Redox reactions in apoplast of growing cells

Redox reactions affecting the cell wall extensibility proceed in the apoplast of growing cells. The reactions involve dozens of oxidoreductases localized in cell walls (Class I and III heme peroxidases, FAD- and Cu-dependent amine oxidases, oxalate oxidase, ascorbate oxidase, superoxide dismutase, etc.) together with NADPH oxidase and quinone reductase of the plasma membrane. The cell wall extensibility decreases due to peroxidase-catalyzed phenolic cross-links of polymers. Cell growth is proven to be directly dependent on production of reactive oxygen species (ROS) in the apoplast. A special value is attached to hydroxyl radical OH?, which is able to locally cleave polysaccharides and, thus, increase wall extensibility. Generation of OH? results from one-electron reduction of H₂O₂ and, consequently, is related to the complex of enzymatic and spontaneous reactions of H₂O₂ turnover in the apoplast. The extensibility also depends on an ascorbate concentration in the apoplast and on a ratio of its oxidized to reduced forms. This dependence is expressed not only in the well-known down-regulation of phenols oxidation but also through pro-oxidant and signal activities. There is only indirect evidence of a role of apoplast-originated redox signaling in the cell growth regulation. In addition to ascorbate, the signaling may supposedly involve ROS, glutathione recycling reactions, numerous redox-sensitive peptides, and proteins localized in the cell wall and the plasma membrane.