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PubMed1987

Imidazolinones and acetohydroxyacid synthase from higher plants: properties of the enzyme from maize suspension culture cells and evidence for the binding of imazapyr to acetohydroxyacid synthase in vivo.

Acetohydroxyacid synthase has been purified from maize (Zea mays, var Black Mexican Sweet) suspension culture cells 49-fold by a combination of ion exchange chromatography, gel filtration, and hydroxyapatite chromatography. Use of the nondenaturing, zwitterionic detergent 3-([3-cholamidopropyl]dimethyl-ammonio)-1-propanesulfonate was necessary to dissociate the enzyme from the heterogeneous, high molecular weight aggregates in which it appears to reside in vitro. The solubilized maize acetohydroxyacid synthase had a relative molecular mass of 440,000. The purified enzyme was highly unstable. Acetohydroxyacid synthase activities in crude extracts of excised maize leaves and suspension cultured cells were reduced 85 and 58%, respectively, by incubation of the tissue with 100 micromolar (excised leaves) and 5 micromolar (suspension cultures) of the imidazolinone imazapyr prior to enzyme extraction, suggesting that the inhibitor binds tightly to the enzyme in vivo. Binding of imazapyr to maize acetohydroxyacid synthase could also be demonstrated in vitro. Evidence is presented which suggests that the interaction between imazapyr and the enzyme is reversible. Imazapyr also exhibited slow-binding properties when incubated with maize cell acetohydroxyacid synthase in extended time course experiments. Initial and final K(i) values for the inhibition were 15 and 0.9 micromolar, respectively. The results suggest that imazapyr is a slow, tight-binding inhibitor of acetohydroxyacid synthase.

Journal Article↗

PubMed1988

Phytochrome mediated regulation of sucrose phosphate synthase activity in maize.

The extractable activity of sucrose phosphate synthase was determined in etiolated seedlings of maize (Zea mays L.), soybean (Glycine max [L.] Merr.), and sugar beet (Beta vulgaris L.) following treatments of changing light quality. A 30-minute illumination of 30 microeinsteins per square meter per second white light produced a three-fold increase in sucrose phosphate synthase activity at 2 hours postillumination when compared to seedlings maintained in total darkness. Etiolated maize seedlings treated with 3.6 microeinsteins per square meter per second of red and far-red light showed a 50% increase and a 50% decrease in sucrose phosphate synthase activity, respectively, when compared to etiolated maize seedlings treated with white light. Maize seedlings exposed for 30 minutes to red followed by 30 minutes to far-red showed an initial increase in sucrose phosphate synthase activity followed by a rapid decrease to control level. Neither soybean or sugar beet sucrose phosphate synthase responded to the 30-minute illumination of white light. Phytochrome is involved in sucrose phosphate synthase regulation in maize, whereas it is not responsible for changes in sucrose phosphate synthase activity in soybean or sugar beet.

Journal Article↗

PubMed1988

Pulse-induced phototropisms in oat and maize coleoptiles.

Phototropisms induced by a pulse (1-30 seconds) of blue light in red-light-grown coleoptiles of oats (Avena sativa L.) and maize (Zea mays L.) were investigated in terms of fluence-response relationships and time courses. Phototropic stimulation was made by a laser beam (457.9 nanometers), allowing application of high-fluence pulses. The phototropic fluence-response curves for oats and maize revealed two peaks in the positive range, thus indicating the occurrence of two separable pulse-induced positive responses. The response at low fluences corresponded to the ;first positive curvature.' The response at high fluences was very small in oats, but was large in maize. Reciprocity was valid in this high-fluence response (tested only for maize), indicating that it is distinct from the so-called ;second positive curvature.' In oats, the trough between the two positive responses fell into the negative range. This negative response, corresponding to the ;first negative curvature,' showed time courses distinct from those of ;first positive curvature:' the negative response was induced after a longer time lag and developed with a more gradual increase of the rate of bending. The maximal rate of the negative response was as high as one-half of that of first positive curvature. In maize, the trough between the two responses was in the positive range, and the time-course result revealed no apparent response counteracting the positive responses. Physiological and ecological implications of the pulse-induced phototropisms are discussed.

Journal Article↗

PubMed1989

Proton and copper adsorption to maize and soybean root cell walls.

