HYDROGEN PEROXIDE-MEDIATED CELL-WALL STIFFENING IN-VITRO IN MAIZE COLEOPTILES

It has recently been proposed that H2O2-dependent peroxidative formati on of phenolic cross-links between cell-wall polymers serves as a mech anism for fixing the viscoelastically extended wall structure and thus confers irreversibility to wall extension during cell growth (M. Hohl et al. 1995, Physiol. Plant. 94: 491-198). In the present paper the i solated cell wall (operationally, frozen/thawed maize coleoptile segme nts) was used as an experimental system to investigate H2O2-dependent cell-wall stiffening in vitro. Hydrogen peroxide inhibited elongation growth (in vivo) and decreased cell-wall extensibility (in vitro) in t he concentration range of 10-10000 mu mol . l(-1). In rheological meas urements with a constant-load extensiometer the stiffening effect of H 2O2 could be observed with both relaxed and stressed cell walls. In-vi tro cell-wall stiffening was a time-dependent reaction that lasted abo ut 60 min in the presence of saturating concentrations of H2O2. The pr esence of peroxidase in the growth-limiting outer epidermal wall of th e coleoptile was shown by histochemical assays. Peroxidase inhibitors (azide, ascorbate) suppressed the wall-stiffening reaction by H2O2 in vitro. Hydrogen peroxide induced the accumulation of a fluorescent, in soluble material in the cell walls of living coleoptile segments. Thes e results demonstrate that primary cell walls of a growing plant organ contain all ingredients for the mechanical fortification of the wall structure by H2O2,-inducible phenolic cross-linking.