UDP-glucose: Glucan Synthetase in Developing Cotton Fibers: I. Kinetic and Physiological Properties

2-NBDG Uptake in Gossypium hirsutum in vitro ovules: exploring tissue-specific accumulation and its impact on hexokinase-mediated glycolysis regulation

Fluorescent glucose derivatives are valuable tools as glucose analogs in plant research to explore metabolic pathways, study enzyme activity, and investigate cellular processes related to glucose metabolism and sugar transport. They allow visualization and tracking of glucose uptake, its utilization, and distribution within plant cells and tissues. This study investigates the phenotypic and metabolic impact of the exogenously fed glucose derivative, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose) (2-NBDG) on the fibers of Gossypium hirsutum (Upland cotton) ovule in vitro cultures. The presence of 2-NBDG in the culture medium did not lead to macroscopic morphological alterations in ovule and fiber development or to the acquisition of fluorescence or yellow coloration. Confocal laser scanning microscope imaging and chromatographic analysis of cotton ovules' outer rim cross-sections showed that the 2-NBDG is transported from the extracellular space and accumulated inside some outer integument cells, epidermal cells, and fertilized epidermal cells (fibers), but is not incorporated into the cell walls. Untargeted metabolic profiling of the fibers revealed significant changes in the relative levels of metabolites involved in glycolysis and upregulation of alternative energy-related pathways. To provide biochemical and structural evidence for the observed downregulation of glycolysis pathways in the fibers containing 2-NBDG, kinetics analysis and docking simulations were performed on hexokinase from G. hirsutum (GhHxk). Notably, the catalytic activity of heterologously expressed recombinant active GhHxk exhibited a five-fold decrease in reaction rates compared to D-glucose. Furthermore, GhHxk exhibited a linear kinetic behavior in the presence of 2-NBDG instead of the Michaelis-Menten kinetics found for D-glucose. Docking simulations suggested that 2-NBDG interacts with a distinct binding site of GhHxk9, possibly inducing a conformational change. These results highlight the importance of considering fluorescent glucose derivatives as ready-to-use analogs for tracking glucose-related biological processes. However, a direct comparison between their mode of action and its extrapolation into biochemical considerations should go beyond microscopic inspection and include complementary analytical techniques.

Conserved Active Site Architecture Between Bacterial Cellulose and Chitin Synthases

Glycosyltransferases (GTs) are a large and diverse group of enzymes responsible for catalyzing the formation of a glycosidic bond between a donor molecule, usually a monosaccharide, and a wide range of acceptor molecules, thus, playing critical roles in various essential biological processes. Chitin and cellulose synthases are two inverting processive integral membrane GTs, belonging to the type-2 family involved in the biosynthesis of chitin and cellulose, respectively. Herein, we report that bacterial cellulose and chitin synthases share an E-D-D-ED-QRW-TK active site common motif that is spatially co-localized. This motif is conserved among distant bacterial evolutionary species despite their low amino acid sequence and structural similarities between them. This theoretical framework offers a new perspective to the current view that bacterial cellulose and chitin synthases are substrate specific and that chitin and cellulose are organism specific. It lays the ground for future in vivo and in silico experimental assessment of cellulose synthase catalytic promiscuity against uridine diphosphate N-acetylglucosamine and chitin synthase against uridine diphosphate glucose, respectively.

Requirement for a membrane potential for cellulose synthesis in intact cells of Acetobacter xylinum

The marked lability in cell-free preparations of the enzyme system involved in cellulose biosynthesis in most organisms studied led us to investigate factors responsible for loss of activity on cellular disruption. Previous studies have led to the suggestion that the existence of a transmembrane electrical potential (DeltaPsi) may be one factor responsible for maintaining an active system in intact cells. In this report, we show that dissipation of the DeltaPsi in metabolizing cells of Acetobacter xylinum results in severe inhibition of cellulose synthesis. The effect can be reversed by restoration of the DeltaPsi. Inhibition of cellulose biosynthesis by dissipation of the DeltaPsi can be observed under conditions in which no substantial impairment of energy metabolism occurs-i.e., under conditions in which a transmembrane pH gradient is of sufficient magnitude to maintain an adequate overall protonmotive force across the membrane. The inhibition of cellulose biosynthesis is specifically related to changes in the DeltaPsi, since the process can proceed normally in the absence of the pH gradient. These results support the suggestion that loss of the DeltaPsi on cellular disruption may be one of the factors responsible for the low capacity for cellulose synthesis in isolated membrane preparations and also raise the possibility that modulation of the DeltaPsi could be one means of regulating the rate of cellulose synthesis in vivo.

