Xyloglucan and beta-glucans: origins and destinations

In ultrastructural studies, cellulose microfibrils can be seen to form an integral part of the primary wall framework along with xyloglucan, which is firmly bound to the wall matrix. Callose (1,3-beta-glucan) is not part of the wall proper but is deposited after wounding between wall and plasmamembrane in the periplasmic space. Of these extracellular end products, only xyloglucan has been detected intracellularly. The four enzymes required for xyloglucan assembly have all been found in Golgi and pulse-chase experiments in vivo show that xyloglucan molecules are completed in Golgi before being secreted via dictyosome vesicles. Cellulose and 1,3-beta-glucan, in contrast, both appear to be formed at the cell surface. Plant membrane preparations assayed immediately after isolation can form a small amount of 1,4-beta-linked product from UDP-glucose, which has a relatively low mol. wt equivalent to dextran 70 000. Only a few glucose units appear to be added to an endogenous acceptor, However, plasmamembrane, isolated from protoplasts after a period of digestion in a mixture of hydrolases, shows no capacity to form 1,4-beta-glucan, but instead forms 1,3-beta-glucan with a high mol. wt, The available data are consistent with the possibility that 1,4-beta-glucan synthase is converted to 1,3-beta-glucan synthase in isolated (perturbed) membranes and in wounded tissues. The question of particular current interest is whether such a conversion takes place in a reversible manner, in which event conditions may be found to isolate 1,3-beta-glucan synthase and reconstitute it into a complex that regains the capacity to form 1,4-beta-glucan. This may never be possible, however, if 1,4-beta-glucan linkages are differentially susceptible to decay in the presence of proteases while the capacity to form 1,3-beta-linkages is stimulated. It may be, therefore, that if membranes were isolated under conditions that completely inhibited endogenous proteases, the yield of 1,4-beta-glucan synthase would be maximized and the development of 1,3-beta-glucan synthase activity minimized.