Hemicellulosic polymers of cell walls of zea coleoptiles

Hemicellulosic polymers comprised about 43% of the primary walls of Zea mays L. cv WF9 × Bear 38 coleoptiles; these polymers were separated by an alkali-gradient into three major fractions. Fraction 1 (GAX I) was solubilized from walls with 0.01 to 0.045 n KOH and consisted of novel glucuronoarabino(galacto)xylans. Nearly six of every seven residues of these xylans were substituted predominantly with single arabinosyl sidegroups. Fraction 2 (GAX II), material released by 0.45 to 0.8 n KOH, was also enriched with glucuronoarabinoxylan, but only two of every three xylose residues was substituted. This xylan was similar to those found in Zea and other Graminaceous species. Both of these xylan fractions contained uronic acid, terminal- and 4-linked galactosyl, and small amounts of 2-, 3-, 5-, and 3,5-linked arabinosyl units. Fraction 3 (MG-GAX) was released by 2.0 to 3.0 n KOH and consisted of about 60% mixed-linked glucan and about 40% glucuronoarabinoxylan. This fraction represented about half of the total hemicellulosic material of the primary walls of these coleoptiles.     

Changes in metabolism of cell wall polysaccharides in oat leaves during senescence: relevance to the senescence-promoting effect of methyl jasmonate

Changes in cell wall polysaccharides in oat (Avena sativa L.) leaf segments during senescence promoted by methyl jasmonate (JA-Me) were studied. During the incubation with water at 25 °C in the dark, the loss of chlorophyll of the segments excised from the primary leaves of 8-day-old green seedlings was found dramatically just after leaf excision, and leaf color completely turned to yellow after the 3- to 4-day incubation in the dark. Application of 10 µM JA-Me substantially promoted the loss of chlorophyll corresponding with the chloroplast degradation. Cell wall polysaccharides in oat leaf segments mainly consisted of hemicellulosic and cellulosic ones. During the process of leaf senescence, the amount of hemicellulosic I and II, and cellulosic polysaccharides decreased, but little in pectic polysaccharides. JA-Me significantly enhanced the decrease in cellulosic polysaccharides, but little in hemicellulosic ones. Arabinose, xylose and glucose were identified as main constituents of neutral sugars of hemicellulosic polysaccharides. The neutral sugar compositions of hemicellulosic polysaccharides changed little during leaf senescence both in the presence or absence of JA-Me. These facts suggest that JA-Me affects sugar metabolism relating to cellulosic polysaccharides during leaf senescence.     

Structural investigations on two hemicellulosic polysaccharides from groundnut (Arachis hypogea) seed endosperm

A highly branched xylan and a linear, β-d-(1→4)-linked glucomannan are the two hemicellulosic components isolated from the endosperms of groundnut (Arachis hypogea). Electrophoretic, sedimentation, and sugar analysis indicate the polysaccharides to be fairly homogeneous. The O-methyl derivatives of the polysaccharides were analysed, after reduction and O-acetylation, by gas-liquid chromatography and g.l.c.-mass spectrometry. 2,3,4-Tri-O-methyl-d-xylose (3.6 mol), 2,3-di-O-methyl-d-xylose (21.0 mol), 3-O-methyl-d-xylose (2.8 mol), and d-xylose (4.2 mol) were detected in the xylan, whereas 2,3,4,6-tetra-O-methyl-d-glucose and/or mannose (1.6 mol), 2,3,6-tri-O-methyl-d-mannose (5.6 mol), and 2,3,6-tri O-methyl-d-glucose (21.2 mol) were found in the glucomannan. Periodate and Smith-degradation studies substantiate the results of methylation analysis on the xylan. A glucose: mannose ratio of 3:1 for the glucomannan, however, suggests that this fraction may be an aggregate of true glucomannan and glucan or degraded cellulose.     

Cell wall development in maize coleoptiles

The physical bases for enhancement of growth rates induced by auxin involve changes in cell wall structure. Changes in the chemical composition of the primary walls during maize (Zea mays L. cv WF9 × Bear 38) coleoptile development were examined to provide a framework to study the nature of auxin action. This report documents that the primary walls of maize cells vary markedly depending on developmental state; polymers synthesized and deposited in the primary wall during cell division are substantially different from those formed during cell elongation.

