The three-dimensional structure of the mega-oligosaccharide rhamnogalacturonan II monomer: a combined molecular modeling and NMR investigation

Polysaccharides from Flos Sophorae Immaturus ameliorates insulin resistance in IR-HepG2 cells by co-regulating signaling pathways of AMPK and IRS-1/PI3K/AKT

Four polysaccharides, named FSIP, FSIP-I, FSIP-II and FSIP-III, were isolated from Flos Sophorae Immaturus. Structure characterization revealed that FSIP-I and FSIP-II were types of AG-II-like polysaccharides while FSIP-III featured a RG-II-like structure with high content of GalpA. In vitro experiments showed that FSIPs upregulated HK and PK activities in glycolysis while downregulated G-6-Pase activities in gluconeogenesis. This increased glucose utilization while decreased the glucose synthesis in IR-HepG2 cells, potentially reducing elevated blood sugar levels induced by excess insulin. In terms of antioxidant system, FSIPs decreased the levels of ROS and MDA, and increased the activities of SOD and CAT, enhancing antioxidant capacity to counteract damage caused by insulin resistance in IR-HepG2 cells. To further explore the mechanism, related genes expressions were analyzed. The results found that FSIPs ameliorated insulin resistance via regulating AMPK and IRS-1/PI3K/AKT signal pathways. In the case of AMPK, glucose can be channeled into oxidative (catabolic) pathway, whereas, in the case of IRS-1/PI3K/AKT, glucose can be stored as glycogen (anabolic). This co-modulation could ameliorate insulin resistance by upregulating the glycolysis and repressing the gluconeogenesis in catabolism, and upregulating the glycogen synthesis in anabolism. Additionally, FSIP-III exhibited better anti-insulin resistance activity, attributed to its high content of GalpA.

Lemon gum: Non-toxic arabinogalactan isolated from Citrus × latifolia with antiproliferative property against human prostate adenocarcinoma cells

Lemon gum (LG) obtained from Citrus × latifolia in Brazil was isolated and characterized. In addition, gum biocompatibility was evaluated in vitro and in vivo by Galleria mellonella and mice model. The cytotoxicity against tumor cells was also evaluated. The ratio of arabinose:galactose: rhamnose:4-OMe-glucuronic acid was 1:0.65:0.06:0.15. Small traces of protein were detected, emphasizing the isolate purity. Molar mass was 8.08 × 10⁵ g/mol, with three different degradation events. LG showed antiproliferative activity against human prostate adenocarcinoma cancer cells, with percentage superior to 50 %. In vivo toxicity models demonstrated that LG is biocompatible polymer, with little difference in the parameters compared to control group. These results demonstrate advance in the study of LG composition and toxicity, indicating a potential for several biomedical and biotechnological future applications.

Multifaceted Computational Modeling in Glycoscience

Glycoscience assembles all the scientific disciplines involved in studying various molecules and macromolecules containing carbohydrates and complex glycans. Such an ensemble involves one of the most extensive sets of molecules in quantity and occurrence since they occur in all microorganisms and higher organisms. Once the compositions and sequences of these molecules are established, the determination of their three-dimensional structural and dynamical features is a step toward understanding the molecular basis underlying their properties and functions. The range of the relevant computational methods capable of addressing such issues is anchored by the specificity of stereoelectronic effects from quantum chemistry to mesoscale modeling throughout molecular dynamics and mechanics and coarse-grained and docking calculations. The Review leads the reader through the detailed presentations of the applications of computational modeling. The illustrations cover carbohydrate-carbohydrate interactions, glycolipids, and N- and O-linked glycans, emphasizing their role in SARS-CoV-2. The presentation continues with the structure of polysaccharides in solution and solid-state and lipopolysaccharides in membranes. The full range of protein-carbohydrate interactions is presented, as exemplified by carbohydrate-active enzymes, transporters, lectins, antibodies, and glycosaminoglycan binding proteins. A final section features a list of 150 tools and databases to help address the many issues of structural glycobioinformatics.

