The bacterial and fungal rhizosphere communities of strawberry (Fragaria ananassa Duch.) and oilseed rape (Brassica napus L.) were analysed using molecular fingerprints. We aimed to determine to what extent the structure of different microbial groups in the rhizosphere is influenced by plant species and sampling site. Total community DNA was extracted from bulk and rhizosphere soil taken from three sites in Germany in two consecutive years. Bacterial, fungal and group-specific (Alphaproteobacteria, Betaproteobacteria and Actinobacteria) primers were used to PCR-amplify 16S rRNA and 18S rRNA gene fragments from community DNA prior to denaturing gradient gel electrophoresis (DGGE) analysis. Bacterial fingerprints of soil DNA revealed a high number of equally abundant faint bands, while rhizosphere fingerprints displayed a higher proportion of dominant bands and reduced richness, suggesting selection of bacterial populations in this environment. Plant specificity was detected in the rhizosphere by bacterial and group-specific DGGE profiles. Different bulk soil community fingerprints were revealed for each sampling site. The plant species was a determinant factor in shaping similar actinobacterial communities in the strawberry rhizosphere from different sites in both years. Higher heterogeneity of DGGE profiles within soil and rhizosphere replicates was observed for the fungi. Plant-specific composition of fungal communities in the rhizosphere could also be detected, but not in all cases. Cloning and sequencing of 16S rRNA gene fragments obtained from dominant DGGE bands detected in the bacterial profiles of the Rostock site revealed that Streptomyces sp. and Rhizobium sp. were among the dominant ribotypes in the strawberry rhizosphere, while sequences from Arthrobacter sp. corresponded to dominant bands from oilseed rape bacterial fingerprints.

Rhizosphere bacterial communities of two transgenic potato lines which produce T4 lysozyme for protection against bacterial infections were analyzed in comparison to communities of wild-type plants and transgenic controls not harboring the lysozyme gene. Rhizosphere samples were taken from young, flowering, and senescent plants at two field sites in three consecutive years. The communities were characterized in a polyphasic approach. Cultivation-dependent methods included heterotrophic plate counts, determination of species composition and diversity based on fatty acid analysis of isolates, and community level catabolic profiling. Cultivation-independent analyses were based on denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments amplified from rhizosphere DNA using primers specific for Bacteria, Actinomycetales, or ␣-or ␤-Proteobacteria. Several bands of the DGGE patterns were further characterized by sequence analysis. All methods revealed that environmental factors related to season, field site, or year but not to the T4 lysozyme expression of the transgenic plants influenced the rhizosphere communities. For one of the T4 lysozymeproducing cultivars, no deviation in the rhizosphere communities compared to the control lines was observed. For the other, differences were detected at some of the samplings between the rhizosphere community structure and those of one or all other cultivars which were not attributable to T4 lysozyme production but most likely to differences observed in the growth characteristics of this cultivar.

Fungi with antagonistic activity toward plant pathogens play an essential role in plant growth and health.To analyze the effects of the plant species and the site on the abundance and composition of fungi with antagonistic activity toward Verticillium dahliae, fungi were isolated from oilseed rape and strawberry rhizosphere and bulk soil from three different locations in Germany over two growing seasons. A total of 4,320 microfungi screened for in vitro antagonism toward Verticillium resulted in 911 active isolates. This high proportion of fungi antagonistic toward the pathogen V. dahliae was found for bulk and rhizosphere soil at all sites. A plant-and site-dependent specificity of the composition of antagonistic morphotypes and their genotypic diversity was found. The strawberry rhizosphere was characterized by preferential occurrence of Penicillium and Paecilomyces isolates and low numbers of morphotypes (n ‫؍‬ 31) and species (n ‫؍‬ 13), while Monographella isolates were most frequently obtained from the rhizosphere of oilseed rape, for which higher numbers of morphotypes (n ‫؍‬ 41) and species (n ‫؍‬ 17) were found. Trichoderma strains displayed high diversity in all soils, but a high degree of plant specificity was shown by BOX-PCR fingerprints. The diversity of rhizosphere-associated antagonists was lower than that of antagonists in bulk soil, suggesting that some fungi were specifically enriched in each rhizosphere. A broad spectrum of new Verticillium antagonists was identified, and the implications of the data for biocontrol applications are discussed.

Rhizobacteria with antagonistic activity towards plant pathogens play an essential role in root growth and plant health and are influenced by plant species in their abundance and composition. To determine the extent of the effect of the plant species and of the site on the abundance and composition of bacteria with antagonistic activity towards Verticillium dahliae, bacteria isolated from the rhizosphere of two Verticillium host plants, oilseed rape and strawberry, and from bulk soil were analysed at three different locations in Germany over two growing seasons. A total of 6732 bacterial isolates screened for in vitro antagonism towards Verticillium resulted in 560 active isolates, among which Pseudomonas (77%) and Serratia (6%) were the most dominant genera. The rhizosphere effect on the antagonistic bacterial community was shown by an enhanced proportion of antagonistic isolates, by enrichment of specific amplified ribosomal DNA restriction analysis types, species and genotypes, and by a reduced diversity in the rhizosphere in comparison to bulk soil. Such an effect was influenced by the plant species and by the site of its cultivation. Altogether, 16S rRNA gene sequencing of 66 isolates resulted in the identification of 22 different species. Antagonists of the genus Serratia were preferentially isolated from oilseed rape rhizosphere, with the exception of one site. For isolates of Pseudomonas and Serratia, plant-specific and site-specific genotypes were found.