Chemical structure of the cell walls of dwarf maize and changes mediated by gibberellin

Dwarf maize (Zea mays L.), a mutant deficient in gibberellin synthesis, provides an excellent model to study the influence of gibberellin on biochemical processes related to plant development. Alterations in the chemical structure of the cell wall mediated by gibberellin were examined in seedlings of this mutant. The composition of the walls of roots, mesocotyl, coleoptile, and primary leaves of dwarf maize was similar to that of normal maize and other cereal grasses. Glucuronoarabinoxylans constituted the principal hemicelluloses, but walls also contained substantial amounts of xyloglucan and mixed-linkage beta-d-glucan. Root growth in dwarf maize was essentially normal, but growth of mesocotyl and primary leaves was severely retarded. Injection of the gibberellin into the cavity of the coleoptile resulted in a marked increase in elongation of the primary leaves. This elongation was accompanied by increases in total wall mass, but the proportion of beta-d-glucan decreased from 20% to 15% of the hemicellulosic polysaccharide. During leaf expansion, the proportion decreased further to only 10%. Through 4 days incubation, the proportion of beta-d-glucan in leaves of control seedlings without gibberellin was nearly constant. Extraction of exo- and endo-beta-d-glucan hydrolases from purified cell walls and assay against a purified oat bran beta-d-glucan demonstrated that gibberellin increased the activity of the endo-beta-d-glucan hydrolase. These and other data support the hypothesis that beta-d-glucan metabolism is central to control of cell expansion in cereal grasses.