Inhibition of Fusarium graminearum growth in flour gel cultures by hexane-soluble compounds from oat (Avena sativa L.) flour

Fusarium head blight, incited by the fungus Fusarium graminearum, primarily affects wheat (Triticum aestivum) and barley (Hordeum vulgarum), while oat (Avena sativa) appears to be more resistant. Although this has generally been attributed to the open panicle of oats, we hypothesized that a chemical component of oats might contribute to this resistance. To test this hypothesis, we created culture media made of wheat, barley, and oat flour gels (6 g of flour in 20 ml of water, gelled by autoclaving) and inoculated these with plugs of F. graminearum from actively growing cultures. Fusarium growth was measured from the diameter of the fungal plaque. Plaque diameter was significantly smaller on oat flour cultures than on wheat or barley cultures after 40 to 80 h of growth. Ergosterol concentration was also significantly lower in oat cultures than in wheat cultures after growth. A hexane extract from oats added to wheat flour also inhibited Fusarium growth, and Fusarium grew better on hexane-defatted oat flour. The growth of Fusarium on oat flour was significantly and negatively affected by the oil concentration in the oat, in a linear relationship. A hexane-soluble chemical in oat flour appears to inhibit Fusarium growth and might contribute to oat's resistance to Fusarium head blight. Oxygenated fatty acids, including hydroxy, dihydroxy, and epoxy fatty acids, were identified in the hexane extracts and are likely candidates for causing the inhibition.

Response of enzymes and storage proteins of maize endosperm to nitrogen supply

To examine the effects of N nutrition upon endosperm development, maize (Zea mays) kernels were grown in vitro with either 0, 3.6, 7.1, 14.3, or 35.7 millimolar N. Kernels were harvested at 20 days after pollination for determination of enzyme activities and again at maturity for quantification of storage products and electrophoretic separation of zeins. Endosperm dry weight, starch, zein-N, and nonzein-N all increased in mature kernels as N supply increased from zero to 14.3 millimolar. The activities of sucrose synthase, aldolase, phosphoglucomutase, glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase, and acetolactate synthase increased from 1- to 2.5-fold with increasing N supply. Adenosine diphosphate-glucose pyrophosphorylase and both ATP- and PPi-dependent phosphofructokinases increased to lesser extents, while no significant response was detected for hexose kinases and glutamine synthetase. Nitrogen-induced changes in enzyme activities were often highly correlated with changes in final starch and/or zein-N contents. Separation of zeins indicated that these peptides were proportionately enhanced by N supply, with the exception of C-zein, which was relatively insensitive to N. These data indicate that at least a portion of the yield increase in maize produced by N fertilization is induced by a modification of kernel metabolism in response to N supply.

Sugar metabolism in germinating soybean seeds: evidence for the sorbitol pathway in soybean axes

Characterization of sugar content and enzyme activity in germinating soybean (Glycine max L. Merrell) seeds led to the discovery of sorbitol accumulating in the axes during germination. The identity of sorbitol was confirmed by relative retention times on high-performance liquid chromatography and gas liquid chromatography and by mass spectra identical with authentic sorbitol. Accumulation of sorbitol in the axes started on day 1 of germination as sucrose decreased and glucose and fructose increased. Sucrose also decreased in the cotyledons, but there was no accumulation of sorbitol, glucose, or fructose. Accumulation of sorbitol and hexoses was highly correlated with increased invertase activity in the axes, but not with sucrose synthase and sucrose phosphate synthase activities. Sucrose synthase activity was relatively high in the axes, whereas the activity of sucrose phosphate synthase was relatively high in the cotyledons. Ketose reductase and aldose reductase were detected in germinating soybean axes, but not in cotyledons. Fructokinase and glucokinase were present in both axes and cotyledons. The data suggest a sorbitol pathway functioning in germinating soybean axes, which allows for the interconversion of glucose and fructose with sorbitol as an intermediate.

Fructokinases from developing maize kernels differ in their specificity for nucleoside triphosphates

A new form of fructokinase has been identified from developing maize (Zea mays L.) kernels that utilizes CTP, UTP, and GTP from four to eight times more effectively than ATP at nonlimiting concentrations. Ten millimolar dithiothreitol was necessary to stabilize activity. A second form of fructokinase was nonspecific for nucleoside triphosphate whereas a third form was fairly specific for ATP.