Ascertaining the biochemical function of an essential pectin methylesterase in the gut microbe Bacteroides thetaiotaomicron

Pectins are a major dietary nutrient source for the human gut microbiota. The prominent gut microbe Bacteroides thetaiotaomicron was recently shown to encode the founding member (BT1017) of a new family of pectin methylesterases essential for the metabolism of the complex pectin rhamnogalacturonan-II (RG-II). However, biochemical and structural knowledge of this family is lacking. Here, we showed that BT1017 is critical for the metabolism of an RG-II-derived oligosaccharide ΔBT1017oligoB generated by a BT1017 deletion mutant (ΔBT1017) during growth on carbohydrate extract from apple juice. Structural analyses of ΔBT1017oligoB using a combination of enzymatic, mass spectrometric, and NMR approaches revealed that it is a bimethylated nonaoligosaccharide (GlcA-β1,4-(2-O-Me-Xyl-α1,3)-Fuc-α1,4-(GalA-β1,3)-Rha-α1,3-Api-β1,2-(Araf-α1,3)-(GalA-α1,4)-GalA) containing components of the RG-II backbone and its side chains. We showed that the catalytic module of BT1017 adopts an α/β-hydrolase fold, consisting of a central twisted 10-stranded β-sheet sandwiched by several α-helices. This constitutes a new fold for pectin methylesterases, which are predominantly right-handed β-helical proteins. Bioinformatic analyses revealed that the family is dominated by sequences from prominent genera of the human gut microbiota, including Bacteroides and Prevotella Our re-sults not only highlight the critical role played by this family of enzymes in pectin metabolism but also provide new insights into the molecular basis of the adaptation of B. thetaiotaomicron to the human gut.

(3, 2)D 1H, 13C BIRDr,X-HSQC-TOCSY for NMR structure elucidation of mixtures: application to complex carbohydrates

Overlap of NMR signals is the major cause of difficulties associated with NMR structure elucidation of molecules contained in complex mixtures. A 2D homonuclear correlation spectroscopy in particular suffers from low dispersion of ¹H chemical shifts; larger dispersion of ¹³C chemical shifts is often used to reduce this overlap, while still providing the proton-proton correlation information e.g. in the form of a 2D ¹H, ¹³C HSQC-TOCSY experiment. For this methodology to work, ¹³C chemical shift must be resolved. In case of ¹³C chemical shifts overlap, ¹H chemical shifts can be used to achieve the desired resolution. The proposed (3, 2)D ¹H, ¹³C BIRD^r,X-HSQC-TOCSY experiment achieves this while preserving singlet character of cross peaks in the F₁ dimension. The required high-resolution in the ¹³C dimension is thus retained, while the cross peak overlap occurring in a regular HSQC-TOCSY experiment is eliminated. The method is illustrated on the analysis of a complex carbohydrate mixture obtained by depolymerisation of a fucosylated chondroitin sulfate isolated from the body wall of the sea cucumber Holothuria forskali.

2-Fluoro-L-Fucose Is a Metabolically Incorporated Inhibitor of Plant Cell Wall Polysaccharide Fucosylation

The monosaccharide L-fucose (L-Fuc) is a common component of plant cell wall polysaccharides and other plant glycans, including the hemicellulose xyloglucan, pectic rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II), arabinogalactan proteins, and N-linked glycans. Mutations compromising the biosynthesis of many plant cell wall polysaccharides are lethal, and as a result, small molecule inhibitors of plant cell wall polysaccharide biosynthesis have been developed because these molecules can be applied at defined concentrations and developmental stages. In this study, we characterize novel small molecule inhibitors of plant fucosylation. 2-fluoro-L-fucose (2F-Fuc) analogs caused severe growth phenotypes when applied to Arabidopsis seedlings, including reduced root growth and altered root morphology. These phenotypic defects were dependent upon the L-Fuc salvage pathway enzyme L-Fucose Kinase/ GDP-L-Fucose Pyrophosphorylase (FKGP), suggesting that 2F-Fuc is metabolically converted to the sugar nucleotide GDP-2F-Fuc, which serves as the active inhibitory molecule. The L-Fuc content of cell wall matrix polysaccharides was reduced in plants treated with 2F-Fuc, suggesting that this molecule inhibits the incorporation of L-Fuc into these polysaccharides. Additionally, phenotypic defects induced by 2F-Fuc treatment could be partially relieved by the exogenous application of boric acid, suggesting that 2F-Fuc inhibits RG-II biosynthesis. Overall, the results presented here suggest that 2F-Fuc is a metabolically incorporated inhibitor of plant cellular fucosylation events, and potentially suggest that other 2-fluorinated monosaccharides could serve as useful chemical probes for the inhibition of cell wall polysaccharide biosynthesis.

