Molecular and Metabolic Aspects of Plant and Bacterial Salt Tolerance

Abstract: Salt stress is one of the historically as well as potentially largest agriculture problem humans have faced. Researchers have investigated and tried to solve this problem for a long time. Over 20 reviews have been written on this topic the last five years. In this thesis the extent of salt stress in today's agriculture, the toxic and damaging effects on plants and bacteria exposed to salt stress, and the molecular responses of plants and bacteria exposed to salt stress have been summarised. More extensive analysis on a osmo-protective and a potentially osmo-protective molecule, glycine betaine (GB) and -aminobutyric acid (GABA) are given. In addition, an overview of modern microarray transcription analysis technology is included. GB is generally accepted as one of the most efficient osmolytes helping organisms to cope with several abiotic stresses such as salt and osmotic stress. An investigation on GBs influence on transcription indicated that GB may have properties resembling hormones in Arabidopsis. Many of the transcriptional effects by GB in Arabidopsis suggest that elevated internal levels of GB may inhibit growth. GB also influenced heat shock proteins transcription levels in NaCl stressed Escherichia coli. This effect support earlier findings that GB functions as a chemical chaperone. GB was also showed to improve growth rate of E. coli and enhance survival of hyperosmotically NaCl shocked Arabidopsis. GABA has been assigned several potential roles important for abiotic stress tolerance such as: function as a signalling molecule, being involved in reactive oxygen species scavenging and being a signalling molecule. Tobacco transformed with a bovine calmodulin gene showed a faster germination on saline media compared to control. This transgenic tobacco also showed enhanced transient levels of GABA during germination. GABA may have been produced by Ca2+/calmodulin-stimulated glutamate decarboxylase. Analysis of germination on media containing combinations of NaCl, LaCl3, EGTA, CaCl2 and GABA in combination with analysis of free amino acid levels in NaCl shock treated mature tobacco plants suggest that GABA rather may work as a signalling molecule than as an osmolyte in salt stressed tobacco. It is well established that high salt level containing soils and water solutions have decreased aeration properties. Therefore growth properties of E. coli expressing haemoglobin growing in high NaCl concentrations were investigated. It was found that E. coli expressing Vitreoscilla haemoglobin probably reached a higher cell density and showed a lower level of lipid peroxidation compared with control. This data suggest that Vitreoscilla haemoglobin may improve reactive oxygen species scavenging in vivo under salt stress.