The embryonal coleoptile wall is comprised of mostly glucuronoarabinoxylan (GAX), xyloglucan, and polymers enriched in 5-arabinosyl linkages. During development, both GAX and xyloglucan are synthesized, but the 5-arabinosyls are not. Rapid coleoptile elongation is accompanied by synthesis of a mixed-linked glucan that is nearly absent from the embryonal wall. A GAX highly substituted with mostly terminal arabinofuranosyl units is also synthesized during elongation and, based on pulse-chase studies, exhibits turnover possibly to xylans with less substitution via loss of the arabinosyl and glucuronosyl linkages.

     

Glycanase activities associated with cell walls ofCicer arietinum L. epicotyls

The effect of the proteins extracted with 1 M LiCl from theCicer arietinum etiolated epicotyl cell walls on the degradation of polysaccharides extracted with alkali was studied. The hemicellulosic polysaccharides were fractionated into three fractions extracted with 4 % KOH, 4 % KOH containing 8 M urea, and 24 % KOH. The protein extract showed exo-glycanase activity against all three hemicellulosic fractions whilst endo-glycanase activity was shown mainly against the hemicellulosic polysaccharides extracted with 4% KOH.     

Structural characterization of cell wall polysaccharides from two plant species endemic to central Africa,Fleurya aestuans and Phragmenthera capitata

Fleurya aestuans (Linnaeus) Miquel and Phragmenthera capitata (Spreng) are two plants endemic to central Africa that are used in traditional medicine. However, information on their molecular constituents is lacking. In the present study and as part of our research on the structure/bioactivity relationship of plant cell wall molecules, we investigated the structure of polysaccharides isolated from leaf cell walls of both plant species. To this end, we used sequential extraction of polysaccharides, gas chromatography, matrix assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS) and immuno-dot assays. Our data indicate the presence of both pectin and hemicellulosic polysaccharides in the cell walls of both plants. In particular, cell wall of F. aestuans leaves appears to contain much more pectin than those of P. capitata. Structural analysis of hemicellulosic polysaccharides revealed differences in the structure of xyloglucan isolated from both species. While only the XXXG-type was found in P. capitata, both XXXG and XXGG types were detected in F. aestuans. No arabinosylated subunits were found in any of the xyloglucan isolated from both plant species. In addition, xylan structure with non methylated-α-d-glucuronic acid on side chains was only detected in F. aestuans leaf cell walls. Finally, structural analysis of rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II) shows that unlike RG-II, RG-I is qualitatively different between F. aestuans and P. capitata leaves.     

Quantitative and qualitative changes in cell wall polysaccharides in relation to growth and cell wall loosening in Lactuca sativa hypocotyls

The correlation between hypocotyl elongation, cell wall loosening and changes in cell wall polysaccharides was studied using intact lettuce seedlings grown in the dark or in light together with gibberellic acid (GA) and/or 5-fluorodeoxyuridine (FUDR). The following results were obtained:
1) The production of pectic, hemicellulosic and cellulosic polysaccharides look place in parallel with hypocotyl elongation, which was substantially affected by different growth conditions.
2) The mole percentage sugar composition of pectic and hemicellulosic polysaccharides changed in response to dark, light, GA, or FUDR treatments.
3) The amounts of xylose and glucose in hemicellulosic polysaccharides and those of galactosc, rhumnose and uronic acid in pectic polysaccharides increased in parallel with hypocotyl elongation.
4) Statistical analysis of the quantitative relationship between sugars composing polysaccharides revealed that the uronic acid content changed in parallel with those of rhamnose and galactose in pectic polysaccharides, and the content of xylose varied in parallel with those of fucose and glucose.
5) The content of hemicellulosic polysaccharides was correlated with cell wall loosening represented by a decrease in the minimum stress-relaxation time. Changes in the stress-relaxation time value were correlated with those in the content of araltinose and galactose in hemicellulosic polysaccharides.
Based on these results, the relationship between hypocotyl elongation, changes in cell wall polysaccharides, and cell wall loosening is discussed with respect to the effect of GA and FUDR on hypocotyl elongation.     

