Effect of elevated tropospheric ozone on the structure of bacterial communities inhabiting the rhizosphere of herbaceous plants native to Germany.

Current elevated concentrations of ozone in the atmosphere, as they are observed during summer seasons, can cause severe effects on plant vegetation. This study was initiated to analyze whether ozone-stressed plants also transfer signals below ground and thereby alter the bacterial community composition in their rhizospheres. Herbaceous plants, native to Germany, with tolerance (Anthoxanthum odoratum, Achillea millefolium, Poa pratensis, Rumex acetosa, and Veronica chamaedrys) and sensitivity (Matricaria chamomilla, Sonchus asper, and Tanacetum vulgare) to ozone, raised in the greenhouse, were exposed in open-top chambers to two different ozone regimes, i.e., "summer stress" and a normal ozone background. DNA of bacterial cells from the rhizospheres was directly extracted, and partial sequences of the 16S rRNA genes were PCR amplified with primers targeting the following phylogenetic groups: Bacteria, alpha-Proteobacteria, Actinobacteria, and Pseudomonas, respectively. The diversity of the amplified products was analyzed by genetic profiling based on single-strand conformation polymorphism (SSCP). Neither the tolerant nor the sensitive plants, the latter with visible above-ground damage, showed ozone-induced differences in any of the SSCP profiles, with the single exception of Actinobacteria-targeted profiles from S. asper. To increase the stress, S. asper was germinated and raised in the continuous presence of an elevated level of ozone. SSCP profiles with Bacteria-specific primers combined with gene probe hybridizations indicated an ozone-related increase in a Xanthomonas-related 16S rRNA gene and a decrease in the respective gene from the plant plastids. The fact that only this latter unrealistic scenario caused a detectable effect demonstrated that ozone stress has a surprisingly small effect on the structural diversity of the bacterial community in rhizospheres.