A surface complexation model which has been used to describe inner-sphere complexation on metal oxide surfaces was applied to the adsorption of Cu by isolated cell walls of 4-day and 28-day-old maize (Zea mays L. cv WF9 x Mo17) and 21-day-old soybean (Glycine max [L.] Merr. cv Dare) roots. Concentration dependence of the titration data prevented the determination of unique pK and capacitance values for the 4-day maize cell walls, though mean values obtained for the intrinsic pK of the titratable carboxyl groups were 3.0 (4-day maize), 3.6 (28-day maize), and 3.0 (21-day soybean) as determined by potentiometric titration with either NaOH or HCl in 20 millimolar NaCl. The constant capacitance model was applied to Cu sorption data from rapid batch equilibrium experiments in an ionic medium of 20 millimolar NaClO(4). Speciation calculations indicated that the formation of a bidentate surface complex was sufficient to describe the experimental data for all three types of plant material, with only one value for the pK and capacitance density. The intrinsic constants of Cu complexation by a neutral site are: log K = -0.3 +/- 0.1, -0.2 +/- 0.3, and 0.9 +/- 0.1 for 4-day and 28-day maize, and 21-day soybean, respectively. The integral capacitance density parameter, which describes the relationship between surface charge density and electrical potential, is several times larger than for crystalline mineral surfaces. This result indicates that the surface electrical potential remains low even when the surface charge density is high. Such behavior is characteristic of gels and porous oxides.

Journal Article↗

PubMed1989

Dependence of Ethanolic Fermentation, Cytoplasmic pH Regulation, and Viability on the Activity of Alcohol Dehydrogenase in Hypoxic Maize Root Tips.

We examined the role of alcohol dehydrogenase (ADH) in the metabolism and survival of hypoxic maize (Zea mays L.) root tips. The dependence of the rate of ethanolic fermentation, cytoplasmic pH, and viability on the activity of ADH in maize root tips during extreme hypoxia was determined. Maize lines with ADH activities differing over about a 200-fold range were studied. Effects of genetic background were controlled by comparing pairs of F4 progeny of crosses between mutant (low ADH activity) and reference inbred lines. The capacity of hypoxic root tips to perform ethanolic fermentation exhibited a dependence on ADH activity only at activities found in Adh 1 nulls. The ability of maize root tips to withstand prolonged and extreme hypoxia was like-wise independent of ADH activity, except at the lowest activities. Root tips that exhibited lower tolerance of hypoxia had more acidic cytoplasm during extreme hypoxia. We conclude that the activity of ADH in normal maize root tips does not limit the capacity for energy production via fermentation, and does not determine viability under extreme hypoxia. The significance of the induction of ADH activity in plants by hypoxia is discussed.

Journal Article↗

PubMed1989

Photorespiratory rates in wheat and maize as determined by o-labeling.

A method was devised to quantify short-term photorespiratory rates in terrestrial plants using (18)O-intermediates of the glycolate pathway, specifically glycolate, glycine, and serine. The pathway intermediates were isolated and analyzed on a GC/MS to determine molecular percent (18)O-enrichment. Rates of glycolate synthesis were determined from (18)O-labeling kinetics of the intermediates, derived rate equations, and nonlinear regression techniques. Glycolate synthesis in wheat (Triticum aestivum L.), a C(3) plant, and maize (Zea mays L.), a C(4) plant, was stimulated by high O(2) concentrations and inhibited by high CO(2) concentrations. The synthesis rates were 7.3, 2.1, and 0.7 micromoles per square decimeter per minute under a 21% O(2) and 0.035% CO(2) atmosphere for leaf tissue of wheat, maize seedlings, and 3-month-old maize, respectively. Photorespiratory CO(2) evolution rates were estimated to be 27, 6, and 2%, respectively, of net photosynthesis for the three groups of plants under the above atmosphere. The results from maize tissue support the hypothesis that C(4) plants photorespire, albeit at a reduced rate in comparison to C(3) plants, and that the CO(2)/O(2) ratio in the bundle sheath of maize is higher in mature tissue than in seedling tissue. The pool size of the three photorespiratory intermediates remained constant and were unaffected by changes in either CO(2) or O(2) concentrations throughout the 10-minute labeling period. This suggests that photorespiratory metabolism is regulated by other mechanism besides phosphoglycolate synthesis by ribulose-1,5-bisphosphate carboxylase/oxygenase, at least under short-term conditions. Other mechanisms could be alternate modes of synthesis of the intermediates, regulation of some of the enzymes of the photorespiratory pathway, or regulation of carbon flow between organelles involved in photorespiration. The glycolate pool became nearly 100% (18)O-labeled under an atmosphere of 40% O(2). This pool failed to become 100% (18)O-enriched under lower O(2) concentrations.

