by Lui Ka Man. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (leaves 143-166). / Abstracts in English and Chinese. / Acknowledgements --- p.i / List of Abbreviations --- p.ii / Abstract --- p.iv / 摘要 --- p.viii / List of Tables --- p.xi / List of Figures --- p.xii / Contents --- p.xvii / Chapter CHAPTER 1 --- INTRODUCTION / Chapter 1.1 --- Sulfur-Containing Amino Acids --- p.1 / Chapter 1.2 --- Metabolism of Sulfur-Containing Amino Acids --- p.3 / Chapter 1.2.1 --- The Metabolic Pathways --- p.3 / Chapter 1.2.2 --- Metabolic Regulation of Homocysteine --- p.7 / Chapter 1.3 --- Sulfur-Containing Amino acids and Health --- p.10 / Chapter 1.3.1 --- Sulfur-Containing Amino Acids and Renal Diseases --- p.11 / Chapter 1.3.2 --- Sulfur-Containing Amino Acids and the Nervous System --- p.13 / Chapter 1.3.3 --- Sulfur-Containing Amino Acids and Heart Diseases --- p.16 / Chapter 1.3.4 --- Sulfur-Containing Amino Acids and Liver Diseases --- p.20 / Chapter 1.4 --- Vitamin C (Ascorbic Acid) --- p.21 / Chapter 1.4.1 --- Biosynthesis 、 --- p.23 / Chapter 1.4.2 --- Vitamin C Transport System --- p.24 / Chapter 1.4.2.1 --- Uptake of Ascorbic Acid --- p.25 / Chapter 1.4.2.2 --- Uptake of Dehydroascorbic Acid --- p.26 / Chapter 1.5 --- Vitamin C and Health --- p.28 / Chapter 1.5.1 --- Vitamin C and Heart Diseases --- p.28 / Chapter 1.5.2 --- Vitamin C and Renal Diseases --- p.29 / Chapter 1.5.3 --- Vitamin C and Brain Diseases --- p.30 / Chapter 1.5.4 --- Vitamin C and Liver Diseases --- p.31 / Chapter 1.6 --- The Aims of Project --- p.33 / Chapter CHAPTER 2 --- MATERIALS AND METHODS / Chapter 2.1 --- Materials --- p.35 / Chapter 2.2 --- Preparation of Materials --- p.37 / Chapter 2.2.1 --- Mobile Phase for Ascorbic Acid Analysis --- p.37 / Chapter 2.2.2 --- Mobile Phase for Amino Acid Analysis --- p.37 / Chapter 2.2.3 --- Reagents for RNA Extraction and Reverse Transcription- Polymerase Chain Reaction (RT-PCR) --- p.38 / Chapter 2.2.4 --- Sense and Antisense Primers --- p.38 / Chapter 2.2.5 --- Reagents for Electrophoresis --- p.40 / Chapter 2.3 --- Animals --- p.40 / Chapter 2.4 --- Experimental Studies --- p.41 / Chapter 2.5 --- Methods --- p.41 / Chapter 2.5.1 --- Ascorbic Acid Analysis --- p.41 / Chapter 2.5.1.1 --- Extraction of Ascorbic Acid --- p.41 / Chapter 2.5.1.2 --- Chromatography --- p.42 / Chapter 2.5.2 --- Amino Acid Analysis --- p.45 / Chapter 2.5.2.1 --- Sample Preparation --- p.45 / Chapter 2.5.2.2 --- Chromatography --- p.45 / Chapter 2.5.3 --- Determination of RNA Expression by RT-PCR Analysis --- p.50 / Chapter 2.5.3.1 --- RNA Isolation --- p.50 / Chapter 2.5.3.2 --- Measurement of RNA Yield --- p.51 / Chapter 2.5.3.3 --- Reverse Transcription --- p.51 / Chapter 2.5.3.4 --- Polymerase Chain Reaction (PCR) --- p.52 / Chapter 2.5.3.5 --- Separation of PCR Products by Agarose Gel Electrophoresis --- p.52 / Chapter 2.5.3.6 --- Quantification of Band Density --- p.53 / Chapter 2.6 --- Statistical Analysis --- p.53 / Chapter CHAPTER 3 --- RESULTS / Chapter 3.1 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Distributions in Mice --- p.54 / Chapter 3.1.1 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Concentrations in the Plasma --- p.55 / Chapter 3.1.2 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Concentrations in the Kidney --- p.57 / Chapter 3.1.3 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Concentrations in the Liver --- p.59 / Chapter 3.1.4 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Concentrations in the Brain --- p.61 / Chapter 3.1.5 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Concentrations in the Heart --- p.63 / Chapter 3.1.6 --- Summary --- p.65 / Chapter 3.2 --- Time-Response --- p.66 / Chapter 3.2.1 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration for Various Periods on Ascorbic Acid Concentrations in the Plasma --- p.67 / Chapter 3.2.2 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration for Various Periods on Ascorbic Acid Concentrations in the Kidney --- p.69 / Chapter 3.2.3 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration for Various Periods on Ascorbic Acid Concentrations in the Liver --- p.71 / Chapter 3.2.4 