Sugar metabolism in developing kernels of starch-deficient endosperm mutants of maize

Sugar metabolism in kernels of starch-deficient endosperm mutants of maize (Zea mays L.) was examined to determine how single locus mutations of carbohydrate metabolism affect carbohydrate metabolism as a whole. Activities of 14 enyzmes were measured in extracts from endosperms from isogenic lines of normal, shrunken, shrunken-2, shrunken-4, brittle-1, and brittle-2 maize in an OH43 background. Nearly every enzyme activity examined was affected in some or all of the mutants. Sucrose synthase and aldolase activities were lower in all mutants compared to normal. ADP-Glc pyrophosphorylase activity in immature kernels was much higher in brittle endosperms than in normal, but absent in brittle-2 and shrunken-2 endosperms. The activity in those genotypes exhibiting activity was positively correlated with sucrose concentration in the kernels. Sucrose may be modulating the coarse control of ADP-Glc pyrophosphorylase activity by affecting the genetic transcription of message for this enzyme. Sorbitol dehydrogenase activity was negatively correlated with its substrate, fructose, supporting the hypothesis that sorbitol dehydrogenase converts fructose produced during sucrose degradation into sorbitol. Glucokinase activity was positively correlated with mature kernel dry weight. This supports the hypothesis that glucokinase activity may limit sucrose utilization. Shrunken-4 extracts had lower activities for a number of enzymes, supporting the view that this mutant may have an impediment to protein synthesis. Elevated sucrose levels were evenly distributed throughout 20-day postpollination shrunken-2 kernels, whereas a sucrose concentration gradient existed in normal kernels between the basal region and the upper endosperm. This gradient is apparently generated by the utilization of sugars and may facilitate the movement of sugars into developing corn kernels.

Separation and characterization of four hexose kinases from developing maize kernels

Four forms of hexose kinase activity from developing maize (Zea mays L.) kernels have been separated by ammonium sulfate precipitation, gel filtration chromatography, blue-agarose chromatography, and ion exchange chromatography. Two of these hexose kinases utilized d-glucose most effectively and are classified as glucokinases (EC 2.7.1.2). The other two hexose kinases utilized only d-fructose and are classified as fructokinases (EC 2.7.1.4). All hexose kinases analyzed had broad pH optima between 7.5 and 9.5 with optimal activity at pH 8.5. The two glucokinases differed in substrate affinities. One form had low K(m) values [K(m)(glucose) = 117 micromolar, K(m)(ATP) = 66 micromolar] whereas the other form had much higher K(m) values [K(m)(glucose) = 750 micromolar, K(m)(ATP) = 182 micromolar]. Both fructokinases had similar substrate saturation responses. The K(m)(fructose) was about 130 micromolar and the K(m)(ATP) was about 700 micromolar. Both exhibited uncompetitive substrate inhibition by fructose [K(i)(fructose) = 1.40 to 2.00 millimolar]. ADP inhibited all four hexose kinase activities, whereas sugar phosphates had little effect on their activities. The data suggest that substrate concentrations are an important factor controlling hexose kinase activity in situ.

Enzymes of sucrose and hexose metabolism in developing kernels of two inbreds of maize

Tissue distribution and activity of enzymes involved in sucrose and hexose metabolism were examined in kernels of two inbreds of maize (Zea mays L.) at progressive stages of development. Levels of sugars and starch were also quantitated throughout development. Enzyme activities studied were: ATP-linked fructokinase, UTP-linked fructokinase, ATP-linked glucokinase, sucrose synthase, UDP-Glc pyrophosphorylase, UDP-Glc dehydrogenase, PPi-linked phosphofructokinase, ATP-linked phosphofructokinase, NAD-dependent sorbitol dehydrogenase, NADP-dependent 6-P-gluconate dehydrogenase, NADP-dependent Glc-6-P dehydrogenase, aldolase, phosphoglucoisomerase, and phosphoglucomutase. Distribution of invertase activity was examined histochemically. Hexokinase and ATP-linked phosphofructokinase activities were the lowest among these enzymes and it is likely that these enzymes may regulate the utilization of sucrose in developing maize kernels. Most of the hexokinase activity was found in the endosperm, but the embryo had high activity on a dry weight basis. The endosperm, which stores primarily starch, contained high PPi-linked phosphofructokinase and low ATP-linked phosphofructokinase activities, whereas the embryo, which stores primarily lipids, had much higher ATP-linked phosphofructokinase activity than did the endosperm. It is suggested that PPi required by UDP-Glc pyrophosphorylase and PPi-linked phosphofructokinase in the endosperm may be supplied by starch synthesis. Sorbitol dehydrogenase activity was largely restricted to the endosperm, whereas 6-P-gluconate and Glc-6-P dehydrogenase activities were highest in the base and pericarp. A possible metabolic pathway by which sucrose is converted into starch is proposed.

