Molecular domains of the cellulose/xyloglucan network in the cell walls ofhigher plants

Cellulose and xyloglucan (XG) assemble to form the cellulose/XG network, wh ich is considered to be the dominant load-bearing structure in the growing cell walls of non-graminaceous land plants. We have extended the most commo nly accepted model for the macromolecular organization of XG in this networ k, based on the structural and quantitative analysis of three distinct XG f ractions that can be differentially extracted from the cell walls isolated from etiolated pea stems. Approximately 8% of the dry weight of these cell walls consists of XG that can be solubilized by treatment of the walls with a XG-specific endoglucanase (XEG). This material corresponds to an enzyme- susceptible XG domain, proposed to form the cross-links between cellulose m icrofibrils. Another 10% of the cell wall consists of XG that can be solubi lized by concentrated KOH after XEG treatment. This material constitutes an other XG domain, proposed to be closely associated with the surface of the cellulose microfibrils. An additional 3% of the cell wall consists of XG th at can be solubilized only when the XEG- and KOH-treated cell walls are tre ated with cellulase. This material constitutes a third XG domain, proposed to be entrapped within or between cellulose microfibrils. Analysis of the t hree fractions indicates that metabolism is essentially limited to the enzy me-susceptible domain. These results support the hypothesis that enzyme-cat alyzed modification of XG cross-links in the cellulose/XG network is requir ed for the growth and development of the primary plant cell wall, and demon strate that the structural consequences of these metabolic events can be an alyzed in detail.