Complexity in the spatial localization and length distribution of plant cell‐wall matrix polysaccharides

Distinct polysaccharide fractions can be obtained from onion cell walls by sequential extraction methods. Pectins and xyloglucan molecules from these fractions have been imaged by electron microscopy and their lengths measured. Unexpectedly, instead of a simple distribution of lengths, both classes of matrix polysaccharide showed clear evidence of length periodicity, suggesting block polymer construction using subunits about 30 nm long, each containing a minimum of about sixty backbone sugar residues. These results are discussed in the light of our observation that the average hemicellulose cross‐link between cellulose microfibrils in the wall is also about 30 nm long, and that pectin forms a co‐extensive network with the cellulose/xyloglucan network. Using monoclonal antibodies to pectin we have further shown that even within a thin primary cell wall, containing only a few lamellae, distinct zones of pectin distribution are detectable, thus reinforcing our notion of the importance of domains in wall architecture. The chemical heterogeneity detected amongst pectins indicates that in a primary cell wall each lamella of cellulose microfibrils may be in a unique matrix environment.