Fourier-Transform Raman and Fourier-Transform Infrared Spectroscopy (An Investigation of Five Higher Plant Cell Walls and Their Components)

Infrared and Raman spectra of sequentially extracted primary cell walls and their pectic polymers were obtained from five angiosperm plants. Fourier-transform Raman spectrometry was shown to be a powerful tool for the investigation of primary cell-wall architecture at a molecular level, providing complementary information to that obtained by Fourier-transform infrared microspectroscopy. The use of an extraction procedure using imidazole instead of cyclohexane trans-1,2-N,N,N[prime],N[prime]-diaminotetraacetate allows the extension of the infrared spectral window for data interpretation from 1300 to 800 cm-1, to 2000 to 800 cm-1, and allows us to obtain Raman spectra from extracted cell-wall material. Wall constituents such as pectins, proteins, aromatic phenolics, cellulose, and hemicellulose have characteristic spectral features that can be used to identify and/or fingerprint these polymers without, in most cases, the need for any physical separation. The Gramineae (rice [Oryza sativa], polypogon [Polypogon fugax steud], and sweet corn [Zea mays]) are spectroscopically very different from the nongraminaceous monocotyledon (onion [Allium cepa]) and the dicotyledon (carrot [Daucus carota]); this reflects differences in chemical composition and cross-linking of the walls. The possibility of a taxonomic classification of plant cell walls based on infrared and Raman spectroscopies and the use of spectral fingerprinting for authentication and detection of adulteration of products rich in cell-wall materials are discussed.     

Characterization of the cell-wall polysaccharides of Arabidopsis thaliana leaves.

The cell-wall polysaccharides of Arabidopsis thaliana leaves have been isolated, purified, and characterized. The primary cell walls of all higher plants that have been studied contain cellulose, the three pectic polysaccharides homogalacturonan, rhamnogalacturonan I and rhamnogalacturonan II, the two hemicelluloses xyloglucan and glucuronoarabinoxylan, and structural glycoproteins. The cell walls of Arabidopsis leaves contain each of these components and no others that we could detect, and these cell walls are remarkable in that they are particularly rich in phosphate buffer-soluble polysaccharides (34% of the wall). The pectic polysaccharides of the purified cell walls consist of rhamnogalacturonan I (11%), rhamnogalacturonon II (8%), and homogalacturonan (23%). Xyloglucan (XG) accounts for 20% of the wall, and the oligosaccharide fragments generated from XG by endoglucanase consist of the typical subunits of other higher plant XGs. Glucuronoarabinoxylan (4%), cellulose (14%) and protein (14%) account for the remainder of the wall. Except for the phosphate buffer-soluble pectic polysaccharides, the polysaccharides of Arabidopsis leaf cell walls occur in proportions similar to those of other plants. The structure of the Arabidopsis cell-wall polysaccharides are typical of those of many other plants.     

Structural features of cell-wall polymers of the apple

Cell-wall material was isolated from ripe-apple cortical tissues by sequential extraction with aqueous 1.5% sodium dodecyl sulphate and aqueous 90% methyl sulphoxide. The wall material, which contained ～1% of protein, with proline and hydroxyproline as the preponderant amino acids, was sequentially extracted with water at 80°, oxalate at 80°, m KOH at 1°, and m and 4m KOH at 20°, to leave a residue of α-cellulose, which was associated with an appreciable amount of arabinose-rich pectic material. The depectinated material was also extracted with 6m guanidinium thiocyanate at 20° to solubilise preferentially polysaccharides rich in mannose. The hot-water-soluble pectic substances were richer in arabinose compared with the oxalate-soluble ones and were resolved into five fractions by anion-exchange chromatography. The bulk of the hemicelluloses, which were xyloglucans, were solubilised by 4m KOH. The alkali-soluble hemicellulose polymers were resolved by anion-exchange chromatography into polysaccharides, mainly xyloglucans, arabinoxylan-pectic-xyloglucan, and arabinoxylan-pectic complexes. Small amounts of polysaccharide-protein-polyphenol complexes (where the polysaccharide moieties were arabinoxylans), pectic substances, and xyloglucans were also present. The glycosidic linkages of the above polymers were determined by methylation analysis. The general structural features of the cell-wall polymers are discussed.     

