Plant species is considered to be one of the most important factors in shaping rhizobacterial communities, but specific plant–microbe interactions in the rhizosphere are still not fully understood. Arabidopsis thaliana, for which a large number of naturally occurring ecotype accessions exist, lacks mycorrhizal associations and is hence an ideal model for rhizobacterial studies. Eight Arabidopsis accessions were found to exert a marked selective influence on bacteria associated with their roots, as determined by terminal-restriction fragment length polymorphism (T-RFLP) and ribosomal intergenic spacer analysis (RISA). Community differences in species composition and relative abundance were both significant (P <0.001). The eight distinct and reproducible accession-dependent community profiles also differed from control bulk soil. Root exudates of these variants were analysed by high performance liquid chromatography (HPLC) to try to establish whether the unique rhizobacterial assemblages among accessions could be attributed to plant-regulated chemical changes in the rhizosphere. Natural variation in root exudation patterns was clearly exhibited, suggesting that differences in exudation patterns among accessions could be influencing bacterial assemblages. Other factors such as root system architecture are also probably involved. Finally, to investigate the Arabidopsis rhizosphere further, the phylogenetic diversity of rhizobacteria from accession Cvi-0 is described.
The rhizosphere is strongly influenced by plant-derived phytochemicals exuded by roots and plant species exert a major selective force for bacteria colonizing the root-soil interface. We have previously shown that rhizobacterial recruitment is tightly regulated by plant genetics, by showing that natural variants of Arabidopsis thaliana support genotype-specific rhizobacterial communities while also releasing a unique blend of exudates at six weeks post-germination. To further understand how exudate release is controlled by plants, changes in rhizobacterial assemblages of two Arabidopsis accessions, Cvi and Ler where monitored throughout the plants' life cycle. Denaturing gradient gel electrophoresis (DGGE) fingerprints revealed that bacterial communities respond to plant derived factors immediately upon germination in an accession-specific manner. Rhizobacterial succession progresses differently in the two accessions in a reproducible manner. However, as plants age, rhizobacterial and control bulk soil communities converge, indicative of an attenuated rhizosphere effect, which coincides with the expected slow down in the active release of root exudates as plants reach the end of their life cycle. These data strongly suggest that exudation changes during plant development are highly genotype-specific, possibly reflecting the unique, local co-evolutionary communication processes that developed between Arabidopsis accessions and their indigenous microbiota. IntroductionPlant roots are an active component of soil, creating the rhizosphere by physically changing soil characteristics as they elongate and branch out, and dramatically altering the chemical milieu through exudation of hundreds of phytochemicals and sloughedoff cells. 1,2 Root-derived compounds mediate a plethora of biological processes. The plant-bacterial relationships that occur at the root-soil interface are probably a combination of opportunistic interactions, as ubiquitous organisms exploit the nutrients present, and highly regulated communication events. The latter interactions are mediated by root-exuded signalling molecules that elicit specific responses. The type and amounts of the various exudate compounds synthesized and released by roots are under the plant's genetic control, making plant genetic make-up one of the most significant determinants of rhizobacterial selection. Even opportunistic use of root exudates as metabolic fuel will attract only those bacteria equipped to make use of the nutrients that are extant and at the same time capable of adapting to the rhizosphere environment. The unique exudate cocktails released by different plant species therefore signal to and attract a specific bacterial assemblage in a reproducible manner, explaining the well established role of plant species in rhizobacterial community selection. 3,4 Using various Arabidopsis thaliana accessions, we have previously shown that the plant genetic control of rhizobacterial recruitment is so tightly regulated that even genetic variants of the same plant species se...
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