Differential Effect of Auxin on Molecular Weight Distributions of Xyloglucans in Cell Walls of Outer and Inner Tissues from Segments of Dark Grown Squash (Cucurbita maxima Duch.) Hypocotyls

Effects of indole-3-acetic acid (IAA) on the mechanical properties of cell walls and structures of cell wall polysaccharides in outer and inner tissues of segments of dark grown squash (Cucurbita maxima Duch.) hypocotyls were investigated. IAA induced the elongation of unpeeled, intact segments, but had no effect on the elongation of peeled segments. IAA induced the cell wall loosening in outer tissues as studied by the stress-relaxation analysis but not in inner tissues. IAA-induced changes in the net sugar content of cell wall fractions in outer and inner tissues were very small. Extracted hemicellulosic xyloglucans derived from outer tissues had a molecular weight about two times as large as in inner tissues, and the molecular weight of xyloglucans in both outer and inner tissues decreased during incubation. IAA substantially accelerated the depolymerization of xyloglucans in outer tissues, while it prevented that in inner tissues. These results suggest that IAA-induced growth in intact segments is due to the cell wall loosening in outer tissues, and that IAA-accelerated depolymerization of hemicellulosic xyloglucans in outer tissues is involved in the cell wall loosening processes.     

Nanoscopic cell-wall architecture of an immunogenic ligand in Candida albicans during antifungal drug treatment

The cell wall of Candida albicans is composed largely of polysaccharides. Here we focus on β-glucan, an immunogenic cell-wall polysaccharide whose surface exposure is often restricted, or “masked,” from immune recognition by Dectin-1 on dendritic cells (DCs) and other innate immune cells. Previous research suggested that the physical presentation geometry of β-glucan might determine whether it can be recognized by Dectin-1. We used direct stochastic optical reconstruction microscopy to explore the fine structure of β-glucan exposed on C. albicans cell walls before and after treatment with the antimycotic drug caspofungin, which alters glucan exposure. Most surface-accessible glucan on C. albicans yeast and hyphae is limited to isolated Dectin-1–binding sites. Caspofungin-induced unmasking caused approximately fourfold to sevenfold increase in total glucan exposure, accompanied by increased phagocytosis efficiency of DCs for unmasked yeasts. Nanoscopic imaging of caspofungin-unmasked C. albicans cell walls revealed that the increase in glucan exposure is due to increased density of glucan exposures and increased multiglucan exposure sizes. These findings reveal that glucan exhibits significant nanostructure, which is a previously unknown physical component of the host–Candida interaction that might change during antifungal chemotherapy and affect innate immune activation.     

Cell Walls of Tobacco Cells and Changes in Composition Associated with Reduced Growth upon Adaptation to Water and Saline Stress

The relative mass of the cell walls of tobacco (Nicotiana tabacum L.) cells adapted to grow in medium containing 30% polyethylene glycol 8000 or 428 millimolar NaCl was reduced to about 50% of that of the walls of unadapted cells. Cellulose synthesis was inhibited substantially in adapted cells. The proportions of total pectin in walls of unadapted and adapted cells were about the same, but substantial amount of uronic acid-rich material from walls of cells adapted to either NaCl or polyethylene glycol was more easily extracted with cold sodium ethylenediamine tetraacetic acid solutions (NM Iraki et al. [1989] Plant Physiol. 91: 39-47). We examined the linkage composition of the pectic and hemicellulosic polysaccharides to ascertain chemical factors that may explain this difference in physical behavior. Adaptation to stress resulted in the formation of a loosely bound shell of polygalacturonic acid and rhamnogalacturonan. Pectins extracted from walls of adapted cells by either cold sodium ethylenediamine tetraacetic acid or hot ammonium oxalate were particularly enriched in rhamnose. Compared to pectins of unadapted cells, rhamnosyl units of the rhamnogalacturonans of adapted cells were more highly substituted with polymers containing arabinose and galactose, but the side groups were of greatly reduced molecular size. Possible functional roles of these modifications in cell wall metabolism related to adaptation to osmotic stress are discussed.     

Biosynthesis of non-cellulosic polysaccharides of plant cell walls

Enzymes that make the polymer backbones of plant cell wall polysaccharides have proven to be recalcitrant to biochemical purification. Availability of mutational genetics and genomic tools paved the way for rapid progress in identifying genes encoding various cell wall glycan synthases. Mutational genetics, the primary tool used in unraveling cellulose biosynthesis, was ineffective in assigning function to any of the hemicellulosic, polymerizing glycan synthases. A combination of comparative genomics and functional expression in a heterologous system allowed identification of various cellulose synthase-like (Csl) sequences as being involved in the formation of β-1,4-mannan, β-1,4-glucan, and mixed-linked glucan. A number of xylose-deficient mutants have led to a variety of genes, none of which thus far possesses the motifs known to be conserved among polymerizing β-glycan synthases. Except for xylan synthase, which appears to be an agglomerate of proteins just like cellulose synthase, Golgi glycan synthases already identified suggest that the catalytic polypeptide by itself is sufficient for enzyme activity, most likely as a homodimer. Several of the Csl genes remain to be assigned a function. The possibility of the involvement of various Csl genes in making more than one product remains.     

