Alkaline extractability of pectic arabinan and galactan and their mobility in sugar beet and potato cell walls

The organisation of sugar beet and potato cell walls was studied using alkaline extractions following a response surface methodology, simultaneously with solid-state ¹³C NMR spectroscopy. The influence of two extraction parameters: NaOH concentration (0.05, 0.275, 0.5 M) and temperature (40, 65, 90 °C) on the composition (neutral and acidic sugars) of the residues recovered was established. Treatments of increasing harshness progressively washed off non-cellulosic polysaccharides from the cell walls. Alkaline treatments applied to sugar beet cell wall material (SB-CWM) revealed the presence of diverse pectin populations. The existence of distinct pectin populations in potato cell wall material (P-CWM) was less outstanding. Solid-state ¹³C NMR applied to SB-CWM and P-CWM and residues after treatment by 0.275 M NaOH at 65 °C revealed two fractions of pectic arabinan and galactan side chains. One fraction was highly mobile, whereas the other one displayed restricted mobility.     

Cell wall polysaccharide distribution in Sandersonia aurantiaca flowers using immuno-detection

The localization of cell wall polysaccharides of the fused petals of monocotyledonous Sandersonia aurantiaca flowers has been identified using antibodies directed to pectin and xyloglucan epitopes and detection by fluorescence microscopy. Cross sections of the petal tissue were taken from cut flowers in bud and at various stages of maturity and senescence. Patterns of esterification in pectin backbones were identified by JIM5 and 2F4 labelling. Pectic galactan and arabinan side branches were detected by LM5 and LM6, respectively, while fucosylated xyloglucan was identified by CCRC-M1. The labelling patterns highlighted compositional differences between walls of the outer/inner epidermis compared to the spongy parenchyma cells of the interior mesophyll for fucosylated xyloglucan and arabinan. Partially esterified homogalacturonan was present in the junction zones of the outer epidermis and points of contact between cells of the mesophyll, and persisted throughout senescence. Pectic galactans were ubiquitous in the outer and inner epidermal cell walls and walls of the interior mesophyll at flower opening, whereas pectic arabinan was found predominantly in the epidermal cells. Galactan was lost from walls of all cells as flowers began to senesce, while fucosylated xyloglucan appeared to increase over this time. Such differences in the location of polysaccharides and the timing of changes suggest distinct combinations of certain polysaccharides offer mechanical and rheological advantages that may assist with flower opening and senescence.     

In muro fragmentation of the rhamnogalacturonan I backbone in potato (Solanum tuberosum L.) results in a reduction and altered location of the galactan and arabinan side-chains and abnormal periderm development

Rhamnogalacturonan (RG) I is a branched pectic polysaccharide in plant cell walls. Rhamnogalacturonan lyase (eRGL) from Aspergillus aculeatus is able to cleave the RG I backbone at specific sites. Transgenic potato (Solanum tuberosum L.) plants were made by the introduction of the gene encoding eRGL, under the control of the granule-bound starch synthase promoter. The eRGL protein was successfully expressed and translated into an active form, demonstrated by eRGL activity in the tuber extracts. The transgenic plants produced tubers with clear morphological alterations, including radial swelling of the periderm cells and development of intercellular spaces in the cortex. Sugar compositional analysis of the isolated cell walls showed a large reduction in galactosyl and arabinosyl residues in transgenic tubers. Immunocytochemical studies using the LM5 (galactan) and LM6 (arabinan) antibodies also showed a large reduction in galactan and arabinan side-chains of RG I. Most of the remaining LM5 epitopes were located in the expanded middle lamella at cell corners of eRGL tubers, which is in contrast to their normal location in the primary wall of wild type tubers. These data suggest that RG I has an important role in anchoring galactans and arabinans at particular regions in the wall and in normal development of the periderm.     

Developmental changes in guard cell wall structure and pectin composition in the moss Funaria: implications for function and evolution of stomata

Background and Aims

In seed plants, the ability of guard cell walls to move is imparted by pectins. Arabinan rhamnogalacturonan I (RG1) pectins confer flexibility while unesterified homogalacturonan (HG) pectins impart rigidity. Recognized as the first extant plants with stomata, mosses are key to understanding guard cell function and evolution. Moss stomata open and close for only a short period during capsule expansion. This study examines the ultrastructure and pectin composition of guard cell walls during development in Funaria hygrometrica and relates these features to the limited movement of stomata.

