MECHANISM OF XYLOGLUCAN BREAKDOWN IN CELL-WALLS OF AZUKI-BEAN EPICOTYLS

Xyloglucans, extracted with 24% KOH from pectinase-treated cell walls of epicotyl segments of azuki bean (Vigna angularis Ohwi et Ohashi), c onsisted of a single peak with an average molecular mass of 160 kDa. X yloglucans with a molecular mass of about 50 kDa and higher exhibited a constant sugar composition, while for those smaller than 50 kDa, the lower the molecular mass was, the lower was the content of fucose. Wh en epicotyl segments of azuki bean were incubated with 10 mu M indole- 3-acetic acid (IAA) for 4 h, the average molecular mass of xyloglucans in the cell walls decreased by 20%. Fucose-binding lectins from Tetra gonolobus purpureus Moench (TPA) and Ulex europaeus L. (UEA I) complet ely inhibited the LAA-induced decrease in molecular mass of xyloglucan s. These results suggest that auxin-induced xyloglucan breakdown in az uki bean epicotyls consists of two major processes, the first step bei ng the partial cleavage of the glucan backbone of xyloglucans into 50k Da products without any stripping of side chains and the second step b eing the degradation of overall molecule, which is triggered by the re moval of the terminal fucose from side chains. Endoglucanases (EC 3.2. 1.4) and alpha-fucosidases (EC 3.2.1.51), which appear to be involved in this two-step breakdown of xyloglucans, were extracted from the cel l walls of azuki bean epicotyls with 1M NaCl.