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration for Various Periods on Ascorbic Acid Concentrations in the Brain --- p.73 / Chapter 3.2.5 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration for Various Periods on Ascorbic Acid / Chapter 3.2.6 --- Summary --- p.77 / Chapter 3.3 --- Effects of Sulfur-Containing Amino Acids and Leucine on SVCT and GLUT3 Gene Expressions --- p.78 / Chapter 3.3.1 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration on SVCT mRNA Expression in the Kidney --- p.79 / Chapter 3.3.2 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration on SVCT mRNA Expression in the Liver --- p.89 / Chapter 3.3.3 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration on SVCT and GLUT3 mRNA Expression in the Brain --- p.98 / Chapter 3.3.4 --- Effects of Sulfur-Containing Amino Acids and Leucine Administration on SVCT mRNA Expression in the Heart --- p.109 / Chapter 3.3.5 --- Summary --- p.115 / Chapter 3.4 --- Sulfur-Containing Amino Acids Concentrations in the Plasma --- p.117 / Chapter 3.4.1 --- Effects of Administration of Sulfur-Containing Amino Acids and Leucine on Methionine Concentrations in the Plasma --- p.117 / Chapter 3.4.2 --- Effects of Administration of Sulfur-Containing Amino Acids and Leucine on Cystine Concentrations in the Plasma --- p.119 / Chapter 3.4.3 --- Effects of Administration of Sulfur-Containing Amino Acids and Leucine on Taurine Concentrations in the Plasma --- p.119 / Chapter 3.4.4 --- Effects of Administration of Sulfur-Containing Amino Acids and Leucine on Leucine Concentrations in the Plasma --- p.122 / Chapter 3.4.5 --- Summary --- p.124 / Chapter CHAPTER 4 --- DISCUSSION AND CONCLUSIONS / Chapter 4.1 --- Effects of Sulfur-Containing Amino Acids and Leucine on Ascorbic Acid Concentrations in Mice --- p.126 / Chapter 4.2 --- Effects of Sulfur-Containing Amino Acids and Leucine on SVCT and GLUT3 Gene Expressions --- p.131 / Chapter 4.3 --- Sulfur-Containing Amino Acids Concentrations in the Plasma --- p.136 / Chapter 4.4 --- Conclusions --- p.140 / REFERENCES --- p.143

Arid and semi-arid regions, mostly found in developing countries with exponentially increasing populations, are in chronic lack of water thereby severely limiting agricultural production. Irrigation with saline water, which is available in large quantities, could be an obvious solution, but current crops are all salt sensitive. Although major efforts are underway to breed salt tolerant crops, no breakthrough results have yet been obtained. One alternative could rely on plant-interacting microbiota communities. Indeed, rhizophere and endosphere microbial communities are distinct from those of the surrounding soils, and these specific communities contribute to plant growth and health by increasing nutrient availability or plant resistance towards abiotic and biotic stresses. Here we show that plant microbe interactions induce plant tolerance to multiple stresses. From a collection of strains isolated from the desert plant Indigofera argentea, we could identify at least four different strategies to induce salt stress tolerance in Arabidopsis thaliana. A deep analysis of Enterobacter sp. SA187 showed that it induces Arabidopsis tolerance to salinity through activation of the ethylene signaling pathway. Interestingly, although SA187 does not produce ethylene as such, the association of SA187 with plants induces the expression of the methionine salvage pathway in SA187 resulting in the conversion of bacterially produced 2-keto-4-methylthiobutyric acid (KMBA) to ethylene. In addition, a metabolic network characterization of both SA187 and Arabidopsis in their free-living and endophytic state revealed that the sulfur metabolic pathways are strongly upregulated in both organisms. Furthermore, plant genetic experiments verified the essential role of the sulfur metabolism and ethylene signaling in plant salt stress tolerance. Our findings demonstrate how successful plant microbes of a given community can help other plants to enhance tolerance to abiotic stress, and reveal a part of the complex molecular communication process during beneficial plant-microbe interaction.

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