Despite the fact that cold shock domain proteins (CSDPs) and glycine-rich RNA-binding proteins (GRPs) have been implicated to play a role during the cold adaptation process, their importance and function in eukaryotes, including plants, are largely unknown. To understand the functional role of plant CSDPs and GRPs in the cold response, two CSDPs (CSDP1 and CSDP2) and three GRPs (GRP2, GRP4 and GRP7) from Arabidopsis thaliana were investigated. Heterologous expression of CSDP1 or GRP7 complemented the cold sensitivity of BX04 mutant Escherichia coli that lack four cold shock proteins (CSPs) and is highly sensitive to cold stress, and resulted in better survival rate than control cells during incubation at low temperature. In contrast, CSDP2 and GRP4 had very little ability. Selective evolution of ligand by exponential enrichment (SELEX) revealed that GRP7 does not recognize specific RNAs but binds preferentially to G-rich RNA sequences. CSDP1 and GRP7 had DNA melting activity, and enhanced RNase activity. In contrast, CSDP2 and GRP4 had no DNA melting activity and did not enhance RNAase activity. Together, these results indicate that CSDPs and GRPs help E.coli grow and survive better during cold shock, and strongly imply that CSDP1 and GRP7 exhibit RNA chaperone activity during the cold adaptation process.

Summary: Although glycine‐rich RNA‐binding protein 2 (GRP2) has been implicated in plant responses to environmental stresses, the function and importance of GRP2 in stress responses are largely unknown. Here, we examined the functional roles of GRP2 in Arabidopsis thaliana under high‐salinity, cold or osmotic stress. GRP2 affects seed germination of Arabidopsis plants under salt stress, but does not influence seed germination and seedling growth of Arabidopsis plants under osmotic stress. GRP2 accelerates seed germination and seedling growth in Arabidopsis plants under cold stress, and contributes to enhancement of cold and freezing tolerance in Arabidopsis plants. No differences in germination between the wild‐type and transgenic plants were observed following addition of abscisic acid (ABA) or glucose, implying that GRP2 affects germination through an ABA‐independent pathway. GRP2 complements the cold sensitivity of an Escherichia coli BX04 mutant and exhibits transcription anti‐termination activity, suggesting that it has an RNA chaperone activity during the cold adaptation process. Mitochondrial respiration and catalase and peroxidase activities were affected by expression of mitochondrial‐localized GRP2 in Arabidopsis plants under cold stress. Proteome analysis revealed that expression of several mitochondrial‐encoded genes was modulated by GRP2 under cold stress. These results provide new evidence indicating that GRP2 plays important roles in seed germination, seedling growth and freezing tolerance of Arabidopsis under stress conditions, and that GRP2 exerts its function by modulating the expression and activity of various classes of genes.

Summary: Glycine‐rich RNA‐binding proteins (GR‐RBPs) have been implicated to play roles in post‐transcriptional regulation of gene expression in plants under various stress conditions, but the functional roles of GR‐RBPs under stress conditions remain to be verified. Here, we examine the biological roles of a GR‐RBP, designated atRZ‐1a, in Arabidopsis thaliana under stress conditions. atRZ‐1a was expressed ubiquitously in various Arabidopsis organs including stems, roots, leaves, flowers, and siliques. The transcript level of atRZ‐1a increased markedly by cold stress, whereas its expression was marginally downregulated by drought stress or abscisic acid treatment. Germination and seedling growth of the loss‐of‐function mutants were retarded remarkably compared with those of the wild type under cold stress. In contrast, the transgenic Arabidopsis plants that overexpress atRZ‐1a displayed earlier germination and better seedling growth than the wild type under cold stress. Moreover, the atRZ‐1a‐overexpressing transgenic Arabidopsis plants were more freezing tolerant than the wild‐type plants. Heterologous expression of atRZ‐1a in Escherichia coli demonstrated that the E. coli cells expressing atRZ‐1a displayed much higher growth rate than the non‐transformed cells after cold shock. These results provide evidence that atRZ‐1a affects seed germination and seedling growth under low temperature and plays a role in the enhancement of freezing tolerance in Arabidopsis plants.