Journal Article↗

PubMed1989

Characteristics of the Mg-ATPase Activity Associated with the Membrane-Bound Maize Coupling Factor.

The membrane-bound coupling factor of maize mesophyll thylakoids is a latent ATPase. Mg(2+)-ATPase activity can be induced in the light with either dithiothreitol or low concentrations of trypsin. Maize thylakoids that are activated with light plus trypsin exhibit considerably higher levels of activity in Na(2)SO(3)-dependent Mg(2+)-ATPase assays compared to thylakoids that are light and dithiothreitol activated (1400 micromoles per milligram of chlorophyll per hour versus 200 micromoles per milligram of chlorophyll per hour). Treatment with light and dithiothreitol or light plus trypsin were also required to demonstrate high levels of octyl glucoside-dependent Mg(2+)-ATPase activity in maize mesophyll thylakoids. Only small differences in octyl glucoside-dependent Mg(2+)-ATPase activity were observed in preparations that were activated in the light with either trypsin or dithiothreitol. Mg(2+)-ATPase activity can also be induced in maize mesophyll chloroplasts by illuminating intact preparations under appropriate conditions. Little or no ATPase activity was observed in the absence of illumination or in the presence of light plus methyl viologen. The active state decayed in the dark with a t((1/2)) of 6 to 7 minutes at room temperature. Based on the effect of the thiol oxidant, o-iodosobenzoate, and the uncoupler, nigericin, on the kinetics of deactivation of ATPase activity in intact maize chloroplasts, it appears that the activation process requires a transmembrane proton gradient and reduction of a key disulfide bridge in the gamma of chloroplast coupling factor one.

Journal Article↗

PubMed1989

Increased gene expression by the first intron of maize shrunken-1 locus in grass species.

The first intron of the shrunken-1 (Sh1) locus of maize was incorporated into constructs containing the chloramphenicol acetyltransferase gene (CAT) coupled with the nopaline synthase 3' polyadenylation signal. Transcription was driven with the 35S promoter of the cauliflower mosaic virus (CaMV) or the Sh1 promoter of maize. Transient gene expression was monitored following electroporation into protoplasts of Panicum maximum (guineagrass), Pennisetum purpureum (napiergrass), or Zea mays (maize). The 1028 base pair intron increased gene expression in cells of each species when transcription was driven with the 35S promoter. Eleven to 91-fold increases were observed. Expression levels observed in maize were two and eight times those observed in napiergrass and guineagrass, respectively. The 35S promoter gave CAT activity 10 to 100 times that observed with the Sh1 promoter. Whereas expression driven by the 35S promoter was reproducible, that observed with the Sh1 promoter proved quite variable. In similar constructs the first intron of the alcohol dehydrogenase-1 (Adh1) gene of maize led to increased gene expression of only 7 to 10% of that observed with the Sh1 first intron. The increased level of gene expression caused by the Sh1 first intron is approximately 10 times higher than that caused by any other plant introns that have been used. Thus, the Sh1 first intron may prove quite useful in increasing expression of foreign genes in monocots and possibly other plants.

Journal Article↗

PubMed1991

Proline is not the primary determinant of chilling tolerance induced by mannitol or abscisic Acid in regenerable maize callus cultures.

Chilling sensitive regenerable maize (Zea mays L.) callus cultures can be induced to survive prolonged exposure to 4 degrees C by treatments with mannitol, abscisic acid (ABA), and/or high levels of proline. Maize callus with a free proline content of about 122 micromoles/grain fresh weight survived longer exposures to 4 degrees C than did callus with a free proline content of about 68 micromoles/grain fresh weight. The addition of 0.53 molar mannitol or 0.1 millimolar ABA to culture medium produced a free proline content in maize callus of about 136 and 145 micromoles/grain fresh weight, respectively, if the medium contained 12 millimolar proline or about 36 and 1 micromoles/grain fresh weight, respectively, if no proline was in the medium. Although these mannitol and ABA treatments produced drastically different free proline levels in maize callus, callus grown on these media survived longer exposures to 4 degrees C than did maize callus grown on any proline treatment alone. Thus, the internal free proline level of treated callus is not the primary factor conferring chilling tolerance on these tissues.

