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Indirect effects of polycyclic aromatic hydrocarbon contamination on microbial communities in legume and grass rhizospheres

Kawasaki, Akitomo, Watson, Erika R., Kertesz, Michael A.
Plant and soil 2012 v.358 no.1-2 pp. 169-182
denaturing gradient gel electrophoresis, ribosomal RNA, Trifolium repens, genes, roots, polluted soils, lentils, rhizosphere, Festuca rubra subsp. rubra, bacterial communities, Actinobacteria, Lolium, plant exudates, grasses, community structure, fungal communities, models, chickpeas, Trifolium pratense, Trifolium subterraneum, community development, root systems, phenanthrenes, microorganisms, biodegradation
BACKGROUND AND AIMS: Biodegradation of polycyclic aromatic hydrocarbons (PAHs) is accelerated in the presence of plants, due to the stimulation of rhizosphere microbes by plant exudates (nonspecific enhancement). However, plants may also recruit specific microbial groups in response to PAH stress (specific enhancement). In this study, plant effects on the development of rhizosphere microbial communities in heterogeneously contaminated soils were assessed for three grasses (ryegrass, red fescue and Yorkshire fog) and four legumes (white clover, chickpea, subterranean clover and red lentil). METHODS: Plants were cultivated using a split-root model with their roots divided between two independent pots containing either uncontaminated soil or PAH-contaminated soil (pyrene or phenanthrene). Microbial community development in the two halves of the rhizosphere was assessed by T-RFLP (bacterial and fungal community) or DGGE (bacterial community), and by 16S rRNA gene tag-pyrosequencing. RESULTS: In legume rhizospheres, the microbial community structure in the uncontaminated part of the split-root model was significantly influenced by the presence of PAH-contamination in the other part of the root system (indirect effect), but this effect was not seen for grasses. In the contaminated rhizospheres, Verrucomicrobia and Actinobacteria showed increased populations, and there was a dramatic increase in Denitratisoma numbers, suggesting that this genus may be important in rhizoremediation processes. CONCLUSION: Our results show that Trifolium and other legumes respond to PAH-contamination stress in a systemic manner, to influence the microbial diversity in their rhizospheres.