Methods

Developing stomata were examined and immunogold-labelled in transmission electron microscopy using monoclonal antibodies to five pectin epitopes: LM19 (unesterified HG), LM20 (esterified HG), LM5 (galactan RG1), LM6 (arabinan RG1) and LM13 (linear arabinan RG1). Labels for pectin type were quantitated and compared across walls and stages on replicated, independent samples.

Key Results

Walls were four times thinner before pore formation than in mature stomata. When stomata opened and closed, guard cell walls were thin and pectinaceous before the striated internal and thickest layer was deposited. Unesterified HG localized strongly in early layers but weakly in the thick internal layer. Labelling was weak for esterified HG, absent for galactan RG1 and strong for arabinan RG1. Linear arabinan RG1 is the only pectin that exclusively labelled guard cell walls. Pectin content decreased but the proportion of HG to arabinans changed only slightly.

Conclusions

This is the first study to demonstrate changes in pectin composition during stomatal development in any plant. Movement of Funaria stomata coincides with capsule expansion before layering of guard cell walls is complete. Changes in wall architecture coupled with a decrease in total pectin may be responsible for the inability of mature stomata to move. Specialization of guard cells in mosses involves the addition of linear arabinans.     

Molecular size and separability features of pea cell wall polysaccharides : implications for models of primary wall structure

Relative molecular size distributions of pectic and hemicellulosic polysaccharides of pea (Pisum sativum cv Alaska) third internode primary walls were determined by gel filtration chromatography. Pectic polyuronides have a peak molecular mass of about 1100 kilodaltons, relative to dextran standards. This peak may be partly an aggregate of smaller molecular units, because demonstrable aggregation occurred when samples were concentrated by evaporation. About 86% of the neutral sugars (mostly arabinose and galactose) in the pectin cofractionate with polyuronide in gel filtration chromatography and diethylaminoethyl-cellulose chromatography and appear to be attached covalently to polyuronide chains, probably as constituents of rhamnogalacturonans. However, at least 60% of the wall's arabinan/galactan is not linked covalently to the bulk of its rhamnogalacturonan, either glycosidically or by ester links, but occurs in the hemicellulose fraction, accompanied by negligible uronic acid, and has a peak molecular mass of about 1000 kilodaltons. Xyloglucan, the other principal hemicellulosic polymer, has a peak molecular mass of about 30 kilodaltons (with a secondary, usually minor, peak of approximately 300 kilodaltons) and is mostly not linked glycosidically either to pectic polyuronides or to arabinogalactan. The relatively narrow molecular mass distributions of these polymers suggest mechanisms of co- or postsynthetic control of hemicellulose chain length by the cell. Although the macromolecular features of the mentioned polymers individually agree generally with those shown in the widely disseminated sycamore cell primary wall model, the matrix polymers seem to be associated mostly noncovalently rather than in the covalently interlinked meshwork postulated by that model. Xyloglucan and arabinan/galactan may form tightly and more loosely bound layers, respectively, around the cellulose microfibrils, the outer layer interacting with pectic rhamnogalacturonans that occupy interstices between the hemicellulose-coated microfibrils.     

Changes in the distribution of cell wall polysaccharides in early fruit pericarp and ovule, from fruit set to early fruit development, in tomato (Solanum lycopersicum)

During fruit development in tomato (Solanum lycopersicum), cell proliferation and rapid cell expansion occur after pollination. Cell wall synthesis, alteration, and degradation play important roles during early fruit formation, but cell wall composition and the extent of cell wall synthesis/degradation are poorly understood. In this study, we used immunolocalization with a range of specific monoclonal antibodies to examine the changes in cell wall composition during early fruit development in tomato. In exploring early fruit development, the ?1 day post-anthesis (DPA) ovary and fruits at 1, 3, and 5 DPA were sampled. Paraffin sections were prepared for staining and immunolabeling. The 5 DPA fruit showed rapid growth in size and an increase in both methyl-esterified pectin and de-methyl-esterified pectin content in the pericarp, suggesting rapid synthesis and de-methyl esterification of pectin during this growth period. Labeling of pectic arabinan with LM6 antibody and galactan with LM5 antibody revealed abundant amounts of both, with unique distribution patterns in the ovule and premature pericarp. These results suggest the presence of rapid pectin metabolism during the early stages of fruit development and indicate a unique distribution of pectic galactan and arabinan within the ovule, where they may be involved in embryogenesis.     