Cloning and characterization of cotton GhBG gene encoding beta-glucosidase

Beta-1,4-glucosidase (BG, EC3.2.1.21), one of three cellulases, is a widespread family of enzymes involved in the metabolism of cell wall polysaccharides in both prokaryocytes and eukaryotes. Here, we report the isolation of a full-length cDNA encoding beta-1,4-glucosidase protein (designated as GhBG) and its putative function in the process of fiber development and in yeast. Through random sequencing of the cotton fiber cDNA library from 7235 germplasm line, with elite fiber quality in Gossypium hirsutum L. and utilizing the 5' rapid amplification of cDNA ends (RACE) technique, a 2133 bp cDNA clone encoding a cotton fiber specifically expressed protein (accession number: DQ103699) was isolated. GhBG was composed of a 1884 bp open reading frame (ORF) encoding 627 amino acid residues. This putative protein had an isoelectric point of 8.17, a calculated molecular weight of 68.78 KD and a signal peptide with 23 amino acid residues at the N-terminal. RT-PCR analysis indicated GhBG was specifically expressed in fiber cells and was highly abundant in 5-17 day post anthesis (DPA). It was not, however, expressed in root, hypocotyls or leaves. Southern blotting analysis showed there were two copies of GhBG in the upland cotton genome; most likely contained in sub-genome A and sub-genome D. GhBG was then integrated into a yeast expression vector, pREP-5N and electro-transformed into fission yeast Schizosaccharomyces pombe Q-01. The results demonstrated that GhBG led to a significant increase in cell length and width and a remarkable decrease of the length/width ratio. Compared to vector control transformants, cells were significantly larger and rounder and their growth velocity was also reduced.

Specificity of binding of beta-glucoside activators of ryegrass (1-->3)-beta-glucan synthase and the synthesis of some potential photoaffinity activators

Structure-activity relationships among glycoside activators of ryegrass (Lolium multiflorum) (1-->3)-beta-glucan synthase were investigated using a number of natural and synthetic glycosides, including some carrying photoaffinity functions. There is an absolute requirement for a beta-D-glycosyl moiety in the activator, both S- and N-glucosides are active, and the position of the glucosidic linkage in beta-glucose disaccharides has a significant effect on the affinity of binding. However, the binding requirement does not extend beyond a single beta-D-glucosyl residue, and beta-D-oligoglucosides are less effective than disaccharides. The nature of the aglycon has a major influence on the binding affinity. Hydrophobic aglycons lower the concentration required for half-maximal stimulation of the enzyme obtained from an Eadie-Hofstee plot of kinetic data (Ka) for activation, but charge aglycons increase Ka. Relative to methyl-beta-D-glucoside and cellobiose (Ka 1.1 mM), the most potent compounds tested were N-[4-(benzoyl)benzoyl]-beta-D-glucosylamine and 2'-[4-azidosalicylamino]ethyl-1-thio-beta-D-glucoside with K(a)s of approximately 30 microM. The latter also was tested for its potential to specifically label the beta-glucoside-binding site on the synthase, but under the conditions used the binding was found to be nonspecific.

[beta]-Glucan Synthesis in the Cotton Fiber (II. Regulation and Kinetic Properties of [beta]-Glucan Synthases

The regulation and kinetic properties of cellulose synthase as well as [beta]-1,3-glucan synthase have been studied. The cellulose was detected using acetic/nitric acid insolubility as an indicator of cellulose (this product contained only [beta]-1,4-linked glucans; K. Okuda, L. Li, K. Kudlicka, S. Kuga, R.M. Brown, Jr. [1993] Plant Physiol 101: 1131-1142). These studies reveal that (a) [beta]-1,3-glucan synthesis is enhanced up to 31-fold by cellobiose with a Ka of 1.16 mM; (b) cellulose synthesis is increased 12-fold by a combination of cellobiose (Ka = 3.26 mM) and cyclic-3[prime]:5[prime]-GMP (Ka = 100 [mu]M); (c) the common components in the reaction mixture required by both enzymes are cellobiose, calcium, and digitonin; (d) cellulose synthase has an essential requirement for magnesium (Ka = 0.89 mM); (e) cellulose synthase also requires a low concentration of calcium (Ka = 90 [mu]M); (f) the optimal pH for cellulose synthase (7.6-8.0) is slightly higher than that for [beta]-1,3-glucan synthase (7.2-7.6); (g) the Km for UGP-Glc for cotton (Gossypium hirsutum) cellulose synthase is 0.40 mM; (h) the Km for UDP-Glc for for [beta]-1,3-glucan synthase is 0.43 mM.

[beta]-Glucan Synthesis in the Cotton Fiber (III. Identification of UDP-Glucose-Binding Subunits of [beta]-Glucan Synthases by Photoaffinity Labeling with [[beta]-32P]5[prime]-N3-UDP-Glucose

Using differential product entrapment and photolabeling under specifying conditions, we identifIed a 37-kD polypeptide as the best candidate among the UDP-glucose-binding polypeptides for the catalytic subunit of cotton (Gossypium hirsutum) cellulose synthase. This polypeptide is enriched by entrapment under conditions favoring [beta]-1,4-glucan synthesis, and it is magnesium dependent and sensitive to unlabeled UDP-glucose. A 52-kD polypeptide was identified as the most likely candidate for the catalytic subunit of [beta]-1,3-glucan synthase because this polypeptide is the most abundant protein in the entrapment fraction obtained under conditions favoring [beta]-1,3-glucan synthesis, is coincident with [beta]-1,3-glucan synthase activity, and is calcium dependent. The possible involvement of other polypeptides in the synthesis of [beta]-1,3-glucan is discussed.

