Pea Xyloglucan and Cellulose

Abstract

Two auxin-induced endo-1,4-β-glucanases (EC 3.2.1.4) were purified from pea (Pisum sativum L. var. Alaska) epicotyls and used to degrade purified pea xyloglucan. Hydrolysis yielded nonasaccharide (glucose/xylose/galactose/fucose, 4:3:1:1) and heptasaccharide (glucose/xylose, 4:3) as the products. The progress of hydrolysis, as monitored viscometrically (with amyloid xyloglucan) and by determination of residual xyloglucan-iodine complex (pea) confirmed that both pea glucanases acted as endohydrolases versus xyloglucan. K_m values for amyloid and pea xyloglucans were approximately the same as those for cellulose derivatives, but V_max values were lower for the xyloglucans. Auxin treatment of epicotyls in vivo resulted in increases in net deposits of xyloglucan and cellulose in spite of a great increase (induction) of endogenous 1,4-β-glucanase activity. However, the average degree of polymerization of the resulting xyloglucan was much lower than in controls, and the amount of soluble xyloglucan increased. When macromolecular complexes of xyloglucan and cellulose (cell wall ghosts) were treated in vitro with pea 1,4-β-glucanase, the xyloglucan component was preferentially hydrolyzed and solubilized. It is concluded that xyloglucan is the main cell wall substrate for pea endo-1,4-β-glucanase in growing tissue.