter alia) hypocotyl elongation, which is an excellent process for studying interactions of signals involved in the regulation of growth and developmental responses. To obtain insights into molecular events underlying differences in seedling growth under these two conditions, we employed liquid chromatography-mass spectrometry (LC-MS) shotgun proteomics (available via the PRIDE deposit PXD001612). In total, we quantified the relative abundances of peptides representing 1,209 proteins detected in all sample replicates of LC-MS analyses. Comparison of MS spectra after manual validation revealed 48 differentially expressed proteins. Functional classification, analysis of available gene expression data and literature searches revealed alterations associated with root illumination (inter alia) in autotrophic CO2 fixation, C compound and carbohydrate metabolism, and nitrogen metabolism. The results also indicate a previously unreported role for cytokinin plant hormones in the escape-tropism response to root illumination. We complemented these results with reverse transcription followed by quantitative PCR (RT-qPCR), chlorophyll fluorescence and detailed cytokinin signaling analyses, detecting in the latter a significant increase in the activity of the cytokinin two-component signaling cascade in roots and implicating the cytokinin receptor AHK3 as the major mediator of root to hypocotyl signaling in responses to root illumination.

Proteome and metabolome profiling of cytokinin action in Arabidopsis identifying both distinct and similar responses to cytokinin down- and up-regulation

In plants, numerous developmental processes are controlled by cytokinin (CK) levels and their ratios to levels of other hormones. While molecular mechanisms underlying the regulatory roles of CKs have been intensely researched, proteomic and metabolomic responses to CK deficiency are unknown. Transgenic Arabidopsis seedlings carrying inducible barley cytokinin oxidase/dehydrogenase (CaMV35S>GR>HvCKX2) and agrobacterial isopentenyl transferase (CaMV35S>GR>ipt) constructs were profiled to elucidate proteome- and metabolome-wide responses to down- and up-regulation of CK levels, respectively. Proteome profiling identified >1100 proteins, 155 of which responded to HvCKX2 and/or ipt activation, mostly involved in growth, development, and/or hormone and light signalling. The metabolome profiling covered 79 metabolites, 33 of which responded to HvCKX2 and/or ipt activation, mostly amino acids, carbohydrates, and organic acids. Comparison of the data sets obtained from activated CaMV35S>GR>HvCKX2 and CaMV35S>GR>ipt plants revealed unexpectedly extensive overlaps. Integration of the proteomic and metabolomic data sets revealed: (i) novel components of molecular circuits involved in CK action (e.g. ribosomal proteins); (ii) previously unrecognized links to redox regulation and stress hormone signalling networks; and (iii) CK content markers. The striking overlaps in profiles observed in CK-deficient and CK-overproducing seedlings might explain surprising previously reported similarities between plants with down- and up-regulated CK levels.

Physiological responses and endogenous cytokinin profiles of tissue-cultured 'Williams' bananas in relation to roscovitine and an inhibitor of cytokinin oxidase/dehydrogenase (INCYDE) treatments

The effect of supplementing either meta-topolin (mT) or N(6)-benzyladenine (BA) requiring cultures with roscovitine (6-benzylamino-2-[1(R)-(hydroxymethyl)propyl]amino-9-isopropylpurine), a cyclin-dependent kinase (CDK) and N-glucosylation inhibitor, and INCYDE (2-chloro-6-(3-methoxyphenyl)aminopurine), an inhibitor of cytokinin (CK) degradation, on the endogenous CK profiles and physiology of banana in vitro was investigated. Growth parameters including multiplication rate and biomass were recorded after 42 days. Endogenous CK levels were quantified using UPLC-MS/MS while the photosynthetic pigment and phenolic contents were evaluated spectrophotometrically. The highest regeneration rate (93 %) was observed in BA + roscovitine while mT + INCYDE plantlets produced most shoots. Treatment with BA + roscovitine had the highest shoot length and biomass. Although not significant, there was a higher proanthocyanidin level in BA + roscovitine treatments compared to the control (BA). The levels of total phenolics and flavonoids were significantly higher in mT + roscovitine treatment than in the mT-treated regenerants. The presence of roscovitine and/or INCYDE had no significant effect on the photosynthetic pigments of the banana plantlets. Forty-seven aromatic and isoprenoid CKs categorized into nine CK-types were detected at varying concentrations. The presence of mT + roscovitine and/or INCYDE increased the levels of O-glucosides while 9-glucosides were higher in the presence of BA. Generally, the underground parts had higher CK levels than the aerial parts; however, the presence of INCYDE increased the level of CK quantified in the aerial parts. From a practical perspective, the use of roscovitine and INCYDE in micropropagation could be crucial in the alleviation of commonly observed in vitro-induced physiological abnormalities.

