CHANGES IN ORGANELLE SUPEROXIDE-DISMUTASE ISOENZYMES DURING AIR ADAPTATION OF SUBMERGED RICE SEEDLINGS - DIFFERENTIAL BEHAVIOR OF ISOENZYMES IN PLASTIDS AND MITOCHONDRIA

Changes in activities and levels of superoxide dismutase (SOD, EC 1.15 .1.1) isoenzymes were studied during air adaptation of submerged rice (Oryza sativa L.) seedlings. Seeds were germinated for 6 d in the dark under water (submerged), and then for another day in air (air-adapted ). For a control, seeds were germinated for 6 d throughout in air (aer obic). Staining for activity of SOD of shoot extracts showed a total o f five major distinct forms of SOD: one mitochondrial Mn enzyme (mtSOD ) and four CuZn enzymes, one of which was plastidic (plSOD) and the ot her three cytosolic (cytSOD). Activity of plSOD was much lower in subm erged seedlings than in aerobic controls and increased after exposure to air. In contrast, mtSOD activity in submerged seedlings was as high as that found in aerobic controls, and did not increase upon exposure to air. One of the cytSODs showed responses similar to those of plSOD . The activities of another two cytSODs were slightly lower in submerg ed seedlings than those in aerobic controls, but decreased after 24 h of air adaptation. Western blot analysis revealed that these changes i n activities of SODs are due to changes in the levels of their enzyme proteins. We also followed changes in the levels of cytochrome c and f erredoxin-NADP(+) reductase (EC 1.6.99.4) as indices of the developmen t of mitochondria and plastids, respectively. Organelle SODs were alwa ys present at higher levels than would be expected in view of the deve lopment of the electron-transport systems of the corresponding organel les during submergence and the subsequent air-adaptation period.