A membrane-bound enzyme complex synthesising glucan and glucomannan in pine tissues

Particulate membrane preparations isolated from cambial cells and differentiating and differentiated xylem cells of pine (Pinus sylvestris L.) trees synthesised [(14)C]glucans using either guanosine 5'-diphosphate (GDP)-D-[U-(14)C]glucose or uridine 5'-diphosphate (UDP)-D-[U-(14)C]glucose as glycosyl donors. Although these glucans had β-(1→3) and β-(1→4) linkages in an approximate ratio 1:1, the distribution of the linkages in the glucan synthesised from GDP-D-glucose was different from that synthesised from UDP-D-glucose. The synthesis of the mixed β-(1→3) and β-(1→4) glucan from GDP-D-[U-(14)C]glucose was changed to that of β-(1→4) glucomannan in the presence of increasing concentrations of GDP-D-mannose. The glucan formed from UDP-D-[U-(14)C]glucose was not affected by any concentration of GDP-D-mannose. The membrane preparations epimerized GDP-D-glucose to GDP-D-mannose; however, the low amount of GDP-D-mannose formed was not incorporated into the polymer becaus the affinity of the synthase for GDP-D-glucose was much greater than that for GDP-D-mannose. The glucan formed from GDP-D-glucose and the glucomannan formed from GDP-D-glucose together with GDP-D-mannose were characterized. The apparent K m and V max of the glucan synthase for GDP-D-glucose were 6.38 μM and 5.08 μM·min(-1), respectively. No lipid intermediates were detected during the synthesis of either glucan or glucomannan. The results indicated that an enzyme complex for the formation of the glucomannan was bound to the membrane.

Modulation of Pea Membrane beta-Glucan Synthase Activity by Calcium, Polycation, Endogenous Protease, and Protease Inhibitor

beta-Glucan synthase activity in plant membranes can be markedly altered by a multiplicity of apparently unrelated factors. In pea epicotyl membranes it is enhanced by low and inhibited by high concentrations of added Ca(2+), trypsin or soluble pea protease. Ca(2+) stimulates preexisting synthase activity, particularly in the presence of polycations (spermidine), but protease treatments activate and, with time, inactivate synthase zymogen. Endogenous pea protease activity is also associated with washed pea membrane and appears to be responsible for the decay observed with time in the beta-glucan synthase activity. Endogenous pea protease activity is inhibited by thiol inhibitors, e.g. iodoacetamide and Hg(2+), and by a heat-stable peptide, molecular weight approximately 10,000, that is found in supernatants of pea extracts. These protease inhibitors have the capacity to protect beta-glucan synthase activity from denaturation or its zymogen from activation due to endogenous or added protease activity. Evidence is described which supports the proposal that 1,4-beta-glucan synthase is destroyed and possibly converted to 1,3-beta-glucan synthase activity by protease action, and that the latter may then be greatly enhanced by Ca(2+) and polycations.

UDP-Glucose: (1-->3)-beta-Glucan Synthases from Mung Bean and Cotton: Differential Effects of Ca and Mg on Enzyme Properties and on Macromolecular Structure of the Glucan Product

A re-examination of the kinetic properties of UDP-glucose: (1-->3)-beta-glucan (callose) synthases from mung bean seedlings (Vigna radiata) and cotton fibers (Gossypium hirsutum) shows that these enzymes have a complex interaction with UDP-glucose and various effectors. Stimulation of activity by micromolar concentrations of Ca(2+) and millimolar concentrations of beta-glucosides or other polyols is highest at low (<100 micromolar) UDP-glucose concentrations. These effectors act both by raising the V(max) of the enzyme, and by lowering the apparent K(m) for UDP-glucose from >1 millimolar to 0.2 to 0.3 millimolar. Mg(2+) markedly enhances the affinity of the mung bean enzyme for Ca(2+) but not for beta-glucoside; with saturating Ca(2+), Mg(2+) only slightly stimulates further production of glucan. However, the presence of Mg(2+) during synthesis, or NaBH(4) treatment after synthesis, changes the nature of the product from dispersed, alkali-soluble fibrils to highly aggregated, alkali-insoluble fibrils. Callose synthesized in vitro by the Ca(2+), beta-glucoside-activated cotton fiber enzyme, with or without Mg(2+), is very similar in size to callose isolated from cotton fibers, but is a linear (1-->3)-beta-glucan lacking the small amount of branches at C-0-6 found in vivo. We conclude that the high degree of aggregation of the fibrils synthesized with Mg(2+)in vitro is caused either by an alteration of the glucan at the reducing end or, indirectly, by an effect of Mg(2+) on the conformation of the enzyme. Rate-zonal centrifugation of the solubilized mung bean callose synthase confirms that divalent cations can affect the size or conformation of this enzyme.

