Glutathione transferases in herbicide-resistant and herbicide-susceptible black-grass (Alopecurus myosuroides )

Glutathione transferase (GST) activities toward the selective herbicide fenoxaprop-ethyl together with thiol contents, have been compared in seedlings of wheat (Triticum aestivum) and two populations of black-grass (Alopecurus myosuroides) which are resistant to a range of herbicides (Peldon and Lines El), and a black-grass population which is susceptible to herbicides (Rothamsted). GST activities toward the non-cereal herbicides metolachlor and fluorodifen were also determined. On the basis of enzyme specific activity, GST activities toward fenoxaprop-ethyl in the leaves were in the order wheat > Peldon = Lines El > Rothamsted, while with fluorodifen and metolachlor the order was Peldon = Lines El > Rothamsted > wheat. Using an antibody raised to the major GST from wheat, which is composed of 25-kDa subunits, it was shown that the enhanced GST activities in both Peldon and Lines El correlated with an increased expression of a 25-kDa polypeptide and the appearance of novel 27-kDa and 28-kDa polypeptides. Leaves of both wheat and black-grass contained glutathione and hydroxymethylglutathione, with the concentrations of glutathione being in the order Peldon > Lines El = Rothamsted = wheat. However, in glasshouse dose-response assays, the Lines El population showed much greater resistance to fenoxaprop-ethyl than Peldon. We conclude that high GST activities and the availability of glutathione may contribute partially to the relative tolerance of black-grass to herbicides detoxified by glutathione conjugation. Although herbicide-resistant populations show enhanced GST expression, in the case of fenoxaprop-ethyl the associated increased detoxifying activities alone cannot explain the differences between populations in the degree of resistance seen at the whole plant level.