Journal Article↗

PubMed1991

In vivo regulatory phosphorylation site in c(4)-leaf phosphoenolpyruvate carboxylase from maize and sorghum.

Reversible seryl-phosphorylation contributes to the light/dark regulation of C(4)-leaf phosphoenolpyruvate carboxylase (PEPC) activity in vivo. The specific regulatory residue that, upon in vitro phosphorylation by a maize-leaf protein-serine kinase(s), leads to an increase in catalytic activity and a decrease in malate-sensitivity of the target enzyme has been recently identified as Ser-15 in (32)P-phosphorylated/activated dark-form maize PEPC (J-A Jiao, R Chollet [1990] Arch Biochem Biophys 283: 300-305). In order to ascertain whether this N-terminal seryl residue is, indeed, the in vivo regulatory phosphorylation site, [(32)P]phosphopeptides were isolated and purified from in vivo(32)P-labeled maize and sorghum leaf PEPC and subjected to automated Edman degradation analysis. The results show that purified light-form maize PEPC contains 14-fold more (32)P-radioactivity than the corresponding dark-form enzyme on an equal protein basis and, more notably, only a single N-terminal serine residue (Ser-15 in maize PEPC and its structural homolog, Ser-8, in the sorghum enzyme) was found to be (32)P-phosphorylated in the light or dark. These in vivo observations, combined with the results from our previous in vitro phosphorylation studies (J-A Jiao, R Chollet [1989] Arch Biochem Biophys 269: 526-535; [1990] Arch Biochem Biophys 283: 300-305), demonstrate that an N-terminal seryl residue in C(4) PEPC is, indeed, the regulatory site that undergoes light/dark changes in phosphorylation-status and, thus, plays a major, if not cardinal role in the light-induced changes in catalytic and regulatory properties of this cytoplasmic C(4)-photosynthesis enzyme in vivo.

Journal Article↗

PubMed1998

Ribozymes targeted to stearoyl-ACP delta9 desaturase mRNA produce heritable increases of stearic acid in transgenic maize leaves.

Ribozymes are RNAs that can be designed to catalyze the specific cleavage or ligation of target RNAs. We have explored the possibility of using ribozymes in maize to downregulate the expression of the stearoyl-acyl carrier protein (Delta9) desaturase gene. Based on site accessibility and catalytic activity, several ribozyme constructs were designed and transformed into regenerable maize lines. One of these constructs, a multimer hammerhead ribozyme linked to a selectable marker gene, was shown to increase leaf stearate in two of 13 maize lines. There were concomitant decreases in Delta9 desaturase mRNA and protein. The plants with the altered stearate phenotype were shown to express ribozyme RNA. The ribozyme-mediated trait was heritable, as evidenced by stearate increases in the leaves of the R1 plants derived from a high-stearate line. The increase in stearate correlated with the presence of the ribozyme gene. A catalytically inactive version of this ribozyme did not produce any significant effect in transgenic maize. This is evidence that ribozymes can be used to modulate the expression of endogenous genes in maize.

Amino Acid Sequence↗

PubMed2005

Mx-rMx, a family of interacting transposons in the growing hAT superfamily of maize.

More than half a century after the discovery of transposable elements, the number of genetically defined autonomous elements that have been isolated and characterized molecularly in any one species remains surprisingly small. Because of its rich genetic history, maize (Zea mays) is, by far, the plant with the largest number of such elements. Yet, even in maize, a maximum of only two autonomous elements have been characterized in any transposon superfamily. This article describes the isolation and molecular and genetic characterization of Mx (for mobile element induced by x-rays), a third autonomous member of the hAT transposon superfamily in maize. Mx is 3731 bp long, ends in 13-bp terminal inverted repeats (TIRs), and causes an 8-bp duplication of the target site. Mx and rMx (for responder to Mx), its 571-bp nonautonomous partner, define a classical family of interacting transposable elements. Surprisingly, the TIRs of Mx and rMx are only 73% identical, and the subterminal sequences are even less so, suggesting that Mx and rMx may represent diverging transposable elements still capable of mobilization by the same transposase. Sequences that are closer to the ends of either Mx or rMx are present in the maize genome. Mx is predicted to encode a 674-amino acid protein that is homologous to the Ac transposase. Although Mx and Ac are closely related, they do not interact. Other data suggest that maize may possess at least five families of hAT transposons that do not interact with each other. The possible origin of noninteracting transposon families within the same superfamily is discussed.

