Degradation of the herbicide dicamba by two sphingomonads via different O-demethylation mechanisms

Abstract

Dicamba, a chlorinated benzoic acid herbicide, is widely used to control broadleaf weeds. In this study, two dicamba-degrading sphingomonads Sphingobium sp. Ndbn-10 and Sphingomonas sp. Ndbn-20 were isolated from activated sludge and compost samples, respectively. Both isolates could completely degrade and utilize dicamba, but strain Ndbn-10 possessed relatively higher dicamba degradation efficiency than strain Ndbn-20. The degradation of dicamba in both isolates was initiated by O-demethylation to generate 3,6-dichlorosalicylic acid (DCSA) but through different mechanisms. For Sphingobium sp. Ndbn-10, O-demethylation is catalyzed by a NADH-dependent dicamba monooxygenase (DMO), whereas in Sphingomonas sp. Ndbn-20, a tetrahydrofolic acid (THF)-dependent O-demethylase is responsible for the O-demethylation. Our results suggest that sphingomonads can rapidly and efficiently adapt to degrading dicamba through the evolution of different metabolic pathway mechanisms and highlight a novel THF-dependent dicamba O-demethylation mechanism in bacteria.