Knock-down of Stress Inducible OsSRFP1 Encoding an E3 Ubiquitin Ligase with Transcriptional Activation Activity Confers Abiotic Stress Tolerance Through Enhancing Antioxidant Protection in Rice

E3 ubiquitin ligases are involved in a variety of physiological processes. This study demonstrated the function of a previously unknown rice RING finger E3 ligase, Oryza sativa Stress-related RING Finger Protein 1 (OsSRFP1) in stress responses in rice. OsSRFP1 was ubiquitously expressed in various rice organs, with the higher expression levels in roots, panicles and culm nodes. The transcript of OsSRFP1 was induced by cold, dehydration, salt, H2O2 and abscisic acid treatments. Interestingly, the OsSRFP1-overexpressing plants were less tolerant to salt, cold and oxidative stresses than wild type plants; while the RNA interference silencing of OsSRFP1 plants were more tolerant than wild type without yield penalty. Compared with the wild type, amounts of free proline and activities of antioxidant enzymes were increased in the RNAi plants but decreased in the overexpression plants under cold stress, which were inversely correlated with the malondialdehyde and hydrogen peroxide (H2O2) levels in the tested lines. Microarray analysis showed that expression of numerous genes involving in ROS homeostasis was altered in the OsSRFP1-overexpressing plants under normal and cold conditions. In vitro ubiquitination assays showed that OsSRFP1 possessed E3 ubiquitin ligase activity and the intact RING domain was essential for the activity. Moreover, OsSRFP1 might function in transcriptional regulation with nuclear localization. Taken together, our results demonstrate that OsSRFP1 may have dual functions in post-translational and transcriptional regulations in modulating abiotic stress responses in rice, at least in part, by negatively regulating antioxidant enzymes-mediated reactive oxygen species removal.