Endo-1,4-beta-glucanase, xyloglucanase, and xyloglucan endo-transglycosylase activities versus potential substrates in ripening tomatoes

In ripening fruits of tomato (Lycopersicon esculentum L. var 83-G-38), the amounts of cellulose and xyloglucan (XG) remained constant during tissue softening, but the relative molecular weight [M(r)] of XG decreased markedly and the M(r) of cellulose declined slightly. These changes could have been due to activities of nonspecific endo-1,4-beta-glucanases and/or buffer-soluble XG endo-transglycosylase both of which increased when tissue firmness declined most rapidly. Tomato extracts also reduced the viscosity of XG solutions, especially in the presence of added XG oligosaccharides. This depolymerizing (XGase) capacity differed from beta-glucanase and XG transglycosylase activity (a) by being almost entirely buffer insoluble, and (b) by declining precipitously during fruit softening. Although it disappeared from ripe fruit, XGase may have functioned in promoting wall loosening at earlier stages of fruit development when its activity was highest. By contrast, during aging of fruit in the ripening-inhibited mutant rin there was no change in M(r) of XG or cellulose, and activities of beta-glucanases and XG transglycosylase were lower than in wild-type tomato. Nevertheless, some softening of the fruit did take place over time and XG amounts declined, possibly because high XGase activity was maintained in the mutant, unlike in wild-type fruit