Changes in Esterification of the Uronic Acid Groups of Cell Wall Polysaccharides during Elongation of Maize Coleoptiles

Cell walls of grasses have two major polysaccharides that contain uronic acids, the hemicellulosic glucuronoarabinoxylans and the galactosyluronic acid-rich pectins. A technique whereby esterified uronic acid carboxyl groups are reduced selectively to yield their respective 6,6-dideuterio neutral sugars was used to determine the extent of esterification and changes in esterification of these two uronic acids during elongation of maize (Zea mays L.) coleoptiles. The glucosyluronic acids of glucuronoarabinoxylans did not appear to be esterified at any time during coleoptile elongation. The galactosyluronic acids of embryonal coleoptiles were about 65% esterified, but this proportion increased to nearly 80% during the rapid elongation phase before returning to about 60% at the end of elongation. Methyl esters accounted for about two-thirds of the total esterified galacturonic acid in cell walls of unexpanded coleoptiles. The proportion of methyl esters decreased throughout elongation and did not account for the increase in the proportion of esterified galactosyluronic acid units during growth. The results indicate that the galactosyluronic acid units of grass pectic polysaccharides may be converted to other kinds of esters or form ester-like chemical interactions during expansion of the cell wall. Accumulation of novel esters or ester-like interactions is coincident with covalent attachment of polymers containing galactosyluronic acid units to the cell wall.     

Molecular weight distribution of hemicellulosic polysaccharides of the cell wall of tall and dwarf rice cultivars, and the effect of GA3

Growth rate, osmotic potential of the cell, cell wall mechanical properties, sugar composition and molecular weight (MW) distribution of water-soluble hemicellulosic polysaccharides of the second leaf sheath of one tall ( Oryza sativa L. cv. Nihonbare) and two dwarf ( Oryza sativa L. cvs. Tan-ginbozu and Waito C) cultivars of rice were compared. The effect of gibberellic acid (GA₃) on the above-mentioned parameters was also studied using the tall (Nihonbare) and one of the dwarf (Waito C) cultivar. The minimum stress-relaxation time (T₀) was higher in the cell wall from the two dwarfs than in the tall cultivar. Furthermore, in the water-soluble hemicellulosic polysaccharides the mass-average MW of β -glucan was higher and that of arabinoxylan was lower in the tall cultivar than in the dwarf ones. Thus, dwarfism of cvs Tan-ginbnozu and Waito C might be correlated with the different MW distributions of β -glucan and arabinoxylan. GA₃ induces growth in the dwarf Waito C cultivar, decreases the T₀ value of the cell wall, and also decreases the average MW of water-soluble hemicelluloses. Changes in β (1–3)(1–4)glucan or arabinoxylan or in both are proposed as part of the cell wall loosening mechanism induced by gibberellin.     

Uptake and Metabolic Fate of Glucose, Arabinose, and Xylose by Zea mays Coleoptiles in Relation to Cell Wall Synthesis

According to the acid-growth hypothesis, auxin-induced secretion of hydrogen ions activate “wall loosening” enzymes that change the rheological properties of the cell wall. The wall loosening process may yield monosaccharides by the enzymic cleavage of load-bearing polysaccharides. Our study was initiated to determine the metabolic fate of such sugars when released from the major hemicellulosic polysaccharides of the cell walls of Zea mays coleoptiles.     

Mango ripening--chemical and structural characterization of pectic and hemicellulosic polysaccharides

Ripening of mango is characterized by a gradual, but natural softening of the fruit, which is due to progressive depolymerization of pectic and hemicellulosic polysaccharides with significant loss of galactose, arabinose and mannose residues at the ripe stage. Structural characterization employing permethylation followed by GC-MS analysis, IR and ¹³C NMR measurements revealed the major CWS fractions of both unripe and ripe mangoes to be of variable molecular weights and having a 1,4-linked galactan/galacturonan backbone, which is occasionally involved in side chain branches consisting of single residues of galactose and arabinose or oligomeric 1,5-linked arabinofuranose residues linked through 1,3-linkages; whereas the major hemicellulosic fractions of unripe mango to be of xyloglucan-type having 1,4-linked glucan backbone with branching by non-reducing terminal arabinose and xylose residues.     

