Gene Expression Profiles Of AtOSR Family And The Evidence For Involvement Of AtOSR2 In Arabidopsis Responses To Drought And Osmotic Stresses

The previous work has demonstrated that, in Arabidopsis genome, there are 10 genes, named as OSR (osmotic-stress responsive) family genes, that are highly homologous to the MscS (mechanosensitive channel with small conductance) gene family in bacteria. The MscS plays critical role in bacterial cell surviving osmotic downshift, which suggest that OSRs in Arabidopsis may play similar roles in responses to osmotic or draught stress. The focuses of this dissertation work include (1) analyzing expression profiles of OSR genes in different tissues and organs and under various osmotic stresses, and (2) identifying particular OSR genes involved in plant responses to draught stress and characterizing their physiological or biochemical functions.Based on the released Arabidopsis cDNA microarray data, the expression profiles of 10 AtOSR genes in different tissues and organs as well as under various stresses were analyzed. The results suggest that the expression level of all AtOSR genes is relatively low although some of them showed significant expression in particular tissues or cell types (such as in stomatal guard cells) under osmotic or salt stresses. The expression of 3 OSR genes, including OSR2, OSR3 and OSR4, was enhanced, while that of another gene OSR9 was inhibited by osmotic stress and high salinity treatments. By constructing OSR-promoter::GUS transformed Arabidopsis lines, the expression patterns of 10 OSR genes were analyzed in different plant organs at various developmental stages using x-gluc staining methods. The results revealed that all OSR genes expressed in stomatal guard cells and the expression of OSR2 genes in stomatal guard cells were significantly induced by osmotic stress.The phenotypes of T-DNA insertion mutants of OSR genes were screened under different abiotic stresses, and the mutants for 4 OSR genes were found to be more sensitive to drought stress than wild type plants. Two mutants (osr2 and osr9, referring to genes OSR2 and OSR9, respectively) were selected for further gene function characterization. Under osmotic and saline stresses, the water loss rates of excised mutant leaves were significantly greater than that of wild-type plants. In consistent with this result, stomatal closure of the mutants under osmotic or saline stress was decreased compared to wild-type plants. The increased water loss and decreased stomatal closure under water stress may well explain the increased sensitivity of the mutants to osmotic or water stress. The RT-PCR experiments showed that the transcriptional expression of OSR2 was significantly enhanced by osmotic and drought stress. Furthermore, the transgenic complementary lines of osr2 mutants showed similar phenotype and stomatal responses compared to wild-type plants under salt or osmotic stress.In summary, this study investigated the expression patterns of AtOSR family genes in different tissues or organs, at various developmental stages, and under water or osmotic stress. The results suggest that some OSRs may involve in plant responses to osmotic or water stress. The involvement of OSR2 and OSR9 in plant responses to water and osmotic stresses were further characterized. Thegenetic and physiological results indicated that OSR2 play important roles in plant responses to water and osmotic stresses, most likely via regulation of stomatal movement.