Celery (Apium graveolens) parenchyma cell walls: cell walls with minimal xyloglucan

The primary walls of celery ( Apium graveolens L.) parenchyma cells were isolated and their polysaccharide components characterized by glycosyl linkage analysis, cross-polarization magic-angle spinning solid-state ¹³C nuclear magnetic resonance (CP/MAS ¹³C NMR) and X-ray diffraction. Glycosyl linkage analysis showed that the cell walls consisted of mainly cellulose (43 mol%) and pectic polysaccharides (51 mol%), comprising rhamnogalacturonan (28 mol%), arabinan (12 mol%) and galactan (11 mol%). The amounts of xyloglucan (2 mol%) and xylan (2 mol%) detected in the cell walls were strikingly low. The small amount of xyloglucan present means that it cannot coat the cellulose microfibrils. Solid-state ¹³C NMR signals were consistent with the constituents identified by glycosyl linkage analysis and allowed the walls to be divided into three domains, based on the rigidity of the polymers. Cellulose (rigid) and rhamnogalacturonan (semi-mobile) polymers responded to the CP/MAS ¹³C NMR pulse sequence and were distinguished by differences in proton spin relaxation time constants. The arabinans, the most mobile polymers, responded to single-pulse excitation (SPE), but not CP/MAS ¹³C NMR. From solid-state ¹³C NMR of the cell walls the diameter of the crystalline cellulose microfibrils was determined to be approximately 3 nm while X-ray diffraction of the cell walls gave a value for the diameter of approximately 2 nm.     

Organization of pectic arabinan and galactan side chains in association with cellulose microfibrils in primary cell walls and related models envisaged

The structure of arabinan and galactan domains in association with cellulose microfibrils was investigated using enzymatic and alkali degradation procedures. Sugar beet and potato cell wall residues (called 'natural' composites), rich in pectic neutral sugar side chains and cellulose, as well as 'artificial' composites, created by in vitro adsorption of arabinan and galactan side chains onto primary cell wall cellulose, were studied. These composites were sequentially treated with enzymes specific for pectic side chains and hot alkali. The degradation approach used showed that most of the arabinan and galactan side chains are in strong interaction with cellulose and are not hydrolysed by pectic side chain-degrading enzymes. It seems unlikely that isolated arabinan and galactan chains are able to tether adjacent microfibrils. However, cellulose microfibrils may be tethered by different pectic side chains belonging to the same pectic macromolecule.     

Multi‐scale spatial heterogeneity of pectic rhamnogalacturonan I (RG–I) structural features in tobacco seed endosperm cell walls

Plant cell walls are complex configurations of polysaccharides that fulfil a diversity of roles during plant growth and development. They also provide sets of biomaterials that are widely exploited in food, fibre and fuel applications. The pectic polysaccharides, which comprise approximately a third of primary cell walls, form complex supramolecular structures with distinct glycan domains. Rhamnogalacturonan I (RG–I) is a highly structurally heterogeneous branched glycan domain within the pectic supramolecule that contains rhamnogalacturonan, arabinan and galactan as structural elements. Heterogeneous RG–I polymers are implicated in generating the mechanical properties of cell walls during cell development and plant growth, but are poorly understood in architectural, biochemical and functional terms. Using specific monoclonal antibodies to the three major RG–I structural elements (arabinan, galactan and the rhamnogalacturonan backbone) for in situ analyses and chromatographic detection analyses, the relative occurrences of RG–I structures were studied within a single tissue: the tobacco seed endosperm. The analyses indicate that the features of the RG–I polymer display spatial heterogeneity at the level of the tissue and the level of single cell walls, and also heterogeneity at the biochemical level. This work has implications for understanding RG–I glycan complexity in the context of cell‐wall architectures and in relation to cell‐wall functions in cell and tissue development.     

Simultaneous in vivo truncation of pectic side chains

Despite the wide occurrence of pectin in nature only a few source materials have been used to produce commercial pectins. One of the reasons for this is that many plant species contain pectins with high levels of neutral sugar side chains or that are highly substituted with acetyl or other groups. These modifications often prevent gelation, which has been a major functional requirement of commercial pectins until recently. We have previously shown that modification of pectin is possible through heterologous expression of pectin degrading enzymes in planta. To test the effect of simultaneous modification of the two main neutral pectic side chains in pectic rhamnogalacturonan I (RGI), we constitutively expressed two different enzymes in Arabidopsis thaliana that would either modify the galactan or the arabinan side chains, or both side chains simultaneously. Our analysis showed that the simultaneous truncation of arabinan and galactan side chains is achievable and does not severely affect the growth of Arabidopsis thaliana.     