Retargeting a maize β-glucosidase to the vacuole--evidence from intact plants that zeatin-O-glucoside is stored in the vacuole

Cytokinin (CK) activity is regulated by the complex interplay of their metabolism, transport, stability and cellular/tissue localization. O-glucosides of zeatin-type CKs are postulated to be storage and/or transport forms. Active CK levels are determined in part by their differential distribution of CK metabolites across different subcellular compartments. We have previously shown that overexpressing chloroplast-localized Zm-p60.1, a maize β-glucosidase capable of releasing active cytokinins from their O- and N3-glucosides, perturbs CK homeostasis in transgenic tobacco. We obtained tobacco (Nicotiana tabacum L., cv Petit Havana SR1) plants overexpressing a recombinant Zm-p60.1 that is targeted to the vacuole. The protein is correctly processed and localized to the vacuole. When grown on medium containing exogenous zeatin, transgenic seedlings rapidly accumulate fresh weight due to ectopic growths at the base of the hypocotyl. The presence of the enzyme in these ectopic structures is shown by histochemical staining. CK quantification reveals that these transgenic seedlings are unable to accumulate zeatin-O-glucoside to levels similar to those observed in the wild type. When crossed with tobacco overexpressing the zeatin-O-glucosyltransferase gene from Phaseolus, the vacuolar variant shows an almost complete reversion in the root elongation assay. This is the first evidence from intact plants that the vacuole is the storage organelle for CK O-glucosides and that they are available to attack by Zm-p60.1. We propose the use of Zm-p60.1 as a robust molecular tool that exploits the reversibility of O-glucosylation and enables delicate manipulations of active CK content at the cellular level.

Engineering the cytokinin-glucoside specificity of the maize β-D-glucosidase Zm-p60.1 using site-directed random mutagenesis

The maize β-D-glucosidase Zm-p60.1 releases active cytokinins from their storage/transport forms, and its over-expression in tobacco disrupts zeatin metabolism. The role of the active-site microenvironment in fine-tuning Zm-p60.1 substrate specificity has been explored, particularly in the W373K mutant, using site-directed random mutagenesis to investigate the influence of amino acid changes around the 373 position. Two triple (P372T/W373K/M376L and P372S/W373K/M376L) and three double mutants (P372T/W373K, P372S/W373K and W373K/M376L) were prepared. Their catalytic parameters with two artificial substrates show tight interdependence between substrate catalysis and protein structure. P372T/W373K/M376L exhibited the most significant effect on natural substrate specificity: the ratio of hydrolysis of cis-zeatin-O-β-D-glucopyranoside versus the trans-zeatin-O-β-D-glucopyranoside shifted from 1.3 in wild-type to 9.4 in favor of the cis- isomer. The P372T and M376L mutations in P372T/W373K/M376L also significantly restored the hydrolytic velocity of the W373K mutant, up to 60% of wild-type velocity with cis-zeatin-O-β-D-glucopyranoside. These findings reveal complex relationships among amino acid residues that modulate substrate specificity and show the utility of site-directed random mutagenesis for changing and/or fine-tuning enzymes. Preferential cleavage of specific isomer-conjugates and the capacity to manipulate such preferences will allow the development of powerful tools for detailed probing and fine-tuning of cytokinin metabolism in planta.

Spatial and temporal profiles of cytokinin biosynthesis and accumulation in developing caryopses of maize

BACKGROUND AND AIMS

Cytokinins are a major group of plant hormones and are associated with various developmental processes. Developing caryopses of maize have high levels of cytokinins, but little is known about their spatial and temporal distribution. The localization and quantification of cytokinins was investigated in maize (Zea mays) caryopsis from 0 to 28 d after pollination together with the expression and localization of isopentenyltransferase ZmIPT1 involved in cytokinin biosynthesis and ZmCNGT, the gene putatively involved in N9-glucosylation.

