INTRACELLULAR AND EXTRACELLULAR LOCALIZATION OF CYTOSOLIC CUZN-SUPEROXIDE DISMUTASE IN SPINACH LEAF AND HYPOCOTYL

Immunogold-electron microscopic analysis of spinach leaves done with t he antibody specific for ''cytosolic'' CuZn-superoxide dismutase (SOD) indicates that SOD is localized in the apoplast, in the nucleus and i n, or near, the tonoplast. The association of CuZn-SOD with the nucleu s indicates it has a role in preventing fatal mutation caused by react ive species of oxygen. The localizing site of CuZn-SOD in the apoplast ic region of spinach leaf tissues corresponds to that of the accumulat ion of lignin. In spinach hypocotyl ''cytosolic'' CuZn-SOD is localize d in vascular tissues where lignification and the generation of supero xide respectively were shown by the phloroglucin-HCl reaction and form ation of formazane from nitroblue tetrazolium. Because hydrogen peroxi de 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 peroxid ase.