Selection of specific endophytic bacterial genotypes by plants in responseto soil contamination

Plant-bacterial combinations can increase contaminant degradation in the rh izosphere, but the role played by indigenous root-associated bacteria durin g plant growth in contaminated soils is unclear, The purpose of this study was to determine if plants had the ability to selectively enhance the preva lence of endophytes containing pollutant catabolic genes in unrelated envir onments contaminated with different pollutants, At petroleum hydrocarbon co ntaminated sites, two genes encoding hydrocarbon degradation, alkane monoox ygenase (alkB) and naphthalene dioxygenase (ndoB), were two and four times more prevalent in bacteria extracted from the root interior (endophytic) th an from the bulk soil and sediment, respectively. In field sites contaminat ed with nitroaromatics, two genes encoding nitrotoluene degradation, 2-nitr otoluene reductase (ntdAa) and nitrotoluene monooxygenase (ntnM), were 7 to 14 times more prevalent in endophytic bacteria. The addition of petroleum to sediment doubled the prevalence of ndoB-positive endophytes in Scirpus p ungens, indicating that the numbers of endophytes containing catabolic geno types were dependent on the presence and concentration of contaminants, Sim ilarly, the numbers of alkB- or ndoB-positive endophytes in Festuca arundin acea were correlated with the concentration of creosote in the soil but not with the numbers of alkB- or ndoB-positive bacteria in the bulk soil. Our results indicate that the enrichment of catabolic genotypes in the root int erior is both plant and contaminant dependent.