In vitro molecular weight increase in xyloglucans by an apoplastic enzyme preparation from epicotyls of Vigna angularis
Nishitani K,Tominaga R
Physiologia Plantarum 82: 490-497
In order to gain insight into the mechanism of cell extension growth, enzymic processes involved in structural modification of cell wall xyloglucans were investigated, using an apoplastic enzyme preparation from epicotyls of dark grown Vigna angularis Ohwi et Ohashi cv. Takara and purified xyloglucans derived from cell walls of Vigna. The reaction of Vigna xyloglucan (mass average molecular weight = 420 kDa) with the apoplastic enzyme preparation gave three fractions: (1) a waterinsoluble high molecular weight (820 kDa) xyloglucan fraction (WI), (2) a watersoluble low molecular weight (149 kDa) xyloglucan fraction (WS), and (3) an 80% ethanolsoluble monosaccharide fraction (ES). WI and WS were chiefly composed of tgalactosyl, txylosyl, 2xylosyl, 4glucosyl and 4,6glucosyl residues, whereas ES was composed of fucose, galactose, glucose and xylose monomers. The data indicate that WI is generated by the linking of xyloglucan molecules by some alkali stable linkages, probably of glycosidic nature. The optimal pH for the WIproducing activity of the apoplastic enzyme preparation was 5.4. Higher WIproducing activity was detected in the upper juvenile than in the lower nonelongating regions of the epicotyl. Our data suggest the possible involvement of a transglycosylation reaction in the structural changes of the xyloglucans that are responsible for cell extension growth of the Vigna angularis epicotyl. The data are also consistent with the idea that the enzymic processes are regulated by hydrogen ions in the apoplastic space.