Partial Purification of Digitonin-Solubilized beta-Glucan Synthase from Red Beet Root

An enriched glucan synthase fraction was obtained from red beet root microsomes by sequential extraction with the detergents 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate and digitonin. The digitonin suspension was centrifuged on a glycerol gradient, where a glucan synthase peak with a specific activity of 30- to 40-fold over microsomes was recovered. Most protein contaminants were found in the gradient pellet. The glucan synthase-containing fraction was largely free of plasma membrane and tonoplast-derived ATPase activity and was enriched with a protein subunit of 68 kilodaltons.

Synthesis of beta-glucans in Prototheca zopfii. Evidence for the existence of a glycoprotein primer

Membrane preparations from the non-photosynthetic alga Prototheca zopfii incorporate glucose from UDP-[3H]glucose into the trichloroacetic-acid-insoluble fraction and the polysaccharides insoluble in hot alkali. Time course and pulse-chase experiments indicate that the acid-insoluble fraction was a precursor of the alkali-insoluble fraction. Isolation of 3H-labeled membrane or soluble fraction showed that only membrane fractions were able to transfer radioactivity into polysaccharides. Treatment of glucosylated membranes with trypsin or cellulase only partially affect their transfer ability, indicating that the precursor was internalized in vesicles. Analysis of the in vitro synthesized polysaccharides by enzymatic and acid hydrolysis showed that glucose and cellobiose were present as radioactive sugars. Permethylation of the polysaccharide indicates that 80% of the glucose was beta-1,4-bonded with 20% in beta-1,3-linkages. This polysaccharide was found to be identical with the cell-wall beta-glucan obtained in vivo [Rivas, L.A. & Pont Lezica, R. (1978) Planta (Berl.) 165, 348-353].

Incorporation of UDPGlucose into Cell Wall Glucans and Lipids by Intact Cotton Fibers

The [(14)C] moiety from [(3)H]UDP[(14)C]glucose was incorporated by intact cotton fibers into hot water soluble, acetic-nitric reagent soluble and insoluble components, and chloroform-methanol soluble lipids; the [(3)H] UDP moiety was not incorporated. The (3)H-label can be exchanged rapidly with unlabeled substrate in a chase experiment. The cell wall apparent free space of cotton fibers was in the order of 30 picomoles per milligram of dry fibers; 25 picomoles per milligram easily exchanged and about 5 picomoles per milligram more tightly adsorbed. At 50 micromolar UDPglucose, 70% of the [(14)C]glucose was found in the lipid fraction after both a short labeling period and chase. The percent of [(14)C]glucose incorporated into total glucan increased slightly with chase, but the fraction of total glucans incorporated into insoluble acetic-nitric reagent (cellulose) did increase within a 30-minute chase period. The data supports the concept that glucan synthesis, including cellulose, as well as the synthesis of steryl glucosides, acetylated steryl glucosides, and glucosyl-phosphoryl-polyprenol from externally supplied UDPglucose occurs at the plasma membrane-cell wall interface. The synthase enzymes for such synthesis must be part of this interfacial membrane system.

(1-->3)-beta-d-Glucan Synthase from Sugar Beet : I. Isolation and Solubilization

A (1-->3)-beta-glucan synthase has been isolated from petiole tissue of sugar beet (Beta vulgaris L.). Enzyme activity is associated with a membrane fraction with a density of 1.03 grams per cubic centimeter when subjected to isopycnic density gradient centrifugation in Percoll. The reaction product was determined to be a linear (1-->3)-beta-glucan by methylation analysis and by glucanase digestion. (1-->3)-beta-Glucan synthase activity is markedly stimulated by Ca(2+); activation is half-maximal at about 50 micromolar Ca(2+) and is nearly saturated at 100 micromolar. Other divalent cations tested, Mg(2+), Mn(2+), and Sr(2+), also stimulate enzyme activity but are less effective. Enzyme activity was also stimulated up to 12-fold by beta-glucosides. Sirofluor, the fluorochrome from aniline blue, inhibited enzyme activity 95% when included at 1 millimolar. The enzyme was solubilized in Zwittergent 3-14; 85% of total enzyme activity was solubilized in 0.03% detergent and the optimal detergent-to-protein ratio was 0.3 at 3 milligrams per milliliter protein.

Nucleotide effects on glucan-synthesis activities of particulate enzymes from Saprolegnia

Membrane-bound β-1-3- and β-1-4-glucan synthetases of Saprolegnia are affected in vitro by the presence of nucleotides. Both enzymatic activities are inhibited by uridine nucleotides. Guanosine 5'-triphosphate and ATP reduce β-1-3-glucan synthesis but stimulate β-1-4-glucan production; they also increase V max without effect on the K m for uridine 5'-diphosphate glucose. The stimulation by ATP could be the result of an activation or stabilization of the enzymes and might have implications for cell-wall construction during hyphal growth.

