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b'The aim of this PhD thesis was to examine the endophytic colonization behavior of Azospirillum brasilense and Herbaspirillum sp. N3 on wheat roots. The application of the FISH method using species specific phylogenetic oligonucleotide probes and GFP tagging facilitated the detection of a differential colonization behavior by the A. brasilense strains Sp7 and Sp245 on three different wheat varieties (Triticum aestivum). For this purpose a confocal laser scanning microscope (CLSM) was used, which enabled three-dimesional analysis of bacterial colonization of the root. Especially GFP tagged strains were well suited for this application, as there was no pretreatment or sectioning of the root sample necessary. Strain Sp7 was only located on the root surface of all wheat cultivars, whereas strain Sp245 was also found inter- and intracellulary in the outer root cortex layers. There was no recognizable connection between the growth stimulating effect of the inoculum (PGPR-effect) and the localization of the bacteria. The most pronounced PGPR-effect could be observed with the Brazilian wheat cultivar, which seemed to gain greatest benefit of its partnership with A. brasilense due to a certain adaptation to the inoculum. \\nAs the production of the auxin IAA (indole-3-acetic acid) plays a major role in stimulating plant growth, the expression of the key gene ipdC (indole-3-pyruvate decarboxylase) was examined. For this, several methods were tested to generate a fusion of the ipdC promoter with a gfp or rfp reporter gene. Constructing a translational promoter fusion with the gfp variant mut3 on plasmid level made expression analyses possible. With this method the promoter region of strain Sp7 located directly upstream of the ipdC start codon was found to differ only in a few bases from strain Sp245. But further upstream a region of about 150 bases was identified in strain Sp245, which was missing in strain Sp7. For Sp245 two different fusions were constructed, which contained the Sp245 promoter region homologous to strain Sp7 and the whole promoter region of Sp245, respectively. With these constructs the importance of the promoter region only present in strain Sp245 for control and intensity of ipdC expression in A. brasilense Sp245 could be demonstrated. Additionally an induction of the corresponding ipdC promoter fusions of Sp7 and Sp245 was achieved when adding phenylalanine or tyrosine. Total promoter activity was higher in strain Sp7 than in strain Sp245, and ipdC expression appeared to be subject to a stricter control in strain Sp245. These results were confirmed, when the strains containing the promoter fusions were used as reporters for ipdC expression on wheat roots. A demonstration of the induction of the ipdC promoter by root exudates in situ was possible.\\nFinally, isolate N3 from surface sterilized wheat roots was characterized in detail. According to the 16S rDNA sequence data the isolate was phylogenetically allocated to the genus Herbaspirillum. But a subsequent DNS-DNS hybridization ruled out, that the strain belonged to any of the known Herbaspirillum species. Thus, the isolate, which might represent a new species, was named Herbaspirillum sp. N3. A specific, 16S rRNA targeted probe was constructed, which facilitated the examination of wheat roots colonization by this bacteria using FISH. Additionally the strain was GFP tagged to enable the detection in uncut root material. By this, an unequivocal demonstration of the endophytic colonization by Herbaspirillum sp. N3 mainly within the intercellular spaces was possible.'