Light-controlled growth of the maize seedling mesocotyl: Mechanical cell-wall changes in the elongation zone and related changes in lignification

Cell extension in the mesocotyl elongation zone (MEZ) of maize ( Zea mays L.) seedlings is inhibited by light. The growth inhibition by blue light in the MEZ was reversible upon transfer to darkness. This experimental system was used for investigating the modification of mechanical cell-wall properties and the role of cell-wall lignification in cell elongation. The occurrence of lignin in the cortex and vascular bundle tissues of the MEZ was demonstrated by the isolation of diagnostic monomers released after thioacidolysis of the cell walls. Concomitantly with the inhibition of growth, blue light induces an increase in cell-wall stiffness (tensile modulus) as well as an increase in extractable lignin in the outer MEZ tissues (cortex+epidermis). Both effects are reversed when growth is resumed in the MEZ in darkness after a period of growth inhibition induced by 3 h light. In the vascular bundle light produces no comparable change in lignin content. Appearance and disappearance of phenylpropanoid material in MEZ cell walls in the light, or in darkness following a brief light treatment, respectively, can be visualized under the fluorescence microscope by characteristic changes in autofluorescence of tissue sections upon excitation with UV radiation. It is concluded from these results that light-induced lignification of primary walls is involved in cell-wall stiffening and thus inhibition of elongation growth in the MEZ of maize seedlings. Resumption of growth upon redarkening may be initiated by wall loosening in the uppermost MEZ region which displaces the lignified cell walls towards the lower mesocotyl region.     

Changes in non-cellulosic cell-wall polysaccharides during the growth of carrot cells in suspension cultures

The cell-wall composition of carrot (Daucus carota L.) cells has been studied during their growth in suspension culture. Pectic and hemicellulosic polymers were fractionated according to molecular size by a Sepharose 4B column. Polyuronides in the pectic fraction were resolved into high- and low-molecular-weight components. The low-molecular-weight polyuronides were relatively free of neutral sugars and showed a marked increase during the growth of the cell wall. Hemicellulosic polysaccharides were of disperse molecular size. As cell expansion proceeded, the contents of glucose and xylose in the high-molecular-weight region increased while those in the low-molecular-weight fraction decreased. Removal of auxin from the medium apparently caused degradation of high-molecular-weight polymers in both the pectic and hemicellulosic fractions.     

Cell-wall synthesis and elongation growth in hypocotyls of Helianthus annuus L.

The relationship between growth and increase in cell-wall material (wall synthesis) was investigated in hypocotyls of sunflower seedlings (Helianthus annuus L.) that were either grown in the dark or irradiated with continuous white light (WL). The peripheral three to four cell layers comprised 30–50% of the entire wall material of the hypocotyl. The increase in wall material during growth in the dark and WL, respectively, was larger in the inner tissues than in the peripheral cell layers. The wall mass per length decreased continuously, indicating that wall thinning occurs during growth of the hypocotyl. When dark-grown seedlings were transfered to WL, a 70% inhibition of growth was observed, but the increase in wall mass was unaffected. Likewise, the composition of the cell walls (cellulose, hemicellulose, pectic substances) was not affected by WL irradiation. Upon transfer of dark-grown seedlings into WL a drastic increase in wall thickness and a concomitant decrease in cell-wall plasticity was measured. The results indicate that cell-wall synthesis and cell elongation are independent processes and that, as a result, WL irradiation of etiolated hypocotyls leads to a thickening and mechanical stiffening of the cell walls.     

Cell-wall polysaccharides and glycoproteins of parenchymatous tissues of runner bean (Phaseolus coccineus).

