Toward a Working Model of the Growing Plant Cell Wall

Abstract

The efficient formation of inter-polymeric cross-links by oxidative coupling of the small number of polymer-bound phenolic side-chains present in the non-lignified, growing plant cell wall requires considerable specificity in the reactions concerned. In this paper we summarize some of the evidence that such specificity exists. We suggest that the cell wall is initially assembled by non-covalent interactions, especially involving the hydrogen-bonding of neutral xyloglucan chains to several microfibrils, thereby tethering these microfibrils. Oxidative coupling of other matrix polymers [acidic polysaccharides and/or basic glycoproteins] via their phenolic side-chains is seen as a subsequent wall-modification reaction whereby xyloglucan chains may be strapped to their current microfibrils so that the existing architecture is rendered more nearly permanent. Efficient strapping (i.e., fastening the maximum amount of material with fewest "buckles") requires chemical specificity—the formation of cross-links at appropriate sites. There is great specificity both in the biosynthetic reactions by which phenolic side-chains become attached to the wall polymers and also in the choice of phenolic partners and orientation during coupling.