Resistance to acetyl CoA carboxylase-inhibiting herbicides in Lolium multiflorum

A Lolium multiflorum Lam. biotype resistant to diclofop-methyl was found in a Triticum aestivum field in France (Normandy) that had been treated for s everal years with diclofop-methyl. Based on plant survival evaluated 21 d a fter herbicide application, the biotype exhibited a high level of resistanc e to diclofop-methyl and moderate resistance to CGA-184927-propargil and PP -604. The resistant biotype exhibited a small increase in tolerance to halo xyfop-methyl, quizalofop-ethyl, sethoxydim, and BAS-517-H, but was controll ed by recommended field races for these herbicides. The mechanism of resist ance was investigated for diclofop-methyl. There was little or no differenc e in diclofop-methyl absorption by leaves of resistant and susceptible biot ypes measured 48 h after treatment. For both biotypes, less than 1% of abso rbed radiolabel was translocated during 48 h following foliar application o f C-14-diclofopmethyl. Metabolism of diclofop-methyl was not significantly altered in the resistant biotype. In both biotypes, diclofop-methyl was rap idly metabolized to diclofop acid followed by a slow rate of formation of a polar conjugate. Two multifunctional acetyl coenzyme A carboxylase isoform s (ACCase I and ACCase II) were isolated from leaf tissue of resistant and susceptible biotypes. Both isoforms exhibited a subunit molecular mass of 2 03 kDa. For both resistant and susceptible biotypes, ACCase I constituted a pproximately 80% of total ACCase activity. Graminicide concentrations requi red to inhibit ACCase activity by 50% (I-50 values) were determined for bot h ACCase isoforms from resistant and susceptible biotypes. The ACCase II is oform was highly resistant to graminicides in both biotypes. In contrast, t he I-50 value for diclofop inhibition of ACCase I was 19-fold greater for t he enzyme isolated from the resistant biotype compared with the susceptible biotype. It is concluded that diclofop resistance in the L. multiflorum bi otype from Normandy is caused by the presence of a resistant form of the AC Case I isoform.