Abiotic Stress Tolerance. Metabolic and Physiological Effects of Compatible Solutes and Calmodulin on E. coli and Tobacco

Abstract: In their natural habitats, bacteria and plants can be exposed to abiotic stresses such as drought, high salt concentrations and freezing, which are factors limiting their capacity for growth and proliferation. To withstand environmental stresses, they have evolved specific stress responses. These responses include physiological adaptations, alterations in gene expression and production of protective enzymes and metabolites. One common mechanism is the accumulation of compatible solutes. These solutes, also called osmoprotectants, help to regulate cell volume under conditions of water deficit and they can to protect proteins and membrane structures in the cell. This thesis gives an overview of some mechanisms that lead to stress adaptation in bacteria and plants. The effects of high salt concentration in combination with glycine betaine, an effective osmoprotectant, on gene expression in Escherichia coli have been examined using microarray technology. Glycine betaine was found to increase the expression of many genes encoding enzymes in metabolism relative to the expression in salt-stressed cells without access to glycine betaine. In contrast, genes involved in cell processes, including adaptation and protection, among them many heat shock proteins, were downregulated. The enzymes responsible for glycine betaine production in E. coli have been studied by construction of a fusion enzyme and the resulting bifunctional enzyme was characterized in vivo and in vitro, and also expressed in plants. Abiotic stress is the major factor limiting crop productivity, and the development of genetically modified plants with improved stress tolerance is an important challenge in plant gene technology. One approach has been to introduce genes for production of compatible solutes. Here, tobacco plants with a stress-induced expression of genes for production of two different osmoprotectants, glycine betaine and trehalose, were generated, and showed an increased tolerance to drought and oxidative stress. Another strategy is the engineering of stress signaling in plants. Calcium signaling is involved in abiotic stress signaling, and overexpression of calmodulin, a Ca2+-dependent regulatory protein, in tobacco, resulted in increased salt tolerance.