Amino Acid Sequence↗

PubMed2005

Regulation by external K+ in a maize inward shaker channel targets transport activity in the high concentration range.

An inward Shaker K(+) channel identified in Zea mays (maize), ZmK2.1, displays strong regulation by external K(+) when expressed in Xenopus laevis (African clawed frog) oocytes or COS cells. ZmK2.1 is specifically activated by K(+) with an apparent K(m) close to 15 mM independent of the membrane hyperpolarization level. In the absence of K(+), ZmK2.1 appears to enter a nonconducting state. Thus, whatever the membrane potential, this maize channel cannot mediate K(+) influx in the submillimolar concentration range, unlike its relatives in Arabidopsis thaliana. Its expression is restricted to the shoots, the strongest signal (RT-PCR) being associated with vascular/bundle sheath strands. Based on sequence and gene structure, the closest relatives of ZmK2.1 in Arabidopsis are K(+) Arabidopsis Transporter 1 (KAT1) (expressed in guard cells) and KAT2 (expressed in guard cells and leaf phloem). Patch-clamp analyses of guard cell protoplasts reveal a higher functional diversity of K(+) channels in maize than in Arabidopsis. Channels endowed with regulation by external K(+) similar to that of ZmK2.1 (channel activity regulated by external K(+) with a K(m) close to 15 mM, regulation independent of external Ca(2+)) constitute a major component of the maize guard cell inward K(+) channel population. The presence of such channels in maize might reflect physiological traits of C4 and/or monocotyledonous plants.

Amino Acid Sequence↗

PubMed1999

Polyadenylation occurs at multiple sites in maize mitochondrial cox2 mRNA and is independent of editing status.

Polyadenylation of nucleus-encoded transcripts has a well-defined role in gene expression. The extent and function of polyadenylation in organelles and prokaryotic systems, however, are less well documented. Recent reports of polyadenylation-mediated RNA destabilization in Escherichia coli and in vascular plant chloroplasts prompted us to look for polyadenylation in plant mitochondria. Here, we report the use of reverse transcription-polymerase chain reaction to map multiple polyadenylate addition sites in maize mitochondrial cox2 transcripts. The lack of sequence conservation surrounding these sites suggests that polyadenylation may occur at many 3' termini created by endoribonucleolytic and/or exoribonucleolytic activities, including those activities involved in 3' end maturation. Endogenous transcripts could be efficiently polyadenylated in vitro by using maize mitochondrial lysates with an activity that added AMP more efficiently than GMP. Polyadenylated substrates were tested for stability in maize mitochondrial S100 extracts, and we found that, compared with nonpolyadenylated RNAs, the polyadenylated substrates were less stable. Taken together with the low abundance of polyadenylated RNAs in maize mitochondria, our results are consistent with a degradation-related process. The fact that polyadenylation does not dramatically destabilize plant mitochondrial transcripts, at least in vitro, is in agreement with results obtained for animal mitochondria but differs from those obtained for chloroplasts and E. coli. Because fully edited, partially edited, and unedited transcripts were found among the cloned polyadenylated cox2 cDNAs, we conclude that RNA editing and polyadenylation are independent processes in maize mitochondria.

Adenosine Monophosphate↗

PubMed1991

Maize rbcS promoter activity depends on sequence elements not found in dicot rbcS promoters.