Changes in the non-structural carbohydrate content of cotton (Gossypium spp.) fibres at different stages of development

The neutral sugars (glucose, fructose, and sucrose) and the sugar phosphates (glucose 6-phosphate, glucose 1-phosphate and fructose 6-phosphate) soluble in hot aqueous 80% methanol from the fibres of cotton - Gossypium arboreum L., G. barbadense L., and G. hirsutum L. - were determined at various stages of fibre development. In addition, the (1→3)-β-D-glucan content was measured and in the case of G. arboreum the rate of (1→3)-β-D-glucan and cellulose synthesis was determined with [(14)C]sucrose as the precursor. For each of the species a similar chronology was obtained for the changes in content of the various non-structural carbohydrates. At the early stages of secondary wall formation, glucose and fructose exhibited a maximum which was closely followed by a maximum in the (1→3)-β-D-glucan content and in the sugar phosphates. On the other hand, the sucrose content increased regularly until fibre maturity. The rates of synthesis of (1→3)-β-D-glucan and of cellulose were highest following the maximum in the (1→3)-β-D-glucan content, when the latter was being depleted.

Factors Influencing beta-Glucan Synthesis by Particulate Enzymes from Suspension-Cultured Lolium multiflorum Endosperm Cells

Particulate enzymes from suspension-cultured ryegrass (Lolium multiflorum Lam.) endosperm cells incorporated glucosyl residues from UDP-glucose and GDP-glucose into beta-glucans. Three types of beta-glucans were produced from UDP-glucose: 1,3-beta-glucan; 1,4-beta-glucan; and mixed-linkage 1,3;1,4-beta-glucan. As in other systems, relatively more 1,4-beta-glucan was produced from a low (10 micromolar) UDP-glucose concentration, and relatively more 1,3-beta-glucan was produced from a high (1 millimolar) UDP-glucose concentration. However, in ryegrass, 1,3;1,4-beta-glucan represented a major proportion of the products at both low and high UDP-glucose concentrations. The arrangement of linkages in the 1,3;1,4-beta-glucan was different at the two concentrations; at the low UDP-glucose concentration, more sequences of three consecutive 1,4-linkages were produced.The effects of pH, temperature, and metal ion concentrations on incorporation were dependent on the UDP-glucose concentration. At the low UDP-glucose concentration, incorporation into all three types of beta-glucan increased with increasing pH. At the high UDP-glucose concentration, 1,3-beta-glucan was the major product at pH 7 and below; 1,4-beta-glucan synthesis was optimal at pH 8; and synthesis of 1,3;1,4-beta-glucan was greatest above pH 8.With 10 micromolar GDP-glucose as substrate, 1,4-beta-glucan, but no 1,3;1,4-beta-glucan, was produced. Incorporation from either UDP-glucose or GDP-glucose was not influenced by the presence of the other.

Stimulation of membrane-associated polysaccharide synthetases by a membrane potential in developing cotton fibers

Conditions which induce a transmembrane electrical potential, positive with respect to the inside of membrane vesicles, result in a substantial (4-12-fold) stimulation of the activity of membrane-associated β-glucan synthetases in a membrane preparation derived from the developing cotton (Gossypium hirsutum L.) fiber. Induction of electrical potentials which are negative with respect to the inside of the membrane vesicle results in little or no stimulation of β-glucan synthesis. Those products whose synthesis is stimulated are mainly β-1,3-glucan, but there is also a considerable increase in β-1,4-glucan. No α-1,4-glucan (starch) was detected in the reaction products. A transmembrane pH gradient was found to have no effect on β-glucan synthesis. The results indicate that a transmembrane electrical potential can influence, either directly or indirectly, the activity of membrane-associated polysaccharide synthetases.

Cellulose and 1,3-glucan synthesis during the early stages of wall regeneration in soybean protoplasts

Protoplasts isolated from cultured soybean cells (Glycine max (L.) Merr., cv. Mandarin) were used to study polysaccharide biosynthesis during the initial stages of cell wall-regeneration. Within minutes after the protoplasts were transferred to a wall-regeneration medium containing [(14)C]glucose, radioactivity was detected in a product which was chemically characterized as cellulose. The onset and accumulation of radioactivity into cellulose coincided with the appearance fibrils on the surface of protoplasts, as seen under the electron microscope. At these early stages, a variety of polysaccharide-containing polymers other than cellulose were also synthesized. Under conditions where the protoplasts were competent to synthesize cellulose from glucose, uridine diphosphate-[(14)C]glucose and guanosine diphosphate-[(14)C]glucose did not serve as effective substrates for cellulose synthesis. However, substantial amounts of label from uridine diphosphate glucose were incorporated into 1,3-glucan.

