Mechanism of Fenoxaprop Resistance in an Accession of Smooth Crabgrass (Digitaria ischaemum)

An accession of smooth crabgrass [Digitaria ischaemum (Schreb.) Muhl] exhibiting resistance to the herbicide fenoxaprop was discovered recently in New Jersey. This accession was highly resistant to fenoxaprop-ethyl (approximate R/S GR50 ratio of 102) and moderately resistant to quizalofop-ethyl (approximate R/S GR50 ratio of 16.3), but exhibited low resistance to cyclohexanediones such as sethoxydim (R/S GR50 ratio of 1.3). The potential mechanism of resistance was investigated by evaluating the effect of fenoxaprop on acetylCoA carboxylase (EC 6.4.1.2, ACCase) activity extracted from shoots of resistant and susceptible plants and comparing the absorption, translocation, and metabolism of radiolabeled fenoxaprop in resistant and susceptible plants. The patterns of absorption, translocation, and metabolism of fenoxaprop were similar in resistant and susceptible smooth crabgrass. ACCase activity from susceptible plants was very sensitive to fenoxaprop-ethyl and fenoxaprop acid with I50 values of 2 and 4.9 muM, respectively. ACCase activity from resistant plants was very resistant to fenoxaprop-ethyl (I50 > 182 muM) and moderately resistant to fenoxaprop acid (I50, 29 muM). ACCase activity from resistant smooth crabgrass was 50-fold less sensitive to quizalofop-ethyl than that extracted from susceptible smooth crabgrass. ACCase activity extracted from either resistant or susceptible plants was inhibited strongly by sethoxydim (I50 of 7.4 muM in R and 3.4 muM in S). These results suggest that a less sensitive form of the target enzyme, acetyl-CoA carboxylase, confers a high degree of resistance to smooth crabgrass toward fenoxaprop and moderate resistance to other aryloxyphenoxypropionate herbicides.