Determination of the degrees of methylation and acetylation of pectins using a C18 column and internal standards

Biochemical and immunohistochemical analysis of pectic polysaccharides in the cell walls of Arabidopsis mutant QUASIMODO 1 suspension-cultured cells: implications for cell adhesion

Mutation in the Arabidopsis thaliana QUASIMODO 1 gene (QUA1), which encodes a putative glycosyltransferase, reduces cell wall pectin content and cell adhesion. Suspension-cultured calli were generated from roots of wild-type (wt) and qua1-1 A. thaliana plants. The altered cell adhesion phenotype of the qua1-1 plant was also found with its suspension-cultured calli. Cell walls of both wt and qua1-1 calli were analysed by chemical, enzymatic and immunohistochemical techniques in order to assess the role of pectic polysaccharides in the mutant phenotype. Compared with the wt, qua1-1 calli cell walls contained more arabinose (23.6 versus 21.6 mol%), rhamnose (3.1 versus 2.7 mol%), and fucose (1.4 versus 1.2 mol%) and less uronic acid (24.2 versus 27.6 mol%), and they were less methyl-esterified (DM: 22.9% versus 30.3%). When sequential pectin extraction of calli cell walls was performed, qua1-1 water-soluble and chelator-soluble extracts contained more arabinose and less uronic acid than wt. Water-soluble pectins were less methyl-esterified in qua1-1 than in wt. Chelator-soluble pectins were more acetyl-esterified in qua1-1. Differences in the cell wall chemistry of wt and mutant calli were supported by a reduction in JIM7 labelling (methyl-esterified homogalacturonan) of the whole wall in small cells and particularly by a reduced labelling with 2F4 (calcium-associated homogalacturonan) in the middle lamella at tricellular junctions of large qua1-1 cells. Differences in the oligosaccharide profile obtained after endopolygalacturonase degradation of alkali extracts from qua1-1 and wt calli indicated variations in the structure of covalently bonded homogalacturonan. About 29% more extracellular polymers rich in pectins were recovered from the calli culture medium of qua1-1 compared with wt. These results show that perturbation of QUASIMODO 1-1 gene expression in calli resulted in alterations of homogalacturonan content and cell wall location. The consequences of these structural variations are discussed with regard to plant cell adhesion.

Distribution of pectic epitopes in cell walls of the sugar beet root

Immunolabelling techniques with antibodies specific to partially methyl-esterified homogalacturonan (JIM5: unesterified residues flanked by methylesterified residues. JIM7: methyl-esterified residues flanked by unesterified residues), a blockwise de-esterified homogalacturonan (2F4), 1,4-galactan (LM5) and 1,5-arabinan (LM6) were used to map the distribution of pectin motifs in cell walls of sugar beet root (Beta vulgaris). PME and alkali treatments of sections were used in conjunction with JIM5-7 and 2F4. The JIM7 epitope was abundant and equally distributed in all cells. In storage parenchyma, the JIM5 epitope was restricted to some cell junctions and the lining of intercellular spaces while in vascular tissues it occurred at cell junctions in some phloem walls and in xylem derivatives. After secondary wall formation, the JIM5 epitope was restricted to inner cell wall regions between secondary thickenings. The 2F4 epitope was not detected without de-esterification treatment. PME treatments prior to the use of 2F4 indicated that HG at cell corners was not acetylated. The LM5 epitope was mainly present in the cambial zone and when present in storage parenchyma, it was restricted to the wall region closest to the plasma membrane. The LM6 epitope was widely distributed throughout primary walls but was more abundant in bundles than in medullar ray tissue and storage parenchyma. These data show that the occurrence of oligosaccharide motifs of pectic polysaccharides are spatially regulated in sugar beet root cell walls and that the spatial patterns vary between cell types suggesting that structural variants of pectic polymers are involved in the modulation of cell wall properties.

Evidence for in vitro binding of pectin side chains to cellulose

Pectins of varying structures were tested for their ability to interact with cellulose in comparison to the well-known adsorption of xyloglucan. Our results reveal that sugar beet (Beta vulgaris) and potato (Solanum tuberosum) pectins, which are rich in neutral sugar side chains, can bind in vitro to cellulose. The extent of binding varies with respect to the nature and structure of the side chains. Additionally, branched arabinans (Br-Arabinans) or debranched arabinans (Deb-Arabinans; isolated from sugar beet) and galactans (isolated from potato) were shown bind to cellulose microfibrils. The adsorption of Br-Arabinan and galactan was lower than that of Deb-Arabinan. The maximum adsorption affinity of Deb-Arabinan to cellulose was comparable to that of xyloglucan. The study of sugar beet and potato alkali-treated cell walls supports the hypothesis of pectin-cellulose interaction. Natural composites enriched in arabinans or galactans and cellulose were recovered. The binding of pectins to cellulose microfibrils may be of considerable significance in the modeling of primary cell walls of plants as well as in the process of cell wall assembly.