METHODS

Biochemical, cellular and molecular approaches resolved the overall cytokinin profiles, and several gene expression assays were used for two critical genes to assess cytokinin cell-specific biosynthesis and conversion to the biologically inactive form. Cytokinins were immunolocalized for the first time in maize caryopses.

KEY RESULTS

During the period 0-28 d after pollination (DAP): (1) large quantities of cytokinins were detected in the maternal pedicel region relative to the filial tissues during the early stages after fertilization; (2) unpollinated ovules did not accumulate cytokinins; (3) the maternal nucellar region showed little or no cytokinin signal; (4) the highest cytokinin concentrations in filial endosperm and embryo were detected at 12 DAP, predominantly zeatin riboside and zeatin-9-glucoside, respectively; and (5) a strong cytokinin immuno-signal was detected in specific cell types in the pedicel, endosperm and embryo.

CONCLUSIONS

The cytokinins of developing maize caryopsis may originate from both local syntheses as well as by transport. High levels of fertilization-dependent cytokinins in the pedicel suggest filial control on metabolism in the maternal tissue; they may also trigger developmental programmed cell death in the pedicel.

Early cytokinin response proteins and phosphoproteins of Arabidopsis thaliana identified by proteome and phosphoproteome profiling

Cytokinins are plant hormones involved in regulation of diverse developmental and physiological processes in plants whose molecular mechanisms of action are being intensely researched. However, most rapid responses to cytokinin signals at the proteomic and phosphoproteomic levels are unknown. Early cytokinin responses were investigated through proteome-wide expression profiling based on image and mass spectrometric analysis of two-dimensionally separated proteins and phosphoproteins. The effects of 15 min treatments of 7-day-old Arabidopsis thaliana seedlings with four main cytokinins representing hydroxyisopentenyl, isopentenyl, aromatic, and urea-derived type cytokinins were compared to help elucidate their common and specific function(s) in regulating plant development. In proteome and phosphoproteome maps, significant differences were reproducibly observed for 53 and 31 protein spots, respectively. In these spots, 96 proteins were identified by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS), providing a snapshot of early links in cytokinin-regulated signalling circuits and cellular processes, including light signalling and photosynthesis, nitrogen metabolism, the CLAVATA pathway, and protein and gene expression regulation, in accordance with previously described cytokinin functions. Furthermore, they indicate novel links between temperature and cytokinin signalling, and an involvement of calcium ions in cytokinin signalling. Most of the differentially regulated proteins and phosphoproteins are located in chloroplasts, suggesting an as yet uncharacterized direct signalling chain responsible for cytokinin action in chloroplasts. Finally, first insights into the degree of specificity of cytokinin receptors on phosphoproteomic effects were obtained from analyses of cytokinin action in a set of cytokinin receptor double mutants.

Cytokinin import rate as a signal for photosynthetic acclimation to canopy light gradients

Plants growing in dense canopies are exposed to vertical light gradients and show photosynthetic acclimation at the whole-plant level, resulting in efficient photosynthetic carbon gain. We studied the role of cytokinins transported through the transpiration stream as one of probably multiple signals for photosynthetic acclimation to light gradients using both tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana). We show that substantial variation in leaf transpiration parallels the light gradient in tobacco canopies and experimental reduction of the transpiration rate of a leaf, independent of light, is sufficient to reduce photosynthetic capacity in both species, as well as transcript levels of the small subunit of Rubisco (rbcS) gene in Arabidopsis. Mass spectrometric analysis of xylem sap collected from intact, transpiring tobacco plants revealed that shaded leaves import less cytokinin than leaves exposed to high light. In Arabidopsis, reduced transpiration rate of a leaf in the light is associated with lower cytokinin concentrations, including the bioactive trans-zeatin and trans-zeatin riboside, as well as reduced expression of the cytokinin-responsive genes ARR7 and ARR16. External application of cytokinin to shaded leaves rescued multiple shade effects, including rbcS transcript levels in both species, as did locally induced cytokinin overproduction in transgenic tobacco plants. From these data, we conclude that light gradients over the foliage of a plant result in reduced cytokinin activity in shaded leaves as a consequence of reduced import through the xylem and that cytokinin is involved in the regulation of whole-plant photosynthetic acclimation to light gradients in canopies.

Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: a factor of potential importance for auxin-cytokinin-regulated development

One of the most long-lived models in plant science is the belief that the long-distance transport and ratio of two plant hormones, auxin and cytokinin, at the site of action control major developmental events such as apical dominance. We have used in vivo deuterium labeling and mass spectrometry to investigate the dynamics of homeostatic cross talk between the two plant hormones. Interestingly, auxin mediates a very rapid negative control of the cytokinin pool by mainly suppressing the biosynthesis via the isopentenyladenosine-5'-monophosphate-independent pathway. In contrast, the effect of cytokinin overproduction on the entire auxin pool in the plant was slower, indicating that this most likely is mediated through altered development. In addition, we were able to confirm that the lateral root meristems are likely to be the main sites of isopentenyladenosine-5'-monophosphate-dependent cytokinin synthesis, and that the aerial tissue of the plant surprisingly also was a significant source of cytokinin biosynthesis. Our demonstration of shoot-localized synthesis, together with data demonstrating that auxin imposes a very rapid regulation of cytokinin biosynthesis, illustrates that the two hormones can interact also on the metabolic level in controlling plant development, and that the aerial part of the plant has the capacity to synthesize its own cytokinin independent of long-range transport from the root system.

Inhibitory effects of elevated endogenous cytokinins on nitrate reductase in ipt-expressing tobacco are eliminated by short-term exposure to benzyladenine

Using a novel system for expressing ipt gene from Agrobacterium tumefaciens in tobacco (Nicotiana tabacum L., cv. Petit Havana SR1), we were able to grow seedlings and teratoma-like tissue with increased content of cytokinins. This material enabled us to investigate new regulatory aspects of nitrate reduction. We grew control plants and plants with elevated cytokinins on MS media, with or without nitrate and benzyladenine (BA). We determined in vitro nitrate reductase (EC 1.6.6.1) activity (NRA) in this plant material. Initially, we found that ipt-expressing plants always displayed lowered levels of NRA when compared to wild-type SR1 plants. We determined that long-term exposure of tobacco plants and tissue to cytokinins caused up to 60% decrease in NRA. Exposure to 40 mM nitrate was able to induce the activity in such plants 3-fold, increasing the activity in SR1 plants more than 5-fold. We were able to restore wild-type levels of NRA in ipt-expressing plants by simultaneous induction of NR with BA and nitrate. Our results suggest that regulation of NR by nitrate and cytokinin is a result of overlaying cytokinin-driven regulatory processes, with those acting in the short-term having a positive effect on NRA, and those acting over extended periods of time having inhibitory effects on NRA.

Auxin cross-talk: integration of signalling pathways to control plant development

Plants sense and respond to endogenous signals and environmental cues to ensure optimal growth and development. Plant cells must integrate the myriad transduction events into a comprehensive network of signalling pathways and responses. The phytohormone auxin occupies a central place within this transduction network, frequently acting in conjunction with other signals, to co-ordinately regulate cellular processes such as division, elongation and differentiation. As a non-cell autonomous signal, auxin also interacts with other signalling pathways to regulate inter-cellular developmental processes. As part of this especially themed edition of Plant Molecular Biology, we will review examples of 'cross-talk' between auxin and other signalling pathways. Given the current state of knowledge, we have deliberately focused our efforts reviewing auxin interactions with other phytohormone and light signalling pathways. We conclude by discussing how new genomic approaches and the Arabidopsis genome sequence are likely to impact this area of research in the future.

