GLUTATHIONE TRANSFERASES IN HERBICIDE-RESISTANT AND HERBICIDE-SUSCEPTIBLE BLACK-GRASS (ALOPECURUS-MYOSUROIDES)

Glutathione transferase (GST) activities toward the selective herbicid e fenoxaprop-ethyl together with thiol contents, have been compared in seedlings of wheat (Triticum aestivum) and two populations of black-g rass (Alopecurus myosuroides) which are resistant to a range of herbic ides (Peldon and Lines El), and a black-grass population which is susc eptible to herbicides (Rothamsted). GST activities toward the non-cere al herbicides metolachlor and fluorodifen were also determined. On the basis of enzyme specific activity, GST activities toward fenoxaprop-e thyl in the leaves were in the order wheat > Peldon = Lines El > Rotha msted, while with fluorodifen and metolachlor the order was Peldon = L ines El > Rothamsted > wheat. Using an antibody raised to the major GS T from wheat, which is composed of 25-kDa subunits, it was shown that the enhanced GST activities in both Peldon and Lines El correlated wit h an increased expression of a 25-kDa polypeptide and the appearance o f novel 27-kDa and 28-kDa polypeptides. Leaves of both wheat and black -grass contained glutathione and hydroxymethylglutathione, with the co ncentrations of glutathione being in the order Peldon > Lines El = Rot hamsted = wheat. However, in glasshouse dose-response assays, the Line s El population showed much greater resistance to fenoxaprop-ethyl tha n Peldon. We conclude that high GST activities and the availability of glutathione may contribute partially to the relative tolerance of bla ck-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 activiti es alone cannot explain the differences between populations in the deg ree of resistance seen at the whole plant level.