Functional analysis of AT4G16830 encoding an RNA-Binding protein involved in plant osmotic stress response

Exposure to osmotic stress induces in plants a wide range of molecular and cellular responses. Recently, to characterize changes in gene expression occurring during stress adaptation, we performed a transcriptome analysis of potato cells exposed to gradual acclimation to water deficit induced by PolyEthyleneGlycole (Ambrosone et al., 2008. Options mediterraneennes). Among the genes consistently induced during long- term water stress, the rgga gene, coding for a novel Glycine-rich RNA-binding protein, was identified and isolated. BLAST analysis revealed that rgga is conserved in several species and contains an RNA binding domain and two RGG box. RNA binding proteins have been involved in the responses to different exogenous signals, including abiotic stresses, but their functions in stress adaptive mechanisms remain largely unknown. To get insight into the biological role of rgga in plant stress response, a functional study of the hortologous gene (Atrgga) in Arabidopsis thaliana was performed. Similarly to the potato gene, qRT-PCR and Northern analyses revealed that Atrgga expression is regulated in response to different exogenous treatments (ABA, NaCl, PEG) in both cell cultures and young plants. In addition a strong rgga expression in guard cells and in vascular tissues was evinced by rgga promoter-driven GUS assay. To investigate the subcellular localization of the RGGA, transgenic plants overexpressing YFP-RGGA fusion protein were obtained. RGGA-YFP was localized prevalently in the cytoplasm and in the perinuclear region with no evidence of protein accumulation in root meristematic cells. A "gain and loss" of function study in Arabidopsis using rgga knockout mutants and transgenic plants overexpressing rgga under the control of 35S promoter was also performed. Overexpressing plants exhibited osmotic stress tolerance with high plant survival rates under stress conditions. Inversely seed germination and plant growth of rgga knockout mutant were severely affected by osmotic stresses. These data taken together provide compelling evidence that RGGA affects the growth and stress tolerance of Arabidopsis plants under high salt and drought stress conditions, suggesting an important role in the complex machinery of plant adaptation to osmotic stress.