SUGAR RELEASE FROM MAIZE COLEOPTILES DURING AUXIN-MEDIATED, FUSICOCCIN-MEDIATED AND ACID-MEDIATED ELONGATION GROWTH

The biochemical mechanism of the cell wall-loosening process underlyin g auxin-induced elongation growth of plant organs is still unresolved. A hypothesis currently under debate stares that the breakdown of cell -wall polysaccharides by hydrolytic enzymes (cell-wall autolysis) is i nvolved in this process. In order to test this hypothesis we have inve stigated the release of soluble carbohydrates by maize coleoptile segm ents and weak acidic buffer (pH 4). As previously shown for isolated c ell walls (Hohl et al., 1991), abraded segments release monomeric, oli gomeric and polymeric carbohydrates into the incubation medium. No oth er monosaccharides besides glucose could be detected in these fraction s after hydrolysis, indicating that no non-glucan polysaccharides are involved in this process. Auxin had no effect on carbohydrate release in vivo. Also under conditions of acid-mediated growth in the presence of pH-4 buffer, no pi-I-dependent enhancement of carbohydrate release could be detected although buffer per se (pH 4 to 7) had a stimulatin g effect. Fusicoccin increased carbohydrate release but this effect ma y not be caused by cell-wall acidification. In contrast to auxin, fusi coccin had no effect on cell-wall extensibility measured in terms of l oad extension curves (hysteresis loops) of killed segments. We conclud e from these results that, although autolysis of cell-wall polysacchar ides takes place in the growing tissue, it is not causally involved in the control of growth.