Molecular Functions of Rice Cytosol-Localized RING Finger Protein 1 in Response to Salt and Drought and Comparative Analysis of Its Grass Orthologs

In higher plants, the post-translational modification of target proteins via the attachment of molecules such as ubiquitin (Ub) mediates a variety of cellular functions via the Ub/26S proteasome system. Here, a really interesting new gene (RING)-H2 type E3 ligase, which regulates target proteins via the Ub/26S proteasome system, was isolated from a rice plant, and its other grass orthologs were examined to determine the evolution of its molecular function during speciation. The gene encoding Oryza sativa cytoplasmic-localized RING finger protein 1 (OsCLR1) was highly expressed under salt and drought stresses. By contrast, the three grass orthologs, SbCLR1 from Sorghum bicolor, ZmCLR1 from Zea mays and TaCLR1 from Triticum aestivum, showed different responses to these stresses. Despite these differences, all four orthologs exhibited E3 ligase activity with cytosol-targeted localization, demonstrating conserved molecular functions. Although OsCLR1-overexpressing plants showed higher survival rates under both salt and drought stresses than that of the wild type (WT) plants, this pattern was not observed in the other orthologs. In addition, OsCLR1-overexpressing plants exhibited lower germination rates in ABA than that of WT plants, whereas the three ortholog CLR1-overexpressing plants showed rates similar to the WT plants. These results indicate the positive regulation of OsCLR1 in response to salt and drought in an ABA-dependent manner. Despite the molecular functions of the three CLR1 orthologs remaining largely unknown, our results provide an insight into the evolutionary fate of CLR1 grass orthologs during speciation after the divergence from a common ancestor.