Changes in glucan synthetase activity and plasma membrane proteins during encystment of the cellular slime mold Polysphondylium pallidum

The activity of glucan synthetase increased dramatically during encystment of Polysphondylium pallidum cells. The majority of activity was present in purified plasma membranes. Activity, measured as glucose incorporation from UDPG into NaOH-insoluble glucan, increased 30-40 fold in the membranes. Increases in activity within the cells preceded plasma membrane increases and the enzyme appeared to be rapidly transported to the plasma membrane. Intracellular activity was relatively low. When cells were incubated with UDPG and when phloretin was included to inhibit glucose uptake, no NaOH-insoluble glucan was synthesized. Hence, the UDPG-binding site was not exposed at the cell-surface. When the NaOH-insoluble glucan was digested with endo-β-1,4-glucanase the products were cellobiose and glucose. The glucan could also be precipitated from Schweizer's reagent with acetic acid. These results suggest that the glucan contained predominantly β-1,4-linkages and may be cellulose. Experiments with cycloheximide confirmed that protein synthesis was required for encystment. Labeling of cells with [1-(14)C]-acetate showed that the synthesis of certain plasma membrane proteins was developmentally regulated. A number of proteins (e.g., myosin heavy chains and actin) were synthesized during the lag phase and their synthesis was subsequently reduced or ceased altogether. Immediately prior to the commencement of cyst wall formation seven new plasma membrane proteins were synthesized. These proteins were not detected intracellularly, indicating rapid transfer to the plasma membrane. The possible relationship between the seven developmentally regulated proteins and a postulated "multi-enzyme-complex" involved in cellulose synthesis is discussed. Their synthesis may be related to the increase in particles in the outer leaflet of the plasma membrane observed during encystment with freeze-etching (G.W. Erdos and H.R. Hohl, 1980, Cytobios, 29, 7-16).

Protection of cellulose synthesis in detached cotton fibers by polyethylene glycol

Detachment of the cotton fiber cell from the ovule results in loss of over 90% of the in vivo capacity for synthesis of [(14)C]cellulose from [(14)C]glucose. However, over 50% of the capacity for cellulose synthesis in the detached fiber population is protected when polyethylene glycol 4000 is present during detachment and incubation. Radioautography shows that approximately full capacity is restored in about half the fibers, whereas the other half of the population are incapable of cellulose synthesis from supplied glucose. The rate of cellulose synthesis in such fibers has a pH optimum of 6 and the optimum polyethylene glycol 4000 concentration is 0.06 molal (-9 bars). Cellulose synthesis in such detached fibers is synergistically stimulated by Ca(2+) and Mg(2+) and inhibited by K(+).Evidence is presented which indicates that the protection by polyethylene glycol 4000 is due to its ability to promote membrane resealing, which seems to be required for protecting cellulose synthesis in the detached fiber; however, the requirement for membrane integrity is not exclusively involved in the maintainence of an energy generating system for the synthesis. The possibility that a membrane potential may be required for maintaining an active cellulose synthesizing system is discussed.

Glucan synthesis by intact cotton fibres fed with different precursors at the stages of primary and secondary wall formation

Seed clusters of individual locules from fruit capsules of Gossypium arboreum L. with adhering intact fibres were fed with radioactive uridinediphosphoglucose (UDPG), guanosinediphosphoglucose (GDPG), glucose and sucrose. The incorporation into high molecular weight glucans of the fibres was studied. For primary wall fibres, UDPG at 1 mM was by far the best precursor, whereas sucrose was the best precursor for secondary wall fibres. No competition was observed between the incorporation of glucose from UDPG and from sucrose when the two were fed simultaneously to secondary wall fibres, indicating that their metabolic pathways are well separated when they are fed from the apoplast. Inhibitors of respiratory ATP-formation strongly inhibited incorporation of sucrose but not that of UDPG. Sucrose incorporation was studied at five different stages of development of the cotton fibres. At the stage of most intense secondary wall formation the incorporation rate was about 300 times that during primary wall formation (24 days post anthesis (DPA)). Incorporation from 1 mM UDPG or GDPG by secondary wall fibres (35 DPA) was less than twice that of primary wall fibres (22 DPA), indicating that the two sugar nucleotides are not readily used as precursors for secondary wall cellulose when they are fed to the exterior of intact cells. The high molecular weight non-cellulosic glucans formed from UDPG and sucrose at 5 and 1,000 μM were solubilized in strongly alkaline solutions or dimethyl-sulfoxide (DMSO) and were partially characterized by degradation with an exo-β-1,3-glucanase. After feeding for one hour, at most 1/3 of the radioactivity in high molecular weight material was found in cellulose and at least 2/3 in β-1,3-glucan. The proportions varied little for fibres in the age range of 30 to 48 DPA when sucrose was the precursor although the total incorporation varied by a factor of about four. The fact that at all stages of secondary wall formation β-1,3-glucan is synthesized at a very high rate, but that the total amount in the cell wall does not exceed 2% in the later stages of wall formation, can be interpreted in terms of a high turnover of this polysaccharide if it is assumed that wound effects are negligible in the system under study.