Pectic epitopes are differentially distributed in the cell walls of potato (Solanum tuberosum) tubers

Six monoclonal antibodies (mAbs) were used to map the distribution of pectic epitopes in the cell walls of potato ( Solanum tuberosum L. cvs Kardal and Karnico) tuber tissue in both light and electron microscopes. Unesterified (mAb JIM 5 epitope) and methyl-esterified (mAb JIM 7 epitope) pectins were abundant and equally distributed in all parenchymal and vascular cell walls. Homogalacturonans (HGAs) involved in Ca²⁺-cross-linking (mAb 2F4 epitope) were localised to the middle lamella and abundant at cell corners. The tuber cortex was densely labelled, but parenchymal cell walls in the perimedullary region contained few epitopes of calcium pectate except at corners and pit fields. In contrast, pectic side-chains were not detectable in the middle lamella of all parenchymal cell walls, except in the cortex where mAb LM6 (arabinan epitope) labelled the entire wall. The galactan epitope (mAb LM5) was localised to a zone very close to the plasmalemma in cortical cell walls and was also less abundant at pit fields and in vascular cell walls. MAb CCRC-M2 (rhamnogalacturonan I epitope) did not cross-react. Our results show that the cell walls of potato tubers are not homogeneous structures and that the pectic composition of the walls is spatially regulated.     

Mobility-resolved 13C-NMR spectroscopy of primary plant cell walls

Primary plant cell walls contain highly hydrated biopolymer networks, whose major chemistry is known but whose relationship to architectural and mechanical properties is poorly understood. Nuclear magnetic resonance spectroscopy has been used to characterize segmental mobilities via relaxation and anisotropy effects in order to add a dynamic element to emerging models for cell wall architecture. For hydrated onion cell wall material, single pulse excitation revealed galactan (pectin side chains), provided that dipolar decoupling was used, and some of the pectin backbone in the additional presence of magic angle spinning. Cross-polarization excitation revealed the remaining pectin backbones, which exhibited greater mobility (contact time dependence, dipolar dephasing) than the cellulose component, whose noncrystalline and crystalline fractions showed no mobility discrimination. ¹HT₂ behavior could be quantitatively interpreted in terms of high resolution observabilities. Mobility-resolved spectroscopy of cell walls from tomato fruit, pea stem, and tobacco leaf showed similar general effects. Nuclear magnetic resonance study of the sequential chemical extraction of onion cell wall material suggests that galactans fill many of the network pores, that extractability of pectins is not dependent on segmental mobility, and that some pectic backbone (and not side chain) is strongly associated with cellulose. Analysis of the state of cellulose in four hydrated cell walls suggests a noncrystalline content of 60–80% and comparable amounts of Iα and Iβ polymorphs in the crystalline fraction. Comparison with micrographs for onion cell walls shows that noncrystalline cellulose does not equate to chains on fibril surfaces, and chemical shifts show that fully solvated cellulose is not a significant component in cell walls. © 1996 John Wiley & Sons, Inc.     

Sugar beet (Beta vulgaris) pectins are covalently cross-linked through diferulic bridges in the cell wall

Arabinan and galactan side chains of sugar beet pectins are esterified by ferulic acid residues that can undergo in vivo oxidative reactions to form dehydrodiferulates. After acid and enzymatic degradation of sugar beet cell walls and fractionation of the solubilized products by hydrophobic interaction chromatography, three dehydrodiferulate-rich fractions were isolated. The structural identification of the different compounds present in these fractions was performed by electrospray-ion trap-mass spectrometry (before and after (18)O labeling) and high-performance anion-exchange chromatography. Several compounds contained solely Ara (terminal or alpha-1-->5-linked-dimer) and dehydrodiferulate. The location of the dehydrodiferulate was assigned in some cases to the O-2 and in others to the O-5 of non-reducing Ara residues. One compound contained Gal (beta-1-->4-linked-dimer), Ara (alpha-1-->5-linked-dimer) and dehydrodiferulate. The location of the dehydrodiferulate was unambiguously assigned to the O-2 of the non-reducing Ara residue and O-6 of the non-reducing Gal residue. These results provide direct evidence that pectic arabinans and galactans are covalently cross-linked (intra- or inter-molecularly) through dehydrodiferulates in sugar beet cell walls. Molecular modeling was used to compute and structurally characterize the low energy conformations of the isolated compounds. Interestingly, the conformations of the dehydrodiferulate-bridged arabinan and galactan fragments selected from an energetic criterion, evidenced very nice agreement with the experimental occurrence of the dehydrodiferulated pectins. The present work combines for the first time intensive mass spectrometry data and molecular modeling to give structural relevance of a molecular cohesion between rhamnogalacturonan fragments.     