Ketose reductase activity in developing maize endosperm

Ketose reductase (NAD-dependent polyol dehydrogenase EC 1.1.1.14) activity, which catalyzes the NADH-dependent reduction of fructose to sorbitol (d-glucitol), was detected in developing maize (Zea mays L.) endosperm, purified 104-fold from this tissue, and partially characterized. Product analysis by high performance liquid chromatography confirmed that the enzyme-catalyzed reaction was freely reversible. In maize endosperm, 15 days after pollination, ketose reductase activity was of the same order of magnitude as sucrose synthase activity, which produces fructose during sucrose degradation. Other enzymes of hexose metabolism detected in maize endosperm were present in activities of only 1 to 3% of the sucrose synthase activity. CaCl(2), MgCl(2), and MnCl(2) stimulated ketose reductase activity 7-, 6-, and 2-fold, respectively, but had little effect on NAD-dependent polyol dehydrogenation (the reverse reaction). The pH optimums for ketose reductase and polyol dehydrogenase reactions were 6.0 and 9.0, respectively. K(m) values were 136 millimolar fructose and 8.4 millimolar sorbitol. The molecular mass of ketose reductase was estimated to be 78 kilodaltons by gel filtration. It is postulated that ketose reductase may function to metabolize some of the fructose produced during sucrose degradation in maize endosperm, but the metabolic fate of sorbitol produced by this reaction is not known.

Phosphate inhibition of spinach leaf sucrose phosphate synthase as affected by glucose-6-phosphate and phosphoglucoisomerase

The inhibition patterns of inorganic phosphate (Pi) on sucrose phosphate synthase activity in the presence and absence of the allosteric activator glucose-6-P was studied, as well as the effects of phosphoglucoisomerase on fructose-6-P saturation kinetics with and without Pi. In the presence of 5 millimolar glucose-6-P, Pi was a partial competitive inhibitor with respect to both substrates, fructose-6-P and uridine diphosphate glucose. In the absence of glucose-6-P, the inhibition patterns were more complex, apparently because of the interaction of Pi at the activation site as well as the catalytic site. In addition, substrate activation by uridine diphosphate glucose was observed in the absence of effectors. The results suggested that Pi antagonizes glucose-6-P activation of sucrose phosphate synthase by competing with the activator for binding to the modifier site.The fructose-6-P saturation kinetics were hyperbolic in the absence of phosphoglucoisomerase activity, but became sigmoidal by the addition of excess phosphoglucoisomerase. The sigmoidicity persisted in the presence of Pi, but sucrose phosphate synthase activity was decreased. The apparent sigmoidal response may represent the physiological response of sucrose phosphate synthase to a change in hexose-P concentration because sucrose phosphate synthase operates in the cytosol in the presence of high activities of phosphoglucoisomerase. Thus, the enzymic production of an activator from a substrate represents a unique mechanism for generating sigmoidal enzyme kinetics.

Regulation of Spinach Leaf Sucrose Phosphate Synthase by Glucose-6-Phosphate, Inorganic Phosphate, and pH

Sucrose phosphate synthase was partially purified from spinach leaves and the effects and interactions among glucose-6-P, inorganic phosphate (Pi), and pH were investigated. Glucose-6-P activated sucrose phosphate synthase and the concentration required for 50% of maximal activation increased as the concentration of fructose-6-P was decreased. Inorganic phosphate inhibited sucrose phosphate synthase activity and antagonized the activation by glucose-6-P. Inorganic phosphate caused a progressive increase in the concentration of glucose-6-P required for 50% maximal activation from 0.85 mm (minus Pi) to 9.9 mm (20 mm Pi). In the absence of glucose-6-P, Pi caused partial inhibition of sucrose phosphate synthase activity (about 65%). The concentration of Pi required for 50% maximal inhibition decreased with a change in pH from 6.5 to 7.5. When the effect of pH on Pi ionization was taken into account, it was found that per cent inhibition increased hyperbolically with increasing dibasic phosphate concentration independent of the pH. Sucrose phosphate synthase had a relatively broad pH optimum centered at pH 7.5. Inhibition by Pi was absent at pH 5.5, but became more pronounced at alkaline pH, whereas activation by glucose-6-P was observed over the entire pH range tested. The results suggested that glucose-6-P and Pi bind to sites distinct from the catalytic site, e.g. allosteric sites, and that the interactions of these effectors with pH and concentrations of substrate may be involved in the regulation of sucrose synthesis in vivo.