Recovery and fine structure variability of RGII sub-domains in wine (Vitis vinifera Merlot)

BACKGROUND AND AIMS

Rhamnogalacturonan II (RGII) is a structurally complex pectic sub-domain composed of more than 12 different sugars and 20 different linkages distributed in five side chains along a homogalacturonan backbone. Although RGII has long been described as highly conserved over plant evolution, recent studies have revealed variations in the structure of the polysaccharide. This study examines the fine structure variability of RGII in wine, focusing on the side chains A and B obtained after sequential mild acid hydrolysis. Specifically, this study aims to differentiate intrinsic structural variations in these RGII side chains from structural variations due to acid hydrolysis.

METHODS

RGII from wine (Vitis vinifera Merlot) was sequentially hydrolysed with trifluoroacetic acid (TFA) and the hydrolysis products were separated by anion-exchange chromatography (AEC). AEC fractions or total hydrolysates were analysed by MALDI-TOF mass spectrometry.

KEY RESULTS

The optimal conditions to recover non-degraded side chain B, side chain A and RGII backbone were 0·1 m TFA at 40 °C for 16 h, 0·48 m TFA at 40 °C for 16 h (or 0·1 m TFA at 60 °C for 8 h) and 0·1 m TFA at 60 °C for 16 h, respectively. Side chain B was particularly prone to acid degradation. Side chain A and the RGII GalA backbone were partly degraded by 0·1 m TFA at 80 °C for 1-4 h. AEC allowed separation of side chain B, methyl-esterified side chain A and non-methyl-esterified side chain A. The structure of side chain A and the GalA backbone were highly variable.

CONCLUSIONS

Several modifications to the RGII structure of wine were identified. The observed dearabinosylation and deacetylation were primarily the consequence of acidic treatment, while variation in methyl-esterification, methyl-ether linkages and oxidation reflect natural diversity. The physiological significance of this variability, however, remains to be determined.

Spatial structure of plant cell wall polysaccharides and its functional significance

Plant polysaccharides comprise the major portion of organic matter in the biosphere. The cell wall built on the basis of polysaccharides is the key feature of a plant organism largely determining its biology. All together, around 10 types of polysaccharide backbones, which can be decorated by different substituents giving rise to endless diversity of carbohydrate structures, are present in cell walls of higher plants. Each of the numerous cell types present in plants has cell wall with specific parameters, the features of which mostly arise from the structure of polymeric components. The structure of polysaccharides is not directly encoded by the genome and has variability in many parameters (molecular weight, length, and location of side chains, presence of modifying groups, etc.). The extent of such variability is limited by the "functional fitting" of the polymer, which is largely based on spatial organization of the polysaccharide and its ability to form supramolecular complexes of an appropriate type. Consequently, the carrier of the functional specificity is not the certain molecular structure but the certain type of the molecules having a certain degree of heterogeneity. This review summarizes the data on structural features of plant cell wall polysaccharides, considers formation of supramolecular complexes, gives examples of tissue- and stage-specific polysaccharides and functionally significant carbohydrate-carbohydrate interactions in plant cell wall, and presents approaches to analyze the spatial structure of polysaccharides and their complexes.

Synthesis of apiose-containing oligosaccharide fragments of the plant cell wall: fragments of rhamnogalacturonan-II side chains A and B, and apiogalacturonan

Fragments of pectic polysaccharides rhamnogalacturonan-II (RG-II) and apiogalacturonan were synthesised using p-tolylthio apiofuranoside derivatives as key building blocks. Apiofuranose thioglycosides can be conveniently prepared by cyclization of the corresponding dithioacetals possessing a 2,3-O-isopropylidene group, which is required for preservation of the correct (3R) configuration of the apiofuranose ring. The remarkable stability of this protecting group in apiofuranose derivatives requires its replacement with a more reactive protecting group, such as a benzylidene acetal which was used in the synthesis of trisaccharide β-Rhap-(1→3')-β-Apif-(1→2)-α-GalAp-OMe. The X-ray crystal structure of the protected precursor of this trisaccharide has been elucidated.