1. Polymers were solubilized from the cell walls of parenchyma from mature runner-bean pods with minimum degradation by successive extractions with cyclohexane-trans-1,2-diamine-NNN'N'-tetra-acetate (CDTA), Na2CO3 and KOH to leave the alpha-cellulose residue, which contained cross-linked pectic polysaccharides and Hyp-rich glycoproteins. These were solubilized with chlorite/acetic acid and cellulase. The polymers were fractionated by anion-exchange chromatography, and fractions were subjected to methylation analysis. 2. The pectic polysaccharides differed in their ease of extraction, and a small proportion were highly cross-linked. The bulk of the pectic polysaccharides solubilized by CDTA and Na2CO3 were less branched than those solubilized by KOH. There was good evidence that most of the pectic polysaccharides were not degraded during extraction. 3. The protein-containing fractions included Hyp-rich and Hyp-poor glycoproteins associated with easily extractable pectic polysaccharides, Hyp-rich glycoproteins solubilized with 4M-KOH+borate, the bulk of which were not associated with pectic polysaccharides, and highly cross-linked Hyp-rich glycoproteins. 4. Isodityrosine was not detected, suggesting that it does not have a (major) cross-linking role in these walls. Instead, it is suggested that phenolics, presumably linked to C-5 of 3,5-linked Araf residues of Hyp-rich glycoproteins, serve to cross-link some of the polymers. 5. There were two main types of xyloglucan, with different degrees of branching. The bulk of the less branched xyloglucans were solubilized by more-concentrated alkali. The anomeric configurations of the sugars in one of the highly branched xyloglucans were determined by 13C-n.m.r. spectroscopy. 6. The structural features of the cell-wall polymers and complexes are discussed in relation to the structure of the cell walls of parenchyma tissues.     

Analysis of extracellular macromolecules of cultured tobacco cells in comparison with cell-wall macromolecules

Rapidly growing, cultured tobacco cells secreted pectic substances, glycoprotein, and 50% ethanol-soluble polysaccharide into the medium. The extracellular pectic substances lacked rhamnose which was present in the cell-wall pectic substances. The protein moieties of the extracellular and cell-wall glycoproteins were hydroxy. proline-rich and similar in amino-acid composition. The sugar moiety of the cell-wall glycoprotein was similar in monosaccharide composition to that of the combined extracellular glycoprotein plus the 50% ethanol soluble polysaccharide.     

Unbalanced cell-wall synthesis in chloramphenicol-grownRhodotorula glutinis

Rhodotorula glutinis grown in the presence of 500g chloramphenicol per ml of medium has an increase in cell-wall material on a dry weight basis. This increase, from 13.4 to 24.5%, is closely paralleled by an increase in thickness as determined from electron micrographs of isolated cell walls. There is no detectable change in chemical composition of the walls. Vesicular elaborations of the plasmalemma were observed in the chloramphenicol-inhibited cells.We wish to thank Miss Doreen Place for doing the sugar chromatography and Miss Lesley A. Croker for technical assistance.     

Changes in Cell Wall Polysaccharides of Green Bean Pods during Development

The changes in cell wall polysaccharides and selected cell wall-modifying enzymes were studied during the development of green bean (Phaseolus vulgaris L.) pods. An overall increase of cell wall material on a dry-weight basis was observed during pod development. Major changes were detected in the pectic polymers. Young, exponentially growing cell walls contained large amounts of neutral, sugar-rich pectic polymers (rhamnogalacturonan), which were water insoluble and relatively tightly connected to the cell wall. During elongation, more galactose-rich pectic polymers were deposited into the cell wall. In addition, the level of branched rhamnogalacturonan remained constant, while the level of linear homogalacturonan steadily increased. During maturation of the pods, galactose-rich pectic polymers were degraded, while the accumulation of soluble homogalacturonan continued. During senescence there was an increase in the amount of ionically complexed pectins, mainly at the expense of freely soluble pectins. The most abundant of the enzymes tested for was pectin methylesterase. Peroxidase, beta-galactosidase, and alpha-arabinosidase were also detected in appreciable amounts. Polygalacturonase was detected only in very small amounts throughout development. The relationship between endogenous enzyme levels and the properties of cell wall polymers is discussed with respect to cell wall synthesis and degradation.     

Fourier transform infrared microspectroscopy is a new way to look at plant cell walls

Highly reproducible Fourier transform infrared (FTIR) spectra from both single onion (Allium cepa) cell walls and their constituent polymers were obtained under a variety of sampling conditions. The specificity of the chemical extraction sequence used in the preparation of the material was confirmed: pectins only are extracted by cyclohexanediaminetetraacetic acid and sodium carbonate, whereas xyloglucans are extracted by increasing concentrations of potassium hydroxide. There was very little contamination of the first potassium hydroxide extract with residual pectin. The low abundance of both phenolics and protein was also confirmed. The first sodium carbonate extraction almost completely removes esters remaining in the cell wall. We have demonstrated that FTIR spectroscopy can detect large conformational changes in pectic polymers on removal from the cell wall and on drying. FTIR spectroscopy provides a powerful and rapid assay for wall components and putative cross-links by identifying polymers and functional groups nondestructively in muro. The availability of micro-sampling and data acquisition techniques that permit subtraction of the blanket absorption of water make FTIR spectroscopy particularly suitable for studies of cell wall architecture. The use of polarizers with the microscope accessory permits determination of the orientation of particular functional groups with respect to the direction of cell elongation in carrot suspension cells.     

