A novel superoxide dismutase with a high isoelectric point in higher plants. Expression, regulation, and protein localization

Several isoforms of superoxide dismutase (SOD) with a high isoelectric poin t (pl) have been identified by isoelectric focusing chromatography in prote in extracts from Scots pine (Pinus sylvestris) needles. One of these isofor ms, a CuZn-SOD with a pI of about 10 and thus denoted hipI-SOD, has been is olated and purified to apparent homogeneity. A cDNA encoding the hipI-SOD p rotein was cloned and sequenced. Northern hybridization of mRNA isolated fr om different organs and tissues showed that hipI-SOD has a markedly differe nt pattern of expression compared with chloroplastic and cytosolic SOD. Fur thermore, the transcript levels of hipI-SOD and cytosolic SOD were found to respond differently to mechanical wounding, treatment with oxidized glutat hione, paraquat, and ozone. Immunogold electron microscopy localized the hi pI-SOD in the plasma membrane of sieve cells and the Golgi apparatus of alb uminous cells. Moreover, high protein density was also detected in extracel lular spaces such as secondary cell wall thickenings of the xylem and scler enchyma and in intercellular spaces of parenchyma cells.