Although the molecular mechanisms of dicot photosynthetic gene regulation have been pursued actively, comparable studies of monocot regulation have been slow to come forth. We show here that monocot (maize and wheat) but not dicot (pea, tobacco, and Arabidopsis) ribulose-1,5-bisphosphate carboxylase small subunit (rbcS) gene promoters are active in maize mesophyll protoplasts. The evolutionarily conserved GT and G boxes of dicot rbcS promoters are not essential for light-responsive expression in monocot leaf cells. Instead, at least six constitutive and light-sensitive regulatory elements are likely important for maize rbcS expression. Synergism between upstream and downstream promoter elements is required. Whereas in dicots, light triggers coupled leaf development and photosynthetic gene expression, in monocots, light regulation of rbcS is uncoupled from leaf development. Light regulation of maize rbcS may be divided into direct and indirect contributions mediated by different regulatory elements. Because wheat and maize rbcS promoters show sequence homologies and similar expression patterns in monocot and dicot leaf cells, it appears likely that monocots share conserved regulatory elements irrespective of whether they utilize the C3 or C4 pathway for carbon fixation.

Base Sequence↗

PubMed1992

Competence of Immature Maize Embryos for Agrobacterium-Mediated Gene Transfer.

Agrobacterium-mediated transfer of viral sequences to plant cells (agroinfection) was applied to study the susceptibility of immature maize embryos to the pathogen. The shoot apical meristem of immature embryos 10 to 20 days after pollination from four different maize genotypes was investigated for competence for agroinfection. There was a direct correlation between different morphological stages of the unwounded immature embryos and their competence for agroinfection. Agroinfection frequency was highest in the embryogenic line A188. All developmental stages tested showed Agrobacterium virulence gene-inducing activity, whereas bacteriocidal substances were produced at stages of the immature embryos competent for agroinfection. The results suggested that Agrobacterium may require differentiated tissue in the maize shoot apical meristem before wounding for successful T-DNA transfer. This requirement for the young maize embryo has implications for the possible use of Agrobacterium for maize transformation.

Journal Article↗

PubMed1992

A Biochemical Phenotype for a Disease Resistance Gene of Maize.

In maize, major resistance to the pathogenic fungus Cochliobolus (Helminthosporium) carbonum race 1 is determined by the dominant allele of the nuclear locus hm. The interaction between C. carbonum race 1 and maize is mediated by a pathogen-produced, low molecular weight compound called HC-toxin. We recently described an enzyme from maize, called HC-toxin reductase, that inactivates HC-toxin by pyridine nucleotide-dependent reduction of an essential carbonyl group. We now report that this enzyme activity is detectable only in extracts of maize that are resistant to C. carbonum race 1 (genotype Hm/Hm or Hm/hm). In several genetic analyses, in vitro HC-toxin reductase activity was without exception associated with resistance to C. carbonum race 1. The results indicate that detoxification of HC-toxin is the biochemical basis of Hm-specific resistance of maize to infection by C. carbonum race 1.

Journal Article↗

PubMed1992

A maize protein associated with the G-box binding complex has homology to brain regulatory proteins.

The G-box element is a moderately conserved component of the promoter of many inducible genes, including the alcohol dehydrogenase genes of Arabidopsis and maize. We used monoclonal antibodies generated against partially purified G-box binding factor (GBF) activity to characterize maize proteins that are part of the DNA binding complex. Antibodies interacted with partially purified maize GBF complexes to produce a slower migrating complex in the gel retardation assay. Immunoprecipitation experiments suggested that the protein recognized by the antibody is not a DNA binding protein in and of itself, but rather is associated with the DNA binding complex. These monoclonal antibodies were used to isolate cDNA clones encoding a protein that we have designated GF14. Maize GF14 contains a region resembling a leucine zipper and acidic carboxy and amino termini, of which the latter can form an amphipathic alpha-helix similar to known transcriptional activators such as VP16 and GAL4. Protein gel blot analysis of cell culture extract showed that a single, major protein of approximately 30 kD is recognized by anti-GF14; the protein is also present predominantly in the kernel and root. The deduced amino acid sequence of maize GF14 is more than 80% identical to Arabidopsis GF14 and Oenothera PHP-O, and is more than 60% identical to a class of mammalian brain proteins described as both protein kinase C inhibitors and activators of tyrosine and tryptophan hydroxylases. GF14 is found in a variety of monocotyledons and dicotyledons, gymnosperms, and yeast. This suggests a deep evolutionary conservation of a potential regulatory protein associated with a core sequence found in the promoter region of many genes.

14-3-3 Proteins↗