DNA Probe Analysis of Microbial Community Selected in Field by Long‐Term 2,4‐D Application

Abstract

AbstractThe 2,4‐D (2,4‐dichlorophenoxyacetic acid)‐degrading microbial populations selected by 2,4‐D amendments to soil were analyzed by DNA probes during a 4‐yr field study. Two were control subplots not treated with 2,4‐D, and the other six were treated with 2,4‐D in replicates at three different concentrations. Total soil bacterial DNA was regularly extracted from each subplot soil and analyzed on slot and Southern blots using the gene probes tfdA and Spa that detect two different 2,4‐D‐degrading populations. Slot blot analyses revealed that the hybridization signal of the probes to total soil bacterial DNA substantially increased with time and continued 2,4‐D application, indicating a shift from a more diverse 2,4‐D‐degrading microflora to populations that shared greater homology to the two probes. More hybridization signal was obtained with the Spa probe, suggesting that microorganisms with this gene sequence were more dominant than degraders with tfdA‐like genes. The Southern blot band patterns indicated that the dominant Spa‐hybridizing strain was Sphingomonas paucimobilis strain 1443 and the dominant tfdA hybridizing strain was Burkholderia (formerly Pseudomonas) pickettii strain 712, similar to the results of a previous laboratory microcosm study. At the highest 2,4‐D application rate (100 mg kg−1), Southern blots probed by 16S rRNA genes showed a limited number of bands, suggesting that the total soil microbial community had been shifted to one or two dominant strains, presumably 2,4‐D degraders.