Epsilon Open Archive

Abstract

Plant growth-promoting bacteria may be used in agriculture to minimize the utilization of chemical pesticides and fertilizers. This thesis studies one plant growth-promoting bacterial strain, Pseudomonas fluorescens SBW25, and its interactions with winter wheat, potato and tomato in both field and laboratory conditions. This bacterium was tagged with a novel marker gene cassette containing gfp (green fluorescent protein), luxAB (bacterial bioluminescence) and telABkilA (potassium tellurite oxide resistance) and the resulting strain SBW25::tgl was released in the first field trial with a genetically modified microorganism in Sweden. Bacterial numbers on wheat plants and in soil were determined by selective plating on media containing potassium tellurite oxide. The impact of SBW25::tgl on wheat plant growth was measured using plant length and weight, and possible effects on resident bacterial and fungal microflora was evaluated throughout the 8 months sampling period using terminal restriction fragment length polymorphism (T-RFLP). The GMM was found in high numbers on all plant parts throughout the 8 months period, but only minor impacts were found on native microflora due to bacterial (GMM or wild-type) inoculation. SBW25 was also evaluated as a biocontrol agent towards the phytopathogenic bacterium Ralstonia solanacearum on tomato and potato plants. By using gfp-tagged R. solanacearum and red fluorescent protein (rfp) -tagged SBW25 it was possible to distinguish these two strains on plant surfaces using microscopic techniques. Plate counting, flow cytometry and luminometry were used to monitor the strains on plants. Some biocontrol effect of SBW25 was detected on tomato plants. The interactions of SBW25 and arbuscular mycorrhizal (AM) fungi Glomus mosseae and G. intraradices were also studied. Very specific interactions were found, and also synergistic biocontrol effects suggesting that a consortium of microorganisms might be a better choice when applied as plant growth-promoters or biocontrol agents.