Activation tagging identifies a gene from Petunia hybrida responsible for the production of active cytokinins in plants

Cytokinins (CKs) are phytohormones that play an important role in plant growth and development. Although the first naturally produced CK, zeatin, was isolated almost four decades ago, no endogenous gene has been shown to produce active CKs in planta. In an activation tagging experiment we have identified a petunia line that showed CK-specific effects including enhanced shooting, reduced apical dominance and delayed senescence and flowering. This phenotype correlated with the enhanced expression of a gene we labelled Sho (Shooting). Sho, which encodes a protein with homology to isopentenyl transferases (IPTs), also causes CK-specific effects when expressed in other plant species. In contrast to the ipt gene from Agrobacterium, which primarily increases zeatin levels, Sho expression in petunia and tobacco especially enhances the levels of certain N6-(delta2-isopentenyl) adenosine (2iP) derivatives. Our data suggest that Sho encodes a plant enzyme whose activity is sufficient to produce active CKs in plants.

Insights into the functional architecture of the catalytic center of a maize beta-glucosidase Zm-p60.1

The maize (Zea mays) beta-glucosidase Zm-p60.1 has been implicated in regulation of plant development by the targeted release of free cytokinins from cytokinin-O-glucosides, their inactive storage forms. The crystal structure of the wild-type enzyme was solved at 2.05-A resolution, allowing molecular docking analysis to be conducted. This indicated that the enzyme specificity toward substrates with aryl aglycones is determined by aglycone aromatic system stacking with W373, and interactions with edges of F193, F200, and F461 located opposite W373 in a slot-like aglycone-binding site. These aglycone-active site interactions recently were hypothesized to determine substrate specificity in inactive enzyme substrate complexes of ZM-Glu1, an allozyme of Zm-p60.1. Here, we test this hypothesis by kinetic analysis of F193I/Y/W mutants. The decreased K(m) of all mutants confirmed the involvement of F193 in determining enzyme affinity toward substrates with an aromatic aglycone. It was unexpected that a 30-fold decrease in k(cat) was found in F193I mutant compared with the wild type. Kinetic analysis and computer modeling demonstrated that the F193-aglycone-W373 interaction not only contributes to aglycone recognition as hypothesized previously but also codetermines catalytic rate by fixing the glucosidic bond in an orientation favorable for attack by the catalytic pair, E186 and E401. The catalytic pair, assigned initially by their location in the structure, was confirmed by kinetic analysis of E186D/Q and E401D/Q mutants. It was unexpected that the E401D as well as C205S and C211S mutations dramatically impaired the assembly of a catalysis-competent homodimer, suggesting novel links between the active site structure and dimer formation.

Control of axillary bud initiation and shoot architecture in Arabidopsis through the SUPERSHOOT gene

The aerial architecture of flowering plants is determined to a large extent by shoot growth and shoot branching arising from the initiation and growth of axillary meristems. We have identified an Arabidopsis mutant, supershoot (sps), which is characterized by a massive overproliferation of shoots, such that a single plant can generate 500 or more inflorescences. Analysis of the mutant plants shows that the primary defect is because of an increase in the number of meristems formed in leaf axils, together with release of bud arrest, resulting in reiterative branch formation from rosette and cauline leaves. The SPS gene is shown here to encode a cytochrome P450, and together with a 3- to 9-fold increase in levels of Z-type cytokinins in sps mutant plants, indicate a role for SPS in modulating hormone levels. The expression pattern of SPS, with strong expression at the leaf axils, correlates well with the phenotypic defects. Our results indicate that control of shoot branching in Arabidopsis may be accomplished in part by suppression of axillary meristem initiation and growth through the localized attenuation of cytokinin levels at sites of bud initiation.

Cytokinin production by plant growth promoting rhizobacteria and selected mutants

One of the proposed mechanisms by which rhizobacteria enhance plant growth is through the production of plant growth regulators. Five plant growth promoting rhizobacterial (PGPR) strains produced the cytokinin dihydrozeatin riboside (DHZR) in pure culture. Cytokinin production by Pseudomonas fluorescens G20-18, a rifampicin-resistant mutant (RIF), and two TnphoA-derived mutants (CNT1, CNT2), with reduced capacity to synthesize cytokinins, was further characterized in pure culture using immunoassay and thin layer chromatography. G20-18 produced higher amounts of three cytokinins, isopentenyl adenosine (IPA), trans-zeatin ribose (ZR), and DHZR than the three mutants during stationary phase. IPA was the major metabolite produced, but the proportion of ZR and DHZR accumulated by CNT1 and CNT2 increased with time. No differences were observed between strain G20-18 and the mutants in the amounts of indole acetic acid synthesized, nor were gibberellins detected in supernatants of any of the strains. Addition of 10(-5) M adenine increased cytokinin production in 96- and 168-h cultures of strain G20-18 by approximately 67%. G20-18 and the mutants CNT1 and CNT2 may be useful for determination of the role of cytokinin production in plant growth promotion by PGPR.

