Possible functions of copper ions in cell wall loosening

Abstract

We suggest that copper, tightly complexed to specific polymeric components within the primary cell wall, can be reduced from Cu²⁺ to Cu⁺ by apoplastic ascorbate and/or other electron donors and that the Cu⁺ thus formed can participate in a Fenton reaction with apoplastic H₂O₂ to form the highly reactive hydroxyl radical (°OH). The site of production of °OH would thus be determined by the location of Cu in the cell wall polymers. The °OH radical is extremely short-lived and appears unlikely to diffuse more than 1 nm from its site of production within the wall matrix: it would therefore be unlikely to damage the plasma membrane or induce mutations. The °OH would react with an organic component, most likely a polysaccharide, within the cell wall matrix itself. When °OH attacks a polysaccharide, the polymer’s backbone is frequently broken. This process could constitute a hitherto unexplored mechanism of non-enzymic wall-loosening, which would play a role in cell expansion, fruit softening and other physiological processes.