A proposed role for copper ions in cell wall loosening

We present the hypothesis that Cu2+, complexed with cell wall polymers (especially histidine-rich glycoproteins), can be reduced to Cu+ by apoplastic electron donors such as ascorbate and superoxide, and that the Cu+ can then undergo a Fenton reaction with apoplastic hydrogen peroxide (H2O2) to generate hydroxyl radicals (.OH). These can cause non-enzymic scission of wall polysaccharides and may loosen the cell wall. In tomato fruit cell walls, .OH radicals cause pectin solubilisation. However, we show that acidic pectins do not sequester Cu2+ sufficiently strongly to prevent .OH-attack on other molecules (xyloglucan or aromatics) present in the same solution. We describe two approaches by which apoplastic .OH can be detected in vivo: (a) detection of a chemical 'fingerprint' characteristic of .OH-attacked cell wall polysaccharides; (b) infiltration into the apoplast of membrane-impermeant [H-3] benzoyl-amides which react with .OH to yield (H2O)-H-3. Data support apoplastic .OH production in vivo, and its enhancement during pear (Pyrus communis) fruit ripening, but have not so far supported its proposed enhancement during auxin-stimulated cell expansion in maize (Zea mays) coleoptiles.