Structure of the Primary Cell Walls of Suspension-Cultured Rosa glauca Cells: II. Multiple Forms of Xyloglucans

Xyloglucans, characteristic hemicellulosic polysaccharides of plant primary walls, have been isolated from Rosa glauca suspension-cultured cells. The cell wall material was fractionated by two sequences of extraction based on solubilization of the hemicelluloses in alkaline and organic solvent systems, respectively. In both cases, only a part (about 50%) of the total xyloglucan could be extracted, the rest remaining tightly associated with cellulose and necessitating the use of acid to be solubilized. Purification of xyloglucans was effected by formation of a gel in appropriate mixtures of dimethyl sulfoxide and water. Further fractionation could be achieved on a cellulose column eluted with chaotropic solvents. This demonstrated the heterogeneity of xyloglucans in the primary cell walls. Analytical data show that all fractions are constituted with the same sugars: l-arabinose, l-fucose, d-galactose, d-xylose, and d-glucose, but their relative proportions differ, particularly the ratio of glucose to xylose which varies from 1.2 to 2 within the different xyloglucans. The structure of these hemicelluloses was established by methylation analysis and shown to consist of a (1 → 4)-linked glucan backbone which carries substituents on the O-6 of glucose. Here again, the multiple forms of xyloglucans was suggested by the various patterns of substitutions found on the different fractions. The configuration of the linkages were established by ¹³C nuclear magnetic resonance spectroscopy and shown to be β for the glucan backbone, α for the xylosyl and fucosyl substituents, and β for the galactosyl substituents. These configurations agree with the specific rotation of the xyloglucan.     

Hemicellulose biosynthesis

One major component of plant cell walls is a diverse group of polysaccharides, the hemicelluloses. Hemicelluloses constitute roughly one-third of the wall biomass and encompass the heteromannans, xyloglucan, heteroxylans, and mixed-linkage glucan. The fine structure of these polysaccharides, particularly their substitution, varies depending on the plant species and tissue type. The hemicelluloses are used in numerous industrial applications such as food additives as well as in medicinal applications. Their abundance in lignocellulosic feedstocks should not be overlooked, if the utilization of this renewable resource for fuels and other commodity chemicals becomes a reality. Fortunately, our understanding of the biosynthesis of the various hemicelluloses in the plant has increased enormously in recent years mainly through genetic approaches. Taking advantage of this knowledge has led to plant mutants with altered hemicellulosic structures demonstrating the importance of the hemicelluloses in plant growth and development. However, while we are on a solid trajectory in identifying all necessary genes/proteins involved in hemicellulose biosynthesis, future research is required to combine these single components and assemble them to gain a holistic mechanistic understanding of the biosynthesis of this important class of plant cell wall polysaccharides.     

Structure of hemicellulosic polysaccharides of Avena sativa coleoptile cell walls

The glycosidic linkage compositions of intact and, in some cases, enzyme-degraded polysaccharides extracted from the cell walls of oat coleoptiles and subsequently purified have been examined. A major component is shown to be a glucuronoarabinoxylan similar in structure to those described for a variety of other monocots. The noncellulosic glucan component is a β-linked polymer containing both 1,4- and 1,3-linked glucosyl residues in a ratio of 2 to 1. Analysis of the oligosaccharide produced by ‘lichenase’ digestion of this β-glucan suggests that the the 1,3- and 1,4-glucosyl linkages repeat in regular fashion. A small amount of xyloglucan polysaccharides like those described for cell walls of dicots was also detected.     

Sugar composition of pollen grain and pollen tube cell walls

The sugar composition of pollen grain and pollen tube cell walls was studied for Camellia japonica, C. sasanqua, C. sinensis, Tulipa gesneriana and Lilium longiflorum. In all species, the main components of pollen grain walls were arabinose, galactose, glucose and uronic acid. On the other hand, the pollen tube walls consisted mostly of glucose. The pollen tube wall of C. japonica was fractionated into hemicellulose, α-cellulose and pectic substance fractions in yields of 61, 19 and 3 %, respectively. The hemicellulose fraction was composed essentially of glucose. The sugar composition of the pollen tube wall was not influenced by the nature of exogenously supplied sugars. Rapid growth of the pollen tube seemed to correlate with the synthesis of hemicellulosic glucan.