This study reports on the development of a rhizosphere model system for investigating rhizobacterial community dynamics. The genetic plant model Arabidopsis thaliana, a plant that lacks mycorrhizal associations, was used to effectively exclude the influence of plant-fungal symbioses on plant-bacterial communication at the root-soil interface. Using molecular methods to fingerprint bacterial communities based on differences in 16S rRNA gene regions (ribotypes), it was shown that natural variants (accessions) of Arabidopsis significantly alter soil bacterial communities in relation to control bulk soil. These changes were reproducible and accession-dependent. Root exudate composition was shown to be unique for all Arabidopsis accessions studied, providing a biochemical basis for rhizobacterial differences. To determine a causal link between these observations, the indolyl glucosinolates (IGs), a group of root-exuded phytochemicals, were investigated with the use of mutants altered in IG accumulations. Increased IG levels had little effect on rhizobacterial community composition and structure in relation to wild-type, while reduced levels produced a divergent community. It appears that these phytochemicals are involved in plant-controlled rhizobacterial selection. To determine the influence of external factors on this process, abiotic stress induced by a xenobiotic, the polycyclic aromatic hydrocarbon phenanthrene, was investigated. Phenanthrene-induced shifts in rhizobacteria were detected. Plant genotypic differences in rhizobacterial selection were not strongly evident, however, indicating that environmental factors can disrupt the accession-mediated selection of bacteria in the rhizosphere.

To determine whether heterogeneous rhizospheric conditions generated by different accession lines are due to natural variation and not a consequence of asynchronous growth among accessions, rhizobacterial succession of two accessions, Cvi-0 and Ler, was compared. Successional shifts in ribotypes present were seen to occur in response to plant developmental stage. Rhizobacterial communities diverged from bulk soil in a plant genotype-specific manner right from the onset of root emergence but appeared to converge during late succession as plants aged and rhizosphere influence dwindled. Finally, a description of the phylogenetic diversity of the Arabidopsis rhizosphere in the accession Cvi-0 is presented for the first time. The most prominent groups for the soil type used were α-Proteobacteria (27%), Acidobacteria (17%), Bacteroidetes (14%) and γ- and β-Proteobacteria (12% and 10%, respectively).