Hydrogen peroxide-mediated cell-wall stiffening in vitro in maize coleoptiles

It has recently been proposed that H₂O₂-dependent peroxidative formation of phenolic cross-links between cell-wall polymers serves as a mechanism for fixing the viscoelastically extended wall structure and thus confers irreversibility to wall extension during cell growth (M. Hohl et al. 1995, Physiol. Plant. 94: 491–498). In the present paper the isolated cell wall (operationally, frozen/thawed maize coleoptile segments) was used as an experimental system to investigate H₂O₂-dependent cell-wall stiffening in vitro. Hydrogen peroxide inhibited elongation growth (in vivo) and decreased cell-wall extensibility (in vitro) in the concentration range of 10–10000 mol·1^–1. In rheological measurements with a constant-load extensiometer the stiffening effect of H₂O₂ could be observed with both relaxed and stressed cell walls. In-vitro cell-wall stiffening was a time-dependent reaction that lasted about 60 min in the presence of saturating concentrations of H₂O₂. The presence of peroxidase in the growth-limiting outer epidermal wall of the coleoptile was shown by histochemical assays. Peroxidase inhibitors (azide, ascorbate) suppressed the wall-stiffening reaction by H₂O₂ in vitro. Hydrogen peroxide induced the accumulation of a fluorescent, insoluble material in the cell walls of living coleoptile segments. These results demonstrate that primary cell walls of a growing plant organ contain all ingredients for the mechanical fortification of the wall structure by H₂O₂-inducible phenolic cross-linking.Supported by Deutsche Forschungsgemeinschaft. I thank Ms. Bärbel Huvermann for expert technical assistance.     

Effect of Abscisic Acid on Cell-Wall Metabolism in Guard Cells of Vicia faba L.

Cell-wall synthesis in guard cells of Vicia faba L. was examinedusing sonicated epidermal strips incubated with [¹⁴C]glucose.The cell walls of the guard cells incorporated [¹⁴C]glucoseat a lower level in the dark than in the light. Stomatal aperturein the epidermal strips was reduced by application of 1 µmabscisic acid (ABA) in the light but not in the dark. The ABAtreatment reduced the incorporation of [¹⁴C]glucose into thecell walls especially in the light. Fractionation of the labeledcell-wall components revealed that ABA inhibited the synthesisof pectic substances and cellulose, but did not affect hemicellulosesynthesis. Microautoradiographs of the cell-wall fraction ofthe epidermal strips showed that a large amount of radioactivitywas distributed at both ends of the guard cells in the absenceof ABA and that removal of pectic substances from the cell-wallfraction resulted in uniform distribution of the radioactivityin the cell walls of the guard cells. These results indicatedthat the synthesis of pectic substances was active at both endsof the guard cells and was inhibited by ABA. Measurement ofspecific activities of neutral sugars in the guard-cell wallsshowed that polymers composed of galactose underwent activeturnover and that synthesis of glucans was inhibited by ABA.These results revealed a strong correlation between the stomatalmovement and the synthesis of pectic substances and cellulosein the guard cells, suggesting that the cell-wall metabolismin the guard cells may play a role in the regulation of stomatalmovement. (Received October 9, 1987; Accepted March 9, 1988)     

Polysaccharide compositions of primary cell walls of the palms Phoenix canariensis and Rhopalostylis sapida

