Characterisation of pectin-xylan complexes in tomato primary plant cell walls.

The primary plant cell wall is composed of a complex network of pectin, hemicellulose and cellulose. Potential interactions between these polysaccharides were studied for carrot, tomato and strawberry, with a focus on the role of pectin. The Chelating agent Unextractable Solids (ChUS), the residue after water- and EDTA extraction, was ball milled and subsequently water extracted. For tomato and strawberry, pectin and substantial amounts of hemicellulose were solubilised. Anion exchange chromatography (AEC) showed co-elution of pectin and acetylated glucuronoxylan in tomato, representing 18% of solubilised uronic acid and 48% of solubilised xylose by ball milling from ChUS. The existence of a covalently linked pectin-xylan complex was proposed since xylan co-precipitated with pectin under mild alkali conditions. It was proposed that pectin links with xylan through the RG-I region since degradation of HG did not alter AEC elution patterns for RG-I and xylan, suggesting RG-I - xylan interactions.

Interactions between pectin and cellulose in primary plant cell walls.

To understand the architecture of the plant cell wall, it is of importance to understand both structural characteristics of cell wall polysaccharides and interactions between these polysaccharides. Interactions between polysaccharides were studied in the residue after water and chelating agent extraction by sequential extractions with H2O and alkali. The 6 M alkali residue still represented 31%, 11% and 5% of all GalA present in carrot, tomato and strawberry, respectively, and these pectin populations were assumed to strongly interact with cellulose. Digestion of the carrot 6 M alkali residue by glucanases released ∼27% of the 6 M residue, mainly representing pectin. In tomato and strawberry alkali residues, glucanases were not able to release pectin populations. The ability of glucanases to release pectin populations suggests that the carrot cell wall contains unique, covalent interactions between pectin and cellulose.

Acetylated pectins in raw and heat processed carrots.

Heat processing results in softening of carrots, changing the pectin structure. The effect of heat processing on pectin was studied, showing that the amount of pectin in water soluble solids (WSS) and chelating agent soluble solids (ChSS) increased substantially upon heat processing of the carrots. Pectin in WSS from both unprocessed and heat processed carrot had a degree of methyl-esterification (DM) of ≈60% and a degree of acetylation (DA) of ≈20%. Enzymatic degradation released methyl-esterified galacturonic acid oligomers of degree of polymerisation ≥6 carrying acetyl groups. Mass spectrometry confirmed acetylation in highly methyl-esterified homogalacturonan (HG) regions, next to known rhamnogalacturonan (RG-I) acetylation. ChSS HGs were un-acetylated. RG-I levels of both heat processed carrot WSS and ChSS increased. Digestion of WSS with RG-I degrading enzymes showed that WSS arabinan became more linear upon heat processing resulting in the release of oligosaccharides, while in ChSS galactan became more linear.