Influence of boron on carrot cell wall structure and its resistance to fracture

Plant cell wall structure integrity and associated tissue mechanical properties is one of key determinants for the perceived texture of plant-based foods. Carrots (Daucus carota) were used to investigate the effect of mineral supply of boron (B) and/or calcium (Ca), during plant growth, on the plant cell wall structure and mechanical properties of matured root tissues. Five commercial cultivars of carrots, Kuroda (orange), Dragon Purple, Kuttiger White, Yellow, and Nutri-Red, were cultivated under controlled glasshouse conditions over two seasons. Significant increases in the accumulation of B and Ca were found for all cultivars of carrots when additional B and Ca were included in the nutrient feeding solutions throughout the plant growth period. Elevated levels of B in carrot root tissue reduced the uptake of Ca and other mineral nutrients and enhanced plant cell wall structural integrity, its resistance to fracture, and the weight and size (both diameter and length) of carrots. Although higher amounts of Ca were accumulated in the plant materials, the additional supply of Ca did not have a significant effect on the mechanical properties of mature plant tissues or on the uptake of B by the plant. The results suggest that B cross-linking of pectin (rhamnogalacturonan II) has a greater influence on mature tissue mechanical properties than Ca cross-linking of pectin (homogalacturonan) when supplied during plant growth.

Structural characterisation of the pectic polysaccharide rhamnogalacturonan II using an acidic fingerprinting methodology

Rhamnogalacturonan II (RG-II) is a structurally complex cell wall pectic polysaccharide. Despite its complexity, both the structure of RG-II and its ability to dimerise via a borate diester are conserved in vascular plants suggesting that RG-II has a fundamental role in primary cell wall organisation and function. The selection and analysis of new mutants affected in RG-II formation represents a promising strategy to unravel these functions and to identify genes encoding enzymes involved in RG-II biosynthesis. In this paper, a novel fingerprinting strategy is described for the screening of RG-II mutants based on the mild acid hydrolysis of RG-II coupled to the analysis of the resulting fragments by mass spectrometry. This methodology was developed using RG-II fractions isolated from citrus pectins and then validated for RG-II isolated from the Arabidopsis mur1 mutant and irx10 irx10-like double mutant.

The structure, function, and biosynthesis of plant cell wall pectic polysaccharides

Plant cell walls consist of carbohydrate, protein, and aromatic compounds and are essential to the proper growth and development of plants. The carbohydrate components make up approximately 90% of the primary wall, and are critical to wall function. There is a diversity of polysaccharides that make up the wall and that are classified as one of three types: cellulose, hemicellulose, or pectin. The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, are a class of molecules defined by the presence of galacturonic acid. The pectic polysaccharides include the galacturonans (homogalacturonan, substituted galacturonans, and RG-II) and rhamnogalacturonan-I. Galacturonans have a backbone that consists of alpha-1,4-linked galacturonic acid. The identification of glycosyltransferases involved in pectin synthesis is essential to the study of cell wall function in plant growth and development and for maximizing the value and use of plant polysaccharides in industry and human health. A detailed synopsis of the existing literature on pectin structure, function, and biosynthesis is presented.

The ins and outs of plant cell walls

New findings reveal that many membrane proteins undergo regulated trafficking between intracellular compartments and the plasma membrane. This also appears to be a common regulatory mechanism in the control of cell wall metabolism.

Arabidopsis thaliana RGXT1 and RGXT2 encode Golgi-localized (1,3)-alpha-D-xylosyltransferases involved in the synthesis of pectic rhamnogalacturonan-II

Two homologous plant-specific Arabidopsis thaliana genes, RGXT1 and RGXT2, belong to a new family of glycosyltransferases (CAZy GT-family-77) and encode cell wall (1,3)-alpha-d-xylosyltransferases. The deduced amino acid sequences contain single transmembrane domains near the N terminus, indicative of a type II membrane protein structure. Soluble secreted forms of the corresponding proteins expressed in insect cells showed xylosyltransferase activity, transferring d-xylose from UDP-alpha-d-xylose to l-fucose. The disaccharide product was hydrolyzed by alpha-xylosidase, whereas no reaction was catalyzed by beta-xylosidase. Furthermore, the regio- and stereochemistry of the methyl xylosyl-fucoside was determined by nuclear magnetic resonance to be an alpha-(1,3) linkage, demonstrating the isolated glycosyltransferases to be (1,3)-alpha-d-xylosyltransferases. This particular linkage is only known in rhamnogalacturonan-II, a complex polysaccharide essential to vascular plants, and is conserved across higher plant families. Rhamnogalacturonan-II isolated from both RGXT1 and RGXT2 T-DNA insertional mutants functioned as specific acceptor molecules in the xylosyltransferase assay. Expression of RGXT1- and RGXT2-enhanced green fluorescent protein constructs in Arabidopsis revealed that both fusion proteins were targeted to a Brefeldin A-sensitive compartment and also colocalized with the Golgi marker dye BODIPY TR ceramide, consistent with targeting to the Golgi apparatus. Taken together, these results suggest that RGXT1 and RGXT2 encode Golgi-localized (1,3)-alpha-d-xylosyltransferases involved in the biosynthesis of pectic rhamnogalacturonan-II.

