The architecture of the primary plant cell wall: the role of pectin reconsidered

Abstract

The primary plant cell wall is composed of the polysaccharide classes pectin, hemicellulose and cellulose, and together, all these polysaccharides form a complex network. However, it is still poorly understood how these polysaccharides together build up the complex primary plant cell wall network. This research aimed at a further understanding of the architecture of the plant cell wall, predominantly focussing on pectin and its potential interactions with (hemi)cellulose. To overcome the insoluble nature of hemicellulose, cellulose and part of all pectin in muro, methods for targeted disruption of the cell wall were developed. The residue after sequential alkali-water extractions still contained 31% of all pectin in carrot. Half of this pectin was released by glucanase digestion of the alkali residue, and was proposed to be present in a covalently linked pectin-cellulose complex. The proposed pectin-cellulose complex was not found in tomato and strawberry, making this a unique characteristic of the carrot cell wall. Planetary ball milling was introduced as a suitable tool to enhance extraction yields with limited depolymerisation of cell wall polysaccharides, allowing a more representative characterisation of cell wall polysaccharides including alkali-labile substituents. Milling followed by water extraction increased the extractability to 75% of all initially insoluble uronic acid and 60% of all initially insoluble xylose. It was proposed that a covalently linked pectin-xylan complex was released from the tomato cell wall, containing 18% of solubilised uronic acid, next to 48% of all xylose. For the sources studied in this research, the proposed pectin-xylan complex was a unique feature of the tomato cell wall. Alternatively, planetary ball milling followed by LiCl-DMSO extraction also solubilised cellulose, next to an increased extractability of pectin and hemicellulose. Whereas LiCl-DMSO solubilised pectin with a low HG:RG-I ratio, the HG:RG-I ratio for the subsequent buffer extractions increased by a factor 2-7. Characterisation of both LiCl-DMSO soluble and insoluble fractions showed methyl-esterified and highly acetylated pectin, showing the potential of characterising methyl-esterification and acetylation of both soluble and insoluble pectin by using LiCl-DMSO. For both carrot and strawberry, pectin homogalacturonan regions were found to be highly acetylated. For carrot, also the additional part of water soluble pectin solubilised by heat processing contained highly methyl-esterified, acetylated HG regions. More firmly associated pectin populations were relatively higher in RG-I, for all different extraction procedures used. This was observed for pectin extracted after heat processing, sequential water-alkaline extractions and milling followed by water or LiCl-DMSO extraction. Finally, the distribution of pectin over different pectin domains was discussed, and compared between sources.