Mechanism of fenoxaprop resistance in an accession of smooth crabgrass (Digitaria ischaemum)

An accession of smooth crabgrass [Digitaria ischaemum (Schreb.) Muhl] exhib iting resistance to the herbicide fenoxaprop was discovered recently in New Jersey. This accession was highly resistant to fenoxaprop-ethyl (approxima te R/S GR(50) ratio of 102) and moderately resistant to quizalofop-ethyl (a pproximate R/S GR(50) ratio of 16.3), but exhibited low resistance to cyclo hexanediones such as sethoxydim (R/S GR(50) ratio of 1.3). The potential me chanism of resistance was investigated by evaluating the effect of fenoxapr op on acetyl-CoA carboxylase (EC 6.4.1.2, ACCase) activity extracted from s hoots of resistant and susceptible plants and comparing the absorption, tra nslocation, and metabolism of radiolabeled fenoxaprop in resistant and susc eptible plants. The patterns of absorption, translocation, and metabolism o f fenoxaprop were similar in resistant and susceptible smooth crabgrass. AC Case activity from susceptible plants was very sensitive to fenoxaprop-ethy l and fenoxaprop acid with I-50 values of 2 and 4.9 mu M, respectively. ACC ase activity from resistant plants was very resistant to fenoxaprop-ethyl ( I-50 > 182 mu M) and moderately resistant to fenoxaprop acid (I-50, 29 mu M ). ACCase activity from resistant smooth crabgrass was 50-fold less sensiti ve to quizalofop-ethyl than that extracted from susceptible smooth crabgras s. ACCase activity extracted from either resistant or susceptible plants wa s inhibited strongly by sethoxydim (I-50 of 7.4 mu M in R and 3.4 mu M in S ). These results suggest that a less sensitive form of the target enzyme, a cetyl-CoA carboxylase, confers a high degree of resistance to smooth crabgr ass toward fenoxaprop and moderate resistance to other aryloxyphenoxypropio nate herbicides. (C)1999 Academic Press.