The range of mobility of the non‐cellulosic polysaccharides is similar in primary cell walls with different polysaccharide compositions

The molecular mobility of the non‐cellulosic polysaccharides in hydrated primary cell walls of three monocotyledons (Italian ryegrass, pineapple and onion) and one dicotyledon (cabbage) was studied using solid‐state ¹³C NMR spectroscopy. These cell walls were chosen as they have different non‐cellulosic polysaccharide compositions. By exploiting proton rotating‐frame and spin‐spin relaxation time constants three different cell wall domains which responded to cross‐polarization experiments were identified. Most of the non‐cellulosic polysaccharides occupied a mobile domain (C), but some occupied a partly rigid domain (B). Crystalline cellulose occupied a highly rigid domain (A). In the cell walls of Italian ryegrass and pineapple, domain C contained mainly glucuronoarabinoxylans and small amounts of rhamnogalacturonans; domain B contained small amounts of xyloglucans and galacturonans. However, in the cell walls of onion and cabbage, domain C contained mainly rhamnogalacturonans with galactans (in onion) or arabinans (in cabbage) as side chains; domain B contained galacturonans and xyloglucans. Single‐pulse excitation was used on Italian ryegrass and cabbage cell walls to reveal signals from a highly mobile fourth domain (D). In Italian ryegrass cell walls domain D contained glucuronoarabinoxylans and small amounts of rhamnogalacturonan, whereas in cabbage cell walls it contained arabinan side chains of rhamnogalacturonans. A novel feature of the research was the use of solid‐state ¹³C NMR spectroscopy to examine the molecular mobilities of the polysaccharides in monocotyledon cell walls that contain glucuronoarabinoxylans.     

5,6-Anhydro-l-ascorbic acid. A reactive intermediate for the formation of 6-substituted derivatives of l-ascorbic acid

Cell-wall material was isolated from the alcohol-insoluble residue of carrot by treatment with Pronase, phenol—acetic acid—water, and aqueous 90% methyl sulphoxide. Some pectic material was solubilised, but the major component was a highly esterified, acidic arabinogalactan. The purified cell-wall material, which contained ～1% of protein, was sequentially extracted with water at 80°, ammonium oxalate at 80°, and m and 4m KOH at 20°, to leave a residue of α-cellulose, which contained some pectic material. From the hot-water-soluble fraction, a major pectic polymer was isolated by anion-exchange chromatography. Methylation analysis showed that it was a rhamnogalacturonan, probably having highly branched arabinan and slightly branched galactan side-chains linked to O-4 of rhamnopyranosyl residues. An unusual feature of this pectic polymer is that it contained a small but significant proportion of 1,4-linked xylopyranosyl residues. From the alkali-soluble fractions, a range of pectic polymers associated with various amounts of xylans and possibly xyloglucans was isolated. The main linkages present in these complexes were 1,4-linked galactopyranosyluronic acid, 1,4-linked galactopyranosyl, and 1,5-linked arabinofuranosyl residues, terminal arabinofuranosyl and galactopyranosyl groups, and, in some fractions, 1,4-linked xylopyranosyl residues. The possible association of some of these polymers with proteins and phenolics is discussed.     

Secondary cell-wall assembly in flax phloem fibres: role of galactans

Non-lignified fibre cells (named gelatinous fibres) are present in tension wood and the stems of fibre crops (such as flax and hemp). These cells develop a very thick S2 layer within the secondary cell wall, which is characterised by (1) cellulose microfibrils largely parallel to the longitudinal axis of the cell, and (2) a high proportion of galactose-containing polymers among the non-cellulosic polysaccharides. In this review, we focus on the role of these polymers in the assembly of gelatinous fibres of flax. At the different stages of fibre development, we analyse in detail data based on sugar composition, linkages of pectic polymers, and immunolocalisation of the β-(1→4)-galactans. These data indicate that high molecular-mass gelatinous galactans accumulate in specialised Golgi-derived vesicles during fibre cell-wall thickening. They consist of RG-I-like polymers with side chains of β-(1→4)-linked galactose. Most of them are short, but there are also long chains containing up to 28 galactosyl residues. At fibre maturity, two types of cross-linked galactans are identified, a C–L structure that resembles the part of soluble galactan with long side chains and a C–S structure with short chains. Different possibilities for soluble galactan to give rise to C–L and C–S are analysed. In addition, we discuss the prospect for the soluble galactan in preventing the newly formed cellulose chains from completing immediate crystallisation. This leads to a hypothesis that firstly the secretion of soluble galactans plays a role in the axial orientation of cellulose microfibrils, and secondly the remodelling and cross-linking of pectic galactans are linked to the dehydration and the assembly of S2 layer.     

