Fig. 1. Schematic representation of the enzymes at the plasma membrane and in the cell wall that are able to form reactive oxygen species (ROS: superoxide anion radical · — hydrogen peroxide hydroxyl radical. into the apoplast. · — is dismutated either enzymatically catalyzed by superoxide dismutase, (O 2 ), (H2O2), ( OH)) O 2 to H2O2 or nonenzymatically in the acidic pH that is typical in the cell wall. ROS play important roles both in cell wall loosening during cell elongation and cross-link formation involved in cell growth restriction..OH is considered as a cell wall-loosening agent formed from H O either non-enzymatically by Fenton reaction involving a transition metal such as 2 2 Fe2+ or Cu +, or enzymatically by peroxidases (not depicted). Di- and oligoferulate bridges, bonds between tyrosine residues abundant in cell wall structural proteins, lignin formation, or cross-linkages between ferulates and lignin are possible cross-links involved in cell growth restriction. For the cross-link formation, oxidative enzymes (peroxidase in a peroxidative cycle using H2O2 as an oxidant, or laccase using O2 as an oxidant) catalyze the oxidation of the phenolic residues after which they make a crosslink. In addition to cell wall modifications, ROS are important signalling components in various biological processes. The left cell shows the enzymes producing ROS, and the right cell shows the enzymes consuming ROS during cell wall cross-linking. Note that all enzymes and the cross-link types mentioned may not be present in the same cell, or in all species. In case of quinone reductases, further studies are needed to find out whether enough quinones are present in the plasma membranes to be able to mediate electron transport from the cytoplasmic reductant to apoplastic molecular oxygen.