The polysaccharide compositions of unlignified primary cell walls from two species of palms were examined. Cell-wall preparations were isolated from the stem apex, including the pre-emergent leaflets and rachides, of Phoenix canariensis (Canary Island date palm), and from leaflets and rachides dissected from pre-emergent leaves in the stem apex of Rhopalostylis sapida (Nikau palm). The non-cellulosic polysaccharides in the cell-wall preparations from both species had similar monosaccharide compositions, with arabinose and galactose being the predominant neutral monosaccharides, together with large amounts of galacturonic acid. These monosaccharide compositions indicated the presence of large proportions of pectic polysaccharides, including homogalacturonans. This was confirmed by linkage analyses of the cell-wall preparations which showed the presence of large proportions of pectic arabinans, together with pectic galactans and/or Type I arabinogalactans. Evidence for rhamnogalacturonan I and small amounts of rhamnogalacturonan II was also obtained. In addition to pectic polysaccharides, the cell-wall preparations contained smaller amounts of xyloglucans and even smaller amounts of heteroxylans, probably glucuronoarabinoxylans, and glucomannans and/or galactoglucomannans; (1→3,1→4)-β-D-glucans were not present. Although palms (Arecaceae) are commelinoid monocotyledons, the polysaccharide compositions of their primary cell walls resemble those of non-commelinoid monocotyledons and dicotyledons. These compositions contrast with those of primary cell walls of other commelinoid families which have glucuronoarabinoxylans rather than pectic polysaccharides as the major non-cellulosic polysaccharides. The results are discussed in relation to the possible evolution of the composition of primary cell walls of monocotyledons.     

Hydroxyl radical-induced cell-wall loosening in vitro and in vivo: implications for the control of elongation growth

Hydroxyl radicals (OH) are capable of unspecifically cleaving cell-wall polysaccharides in a site-specific reaction. I investigated the hypothesis that cell-wall loosening underlying the elongation growth of plant organs is controlled by apoplastically produced OH attacking load-bearing cell-wall matrix polymers. Isolated cell walls (operationally, frozen/thawed, abraded segments from coleoptiles or hypocotyls, respectively) from maize, cucumber, soybean, sunflower or Scots pine seedlings were pre-loaded with catalytic Cu or Fe ions and then incubated in a mixture of ascorbate + H2O2 for generating OH in the walls. This treatment induced irreversible wall extension (creep) in walls stretched in an extensiometer. The reaction could be promoted by acid pH and inhibited by several OH scavengers. Generation of OH by the same reaction in living coleoptile or hypocotyl segments caused elongation growth. Auxin-induced elongation growth of maize coleoptiles could be inhibited by OH scavengers. Auxin promoted the production of superoxide radicals (O2(-)), an OH precursor, in the growth-controlling outer epidermis of maize coleoptiles. It is concluded that OH fulfils basic criteria for a wall-loosening factor acting in auxin-mediated elongation growth of plant species with widely differing cell-wall polysaccharide compositions.     

Fermentation characteristics of cell-wall sugars from soya bean meal,and from separated endosperm and hulls of soya beans

Two experiments were conducted to investigate the degradation of cell-wall sugars from soya bean meal (in situ), and soya bean endosperm and hulls (in vitro). Soya bean meal, soya bean endosperm, and soya bean hulls were extracted with different chemicals to obtain the cell-wall fraction. Soya bean meal cell walls were incubated in the rumen of a fistulated cow. The individual cell-wall sugars were degraded at different rates: galactose (13.6% h⁻¹), arabinose (7.8% h⁻¹), uronic acids (5.1% h⁻¹), xylose (3.5% h⁻¹) and glucose (3.2% h⁻¹). Microscopic evaluation of the cell walls and degraded material revealed the presence of two cell wall types, with distinctly different degradation characteristics: one originating from the hull (thick, slowly degraded) and one from the endosperm (thin, rapidly degraded). Furthermore, the cell-wall sugar composition of endosperm and hull cell walls was different, most markedly for galactose (281 vs. 12 g kg⁻¹) and glucose (132 vs. 508 g kg⁻¹). The degradation of endosperm and hull cell walls was measured in vitro by use of in vitro cumulative gas production. Degradation rates of the individual cell-wall sugars for hull cell walls were similar (ranging from 2.4% to 4.6% h⁻¹). For endosperm cell walls, the degradation rates of the individual sugars were different but with the same ranking as in the in situ experiment (ranging from 20.9% to 7.0% h⁻¹). It was concluded that for soya bean meal cell walls, the cell-wall sugar degradation pattern is influenced by the presence of two cell-wall types (hull and endosperm cell-wall), which differ in their rate of degradation and sugar composition. The difference in cell-wall sugar degradation pattern between hull and endosperm cell walls is likely to be caused by a combined effect of particle size and cell-wall thickness.     