A complement fixing polysaccharide from Biophytum petersianum Klotzsch, a medicinal plant from Mali, West Africa

Biophytum petersianum Klotzsch (syn. Biophytum sensitivum (L.) DC) is a medicinal plant having a traditional use, among others, as a wound healing remedy in Mali and other countries. As a water extract of the aerial parts of the plant is a frequently used preparation, we decided to look for a bioactive polysaccharide in this extract. One of the obtained polysaccharide fractions, BP100 III, isolated from a 100 degrees C water extract from the aerial parts of B. petersianum and having a monosaccharide composition typical for pectic substances, was shown to exhibit potent dose-dependent complement fixating activity. The BP100 III fraction was subjected to degradation by endo-alpha-d-(1-->4)-polygalacturonase, and three fractions were obtained by gel filtration. The highest molecular weight fraction, BP100 III.1, had a more potent activity in the complement test system than the native polymer, while the two lower molecular weight fractions were less active than the native polymer. The major part of BP100 III.1 consists of galacturonic acid and rhamnose, with branches being present on both the rhamnose and galacturonic acid residues. Arabinogalactan type II is also present in the polymer, indicating that BP100 III.1 has a structure typical of the hairy region of pectins. The major part of the two other fractions is a galacturonan, containing a strikingly high number of branch points, some to which xylose is attached. These results indicate that the pectic substance in B. petersianum contains both rhamnogalacturonan and xylogalacturonan regions.

Prospects for glycoinformatics

High-throughput and automated techniques (mass spectrometry in particular) allow increasingly rapid structural analysis of complex glycans. Information concerning the primary structure (composition, sequence and linkages), three-dimensional structure (including dynamics) and interactions of glycans is now available in sufficient quantity to justify the maintenance of databases and search facilities. Several such resources (both commercial and open access) are now available as web tools. To derive the full value of glycan databases, it will be necessary to develop a universally accepted machine-readable structural representation of glycans.

Synthesis of the Branched-Chain Sugar Aceric Acid: A Unique Component of the Pectic Polysaccharide Rhamnogalacturonan-II

Described herein is the synthesis of 3-C-carboxy-5-deoxy-L-xylose (aceric acid), a rare branched-chain sugar found in the complex pectic polysaccharide rhamnogalacturonan-II. The key synthetic step in the construction of aceric acid was the stereoselective addition of 2-trimethylsilyl thiazole to 5-deoxy-1,2-O-isopropylidene-alpha-L-erythro-pentofuran-3-ulose (2), which was prepared from L-xylose. The thiazole group was efficiently converted into the required carboxyl group via conventional transformations. Aceric acid was also synthesized by dihydroxylation of a 3-C-methylene derivative of 2 followed by oxidation of the resulting hydroxylmethyl group. The C-2 epimer of aceric acid was also synthesized using thiazole addition chemistry, starting from L-arabinose.

Synthesis of a 2,3,4-Triglycosylated Rhamnoside Fragment of Rhamnogalacturonan-II Side Chain A Using a Late Stage Oxidation Approach

Pectic polysaccharide RG-II, a key component of plant primary cell walls, is known to exist as a dimer formed by means of borate diester cross-links between apiosyl residues of one of its constituent side-chain oligosaccharides. Described herein is the strategy for the synthesis of the branched tetrasaccharide alpha-d-GalA-(1-->2)-[beta-D-GalA-(1-->3)]-[alpha-L-Fuc-(1-->4)]-alpha-L-Rha-OMe, an RG-II fragment that is linked to the apiosyl residue that is thought to be responsible for the borate complexation in RG-II dimer. Iterative glycosylation of the rhamnoside acceptors derived from the key 2,3-orthoacetate of methyl 4-O-methoxybenzyl-alpha-d-rhamnopyranoside afforded the protected tetrasaccharide. The target dicarboxylic acid saccharide was subsequently prepared by removal of protecting groups followed by TEMPO-mediated oxidation of galactopyranosyl residues to galactopyranosyluronic acids.

