Sugar release from maize coleoptiles during auxin-, fusicoccin- and acid-mediated elongation growth

Abstract

Summary The biochemical mechanism of the cell wall-loosening process underlying auxin-induced elongation growth of plant organs is still unresolved. A hypothesis currently under debate states that the breakdown of cell-wall polysaccharides by hydrolytic enzymes (cell-wall autolysis) is involved in this process. In order to test this hypothesis we have investigated the release of soluble carbohydrates by maize coleoptile segments during the initial period of the growth responses induced by auxin (indole-3-acetic acid), fusicoccin, and weak acidic buffer (pH 4). As previously shown for isolated cell walls (Hohl et al., 1991), abraded segments release monomeric, oligomeric and polymeric carbohydrates into the incubation medium. No other monosaccharides besides glucose could be detected in these fractions 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 pH-dependent enhancement of carbohydrate release could be detected although buffer per se (pH 4 to 7) had a stimulating effect. Fusicoccin increased carbohydrate release but this effect may not be caused by cell-wall acidification. In contrast to auxin, fusicoccin had no effect on cell-wall extensibility measured in terms of load-extension curves (hysteresis loops) of killed segments. We conclude from these results that, although autolysis of cell-wall polysaccharides takes place in the growing tissue, it is not causally involved in the control of growth.