Abstract

Nine numerically dominant 2,4-dichlorophenocyacetic acid (2,4-D)-degrading bacteria were isolated from rice field soils. Most of the isolates were identified as Brukholderia of Sphingomonas species by fatty acid methyl ester (FAME) analysis, and they exhibited diverse chromosomal DNA patterns in polymerase chain reaction (PCR)amplification of repetitive extragenic palindromic (REP) sequences. The isolates utilized 2,4-D as the sole source of carbon and two Sphingomonas species were capable of mineralizing both 3-chlorobenzoate (3-CB) and 4-chlorobenzoate (4-CB), in addition to 2,4-D. Plasmid DNAs were detected from all of the isolates, and conjugation analysis revealed that 2,4-D degradative genes were located on transferable plasmids in most of the isolates. PCR analysis with specific primers selected from tfd genes showed that 67% of the isolates had DNA sequences homologous to the five tfd genes of the 2,4-D degradative plasmid pJP4 of Alcaligenes eutrophus JMP134. Among the isolates, strain TFD7 appeared to be a new genotype in that it contained a transmissible 2,4-D degradative plasmid nonhomologous to the tfd genes. 2,4-D was persistent in natural paddy soils which contained no indigenous 2,4-D-degrading microorganisms, but the application of the 2,4-D-degrading isolates resulted in rapid decline of the soil 2,4-D residues.

• Keywords: Biodegradation; 2;4-dichlorophenoxyacetic acid; paddy soil; bacteria