Isoform-Specific Responses of Superoxide Dismutase to Oxidative Stresses and Hormones in Parquat-Tolerant Rehmannia glutinosa

All accessions of Rehmannia glutinosa show the unique characteristic of intrinsic tolerance to paraquat. The higher level of endogenous superoxide dismutase(SOD) activity and its increase upon paraquat treatment indicated the involvement of SOD in the tolerance mechanism to paraquat in R. glutinosa. In this study, we examined the isoform-specific response of SOD to oxidative stresses and hormones. Six SOD isoforms were found in the leaf, and they were identified as two MnSODs(named MnSOD I and MnSOD II, in order of increasing mobility), one FeSOD and three Cu/ZnSODs(named Cu/ZnSOD I, Cu/ZnSOD II, and Cu/ZnSOD III, in order of increasing mobility). MnSOD I, MnSOD II, FeSOD, Cu/ZnSOD I, Cu/ZnSOD II, and Cu/ZnSOD III, contributed to 4, 11, 7, 15, 30, and 32% of the total SOD activity, respectively. Total SOD activity levels in the leaf were increased by 4, 24, and 21% by paraquat, salicylic acid(SA), and yeast extract(YE), respectively, but little by ethephon. Six SOD isoforms responded differentially to these stresses and hormones. The activities of all the isoforms were increased by YE and SA except that of MnSOD I which was decreased by SA. The activities of MnSOD I, FeSOD, and CuZnSOD I were increased by paraquat. These results suggest that amelioration of oxidative stresses by SOD is fine-tuned by the differential expression of isoforms in R. glutinosa.