Characterization of chlortetracycline-induced glutathione S-transferase to conjugate chloroacetanilide and chlorotriazine herbicides

Glutathione S-transferases (GST) induced in maize by chlortetracycline (CTC) appear to inhibit the ability to detoxify chloroacetanilide herbicides, as determined by liquid chromatography/mass spectrometry. Furthermore, the same CTC-induced GSTs were able to conjugate chlorotriazine herbicides at a rate undistinguishable from GSTs isolated from untreated maize plants. This data suggests that CTC, while not toxic to maize, may have indirect effects on herbicide dtoxification in a class-specific manner. Chlortetracycline (CTC) is a commonly used antibiotic in animal husbandry. A majority of the antibiotic passes through the animal nonmetabolized and the manure is applied to crop fields as fertilizer where it has been shown to have phytotoxic tendencies in certain crops, as well as accumulate in others. Soil amended with CTC has been shown to induce expression of glutathione S-transferases (GST) in maize and subsequent analysis by liquid chromatography/ion-trap mass spectrometry (LC/IT-MS) identified stable CTC products conjugated with glutathione (GSH). Purified GSTs isolated from maize treated with CTC were able to produce nearly twice as much conjugated product relative to the GSTs isolated from nontreated control plants. Due to the prevalence of antibiotics in the environment, this work has raised concerns with regards to inducing herbicide resistance among target weeds or susceptibility among non-target crop plants. Herbicide conjugation by GSTs has been well characterized and is one of the main determinants of plant susceptibility. When GSTs isolated from maize control and CTC-treated plants were added to separate in vitro reactions containing three chloroacetanilide herbicides (metolachlor, propachlor, and alachlor) and one chlorotriazine herbicide (atrazine), the GSTs from the CTC-treated maize showed a reduced ability to conjugate the herbicides. In addition, analysis via LC/IT-MS has made it possible, for the first time, to detect chloroacetanilide herbicides that have been conjugated with two GSH molecules, in addition to the single GSH conjugate.