Hypergravity-induced increase in the apoplastic pH and its possible involvement in suppression of beta-glucan breakdown in maize seedlings

Elongation growth of both coleoptiles and mesocotyls of maize (Zea mays L. cv. Cross Bantam T51) seedlings was inhibited under basipetal hypergravity (300 g) conditions. Hypergravity increased the pH of the apoplastic fluid o f coleoptiles from 5.0 to 5.5 and mesocotyls from 5.2 to 5.7. When beta-1,3 : 1,4-D- glucanases (beta-glucanases) extracted from cell walls of the 1-g control coleoptiles and mesocotyls were assayed at pH 5.0 and 5.5 for coleo ptiles, and at 5.2 and 5.7 for mesocotyls, respectively, the activity in th e increased pH conditions was significantly lower than that in the control pH conditions. During the autolysis of the enzymically active cell wall pre parations obtained from 1-g control organs, a molecular mass downshift of h emicellulosic polysaccharides occurred in cell walls. This downshift was su ppressed in the increased pH conditions as compared with the control pH con ditions. It was reported that hypergravity increased the molecular mass of hemicellulosic polysaccharides by decreasing the beta-glucanase activity, a nd thereby decreased the mechanical extensibility of cell walls in maize co leoptiles and mesocotyls. These results suggest that, in maize coleoptiles and mesocotyls, hypergravity- induced increase in the pH in the apoplastic fluid is involved in the reduction of the activity of beta-glucanases which , in turn, causes an increase in the molecular mass of hemicellulosic polys accharides and inhibits elongation growth.