Specific Determination of alpha-Amylase Activity in Crude Plant Extracts Containing beta-Amylase

The specific measurement of alpha-amylase activity in crude plant extracts is difficult because of the presence of beta-amylases which directly interfere with most assay methods. Methods compared in this study include heat treatment at 70 degrees C for 20 min, HgCl(2) treatment, and the use of the alpha-amylase specific substrate starch azure. In comparing alfalfa (Medicago sativa L.), soybeans (Glycine max [L.] Merr.), and malted barley (Hordeum vulgare L.), the starch azure assay was the only satisfactory method for all tissues. While beta-amylase can liberate no color alone, over 10 International units per milliliter beta-amylase activity has a stimulatory effect on the rate of color release. This stimulation becomes constant (about 4-fold) at beta-amylase activities over 1,000 International units per milliliter. Two starch azure procedures were developed to eliminate beta-amylase interference: (a) the dilution procedure, the serial dilution of samples until beta-amylase levels are below levels that interfere; (b) the beta-amylase saturation procedure, addition of exogenous beta-amylase to increase endogenous beta-amylase activity to saturating levels. Both procedures yield linear calibrations up to 0.3 International units per milliliter. These two procedures produced statistically identical results with most tissues, but not for all tissues. Differences between the two methods with some plant tissues was attributed to inaccuracy with the dilution procedure in tissues high in beta-amylase activity or inhibitory effects of the commercial beta-amylase. The beta-amylase saturation procedure was found to be preferable with most species. The heat treatment was satisfactory only for malted barley, as alpha-amylases in alfalfa and soybeans are heat labile. Whereas HgCl(2) proved to be a potent inhibitor of beta-amylase activity at concentrations of 10 to 100 micromolar, these concentrations also partially inhibited alpha-amylase in barley malt. The reported alpha-amylase activities in crude enzyme extracts from a number of plant species are apparently the first specific measurements reported for any plant tissues other than germinating cereals.

Beta-Amylases from Alfalfa (Medicago sativa L.) Roots

Amylase was found in high activity (193 international units per milligram protein) in the tap root of alfalfa (Medicago sativa L. cv. Sonora). The activity was separated by gel filtration chromatography into two fractions with molecular weights of 65,700 (heavy amylase) and 41,700 (light amylase). Activity staining of electrophoretic gels indicated the presence of one isozyme in the heavy amylase fraction and two in the light amylase fraction. Three amylase isozymes with electrophoretic mobilities identical to those in the heavy and the light amylase fractions were the only amylases identified in crude root preparations. Both heavy and light amylases hydrolyzed amylopectin, soluble starch, and amylose but did not hydrolyze pullulan or beta-limit dextrin. The ratio of viscosity change to reducing power production during starch hydrolysis was identical for both alfalfa amylase fractions and sweet potato beta-amylase, while that of bacterial alpha-amylase was considerably higher. The identification of maltose and beta-limit dextrin as hydrolytic end-products confirmed that these alfalfa root amylases are all beta-amylases.The pH optimum for both beta-amylase fractions was 6.0. Both light and heavy beta-amylases showed normal Michaelis-Menten kinetics, with soluble starch as substrate, and had respectively K(m) values of 5.9 and 6.8 milligrams starch per milliliter and V(max) of 640 and 130 international units per milligram protein. Arrhenius plots indicated that the energy of activation for the heavy beta-amylase remained relatively unchanged (12.7 to 13.0 kilocalories per mole) from 0 to 30 degrees C, whereas the energy of activation for the light amylase increased from 12.0 to about 28.0 kilocalories per mole at 8.7 degrees C as temperature was lowered. The light amylase was shown to be inhibited by maltose.