The role of cysteine residues in structure and enzyme activity of a maize beta-glucosidase

The maize Zm-p60.1 gene encodes a beta-glucosidase that can release active cytokinins from their storage forms, cytokinin-O-glucosides. Mature catalytically active Zm-p60.1 is a homodimer containing five cysteine residues per a subunit. Their role was studied by mutating them to alanine (A), serine (S), arginine (R) or aspartic acid (D) using site-directed mutagenesis, and subsequent heterologous expression in Escherichia coli. All substitutions of C205 and C211 resulted in decreased formation and/or stability of the homodimer, manifested as accumulation of high levels of monomer in the bacterial expression system. Examination of urea- and glutathione-induced dissociation patterns of the homodimer to the monomers, HPLC profiles of hydrolytic fragments of reduced and oxidized forms, and a homology-based three-dimensional structural model revealed that an intramolecular disulfide bridge formed between C205 and C211 within the subunits stabilized the quaternary structure of the enzyme. Mutating C52 to R produced a monomeric enzyme protein, too. No detectable effects on homodimer formation were apparent in C170 and C479 mutants. Given the Km values for C170A/S mutants were equal to that for the wild-type enzyme, C170 cannot participate in enzyme-substrate interactions. Possible indirect effects of C170A/S mutations on catalytic activity of the enzyme were inferred from slight decreases in the apparent catalytic activity, k'cat. C170 is located on a hydrophobic side of an alpha-helix packed against hydrophobic amino-acid residues of beta-strand 4, indicating participation of C170 in stabilization of a (beta/alpha)8 barrel structure in the enzyme. In C479A/D/R/S mutants, Km and k'cat were influenced more significantly suggesting a role for C479 in enzyme catalytic action.

Sucrose and Cytokinin Modulation of WPK4, a Gene Encoding a SNF1-Related Protein Kinase from Wheat

WPK4, a gene encoding a putative protein kinase, was initially identified in wheat (Triticum aestivum) and shown to be up-regulated by light, nutrient deprivation, and cytokinins. To confirm that WPK4 has protein kinase activity, the protein was produced in Escherichia coli as a fusion protein with glutathione S-transferase. The purified protein exhibited autophosphorylation activity and phosphorylated both myelin basic protein and a peptide fragment of rice 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Levels of WPK4 transcripts in wheat seedlings were increased and decreased by the removal and addition of sucrose (Suc), respectively, to the culture medium. The introduction of the N-terminal kinase region of WPK4 into the yeast snf1 mutant cells, which cannot utilize Suc as a carbon source, rescued growth in Suc-containing medium. Cytokinins up-regulated the accumulation of WPK4 transcripts, but their effects were cancelled by the addition of Suc. Our results suggest that Suc negatively regulates the signaling pathway in which transcriptional activation of WPK4 is mediated by cytokinins.

Use of rifampicin in T7 RNA polymerase-driven expression of a plant enzyme: rifampicin improves yield and assembly

Expression systems based on high selectivity and activity of T7 RNA polymerase and presence of a strong T7 promoter have been commonly used for cloning and expression of various recombinant proteins in Escherichia coli. When the expression system is designed in such a way that the produced protein is not being transferred into periplasm, bacterial cells must be lysed in order to isolate and purify the protein. The final yield and quality of the synthesized protein then depend on various factors, protein size, amino acid sequence, solubility in cytoplasm, and folding requirements among them. The yield in the T7 RNA polymerase/promoter system can be positively influenced by use of rifampicin. In this report we demonstrate usefulness of the antibiotic in detail. We describe rifampicin-enhanced expression of a plant cytokinin-specific beta-glucosidase. Two bacterial cultures are compared, one expressing the enzyme without and one in the presence of rifampicin. The antibiotic not only increased the yield of the recombinant protein, which seems to be a general phenomenon, but also favored the final assembly of the protein's subunits into a catalytically active dimer form.