Effect of Boron on the Incorporation of Glucose from UDP-Glucose into Cotton Fibers Grown in Vitro

Boron is required for fiber growth and development in cotton ovules cultured in vitro. Incorporation of [(14)C]glucose by such fiber from supplied UDP-[(14)C]glucose into the hot alkali-insoluble fraction is rapid and linear for about 30 minutes. Incorporation of [(14)C]glucose from such substrate by fibers grown in boron-deficient ovule cultures is much less than in the case with fibers from ovules cultured with boron in the medium. Total products (alkali-soluble plus alkali-insoluble fractions) were also greater in fibers from ovules cultured with boron. The fraction insoluble in acetic-nitric reagent was a small part of the total glucans; however, in the boron-sufficient fibers, there was significantly more of this fraction than in fibers from boron-deficient ovule cultures. The hot water-soluble glucose polymers from the labeled fibers had a significant fraction of the total [(14)C]glucose incorporated from UDP-[(14)C]glucose. Both beta-1,4- and beta-1,3- water-soluble polymers were formed in the boron-sufficient fibers, whereas the same water-soluble fraction from the boron-deficient fibers was predominantly beta-1,3-polymers. The incorporation of [(14)C]glucose from GDP-[(14)C]glucose by the fibers attached to the ovules was insignificant.

beta-1,3-Glucan in Developing Cotton Fibers: Structure, Localization, and Relationship of Synthesis to That of Secondary Wall Cellulose

Evidence is presented for the existence of a noncellulosic beta-1,3-glucan in cotton fibers. The glucan can be isolated as distinct fractions of varying solubility. When fibers are homogenized rigorously in aqueous buffer, part of the total beta-1,3-glucan is found as a soluble polymer in homogenates freed of cell walls. The proportion of total beta-1,3-glucan which is found as the soluble polymer varies somewhat as a function of fiber age. The insoluble fraction of the beta-1,3-glucan remains associated with the cell wall fraction. Of this cell wall beta-1,3-glucan, a variable portion can be solubilized by treatment of walls with hot water, a further portion can be solubilized by alkaline extraction of the walls, and 17 to 29% of the glucan remains associated with cellulose even after alkaline extraction. A portion of this glucan can also be removed from the cell walls of intact cotton fibers by digestion with an endo-beta-1,3-glucanase. The glucan fraction which can be isolated as a soluble polymer in homogenates freed of cell walls is not associated with membranous material, and we propose that it represents glucan which is also extracellular but not tightly associated with the cell wall. Enzyme digestion studies indicate that all of the cotton fiber glucan is beta-linked, and methylation analyses and enzyme studies both show that the predominant linkage in the glucan is 1 --> 3. The possibility of some minor branching at C-6 can also be deduced from the methylation analyses. The timing of deposition of the beta-1,3-glucan during fiber development coincides closely with the onset of secondary wall cellulose synthesis. Kinetic studies performed with ovules and fibers cultured in vitro show that incorporation of radioactivity from [(14)C]glucose into beta-1,3-glucan is linear with respect to time almost from the start of the labeling period; however, a lag is observed before incorporation into cellulose becomes linear with time, suggesting that these two different glucans are not polymerized directly from the same substrate pool. Pulse-chase experiments indicate that neither the beta-1,3-glucan nor cellulose exhibits significant turnover after synthesis.

Synthesis of beta-(1-->3)-Glucan from Extracellular Uridine Diphosphate Glucose as a Wound Response in Suspension-cultured Soybean Cells

Soybean (Glycine max) suspension-cultured cells were incubated with 600 micromolar uridine diphosphate [(14)C]glucose, and the incorporation into alkali-insoluble material was studied. When the cells were kept in suspension by shaking on a linear shaker, the incorporation was very low. The incorporation was stimulated 30-fold when the cells were continually resuspended by stirring with a narrow glass rod. The stirring procedure was shown to damage some of the cells, and the incorporation appeared to be a wound response. The alkali-insoluble material formed was a beta-(1-->3)-glucan, and it was synthesized from uridine diphosphate glucose which did not penetrate through the plasma membrane of intact cells. The synthetase activity was probably induced by the stirring procedure. No evidence for cellulose synthesis from extracellular uridine diphosphate glucose was obtained.

