Enhanced degradation of phenoxyacetic acid in soil by horizontal transfer of the tfdA gene encoding a 2,4-dichlorophenoxyacetic acid dioxygenase

Few studies have investigated the possible impact of in situ gene transfer on the degradation of xenobiotic compounds in natural environments. In this work we showed that horizontal transfer of the tfdA gene, carried on plasmid pRO103, to phenol degrading recipient strains significantly increased the degradation rate of phenoxyacetic acid in sterile and non-sterile soil microcosms. The tfdA gene encodes a 2,4-dichlorophenoxyacetic acid/2-oxoglutarate dioxygenase and by complementation with the phenol degradation pathway an expanded catabolic substrate range, now including phenoxyacetic acid, is evolved. Presence of selective pressure had a positive effect on the emergence of transconjugants. However, even in the absence of phenoxyacetic acid transconjugant populations were detected and were kept at a constant level throughout the experimental period. The residuesphere (interface between decaying plant material and soil matrix) of dry leaves of barley was shown to be a hot-spot for gene transfer and presence of barley straw increased the conjugation frequencies in soil microcosms to the same extent as presence of organic nutrients. The results of this study indicate that dissemination of catabolic plasmids is a possible mechanism of genetic adaptation to degradation of xenobiotic compounds in natural environments, and that complementation of catabolic pathways possibly plays an important role in the evolution of new degradative capabilities. The application of horizontal gene transfer as a possible tool in bioremediation of contaminated sites is discussed.