Ripening-related changes in the cell walls of Spanish pear (Pyrus communis)

Unripe Spanish pears ( Pyras commanis L. ev. Blanquilla ) were ripened at 18°C for 5 and 10 days. Softening of the cortical tissues was associated with swelling of parenchyma cell walls from 1 to more than 5 μm in 10 day ripe pears, by which time the pears were over ripe. However, there was little indication of cell separation and the middle lamella could be detected between most cell walls. Furthermore, cell separation was constrained by regions rich in plasmodesmata where wall swelling was prevented. Parenchyma cells in the 500 μm of tissue underlying the epidermis did not undergo ripening-related changes to the same extent as those of the cortex. These cells, in combination with a sub-epidermal layer of lignified sclereid clusters, constituted a relatively tough and protective skin. Ripening of the cortical tissues was associated with a depletion of alcohol-insoluble pectic polysaccharides, as indicated by the decrease in arabinose and uronic acid. Analysis of alcohol-insoluble cell wall preparations enriched in either parenchyma or sclereid cell walls indicated that this change was predominantly associated with the parenchyma walls. Such changes were less prominent in the peel. The decrease in pectic polysaccharides was accompanied by an increase in their solubility. During ripening, the sclereid clusters of the cortex continued in develop, as indicated by an increase in their size and yield of cell wall xylose and glucose. Cortical parenchyma cells radiating from the sclereids were firmly attached to the lignified cells. This was due to lignification extending from the sclereids into the primary walls of the parenchyma cells. We conclude that dissolution of pectic polysaccharides is one of several factors which determine softening during ripening of Spanish pears.     

Pectic Enzyme Production by Fusarium oxysporum f. sp, ccpac: Induction by Cell Walls from Different Parts of Onion Bulbs at Different Growth Stages

Fusarium oxysporum f. sp. cepae produced significantly different amounts of pectic enzymes when grown on cell walls from morphologically different parts of onion bulbs. Cell walls from stem plate tissue of both tolerant and susceptible onion genotypes allowed a rapid and high production of both exo-polygalacturonase and endo-pectin-frans-eliminase. Bulb scale cell walls from susceptible genotypes induced synthesis of these enzymes at much lower rates and levels, whereas bulb scale cell walls from tolerant genotypes gave poor induction of pectic enzyme synthesis. Leaf sheath cell walls from both susceptible and tolerant genotypes were poor inducers of enzyme synthesis. Enzyme induction by cell walls from leaf sheaths and bulb scales of tolerant genotypes increased dramatically during ageing. Differences in pectic enzyme accumulation on cell walls were not related to fungal growth. These patterns of enzyme induction could help to explain susceptibility or tolerance of bulb scale and leaf sheath tissue of the different genotypes.     

Biological properties of streptococcal cell-wall particles. 3. Dermonecrotic reaction to cell-wall mucopeptides

Abdulla, Essa M. (University of North Carolina School of Medicine, Chapel Hill), and John H. Schwab. Biological properties of streptococcal cell-wall particles. III. Dermonecrotic reaction to cell-wall mucopeptides. J. Bacteriol. 91:374-383. 1966.-Intradermal injection of rabbits and guinea pigs with mucopeptide suspensions produced an acute necrotic lesion which reached maximal severity within 24 hr and gradually subsided with scar formation. Necrosis was evident within 4 hr after injection of 100 mug, and an indurated area (10 x 10 mm) was produced with as little as 5.0 mug. Mucopeptides from six bacterial strains were studied. Comparison of cell walls and derived mucopeptides showed that the acute necrotic lesion tended to be more severe as the residual polysaccharide was decreased. Hyperimmunization with mucopeptide reduced the acute reaction, with evidence of immunological specificity. Incubation with lysozyme also modified the reaction in relation to extent of digestion. Toxicity was related to particle size, since extended sonic vibration decreased activity. Histological sections showed intense accumulations of polymorphonuclear leukocytes, along with altered collagen. A chronic nodular lesion appeared about 7 days after injection of the intact cell-wall fragments. In contrast to the acute necrotic reaction, this lesion was rarely produced by the mucopeptide separated from polysaccharide.     

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.