Covalent Cross-Links in the Cell Wall.

In current models of the organization of polymers in primary cell walls of plants it is proposed that cellulosic microfibrils are embedded in a matrix of intenvoven noncellulosic polysaccharides and proteins (Talbott and Ray, 1992; Carpita and Gibeaut, 1993). There is good evidence that the microfibril surfaces are coated with noncellulosic polysaccharides such as xyloglucans and, possibly, arabinoxylans and glucomannans that are cellulose-like in their conformations. Further, it is envisaged that a proportion of these polysaccharides are hydrogen bonded to the surfaces of cellulose microfibrils and, by virtue of their length, are able to interact with surfaces of more than one microfibril and so act as an adhesive between them. Within the matrix there are other less welldefined hydrogen-bonding possibilities between component polysaccharides, as well as possibilities for ionic and salt interactions between polysaccharides, proteins, and one another. In meristematic and differentiating cells, walls have to withstand osmotically generated turgor pressures that may reach values around 3 to 10 bar (0.3-1 MPa) (Carpita and Gibeaut, 1993; Cosgrove, 1993). Walls must be constructed so as not to fail under these conditions. The aggregate strength of noncovalent forces between wall polymers appears to make this possible. There is little evidence that covalent cross-linking between wall polymers is necessary to achieve this stability (see Talbott and Ray, 1992, for a discussion). The results with 2,6-dichlorobenzonitle-adapted tomato and tobacco cells in suspension culture indicate that although the cellulose-xyloglucan network in these walls is greatly reduced, the cells remain viable under normal osmotic conditions (Shedletzky et al., 1992). In these adapted cells the integrity of the walls appears to be dependent on increased amounts of Caz+-bridged pectates. The altered walls have a lower tensile strength, but their porosity is the same as for walls of nonadapted cells. The situation is somewhat different in the case of barley, a graminaceous monocotyledon. Here, the adapted cells have elevated glucuronoarabinoxylan and (1+3,1+4)-P-glucan contents and a lowered cellulose content (Shedletzky et al., 1992). In addition, there is an increase in ester-linked phenolic acids, especially PCA, and an increase in polysaccharides released after esterase treatment. Covalent cross-linking could be involved and extension might involve transesterification. Walls must not only resist turgor pressure, they must also allow wall expansion during cell growth. Cosgrove (1993) has discussed a mechanism by which stress relaxation in