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Immunogold-electron microscopic analysis of spinach leaves done with the antibody specific for “cytosolic” CuZn-superoxide dismutase (SOD) indicates that SOD is localized in the apoplast, in the nucleus and in, or near, the tonoplast. The association of CuZn-SOD with the nucleus indicates it has a role in preventing fatal mutation caused by reactive species of oxygen. The localizing site of CuZn-SOD in the apoplastic region of spinach leaf tissues corresponds to that of the accumulation of lignin. In spinach hypocotyl “cytosolic” CuZn-SOD is localized in vascular tissues where lignification and the generation of superoxide respectively were shown by the phloroglucin-HCl reaction and formation of formazane from nitroblue tetrazolium. Because hydrogen peroxide is required for lignification via the peroxidase-catalyzed reaction, the CuZn-SOD in the apoplast appears to function in the biosynthesis of lignin by causing rapid disproportionation of the superoxide anion radical prior to its interaction with cellular components and peroxidase.
Ogawa et al. (Sun,) studied this question.