Polypotency of the immunomodulatory effect of pectins

Pectins are the major component of plant cell walls, and they display diverse biological activities including immunomodulation. The pectin macromolecule contains fragments of linear and branched regions of polysaccharides such as homogalacturonan, rhamnogalacturonan-I, xylogalacturonan, and apiogalacturonan. These structural features determine the effect of pectins on the immune system. The backbones of pectic macromolecules have immunosuppressive activity. Pectins containing greater than 80% galacturonic acid residues were found to decrease macrophage activity and inhibit the delayed-type hypersensitivity reaction. Branched galacturonan fragments result in a biphasic immunomodulatory action. The branched region of pectins mediates both increased phagocytosis and antibody production. The fine structure of the galactan, arabinan, and apiogalacturonan side chains determines the stimulating interaction between pectin and immune cells. This review summarizes data regarding the relationship between the structure and immunomodulatory activity of pectins isolated from the plants of the European north of Russia and elucidates the concept of polypotency of pectins in native plant cell walls to both stimulate and suppress the immune response. The possible mechanisms of the immunostimulatory and anti-inflammatory effects of pectins are also discussed.     

A cross-polarization, magic-angle-spinning, 13C-nuclear-magnetic-resonance study of polysaccharides in sugar beet cell walls

Solid-state nuclear magnetic resonance relaxation experiments were used to study the rigidity and spatial proximity of polymers in sugar beet (Beta vulgaris) cell walls. Proton T_1ρ decay and cross-polarization patterns were consistent with the presence of rigid, crystalline cellulose microfibrils with a diameter of approximately 3 nm, mobile pectic galacturonans, and highly mobile arabinans. A direct-polarization, magic-angle-spinning spectrum recorded under conditions adapted to mobile polymers showed only the arabinans, which had a conformation similar to that of beet arabinans in solution. These cell walls contained very small amounts of hemicellulosic polymers such as xyloglucan, xylan, and mannan, and no arabinan or galacturonan fraction closely associated with cellulose microfibrils, as would be expected of hemicelluloses. Cellulose microfibrils in the beet cell walls were stable in the absence of any polysaccharide coating.     

Metabolism of cell wall polysaccharides in cell suspension cultures of Populus alba in relation to cell growth

Changes in the composition of cell walls and extracellular polysaccharides (ECP) were studied during the growth of suspension-cultured Populus alba cells. Three growth phases, namely the cell division phase, cell elongation phase and stationary phase, were distinguished. The active deposition of polysaccharides in cell wall fractions (50 m M Na₂CO₃-, 1 M KOH-, 4 M KOH-soluble and 4 M KOH-insoluble) was observed during the elongation phase. A 50 m M Na₂CO₃-soluble pectic fraction mainly composed of 1,4-linked galactan and arabinan except acidic sugars. The 1,4-linked galactan decreased markedly during elongation. In 1 and 4 M KOH-soluble hemicellulosic fractions, non-cellulosic 1,4-glucan and xyloglucan were observed as major components, respectively. These polysaccharides also decreased during elongation. A large amount of polysaccharides was secreted into the medium as ECP. Neutral sugars were detected predominantly by sugar composition analysis. Acidic sugars, such as galacturonic acid, were less than 12% of total. In this study, active metabolism of pectic polysaccharides in addition to hemicellulosic polysaccharides, especially neutral side chains of pectin, during cell growth, was clarified.     

Cell wall polysaccharides from onions

Onion (Allium cepa) cell walls were fractionated by successive extraction with oxalate-citrate buffer and with alkali. The substantial oxalate-citrate extracted fraction comprised a range of pectic polysaccharides with varying proportions of neutral side-chains. Methylation analysis of the alkali extract indicated that (1,4′)-linked galactans and a substituted xyloglucan were probably major components. Onions thus resemble dicotyledonous plants more than the Gramineae in their cell wall composition.