Mapping sugar beet pectin acetylation pattern

Homogalacturonan-derived partly methylated and/or acetylated oligogalacturonates were recovered after enzymatic hydrolysis (endo-polygalacturonase+pectin methyl esterase+side-chain degrading enzymes) of sugar beet pectin followed by anion-exchange and size exclusion chromatography. Around 90% of the GalA and 75% of the acetyl groups present in the initial sugar beet pectin were recovered as homogalacturonan-derived oligogalacturonates, the remaining GalA and acetyl belonging to rhamnogalacturonic regions. Around 50% of the acetyl groups present in sugar beet homogalacturonans were recovered as partly methylated and/or acetylated oligogalacturonates of degree of polymerisation 5 whose structures were determined by electrospray ionization ion trap mass spectrometry (ESI-IT-MSn). 2-O-acetyl- and 3-O-acetyl-GalA were detected in roughly similar amounts but 2,3-di-O-acetylation was absent. Methyl-esterified GalA residues occurred mainly upstream 2-O-acetyl GalA. Oligogalacturonates containing GalA residues that are at once methyl- and acetyl-esterified were recovered in very limited amounts. A tentative mapping of the distribution of acetyl and methyl esters within sugar beet homogalacturonans is proposed. Unsubstituted GalA residues are likely to be present in limited amounts (approximately 10% of total GalA residues), due to the fact that methyl and acetyl groups are assumed to be most often not carried by the same residues.

Gas chromatography–mass spectrometry method for determining the methanol and acetic acid contents of pectin using headspace solid-phase microextraction and stable isotope dilution

A simple, fast, and direct procedure was developed for the simultaneous determination of the methanol and acetic acid present as esters in the plant cell wall polysaccharide pectin. After base-hydrolysis of esters and acidification of pectin samples, headspace solid-phase microextraction (SPME) was performed using a Carboxen-PDMS fiber assembly. Methanol and acetic acid were separated by gas chromatography with a Chrompak PoraPlot Q capillary column and detected using electron impact mass spectrometry with selected ion monitoring. Stable deuterated isotopomers (d3-methanol and d3-acetic acid) were used as internal standards and for constructing calibration curves, providing accurate and absolute quantification of analytes. The methanol and acetic acid contents in 1 mg quantities of fruit and vegetable pectins were readily quantified by this procedure.

A new approach for studying interaction of the polygalacturonase-inhibiting proteins with pectins

A method for determination of the interaction between pectins and proteins was developed using cross-linked polygalacturonic acid (CLPG) as the pectic substrate and polygalacturonase-inhibiting proteins (PGIPs). Defined water-insoluble pectins were prepared by chemical substitutions with acetyl or methoxyl groups on CLPG. In the presence of 0.1 M NaCl, PGIPs fully bound to CLPG but not to cross-linked alginic acid (CLAL), which had a similar pK(a) to CLPG, suggesting that the inhibitor was not simply bound to the substrate by nonspecific electrostatic interaction. Optimum binding of PGIPs to CLPG occurred at pH 2.4 to 4.7. The binding ability of the inhibitor to CLPGs with degree of methylation (DM) of 66% or degree of acetylation (DAc) of 133% was not significantly changed. In contrast, the DM of 82% or 95% decreased the binding. These results indicated that the carboxylic groups of galacturonic acid residues were involved in the recognition of the substrate by PGIPs.

Assignment of acetyl groups to O-2 and/or O-3 of pectic oligogalacturonides using negative electrospray ionization ion trap mass spectrometry

Partially acetylated and methylated oligogalacturonides produced by enzymatic hydrolysis of sugar beet pectin were analysed by negative electrospray ionization ion trap mass spectrometry (ESI-ITMS). The (18)O labelling of the oligomer reducing end allowed the precise assignment of the fragments resulting from glycosidic bond and cross-ring cleavages. The collisional-induced dissociation of the C(i) and Z(j) fragment ions through sequential MS(n) experiments always displayed (0, 2)A-type cross-ring cleavage ions which were related to C(2)H(4)O(2) losses. These (0, 2)A ions appeared to be highly diagnostic ions allowing the precise location of the acetyl groups to the O-2 and/or O-3 of the acetylated galacturonic acid residues.

Application of CP-MAS and liquid-like solid-state NMR experiments for the study of the ripening-associated cell wall changes in tomato

13C and 1H NMR spectra of an ethanol insoluble material (EIM) prepared from the pericarp of mature-green (MG) and red-ripe (RR) tomato fruits were acquired in 'liquid-like' and cross-polarisation with dipolar decoupling and magic angle spinning (CPMAS) conditions using the same triple resonance probe. Such a strategy allowed acquisitions of various NMR experiments aimed at detecting compositional differences as well as distinguishing differences in molecular mobility for various constituent polysaccharides related with the two ripening stages. Increase of the proton dipolar decoupling power levels from 3 to 50-55 kHz during single pulse 13C acquisition, led to more intense signals for pectic and hemicellulosic polysaccharides. This behaviour was interpreted as reflecting motional restrictions of these polysaccharides inside the porous cell wall network. Measurements of the proton rotating frame relaxation times T(1rho) in the 'liquid-like' conditions and of the proton transverse relaxation times T(2) from CPMAS spectra, revealed changes in mobilities for some pectic polysaccharides in relation with ripening, particularly for the H1 and H5 protons of alpha-1,5 arabinan (Ara) side chains of rhamnogalacturonans. These data are discussed in relation with known pectic modifications occurring during ripening and associated with the tomato fruit softening.