Isolation and characterization of Arabidopsis mutants defective in the induction of ethylene biosynthesis by cytokinin

Cytokinins elevate ethylene biosynthesis in etiolated Arabidopsis seedlings via a post-transcriptional modification of one isoform of the key biosynthetic enzyme ACC synthase. In order to begin to dissect the signaling events leading from cytokinin perception to this modification, we have isolated a series of mutants that lack the ethylene-mediated triple response in the presence of cytokinin due to their failure to increase ethylene biosynthesis. Analysis of genetic complementation and mapping revealed that these Cin mutants (cytokinin-insensitive) represent four distinct complementation groups, one of which, cin4, is allelic to the constitutive photomorphogenic mutant fus9/cop10. The Cin mutants have subtle effects on the morphology of adult plants. We further characterized the Cin mutants by analyzing ethylene biosynthesis in response to various other inducers and in adult tissues, as well as by assaying additional cytokinin responses. The cin3 mutant did not disrupt ethylene biosynthesis under any other conditions, nor did it disrupt any other cytokinin responses. Only cin2 disrupted ethylene biosynthesis in multiple circumstances. cin1 and cin2 made less anthocyanin in response to cytokinin. cin1 also displayed reduced shoot initiation in tissue culture in response to cytokinin, suggesting that it affects a cytokinin signaling element.

Recessive and dominant mutations in the ethylene biosynthetic gene ACS5 of Arabidopsis confer cytokinin insensitivity and ethylene overproduction, respectively

We identified a set of cytokinin-insensitive mutants by using a screen based on the ethylene-mediated triple response observed after treatment with low levels of cytokinins. One group of these mutants disrupts ACS5, a member of the Arabidopsis gene family that encodes 1-aminocyclopropane-1-carboxylate synthase, the first enzyme in ethylene biosynthesis. The ACS5 isoform is mainly responsible for the sustained rise in ethylene biosynthesis observed in response to low levels of cytokinin and appears to be regulated primarily by a posttranscriptional mechanism. Furthermore, the dominant ethylene-overproducing mutant eto2 was found to be the result of an alteration of the carboxy terminus of ACS5, suggesting that this domain acts as a negative regulator of ACS5 function.

Auxin-cytokinin interactions in higher plants: old problems and new tools

The plant hormones auxin and cytokinin interact in a complex manner to control many aspects of growth and differentiation. Recent advances in understanding their metabolism and the cell cycle are now helping to clarify the mechanisms by which these hormones act together to control various physiological and developmental responses. Molecular and genetic tools are being used to reveal interactions between auxin and cytokinin at multiple levels, including mutual regulation of active hormone availability, input into multiple signal transduction pathways, alteration of gene expression, post-translational modifications and direct modulation of enzyme activity.

Cytokinin induction of RNA polymerase I transcription in Arabidopsis thaliana

RNA polymerase I (pol I) transcribes the repeated genes that encode the precursor of 17-18, 5.8, and 25-28 S ribosomal RNA (rRNA). Pol I transcription is up-regulated in growing cells and down-regulated in quiescent cells, presumably reflecting the demand for ribosomes and protein synthesis. However, the signal transduction pathways responsible for pol I regulation are poorly understood. We tested the effects of exogenously applied plant hormones on promoter-dependent rRNA transcription in Arabidopsis thaliana. Gibberellic acid, abscisic acid, auxin, and ethylene had no detectable effect on rRNA transcription, but kinetin (a cytokinin) stimulated rRNA transcription within 1 h of treatment. Increased steady-state levels of accurately initiated rRNA transcripts, detected by S1 nuclease protection, were paralleled by increased levels of nascent rRNA transcripts in isolated nuclei. Therefore, the primary effect of cytokinin appears to be at the level of transcription initiation rather than rRNA stability. Pol I accounts for approximately 34% of total nuclear transcription in untreated plants and approximately 60% following cytokinin treatment. The specific responsiveness of pol I transcription to kinetin suggests that cytokinins may act as general regulators of protein synthetic capacity and growth status in plant cells.