Synthesis of a disaccharide fragment of rhamnogalacturonan II

A disaccharide portion of the A-side chain of the rhamnogalacturonan II oligosaccharide has been prepared. Glycosylation of methyl (methyl 3,4-O-isopropylidene-alpha-D-galactopyranosid)uronate with p-tolyl 2,3-di-O-acetyl-3-C-(benzyloxymethyl)-1-thio-alpha/beta-D-erythrofuranoside was carried out using N-iodosuccinimide as promoter and silver trifluoromethanesulfonate as catalyst. Removal of the protecting groups gave the beta-d-Apif-(1-->2)-alpha-D-GalpA-OMe disaccharide.

Characterization of Metal Binding Properties of Rhamnogalacturonan II from Plant Cell Walls by Size-Exclusion HPLC/ICP-MS

The binding properties of metal ions to a pectic polysaccharide, rhamnogalacturonan II (RG-II), from plant cell walls were analyzed by size-exclusion HPLC/ICP-MS. The dimeric RG-II borate complex (dRG-II-B) from sugar beet and red wine contained 0.8 - 1.1 mol/mol of B, 0.8 - 1.1 mol/mol of Ca, 0.1 - 0.3 mol/mol of Sr, and 0.03 - 0.07 mol/mol of Ba. The treatment of dRG-II-B with Sr2+, Ba2+, Pb2+ or La3+ exchanged the originally present Ca, Sr and Ba. In contrast, monomeric RG-II (mRG-II), which contained approximately 0.1 mol/mol of Ca, formed complexes with La3+, Eu3+, and Lu3+ added to the solution, but did not do so with Ca2+, Sr2+, Ba2+ and Pb2+. The HPLC/ICP-MS and HPLC/RI (refractive index detector) analysis of the partially hydrolyzed mRG-II that was treated with La3+ indicated that the side chains and backbone of mRG-II together form a lanthanoid binding site.

Synthesis of an apiose-containing disaccharide fragment of rhamnogalacturonan-II and some analogues

Beta-rhamnosylation of methyl 2-C-hydroxymethyl-2,3-O-isopropylidene-beta-D-erythrofuranoside and methyl 2,3-O-isopropylidene-beta-D-ribofuranoside was achieved using 4-O-acetyl-2,3-O-carbonyl-alpha-L-rhamnopyranosyl bromide and Ag2O as a promoter. Deprotected disaccharides beta-L-Rhap-(1-->3')-beta-D-Apif-OMe and beta-L-Rhap-(1-->3')-beta-D-Ribf-OMe were compared to their alpha-rhamnosyl isomers which were prepared using conventional Helferich glycosylation.

Rhamnogalacturonan II: structure and function of a borate cross-linked cell wall pectic polysaccharide

Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide that was first identified in 1978 as a quantitatively minor component of suspension-cultured sycamore cell walls. Subsequent studies have shown that RG-II is present in the primary walls of angiosperms, gymnosperms, lycophytes, and pteridophytes and that its glycosyl sequence is conserved in all vascular plants examined to date. This is remarkable because RG-II is composed of at least 12 different glycosyl residues linked together by more than 20 different glycosidic linkages. However, only a few of the genes and proteins required for RG-II biosynthesis have been identified. The demonstration that RG-II exists in primary walls as a dimer that is covalently cross-linked by a borate diester was a major advance in our understanding of the structure and function of this pectic polysaccharide. Dimer formation results in the cross-linking of the two homogalacturonan chains upon which the RG-II molecules are constructed and is required for the formation of a three-dimensional pectic network in muro. This network contributes to the mechanical properties of the primary wall and is required for normal plant growth and development. Indeed, changes in wall properties that result from decreased borate cross-linking of pectin may lead to many of the symptoms associated with boron deficiency in plants.