Ubiquitination-mediated protein degradation via Really Interesting New Gene (RING) E3 ligase plays an important role in plant responses to abiotic stress conditions. Many plant studies have found that RING proteins regulate the perception of various abiotic stresses and signal transduction. In this study, Oryza sativa salt-induced RING Finger Protein 1 (OsSIRP1) gene was selected randomly from 44 Oryza sativa RING Finger Proteins (OsRFPs) genes highly expressed in rice roots exposed to salinity stress. Transcript levels of OsSIRP1 in rice leaves after various stress treatments, including salt, heat, drought and hormone abscisic acid (ABA), were observed. Poly-ubiquitinated products of OsSIRP1 were investigated via an in vitro ubiquitination assay.35S:OsSIRP1-EYFP was distributed in the cytosol of untreated and salt-treated rice protoplasts. Heterogeneous overexpression of OsSIRP1 in Arabidopsis reduced tolerance for salinity stress during seed germination and root growth. Our findings indicate that OsSIRP1 acts as a negative regulator of salinity stress tolerance mediated by the ubiquitin 26S proteasome system.

In this study, our findings regarding the regulation of GA irradiation-induced OsGIRP1 in relation to the levels of photosynthesis-related proteins such as OsrbcL1 and OsrbcS1 and hypersensitive responses of overexpressing plants to GR irradiation provide insight into the molecular functions of OsGIRP1 as a negative regulator in response to the stress of radiation. The RING (Really Interesting New Gene) finger proteins are known to play crucial roles in various abiotic stresses in plants. Here, we report on RING finger E3 ligase, Oryza sativa gamma rays-induced RING finger protein1 gene (OsGIRP1), which is highly induced by gamma rays (GR) irradiation. In vitro ubiquitination assay demonstrated that a single amino acid substitution (OsGIRP1(C196A)) of the RING domain showed no E3 ligase activity, supporting the notion that the activity of most E3s is specified by a RING domain. We isolated at least 6 substrate proteins of OsGIRP1, including 2 Rubisco subunits, OsrbcL1 and OsrbcSl, via yeast two-hybridization and bimolecular fluorescence complementation assays. OsGIRP1 and its partner proteins were targeted to the cytosol and the cytosol or chloroplasts, respectively; however, florescence signals of the complexes with OsGIPR1 were observed in the cytosol. Protein degradation in cell extracts showed that OsGIRP1 mediates proteolysis of 2 substrates, OsrbcS1 and OsrbcL1, via the 26S proteasome degradation pathway. The Arabidopsis plants overexpressing OsGIRP1 clearly exhibited increased sensitivity to GR irradiation. These results might suggest that OsGIRP1 acts as a negative regulator of GR response to mediate the degradation of photosynthesis-related proteins.

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