Soluble Factors in Pisum Extracts Which Moderate Pisum beta-Glucan Synthetase Activity

Homogenates of growing regions of the pea (Pisum sativum L.) epicotyl contain soluble factors (130,000g supernatant) which alter pea beta-glucan synthetase activity, as assayed using the substrate UDP-glucose and either particulate fractions or tissue slices as source of enzyme. A heat-stable dialyzable component is present which enhances as much as 3-fold the synthesis of alkali-soluble and -insoluble products from millimolar levels of substrate. A heat-labile nondialyzable component is also present which suppresses synthesis. This component dominates (the net effect of total crude extract) when low (mum) levels of substrate are employed. Methylation analysis shows that both components primarily affect the proportion of beta-1,4 rather than beta-1,3 linkages which are synthesized. The enhancing factor increases V(max) of the synthetase system and only activates in the presence of high levels of substrate. The suppressing factor appears to inactivate the synthetase, since losses of product or substrate are not significant during brief incubation with extract, the factor acts progressively with time with a pH optimum, and it destroys activity during preincubation with particles or slices. It co-precipitates with a protease (gelatinase) at between 20% and 40%-saturated (NH(4))(2)SO(4), and it co-fractionates with a major component of total protease on Sephadex gel columns (G-200) with an elution volume corresponding to molecular weight 65,000. The concentrations of these factors are such that they could be natural moderators of synthetase activity in vivo if the two were ever brought in contact, and the inactivator could account for the lability of beta1,4-glucan synthetase which occurs upon tissue homogenization.

Tissue Slice and Particulate beta-Glucan Synthetase Activities from Pisum Epicotyls

beta-Glucan synthetase activity in growing regions of pea (Pisum sativum L.) epicotyls was assayed by supplying UDP-glucose to particulate fractions of tissue homogenates or to thin tissue slices. Particulate fractions are less active in forming alkali-insoluble glucan than slices from the same tissue, although many kinetic characteristics (pH and Mg(2+) optimum, apparent K(m)) are similar for the two systems. Synthesis by tissue slices progresses linearly without lag period for at least an hour and is proportional to cut surface area. It is much more rapid from UDP-glucose than from glucose, glucose-1-P, or sucrose. Tests with plasmolyzing agents and trypsin support the conclusion that synthesis from UDP-glucose by slices occurs at accessible surfaces of cut cells. Analyses of glucan products by GLC of partially methylated and acetylated derivatives and by hydrolysis with various beta-glucanases all show that both beta-1,3 and beta-1,4 linkages are formed by particulate fractions and slices at substrate concentrations ranging from micro- to millimolar. beta-1,4 Linkages predominate at low substrate (5 mum) concentration. Kinetic data indicate that the capacity to synthesize beta-1,3-glucan is substrate-activated, and this product predominates in preparations supplied with high (5 mm) substrate.

Changes in biochemical composition of the cell wall of the cotton fiber during development

The composition of the cell wall of the cotton fiber (Gossypium hirsutum L. Acala SJ-1) has been studied from the early stages of elongation (5 days postanthesis) through the period of secondary wall formation, using cell walls derived both from fibers developing on the plant and from fibers obtained from excised, cultured ovules. The cell wall of the elongating cotton fiber was shown to be a dynamic structure. Expressed as a weight per cent of the total cell wall, cellulose, neutral sugars (rhamnose, fucose, arabinose, mannose, galactose, and noncellulosic glucose), uronic acids, and total protein undergo marked changes in content during the elongation period. As a way of analyzing absolute changes in the walls with time, data have also been expressed as grams component per millimeter of fiber length. Expressed in this way for plant-grown fibers, the data show that the thickness of the cell wall is relatively constant until about 12 days postanthesis; after this time it markedly increases until secondary wall cellulose deposition is completed. Between 12 and 16 days postanthesis increases in all components contribute to total wall increase per millimeter fiber length. The deposition of secondary wall cellulose begins at about 16 days postanthesis (at least 5 days prior to the cessation of elongation) and continues until about 32 days postanthesis. At the time of the onset of secondary wall cellulose deposition, a sharp decline in protein and uronic acid content occurs. The content of some of the individual neutral sugars changes during development, the most prominent change being a large increase in noncellulosic glucose which occurs just prior to the onset of secondary wall cellulose deposition. Methylation analyses indicate that this glucose, at least in part, is 3-linked. In contrast to the neutral sugars, no significant changes in cell wall amino acid composition are observed during fiber development.Compositional analyses of cell walls derived from culture-grown fibers indicate that these walls are remarkably similar to those derived from fibers grown on the plant, both in terms of composition and in terms of relative changes in composition during development.A comparison of our results on total cell wall composition and linkages of sugars as determined by a preliminary methylation analysis of unfractionated fiber walls indicates that the primary cell wall of cotton fibers is similar to that of primary cell walls of other dicotyledons and of gymnosperms as reported in the literature.

UDP-glucose: Glucan Synthetase in Developing Cotton Fibers: II. Structure of the Reaction Product

The solubility properties, composition, and structure of the radioactive product synthesized from UDP-[(14)C]glucose by a highly active cotton fiber glucan synthetase have been determined. Product obtained under the following three different conditions was analyzed: at high and low substrate concentrations by detached fibers, and at high substrate concentrations with an isolated particulate preparation. The results of acetic and nitric acid digestion, enzyme digestion, total acid hydrolyses, periodate oxidation, partial acid hydrolyses, and methylation analyses all support the conclusion that the product of the glucan synthetase produced under all three assay conditions is a linear beta-(1-->3)-glucan.