Akira Tabuchi

Mechanism of Xyloglucan Breakdown in Cell Walls of Azuki Bean Epicotyls

Summary Xyloglucans, extracted with 24% KOH from pectinase-treated cell walls of epicotyl segments of azuki bean (Vigna angularis Ohwi et Ohashi), consisted of a single peak with an average molecular mass of 160 kDa. Xyloglucans 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. When epicotyl segments of azuki bean were incubated with 10 μM 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 Tetragonolobus purpureus Moench (TPA) and Ulex europaeus L. (UEA I) completely inhibited the IAA-induced decrease in molecular mass of xyloglucans. These results suggest that auxininduced xyloglucan breakdown in azuki bean epicotyls consists of two major processes, the first step being the partial cleavage of the glucan backbone of xyloglucans into 50 kDa products without any stripping of side chains and the second step being the degradation of overall molecule, which is triggered by the removal of the terminal fucose from side chains. Endoglucanases (EC 3.2.1.4) and α-fucosidases (EC 3.2.1.51), which appear to be involved in this two-step breakdown of xyloglucans, were extracted from the cell walls of azuki bean epicotyls with 1M NaCl.